Eversource Energy (ES) 10-K 25 Feb 15 2014 FY Annual report Financial data

DOCUMENT_AND_ENTITY_INFORMATIO

DOCUMENT AND ENTITY INFORMATION (USD $)	12 Months Ended
	Dec. 31, 2014	Jan. 31, 2015	Jun. 30, 2014
Document And Entity [Line Items]
Entity Registrant Name	NORTHEAST UTILITIES
Document Period End Date	31-Dec-14
Document Type	10-K
Entity Central Index Key	72741
Current Fiscal Year End Date	-19
Entity Common Stock, Shares Outstanding		317,203,765
Entity Public Float			$14,947,688,864
Entity Filer Category	Large Accelerated Filer
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	Yes
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Amendment Flag	FALSE
The Connecticut Light And Power Company [Member]
Document And Entity [Line Items]
Entity Registrant Name	The Connecticut Light and Power Company
Entity Central Index Key	23426
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Public Service Company Of New Hampshire [Member]
Document And Entity [Line Items]
Entity Registrant Name	Public Service Company of New Hampshire
Entity Central Index Key	315256
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Western Massachusetts Electric Company [Member]
Document And Entity [Line Items]
Entity Registrant Name	Western Massachusetts Electric Company
Entity Central Index Key	106170
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
NSTAR Electric Company [Member]
Document And Entity [Line Items]
Entity Registrant Name	NSTAR Electric Company
Entity Central Index Key	13372
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current Assets:
Cash	$38,703	$43,364
Receivables, Net	856,346	765,391
Unbilled Revenues	211,758	224,982
Taxes Receivable	337,307	16,629
Fuel, Materials and Supplies	349,664	303,233
Regulatory Assets Current	672,493	535,791
Prepayments and Other Current Assets	226,194	197,659
Total Current Assets	2,692,465	2,087,049
Property, Plant and Equipment, Net	18,647,041	17,576,186
Deferred Debits and Other Assets:
Regulatory Assets Long Term	4,054,086	3,758,694
Goodwill	3,519,401	3,519,401
Marketable Securities	515,025	488,515
Other Long-Term Assets	349,957	365,692
Total Deferred Debits and Other Assets	8,438,469	8,132,302
Total Assets	29,777,975	27,795,537
Current Liabilities:
Notes Payable	956,825	1,093,000
Long-Term Debt - Current Portion	245,583	533,346
Accounts Payable - Current	868,231	742,251
Regulatory Liability Current	235,022	204,278
Other Liabilities - Current	828,720	702,776
Total Current Liabilities	3,134,381	3,275,651
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	4,467,473	4,029,026
Regulatory Liabilities Long-Term	515,144	502,984
Long-Term Derivative Liabilities	409,632	624,050
Accrued Pension, PBOP and SERP	1,638,558	896,844
Other Long-Term Liabilities	874,387	923,053
Total Deferred Credits and Other Liabilities	7,905,194	6,975,957
Capitalization:
Long-Term Debt	8,606,017	7,776,833
Noncontrolling Interest in Consolidated Subsidiary:
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	155,568	155,568
Common Shareholders' Equity:
Common Stock	1,666,796	1,665,351
Capital Surplus, Paid In	6,235,834	6,192,765
Retained Earnings	2,448,661	2,125,980
Accumulated Other Comprehensive Loss	-74,009	-46,031
Treasury Stock	-300,467	-326,537
Common Shareholders' Equity	9,976,815	9,611,528
Total Capitalization	18,738,400	17,543,929
Total Liabilities and Capitalization	29,777,975	27,795,537
The Connecticut Light And Power Company [Member]
Current Assets:
Cash	2,356	7,237
Receivables, Net	355,140	319,670
Accounts Receivable from Affiliated Companies	16,757	13,777
Unbilled Revenues	102,137	92,401
Taxes Receivable	116,148	20,041
Regulatory Assets Current	220,344	150,943
Materials and Supplies	46,664	54,606
Prepayments and Other Current Assets	37,822	33,041
Total Current Assets	897,368	691,716
Property, Plant and Equipment, Net	6,809,664	6,451,259
Deferred Debits and Other Assets:
Regulatory Assets Long Term	1,475,508	1,663,147
Other Long-Term Assets	177,568	174,380
Total Deferred Debits and Other Assets	1,653,076	1,837,527
Total Assets	9,360,108	8,980,502
Current Liabilities:
Notes Payable To Affiliated Companies	133,400	287,300
Long-Term Debt - Current Portion	162,000	150,000
Accounts Payable - Current	272,971	201,047
Accounts Payable to Affiliated Companies	65,594	56,531
Obligations to Third Party Suppliers	73,624	73,914
Regulatory Liability Current	124,722	93,961
Derivative Liabilities - Current	88,459	92,233
Other Liabilities - Current	153,420	134,716
Total Current Liabilities	1,074,190	1,089,702
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	1,642,805	1,510,586
Regulatory Liabilities Long-Term	81,298	93,757
Long-Term Derivative Liabilities	406,199	617,072
Accrued Pension, PBOP and SERP	273,854	95,895
Other Long-Term Liabilities	148,844	163,588
Total Deferred Credits and Other Liabilities	2,553,000	2,480,898
Capitalization:
Long-Term Debt	2,679,951	2,591,208
Noncontrolling Interest in Consolidated Subsidiary:
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	116,200	116,200
Common Shareholders' Equity:
Common Stock	60,352	60,352
Capital Surplus, Paid In	1,804,869	1,682,047
Retained Earnings	1,072,477	961,482
Accumulated Other Comprehensive Loss	-931	-1,387
Common Shareholders' Equity	2,936,767	2,702,494
Total Capitalization	5,732,918	5,409,902
Total Liabilities and Capitalization	9,360,108	8,980,502
NSTAR Electric Company [Member]
Current Assets:
Cash	12,773	8,021
Receivables, Net	234,481	209,711
Accounts Receivable from Affiliated Companies	40,353	27,264
Unbilled Revenues	29,741	41,368
Taxes Receivable	144,601	25,590
Regulatory Assets Current	198,710	204,144
Materials and Supplies	74,179	44,236
Prepayments and Other Current Assets	10,815	11,120
Total Current Assets	745,653	571,454
Property, Plant and Equipment, Net	5,335,436	5,043,887
Deferred Debits and Other Assets:
Regulatory Assets Long Term	1,179,100	1,235,156
Other Long-Term Assets	73,051	60,624
Total Deferred Debits and Other Assets	1,252,151	1,295,780
Total Assets	7,333,240	6,911,121
Current Liabilities:
Notes Payable	302,000	103,500
Long-Term Debt - Current Portion	4,700	301,650
Accounts Payable - Current	217,311	202,100
Accounts Payable to Affiliated Companies	63,517	75,707
Deferred Tax Liabilities Current	55,136	50,128
Regulatory Liability Current	49,611	53,958
Other Liabilities - Current	186,513	123,869
Total Current Liabilities	878,788	910,912
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	1,527,667	1,466,835
Regulatory Liabilities Long-Term	262,738	253,108
Accrued Pension, PBOP and SERP	235,529	118,010
Other Long-Term Liabilities	129,279	206,386
Total Deferred Credits and Other Liabilities	2,155,213	2,044,339
Capitalization:
Long-Term Debt	1,792,712	1,499,417
Noncontrolling Interest in Consolidated Subsidiary:
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	43,000	43,000
Common Shareholders' Equity:
Common Stock	0	0
Capital Surplus, Paid In	994,130	992,625
Retained Earnings	1,468,955	1,420,828
Accumulated Other Comprehensive Loss	442
Common Shareholders' Equity	2,463,527	2,413,453
Total Capitalization	4,299,239	3,955,870
Total Liabilities and Capitalization	7,333,240	6,911,121
Public Service Company Of New Hampshire [Member]
Current Assets:
Cash	489	130
Receivables, Net	80,151	76,331
Accounts Receivable from Affiliated Companies	3,194	90
Unbilled Revenues	40,181	38,344
Fuel, Materials and Supplies	148,139	128,736
Regulatory Assets Current	111,705	92,194
Prepayments and Other Current Assets	42,392	24,100
Total Current Assets	426,251	359,925
Property, Plant and Equipment, Net	2,635,844	2,467,556
Deferred Debits and Other Assets:
Regulatory Assets Long Term	293,115	219,346
Other Long-Term Assets	39,228	39,891
Total Deferred Debits and Other Assets	332,343	259,237
Total Assets	3,394,438	3,086,718
Current Liabilities:
Notes Payable To Affiliated Companies	90,500	86,500
Long-Term Debt - Current Portion	0	50,000
Accounts Payable - Current	93,349	82,920
Accounts Payable to Affiliated Companies	33,734	22,040
Deferred Tax Liabilities Current	36,164	28,596
Regulatory Liability Current	16,044	20,643
Other Liabilities - Current	38,969	51,729
Total Current Liabilities	308,760	342,428
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	587,292	500,166
Regulatory Liabilities Long-Term	51,372	51,723
Accrued Pension, PBOP and SERP	93,243	15,272
Other Long-Term Liabilities	50,155	46,247
Total Deferred Credits and Other Liabilities	782,062	613,408
Capitalization:
Long-Term Debt	1,076,286	999,006
Common Shareholders' Equity:
Common Stock	0	0
Capital Surplus, Paid In	748,240	701,911
Retained Earnings	486,459	438,515
Accumulated Other Comprehensive Loss	-7,369	-8,550
Common Shareholders' Equity	1,227,330	1,131,876
Total Capitalization	2,303,616	2,130,882
Total Liabilities and Capitalization	3,394,438	3,086,718
Western Massachusetts Electric Company [Member]
Current Assets:
Receivables, Net	51,066	49,018
Accounts Receivable from Affiliated Companies	7,851	47,607
Unbilled Revenues	15,146	16,562
Taxes Receivable	18,126	432
Regulatory Assets Current	51,923	43,024
Marketable Securities - Current	28,658	26,628
Prepayments and Other Current Assets	7,607	10,479
Total Current Assets	180,377	193,750
Property, Plant and Equipment, Net	1,461,321	1,381,060
Deferred Debits and Other Assets:
Regulatory Assets Long Term	146,307	146,088
Marketable Securities	29,452	31,243
Other Long-Term Assets	22,018	40,679
Total Deferred Debits and Other Assets	197,777	218,010
Total Assets	1,839,475	1,792,820
Current Liabilities:
Notes Payable To Affiliated Companies	21,400	0
Long-Term Debt - Current Portion	50,000	0
Accounts Payable - Current	53,732	62,961
Accounts Payable to Affiliated Companies	14,328	9,230
Accrued Interest	7,526	7,525
Deferred Tax Liabilities Current	18,089	13,098
Regulatory Liability Current	22,486	19,858
Counterparty Deposit	3,376	7,688
Other Liabilities - Current	13,178	20,629
Total Current Liabilities	204,115	140,989
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	416,822	396,933
Regulatory Liabilities Long-Term	10,835	13,873
Accrued Pension, PBOP and SERP	17,705	3,911
Other Long-Term Liabilities	33,747	28,619
Total Deferred Credits and Other Liabilities	479,109	443,336
Capitalization:
Long-Term Debt	578,471	629,389
Common Shareholders' Equity:
Common Stock	10,866	10,866
Capital Surplus, Paid In	391,256	390,743
Retained Earnings	178,834	181,014
Accumulated Other Comprehensive Loss	-3,176	-3,517
Common Shareholders' Equity	577,780	579,106
Total Capitalization	1,156,251	1,208,495
Total Liabilities and Capitalization	$1,839,475	$1,792,820

CONSOLIDATED_STATEMENTS_OF_INC

CONSOLIDATED STATEMENTS OF INCOME (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Revenues	$7,741,856	$7,301,204	$6,273,787
Operating Expenses:
Purchased Power and Transmission	3,021,550	2,482,954	2,084,364
Operations and Maintenance	1,427,589	1,514,986	1,583,070
Depreciation	614,657	610,777	519,010
Amortization of Regulatory Assets/(Liabilities), Net	10,704	206,322	79,762
Amortization of Rate Reduction Bonds	0	42,581	142,019
Energy Efficiency Programs	473,127	401,919	313,149
Taxes Other Than Income Taxes	561,380	512,230	434,207
Total Operating Expenses	6,109,007	5,771,769	5,155,581
Operating Income	1,632,849	1,529,435	1,118,206
Interest Expense:
Interest on Long-Term Debt	345,001	340,970	316,987
Interest on Rate Reduction Bonds	0	422	6,168
Other Interest	17,105	-2,693	6,790
Total Interest Expense	362,106	338,699	329,945
Other Income/(Loss), Net	24,619	29,894	19,742
Income Before Income Tax Expense	1,295,362	1,220,630	808,003
Income Tax Expense	468,297	426,941	274,926
Net Income	827,065	793,689	533,077
Net Income Attributable to Noncontrolling Interests	7,519	7,682	7,132
Net Income Attributable to Controlling Interests	819,546	786,007	525,945
Basic Earnings Per Common Share	$2.59	$2.49	$1.90
Diluted Earnings Per Common Share	$2.58	$2.49	$1.89
Weighted Average Common Shares Outstanding:
Basic	316,136,748	315,311,387	277,209,819
Diluted	317,417,414	316,211,160	277,993,631
The Connecticut Light And Power Company [Member]
Operating Revenues	2,692,582	2,442,341	2,407,449
Operating Expenses:
Purchased Power and Transmission	982,876	872,769	858,231
Operations and Maintenance	494,578	523,247	635,733
Depreciation	188,837	177,603	166,853
Amortization of Regulatory Assets/(Liabilities), Net	59,336	4,870	14,372
Energy Efficiency Programs	156,335	89,858	89,299
Taxes Other Than Income Taxes	255,370	234,418	215,972
Total Operating Expenses	2,137,332	1,902,765	1,980,460
Operating Income	555,250	539,576	426,989
Interest Expense:
Interest on Long-Term Debt	135,656	130,620	124,894
Other Interest	11,765	3,030	8,233
Total Interest Expense	147,421	133,650	133,127
Other Income/(Loss), Net	13,376	15,149	10,300
Income Before Income Tax Expense	421,205	421,075	304,162
Income Tax Expense	133,451	141,663	94,437
Net Income	287,754	279,412	209,725
NSTAR Electric Company [Member]
Operating Revenues	2,536,677	2,493,479	2,300,997
Operating Expenses:
Purchased Power and Transmission	1,122,298	849,149	788,252
Operations and Maintenance	326,972	376,360	431,802
Depreciation	188,693	180,298	171,070
Amortization of Regulatory Assets/(Liabilities), Net	-6,330	230,148	117,682
Amortization of Rate Reduction Bonds	0	15,054	90,322
Energy Efficiency Programs	193,516	206,536	201,234
Taxes Other Than Income Taxes	133,072	127,778	119,219
Total Operating Expenses	1,958,221	1,985,323	1,919,581
Operating Income	578,456	508,156	381,416
Interest Expense:
Interest on Long-Term Debt	77,140	79,088	87,100
Interest on Rate Reduction Bonds	0	399	3,585
Other Interest	738	-9,104	-20,631
Total Interest Expense	77,878	70,383	70,054
Other Income/(Loss), Net	4,491	3,639	2,846
Income Before Income Tax Expense	505,069	441,412	314,208
Income Tax Expense	201,981	172,866	123,966
Net Income	303,088	268,546	190,242
Public Service Company Of New Hampshire [Member]
Operating Revenues	959,500	935,402	988,013
Operating Expenses:
Purchased Power and Transmission	313,732	269,754	319,253
Operations and Maintenance	261,848	267,797	263,234
Depreciation	98,436	91,581	87,602
Amortization of Regulatory Assets/(Liabilities), Net	-29,602	-20,387	-24,086
Amortization of Rate Reduction Bonds	0	19,748	56,645
Energy Efficiency Programs	14,286	14,494	14,245
Taxes Other Than Income Taxes	71,417	67,196	66,025
Total Operating Expenses	730,117	710,183	782,918
Operating Income	229,383	225,219	205,095
Interest Expense:
Interest on Long-Term Debt	45,116	44,370	46,228
Interest on Rate Reduction Bonds	0	-154	2,687
Other Interest	233	1,960	1,313
Total Interest Expense	45,349	46,176	50,228
Other Income/(Loss), Net	2,045	3,455	3,008
Income Before Income Tax Expense	186,079	182,498	157,875
Income Tax Expense	72,135	71,101	60,993
Net Income	113,944	111,397	96,882
Western Massachusetts Electric Company [Member]
Operating Revenues	493,423	472,724	441,164
Operating Expenses:
Purchased Power and Transmission	172,876	147,059	136,086
Operations and Maintenance	89,406	96,194	97,031
Depreciation	41,886	37,568	29,971
Amortization of Regulatory Assets/(Liabilities), Net	-6,228	-3,206	410
Amortization of Rate Reduction Bonds	0	7,780	17,632
Energy Efficiency Programs	42,937	39,524	27,802
Taxes Other Than Income Taxes	34,907	28,458	21,458
Total Operating Expenses	375,784	353,377	330,390
Operating Income	117,639	119,347	110,774
Interest Expense:
Interest on Long-Term Debt	24,245	23,625	23,462
Interest on Rate Reduction Bonds	0	177	1,229
Other Interest	686	1,049	1,943
Total Interest Expense	24,931	24,851	26,634
Other Income/(Loss), Net	2,379	3,310	2,503
Income Before Income Tax Expense	95,087	97,806	86,643
Income Tax Expense	37,268	37,368	32,140
Net Income	$57,819	$60,438	$54,503

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Income	$827,065	$793,689	$533,077
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	2,037	2,049	1,971
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	315	-940	217
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	-30,330	25,714	-4,356
Other Comprehensive Income (Loss), Net of Tax, Total	-27,978	26,823	-2,168
Comprehensive Income Attributable to Noncontrolling Interests	7,519	7,682	7,132
Comprehensive Income	791,568	812,830	523,777
The Connecticut Light And Power Company [Member]
Net Income	287,754	279,412	209,725
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	444	444	444
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	12	-31	7
Other Comprehensive Income (Loss), Net of Tax, Total	456	413	451
Comprehensive Income	288,210	279,825	210,176
NSTAR Electric Company [Member]
Net Income	303,088	268,546	190,242
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	442
Other Comprehensive Income (Loss), Net of Tax, Total	442
Comprehensive Income	303,530	268,546	190,242
Public Service Company Of New Hampshire [Member]
Net Income	113,944	111,397	96,882
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	1,162	1,162	1,162
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	19	-54	13
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	0	-3	2
Other Comprehensive Income (Loss), Net of Tax, Total	1,181	1,105	1,177
Comprehensive Income	115,125	112,502	98,059
Western Massachusetts Electric Company [Member]
Net Income	57,819	60,438	54,503
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	338	338	338
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	3	-9	2
Other Comprehensive Income (Loss), Net of Tax, Total	341	329	340
Comprehensive Income	$58,160	$60,767	$54,843

CONSOLIDATED_STATEMENT_OF_CASH

CONSOLIDATED STATEMENT OF CASH FLOWS (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Activities:
Net Income	$827,065	$793,689	$533,077
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Depreciation	614,657	610,777	519,010
Deferred Income Tax Expense (Benefit)	443,259	431,413	292,000
Pension, SERP and PBOP Expense	99,056	195,698	218,540
Pension and PBOP Contributions	-211,649	-342,184	-295,028
Regulatory Over/(Under) Recoveries, Net	6,853	-24,276	-259,853
Amortization of Regulatory Assets/(Liabilities), Net	10,704	206,322	79,762
Amortization of Rate Reduction Bonds	0	42,581	142,019
Proceeds from DOE Damages Claim, Net	132,138
Other Noncash Income (Expense)	39,523	56,071	42,852
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	-122,139	-163,549	-20,214
Increase Decrease in Fuel, Materials and Supplies	-41,310	-14,811	34,321
Taxes Receivable/(Accrued)	-323,224	-50,950	-5,450
Accounts Payable	144,743	-54,619	-128,339
Other Current Assets and Liabilities	15,797	-22,623	8,532
Net Cash Flows Provided by Operating Activities	1,635,473	1,663,539	1,161,229
Investing Activities:
Investments in Property, Plant and Equipment	-1,603,744	-1,456,787	-1,472,272
Proceeds from Sales of Marketable Securities	488,789	627,532	317,294
Purchases of Marketable Securities	-491,220	-679,784	-348,629
Other Investing Activities	14,380	67,816	35,683
Net Cash Flows Used in Investing Activities	-1,591,795	-1,441,223	-1,467,924
Financing Activities:
Cash Dividends on Common Stock	-475,227	-462,741	-375,047
Cash Dividends on Preferred Stock	-7,519	-7,682	-7,029
Increase/(Decrease) in Short-Term Debt	285,075	-397,000	825,000
Issuance of Long-Term Debt	725,000	1,680,000	850,000
Retirements of Long-Term Debt	-576,551	-929,885	-839,136
Retirements of Rate Reduction Bonds	0	-82,139	-114,433
Other Financing Activities	883	-25,253	6,529
Net Cash Flows Provided by/(Used in) Financing Activities	-48,339	-224,700	345,884
Net Increase/(Decrease) in Cash	-4,661	-2,384	39,189
Cash - Beginning of Year	43,364	45,748	6,559
Cash - End of Year	38,703	43,364	45,748
The Connecticut Light And Power Company [Member]
Operating Activities:
Net Income	287,754	279,412	209,725
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Depreciation	188,837	177,603	166,853
Deferred Income Tax Expense (Benefit)	130,949	130,038	140,993
Pension, SERP and PBOP Expense	14,992	24,416	24,062
Regulatory Over/(Under) Recoveries, Net	-20,502	28,298	-100,505
Amortization of Regulatory Assets/(Liabilities), Net	59,336	4,870	14,372
Proceeds from DOE Damages Claim, Net	68,610
Other Noncash Income (Expense)	-1,342	-3,478	-28,952
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	-78,631	-56,593	-7,741
Increase Decrease in Fuel, Materials and Supplies	13,063	9,997	-4,573
Taxes Receivable/(Accrued)	-126,376	-41,594	15,702
Accounts Payable	68,891	-66,225	-190,240
Other Current Assets and Liabilities	6,838	8,513	-27,803
Net Cash Flows Provided by Operating Activities	612,419	495,257	211,893
Investing Activities:
Investments in Property, Plant and Equipment	-515,710	-434,934	-449,137
Other Investing Activities	12,653	2,650	32,009
Net Cash Flows Used in Investing Activities	-503,057	-432,284	-417,128
Financing Activities:
Cash Dividends on Common Stock	-171,200	-151,999	-100,486
Cash Dividends on Preferred Stock	-5,559	-5,559	-5,559
Increase/(Decrease) in Short-Term Debt	0	-89,000	58,000
Issuance of Long-Term Debt	250,000	400,000	0
Increase/(Decrease) in Notes Payable to Affiliate	-153,900	-117,800	346,575
Retirements of Long-Term Debt	-150,000	-125,000	-116,400
Capital Contributions from NU Parent	120,000	40,000	25,000
Other Financing Activities	-3,584	-6,379	-1,895
Net Cash Flows Provided by/(Used in) Financing Activities	-114,243	-55,737	205,235
Net Increase/(Decrease) in Cash	-4,881	7,236	0
Cash - Beginning of Year	7,237	1	1
Cash - End of Year	2,356	7,237	1
NSTAR Electric Company [Member]
Operating Activities:
Net Income	303,088	268,546	190,242
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Bad Debt Expense	24,740	28,108	40,301
Depreciation	188,693	180,298	171,070
Deferred Income Tax Expense (Benefit)	108,133	48,808	4,264
Pension, SERP and PBOP Expense	6,760	35,731	66,010
Pension Contribution	-120,306	-82,000	-25,000
Regulatory Over/(Under) Recoveries, Net	57,696	-119,433	-16,129
Amortization of Regulatory Assets/(Liabilities), Net	-6,330	230,148	117,682
Amortization of Rate Reduction Bonds	0	15,054	90,322
Proceeds from DOE Damages Claim, Net	30,193
Other Noncash Income (Expense)	-51,478	4,428	-32,048
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	-18,853	-45,405	-10,496
Increase Decrease in Fuel, Materials and Supplies	-29,943	3,227	1,813
Taxes Receivable/(Accrued)	-122,746	-38,003	29,899
Accounts Payable	9,753	31,875	2,662
Other Current Assets and Liabilities	38,535	-6,468	22,568
Accounts Payable Related Parties	115,092	-44,491	-61,879
Net Cash Flows Provided by Operating Activities	533,027	510,423	591,281
Investing Activities:
Investments in Property, Plant and Equipment	-465,028	-476,600	-414,089
Decrease in Restricted Cash	0	37,604	3,060
Other Investing Activities	0	400	400
Net Cash Flows Used in Investing Activities	-465,028	-438,596	-410,629
Financing Activities:
Cash Dividends on Common Stock	-253,000	-56,000	-217,000
Cash Dividends on Preferred Stock	-1,961	-2,123	-1,960
Increase/(Decrease) in Short-Term Debt	198,500	-172,500	134,500
Issuance of Long-Term Debt	300,000	200,000	400,000
Retirements of Long-Term Debt	-301,650	-1,650	-401,650
Retirements of Rate Reduction Bonds	0	-43,493	-84,367
Other Financing Activities	-5,136	-1,735	-5,853
Net Cash Flows Provided by/(Used in) Financing Activities	-63,247	-77,501	-176,330
Net Increase/(Decrease) in Cash	4,752	-5,674	4,322
Cash - Beginning of Year	8,021	13,695	9,373
Cash - End of Year	12,773	8,021	13,695
Public Service Company Of New Hampshire [Member]
Operating Activities:
Net Income	113,944	111,397	96,882
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Depreciation	98,436	91,581	87,602
Deferred Income Tax Expense (Benefit)	94,813	75,693	58,552
Pension, SERP and PBOP Expense	7,197	26,846	26,312
Pension and PBOP Contributions	-2,482	-112,964	-96,880
Regulatory Over/(Under) Recoveries, Net	-11,875	-8,481	-183
Amortization of Regulatory Assets/(Liabilities), Net	-29,602	-20,387	-24,086
Amortization of Rate Reduction Bonds	0	19,748	56,645
Proceeds from DOE Damages Claim, Net	14,453
Other Noncash Income (Expense)	10,095	16,079	11,205
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	-15,576	2,412	-84
Increase Decrease in Fuel, Materials and Supplies	-19,403	-33,391	25,897
Taxes Receivable/(Accrued)	-23,857	26,462	-9,752
Accounts Payable	17,796	2,632	-15,248
Other Current Assets and Liabilities	-5,972	-9,520	13,436
Net Cash Flows Provided by Operating Activities	247,967	188,107	230,298
Investing Activities:
Investments in Property, Plant and Equipment	-256,159	-186,009	-203,902
(Increase) Decrease In Notes Receivable From Affiliated Company		0	55,900
Decrease in Restricted Cash	-1,013	22,040	4,200
Other Investing Activities	-139	-88	-135
Net Cash Flows Used in Investing Activities	-257,311	-164,057	-143,937
Financing Activities:
Cash Dividends on Common Stock	-66,000	-68,000	-90,674
Increase/(Decrease) in Short-Term Debt	4,000	23,200	0
Issuance of Long-Term Debt	75,000	250,000	0
Increase/(Decrease) in Notes Payable to Affiliate	0	0	63,300
Retirements of Long-Term Debt	-50,000	-198,235	0
Retirements of Rate Reduction Bonds	0	-29,294	-56,074
Capital Contributions from NU Parent	45,000	0	0
Other Financing Activities	1,703	-4,084	-476
Net Cash Flows Provided by/(Used in) Financing Activities	9,703	-26,413	-83,924
Net Increase/(Decrease) in Cash	359	-2,363	2,437
Cash - Beginning of Year	130	2,493	56
Cash - End of Year	489	130	2,493
Western Massachusetts Electric Company [Member]
Operating Activities:
Net Income	57,819	60,438	54,503
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Depreciation	41,886	37,568	29,971
Deferred Income Tax Expense (Benefit)	34,108	87,028	53,942
Regulatory Over/(Under) Recoveries, Net	1,925	8,458	-19,152
Amortization of Regulatory Assets/(Liabilities), Net	-6,228	-3,206	410
Amortization of Rate Reduction Bonds	0	7,780	17,632
Proceeds from DOE Damages Claim, Net	18,883
Other Noncash Income (Expense)	-2,005	3,381	-3,954
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	39,872	-53,292	-8,896
Increase Decrease in Fuel, Materials and Supplies	-627	865	-2,882
Taxes Receivable/(Accrued)	-22,454	19,840	-8,311
Accounts Payable	1,269	7,456	-19,297
Other Current Assets and Liabilities	-11,169	2,491	581
Net Cash Flows Provided by Operating Activities	153,279	178,807	94,547
Investing Activities:
Investments in Property, Plant and Equipment	-116,205	-128,786	-264,175
Proceeds from Sales of Marketable Securities	73,198	70,778	79,769
Purchases of Marketable Securities	-73,888	-71,390	-80,529
(Increase) Decrease In Notes Receivable From Affiliated Company	0	0	11,000
Other Investing Activities	3,200	7,401	-28
Net Cash Flows Used in Investing Activities	-113,695	-121,997	-253,963
Financing Activities:
Cash Dividends on Common Stock	-59,999	-40,001	-9,432
Issuance of Long-Term Debt	0	80,000	150,000
Increase/(Decrease) in Notes Payable to Affiliate	21,400	-31,900	31,900
Retirements of Long-Term Debt	0	-55,000	-53,800
Retirements of Rate Reduction Bonds	0	-9,352	-17,540
Capital Contributions from NU Parent	0	0	50,000
Other Financing Activities	-985	-558	8,288
Net Cash Flows Provided by/(Used in) Financing Activities	-39,584	-56,811	159,416
Net Increase/(Decrease) in Cash	0	-1	0
Cash - Beginning of Year		1	1
Cash - End of Year			$1

CONSOLIDATED_STATEMENTS_OF_COM1

CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS EQUITY (USD $)	3 Months Ended		12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Common Stock, Shares, Outstanding	316,983,337	315,273,559	316,983,337	315,273,559
Net Income	$223,600	$179,200	$827,065	$793,689	$533,077
Other Comprehensive Income (Loss), Net of Tax			-27,978	26,823	-2,168
Net Income Attributable to Noncontrolling Interests			7,519	7,682	7,132
SharesIssuedInConnectionWithNstarMerger
SharesIssuedValueInConectionWithNstarMerger
Common Stock [Member]
Common Stock, Shares, Outstanding	316,983,337	315,273,559	316,983,337	315,273,559	314,053,634	177,158,692
Common Stock, Value, Outstanding	1,666,796	1,665,351	1,666,796	1,665,351	1,662,547	980,264
Issuance of Common Shares			1,445		2,040
Stock Issued During Period, Shares, New Issues			288,941	659,077	408,018
Stock Issued During Period, Shares, Treasury Stock Reissued			1,420,837		438,329
Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	6,235,834	6,192,765	6,235,834	6,192,765	6,183,267	1,797,884
Issuance of Common Shares			5,164	17,381	11,287
Long-Term Incentive Plan Activity			-9,569	-5,409	-3,897
Stock Issued During Period, Value, Treasury Stock Reissued			37,817		8,454
SharesIssuedValueInConectionWithNstarMerger
OtherChangesInShareholdersEquity			9,657		8,574
OtherEquityImpactsOfMergerWithNstar
Retained Earnings [Member]
Common Stock, Value, Outstanding	2,448,661	2,125,980	2,448,661	2,125,980	1,802,714	1,651,875
Net Income			827,065	793,689	421
Dividends on Common Shares			-496,524		-375,527
Dividends on Preferred Stock			-7,519		-7,029
Net Income Attributable to Noncontrolling Interests					-103
OtherEquityImpactsOfMergerWithNstar		-7,682		-7,682
Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Value, Outstanding	-74,009	-46,031	-74,009	-46,031	-72,854	-70,686
Other Comprehensive Income (Loss), Net of Tax			-27,978		-2,168
Treasury Stock [Member]
Common Stock, Value, Outstanding	-300,467	-326,537	-300,467	-326,537	-338,624	-346,667
Stock Issued During Period, Value, Treasury Stock Reissued			26,070		8,043
Stockholders Equity Total Member
SharesIssuedValueInConectionWithNstarMerger				-462,741
OtherEquityImpactsOfMergerWithNstar		-7,682		-7,682
Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	9,976,815	9,611,528	9,976,815	9,611,528	9,237,050	4,012,670
Net Income			827,065	793,689	3,359
Dividends on Common Shares			-496,524	11,078	-375,527
Issuance of Common Shares			6,609	29,468	13,327
Dividends on Preferred Stock			-7,519	-10,748	-7,029
Long-Term Incentive Plan Activity			-9,569	-5,409	-3,897
Other Comprehensive Income (Loss), Net of Tax			-27,978		-2,168
Stock Issued During Period, Value, Treasury Stock Reissued			63,887	26,823	16,497
Net Income Attributable to Noncontrolling Interests					-103
OtherChangesInShareholdersEquity			9,316		8,574
The Connecticut Light And Power Company [Member]
Net Income	87,200	60,200	287,754	279,412	209,725
Other Comprehensive Income (Loss), Net of Tax			456	413	451
Capital Contributions from NU Parent			120,000	40,000	25,000
The Connecticut Light And Power Company [Member] | Common Stock [Member]
Common Stock, Shares, Outstanding	6,035,205	6,035,205	6,035,205	6,035,205	6,035,205	6,035,205
Common Stock, Value, Outstanding	60,352	60,352	60,352	60,352	60,352	60,352
The Connecticut Light And Power Company [Member] | Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	1,804,869	1,682,047	1,804,869	1,682,047	1,640,149	1,613,503
Stock Issued During Period, Value, Employee Stock Ownership Plan			2,771	1,847	1,595
Payments of Stock Issuance Costs			51	51	51
Capital Contributions from NU Parent			120,000	40,000	25,000
The Connecticut Light And Power Company [Member] | Retained Earnings [Member]
Common Stock, Value, Outstanding	1,072,477	961,482	1,072,477	961,482	839,628	735,948
Net Income			287,754	279,412	209,725
Dividends on Common Shares			-171,200	-151,999	-100,486
Dividends on Preferred Stock			-5,559	-5,559	-5,559
The Connecticut Light And Power Company [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Value, Outstanding	-931	-1,387	-931	-1,387	-1,800	-2,251
Other Comprehensive Income (Loss), Net of Tax			456	413
The Connecticut Light And Power Company [Member] | Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	2,936,767	2,702,494	2,936,767	2,702,494	2,538,329	2,407,552
Net Income			287,754	279,412	209,725
Dividends on Common Shares			-171,200	-151,999	-100,486
Dividends on Preferred Stock			-5,559	-5,559	-5,559
Stock Issued During Period, Value, Employee Stock Ownership Plan			2,771	1,847	1,595
Other Comprehensive Income (Loss), Net of Tax			456	413
Payments of Stock Issuance Costs			51	51	51
Capital Contributions from NU Parent			120,000	40,000	25,000
NSTAR Electric Company [Member]
Net Income	69,300	55,400	303,088	268,546	190,242
Other Comprehensive Income (Loss), Net of Tax			442
NSTAR Electric Company [Member] | Common Stock [Member]
Common Stock, Shares, Outstanding	100	100	100	100	100	100
Common Stock, Value, Outstanding	0	0	0	0	0	0
NSTAR Electric Company [Member] | Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	994,130	992,625	994,130	992,625	992,625	992,625
NSTAR Electric Company [Member] | Retained Earnings [Member]
Common Stock, Value, Outstanding	1,468,955	1,420,828	1,468,955	1,420,828	1,210,405	1,239,123
Net Income				268,546	190,242
Dividends on Common Shares			-253,000	-56,000	-217,000
Dividends on Preferred Stock			-1,961	-2,123	-1,960
NSTAR Electric Company [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Other Comprehensive Income (Loss), Net of Tax			442
NSTAR Electric Company [Member] | Stockholders Equity Total Member
Other Comprehensive Income (Loss), Net of Tax			442
NSTAR Electric Company [Member] | Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	2,463,527	2,413,453	2,463,527	2,413,453	2,203,030	2,231,748
Net Income			303,088	268,546	190,242
Dividends on Common Shares			-253,000	-56,000	-217,000
Dividends on Preferred Stock			-1,961	-2,123	-1,960
Public Service Company Of New Hampshire [Member]
Net Income	29,000	26,800	113,944	111,397	96,882
Other Comprehensive Income (Loss), Net of Tax			1,181	1,105	1,177
Capital Contributions from NU Parent			45,000	0	0
Public Service Company Of New Hampshire [Member] | Common Stock [Member]
Common Stock, Shares, Outstanding	301	301	301	301	301	301
Common Stock, Value, Outstanding	0	0	0	0	0	0
Public Service Company Of New Hampshire [Member] | Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	748,240	701,911	748,240	701,911	701,052	700,285
Stock Issued During Period, Value, Employee Stock Ownership Plan			1,329	859	767
Public Service Company Of New Hampshire [Member] | Retained Earnings [Member]
Common Stock, Value, Outstanding	486,459	438,515	486,459	438,515	395,118	388,910
Net Income			113,944	111,397	96,882
Dividends on Common Shares			-66,000	-68,000	-90,674
Public Service Company Of New Hampshire [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Value, Outstanding	-7,369	-8,550	-7,369	-8,550	-9,655	-10,832
Other Comprehensive Income (Loss), Net of Tax			1,181	1,105	1,177
Public Service Company Of New Hampshire [Member] | Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	1,227,330	1,131,876	1,227,330	1,131,876	1,086,515	1,078,363
Net Income			113,944	111,397	96,882
Dividends on Common Shares			-66,000	-68,000	-90,674
Stock Issued During Period, Value, Employee Stock Ownership Plan			1,329	859	767
Other Comprehensive Income (Loss), Net of Tax			1,181	1,105	1,177
Western Massachusetts Electric Company [Member]
Net Income	18,000	10,400	57,819	60,438	54,503
Other Comprehensive Income (Loss), Net of Tax			341	329	340
Capital Contributions from NU Parent			0	0	50,000
Western Massachusetts Electric Company [Member] | Common Stock [Member]
Common Stock, Shares, Outstanding	434,653	434,653	434,653	434,653	434,653	434,653
Common Stock, Value, Outstanding	10,866	10,866	10,866	10,866	10,866	10,866
Western Massachusetts Electric Company [Member] | Additional Paid-in Capital [Member]
Common Stock, Value, Outstanding	391,256	390,743	391,256	390,743	390,412	340,115
Stock Issued During Period, Value, Employee Stock Ownership Plan			513	331	297
Capital Contributions from NU Parent					50,000
Western Massachusetts Electric Company [Member] | Retained Earnings [Member]
Common Stock, Value, Outstanding	178,834	181,014	178,834	181,014	160,577	115,506
Net Income			57,819	60,438	54,503
Dividends on Common Shares			-59,999	-40,001	-9,432
Western Massachusetts Electric Company [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Common Stock, Value, Outstanding	-3,176	-3,517	-3,176	-3,517	-3,846	-4,186
Other Comprehensive Income (Loss), Net of Tax			341		340
Western Massachusetts Electric Company [Member] | Common Shareholders Equity [Member]
Common Stock, Value, Outstanding	577,780	579,106	577,780	579,106	558,009	462,301
Net Income			57,819	60,438	54,503
Dividends on Common Shares			-59,999	-40,001	-9,432
Stock Issued During Period, Value, Employee Stock Ownership Plan			513	331	297
Other Comprehensive Income (Loss), Net of Tax			341		340
Capital Contributions from NU Parent				$329	$50,000

SUMMARY_OF_SIGNIFICANT_ACCOUNT

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Significant Accounting Policies [Text Block]	NORTHEAST UTILITIES AND SUBSIDIARIES
	THE CONNECTICUT LIGHT AND POWER COMPANY
	NSTAR ELECTRIC COMPANY AND SUBSIDIARY
	PUBLIC SERVICE COMPANY OF NEW HAMPSHIRE AND SUBSIDIARY
	WESTERN MASSACHUSETTS ELECTRIC COMPANY
	COMBINED NOTES TO FINANCIAL STATEMENTS
	Refer to the Glossary of Terms included in this combined Annual Report on Form 10-K for abbreviations and acronyms used throughout the combined notes to the financial statements.
	1.       SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
	A.       About NU, CL&P, NSTAR Electric, PSNH and WMECO
	NU Consolidated: NU is a public utility holding company primarily engaged through its wholly owned regulated utility subsidiaries in the energy delivery business. On April 10, 2012, NU acquired NSTAR and its subsidiaries. See Note 21, "Merger of NU and NSTAR," for further information regarding the merger. NU's wholly owned regulated utility subsidiaries consist of CL&P, NSTAR Electric, PSNH, WMECO, Yankee Gas and NSTAR Gas. NU provides energy delivery service to approximately 3.6 million electric and natural gas customers through these six regulated utilities in Connecticut, Massachusetts and New Hampshire. On February 2, 2015, NU, CL&P, NSTAR Electric, PSNH and WMECO commenced doing business as Eversource Energy.
	NU, CL&P, NSTAR Electric, PSNH and WMECO are reporting companies under the Securities Exchange Act of 1934. NU is a public utility holding company under the Public Utility Holding Company Act of 2005. Arrangements among the regulated electric companies and other NU companies, outside agencies and other utilities covering interconnections, interchange of electric power and sales of utility property are subject to regulation by the FERC. The Regulated companies are subject to regulation of rates, accounting and other matters by the FERC and/or applicable state regulatory commissions (the PURA for CL&P and Yankee Gas, the DPU for NSTAR Electric, WMECO and NSTAR Gas, and the NHPUC for PSNH).
	Regulated Companies: CL&P, NSTAR Electric, PSNH and WMECO furnish franchised retail electric service in Connecticut, Massachusetts and New Hampshire. Yankee Gas and NSTAR Gas are engaged in the distribution and sale of natural gas to customers within Connecticut and central and eastern Massachusetts, respectively. CL&P, NSTAR Electric, PSNH and WMECO's results include the operations of their respective distribution and transmission businesses. PSNH and WMECO's distribution results include the operations of their respective generation businesses. NU also has a regulated subsidiary, NPT, which was formed to construct, own and operate the Northern Pass line, a HVDC transmission line from Québec to New Hampshire under development that will interconnect with a new HVDC transmission line being developed by a transmission subsidiary of HQ.
	Other: NUSCO, NU's service company, Rocky River Realty Company, a wholly-owned real estate subsidiary of NU, Renewable Properties, Inc., a wholly-owned subsidiary of EETV, and Properties, Inc., a wholly-owned subsidiary of PSNH, provide support services to NU, including its regulated companies. Harbor Electric Energy Company, a wholly-owned subsidiary of NSTAR Electric, provides distribution service and ongoing support to the Massachusetts Water Resources Authority. Hopkinton LNG Corp, an indirect, wholly-owned subsidiary of NU, provides natural gas liquefaction, vaporization, and storage services for NSTAR Gas.
	B.       Basis of Presentation
	The consolidated financial statements of NU, NSTAR Electric and PSNH include the accounts of each of their respective subsidiaries. Intercompany transactions have been eliminated in consolidation. The accompanying consolidated financial statements of NU, NSTAR Electric and PSNH and the financial statements of CL&P and WMECO are herein collectively referred to as the "financial statements."
	The preparation of the financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
	NU's consolidated financial information includes NSTAR and its subsidiaries' results of operations beginning April 10, 2012. The information disclosed for the year ended December 31, 2012 for NSTAR Electric is presented on a comparable basis.
	NU consolidates CYAPC and YAEC because CL&P's, NSTAR Electric's, PSNH's and WMECO's combined ownership interest in each of these entities is greater than 50 percent. Intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation of the NU financial statements.
	NU's utility subsidiaries are subject to the application of accounting guidance for entities with rate-regulated operations that considers the effect of regulation resulting from differences in the timing of the recognition of certain revenues and expenses from those of other businesses and industries. NU's utility subsidiaries' distribution, generation and transmission businesses and NPT are subject to rate-regulation that is based on cost recovery and meets the criteria for application of rate-regulated accounting. See Note 2, "Regulatory Accounting," for further information.
	Certain reclassifications of prior year data were made in the accompanying balance sheets for NU, CL&P, NSTAR Electric and PSNH. These reclassifications were made to conform to the current year presentation.
	In accordance with accounting guidance on noncontrolling interests in consolidated financial statements, the Preferred Stock of CL&P and the Preferred Stock of NSTAR Electric, which are not owned by NU or its consolidated subsidiaries and are not subject to mandatory redemption, have been presented as noncontrolling interests in the financial statements of NU.  The Preferred Stock of CL&P and the Preferred Stock of NSTAR Electric are considered to be temporary equity and have been classified between liabilities and permanent shareholders' equity on the balance sheets of NU, CL&P and NSTAR Electric due to a provision in the preferred stock agreements of both CL&P and NSTAR Electric that grant preferred stockholders the right to elect a majority of the CL&P and NSTAR Electric Board of Directors, respectively, should certain conditions exist, such as if preferred dividends are in arrears for a specified amount of time.  The Net Income reported in the statements of income and cash flows represents net income prior to apportionment to noncontrolling interests, which is represented by dividends on preferred stock of CL&P and NSTAR Electric.
	As of December 31, 2014 and 2013, NU's carrying amount of goodwill was approximately $3.5 billion. NU performs an assessment for possible impairment of its goodwill at least annually. NU completed its annual goodwill impairment test for each of its reporting units as of October 1, 2014 and determined that no impairment exists. See Note 22, "Goodwill," for further information.
	C.       Accounting Standards
	Recently Adopted Accounting Standards: On January 1, 2014, as required, NU prospectively adopted the Financial Accounting Standards Board's (FASB) final Accounting Standards Updates (ASU) that required presentation of certain unrecognized tax benefits as reductions to deferred tax assets. Implementation of this guidance had an immaterial impact on the balance sheets and no impact on the results of operations or cash flows of NU, CL&P, NSTAR Electric, PSNH and WMECO.
	Accounting Standards Issued but not Yet Adopted: In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers, effective January 1, 2017, which amends existing revenue recognition guidance and is required to be applied retrospectively (either to each reporting period presented or cumulatively at the date of initial application). Management is reviewing the requirements of the ASU. The ASU's impact is not expected to have a material impact on the financial statements of NU, CL&P, NSTAR Electric, PSNH and WMECO.
	D.       Cash and Cash Equivalents
	Cash and cash equivalents include cash on hand and short-term cash investments that are highly liquid in nature and have original maturities of three months or less. At the end of each reporting period, any overdraft amounts are reclassified from Cash and Cash Equivalents to Accounts Payable on the balance sheets.
	E.       Provision for Uncollectible Accounts
	NU, including CL&P, NSTAR Electric, PSNH and WMECO, presents its receivables at estimated net realizable value by maintaining a provision for uncollectible accounts. This provision is determined based upon a variety of judgments and factors, including the application of an estimated uncollectible percentage to each receivable aging category. The estimate is based upon historical collection and write-off experience and management's assessment of collectability from customers. Management continuously assesses the collectability of receivables and adjusts collectability estimates based on actual experience. Receivable balances are written off against the provision for uncollectible accounts when the accounts are terminated and these balances are deemed to be uncollectible.
	The PURA allows CL&P and Yankee Gas to accelerate the recovery of accounts receivable balances attributable to qualified customers under financial or medical duress (uncollectible hardship accounts receivable) outstanding for greater than 90 days. The DPU allows WMECO to also recover in rates amounts associated with certain uncollectible hardship accounts receivable. Uncollectible customer account balances, which are expected to be recovered in rates, are included in Regulatory Assets or Other Long-Term Assets.
	The total provision for uncollectible accounts and for uncollectible hardship accounts, which is included in the total provision, are included in Receivables, Net on the balance sheets, and were as follows:
			Total Provision for Uncollectible Accounts						Uncollectible Hardship
			As of December 31,						As of December 31,
	(Millions of Dollars)		2014			2013			2014			2013
	NU		$	175.3		$	171.3		$	91.5		$	81.2
	CL&P			84.3			82			74			67.3
	NSTAR Electric			40.7			41.7			-			-
	PSNH			7.7			7.4			-			-
	WMECO			9.9			10			6.2			5.5
	F.       Fuel, Materials and Supplies and Allowance Inventory
	Fuel, Materials and Supplies include natural gas, coal, biomass and oil inventories as well as materials purchased primarily for construction or operation and maintenance purposes. Natural gas, coal, biomass and oil inventories are valued at their respective weighted average cost. Materials and supplies are valued at the lower of average cost or market. As of December 31, 2014, NU and PSNH had $164.3 million and $95.1 million, respectively, of fuel and $185.4 million and $53 million, respectively, of materials and supplies. As of December 31, 2013, NU and PSNH had $139.5 million and $74.2 million, respectively, of fuel and $163.7 million and $54.5 million, respectively, of materials and supplies.
	Fuel, Materials and Supplies also include Renewable Energy Certificates (RECs), which are purchased from suppliers of renewable sources of generation. RECs are used to meet state mandated Renewable Portfolio Standards requirements. As of December 31, 2014 and 2013, NSTAR Electric had $25.1 million and $4.9 million, respectively, of RECs classified as Materials and Supplies on the balance sheets.
	PSNH is subject to federal and state laws and regulations that regulate emissions of air pollutants, including SO2, CO2, and NOx related to its regulated generation units, and uses SO2, CO2, and NOx emissions allowances. At the end of each compliance period, PSNH is required to relinquish SO2, CO2, and NOx emissions allowances corresponding to the actual respective emissions emitted by its generating units over the compliance period. SO2 and NOx emissions allowances are obtained through an annual allocation from the federal and state regulators that are granted at no cost and through purchases from third parties. CO2 emissions allowances are obtained through an annual allocation from the state regulator that are granted at no cost and are acquired through auctions and through purchases from third parties. SO2, CO2, and NOx emissions allowances are charged to expense based on their weighted average cost as they are utilized against emissions volumes at PSNH's generating units.
	SO2, CO2, and NOx emissions allowances are recorded within Fuel, Materials and Supplies and are classified on the balance sheet as short-term or long-term depending on the period in which they are expected to be utilized against actual emissions. As of December 31, 2014 and 2013, PSNH had $20.1 million and $19.4 million, respectively, of long-term SO2 and CO2 emissions allowances classified as Other Long-Term Assets on the balance sheets.
	G.       Restricted Cash and Other Deposits
	As of December 31, 2014, NU, CL&P and PSNH had $3.2 million, $2.1 million, and $1 million respectively, of restricted cash relating to amounts held in escrow, which were included in Prepayments and Other Current Assets on the balance sheets. As of December 31, 2013, these amounts were $1.7 million and $1.4 million for NU and CL&P, respectively.
	As of December 31, 2014, NU, CL&P and PSNH had $9.9 million, $1.2 million and $2.5 million, respectively, of cash collateral posted not subject to master netting agreements, primarily with ISO-NE, which were included in Prepayments and Other Current Assets on the balance sheets. As of December 31, 2013, these amounts were $17.9 million and $9 million for NU and NSTAR Electric, respectively.
	H.       Fair Value Measurements
	Fair value measurement guidance is applied to derivative contracts that are not elected or designated as "normal purchases or normal sales" (normal) and to the marketable securities held in trusts. Fair value measurement guidance is also applied to valuations of the investments used to calculate the funded status of pension and PBOP plans and nonrecurring fair value measurements of nonfinancial assets such as goodwill and AROs, and is also used to estimate the fair value of preferred stock and long-term debt.
	Fair Value Hierarchy: In measuring fair value, NU uses observable market data when available and minimizes the use of unobservable inputs. Inputs used in fair value measurements are categorized into three fair value hierarchy levels for disclosure purposes. The entire fair value measurement is categorized based on the lowest level of input that is significant to the fair value measurement. NU evaluates the classification of assets and liabilities measured at fair value on a quarterly basis, and NU's policy is to recognize transfers between levels of the fair value hierarchy as of the end of the reporting period. The three levels of the fair value hierarchy are described below:
	Level 1 - Inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities as of the reporting date.  Active markets are those in which transactions for the asset or liability occur in sufficient frequency and volume to provide pricing information on an ongoing basis.
	Level 2 - Inputs are quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-derived valuations in which all significant inputs are observable.
	Level 3 - Quoted market prices are not available. Fair value is derived from valuation techniques in which one or more significant inputs or assumptions are unobservable. Where possible, valuation techniques incorporate observable market inputs that can be validated to external sources such as industry exchanges, including prices of energy and energy-related products.
	Determination of Fair Value: The valuation techniques and inputs used in NU's fair value measurements are described in Note 4, "Derivative Instruments," Note 5, "Marketable Securities," Note 6, "Asset Retirement Obligations," Note 9A, "Employee Benefits – Pension Benefits and Postretirement Benefits Other Than Pensions," Note 13, "Fair Value of Financial Instruments," and Note 21, "Merger of NU and NSTAR," to the financial statements.
	I.       Derivative Accounting
	Many of the Regulated companies' contracts for the purchase and sale of energy or energy-related products are derivatives. The accounting treatment for energy contracts entered into varies and depends on the intended use of the particular contract and on whether or not the contract is a derivative. For the Regulated companies, regulatory assets or regulatory liabilities are recorded to offset the fair values of derivative contracts, as contract settlements are recovered from, or refunded to, customers in future rates.
	The application of derivative accounting is complex and requires management judgment in the following respects: identification of derivatives and embedded derivatives, election and designation of the normal exception, and determination of the fair value of derivative contracts. All of these judgments can have a significant impact on the financial statements.
	The judgment applied in the election of the normal exception (and resulting accrual accounting) includes the conclusion that it is probable at the inception of the contract and throughout its term that it will result in physical delivery of the underlying product and that the quantities will be used or sold by the business in the normal course of business. If facts and circumstances change and management can no longer support this conclusion, then the normal exception and accrual accounting is terminated, and fair value accounting is applied prospectively.
	The fair value of derivative contracts is based upon the contract terms and conditions and the underlying market price or fair value per unit. When quantities are not specified in the contract, the Company determines whether the contract has a determinable quantity by using amounts referenced in default provisions and other relevant sections of the contract. The fair value of derivative assets and liabilities with the same counterparty are offset and recorded as a net derivative asset or liability on the balance sheets. Changes in the fair value of derivative contracts are recorded as regulatory assets or liabilities and do not impact net income.
	For further information regarding derivative contracts, see Note 4, "Derivative Instruments," to the financial statements.
	J.       Equity Method Investments
	Regional Decommissioned Nuclear Companies: CL&P, NSTAR Electric, PSNH and WMECO own common stock in three regional nuclear generation companies (CYAPC, YAEC and MYAPC, collectively referred to as the Yankee Companies), each of which owned a single nuclear generating facility that has been decommissioned. For CL&P, NSTAR Electric, PSNH and WMECO, the respective investments in CYAPC, YAEC and MYAPC are accounted for under the equity method. NU consolidates CYAPC and YAEC because CL&P's, NSTAR Electric's, PSNH's and WMECO's combined ownership interest in each of these entities is greater than 50 percent. Intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation of the NU financial statements.
	Ownership interests in the Yankee Companies as of December 31, 2014 and 2013 were as follows:
	(Percent)	CYAPC			YAEC			MYAPC
CL&P	34.5	%		24.5	%		12	%
NSTAR Electric	14			14			4
PSNH	5			7			5
WMECO	9.5			7			3
The total carrying values of CL&P's, NSTAR Electric's, PSNH's and WMECO's ownership interests in CYAPC, YAEC and MYAPC, which are included in Other Long-Term Assets on their respective balance sheets, were as follows:
	As of December 31,
(Millions of Dollars)	2014			2013
CL&P	$	1.2		$	1.2
NSTAR Electric		0.5			0.5
PSNH		0.3			0.3
WMECO		0.3			0.3
For further information on the Yankee Companies, see Note 11C, "Commitments and Contingencies - Contractual Obligations - Yankee Companies," to the financial statements.
Other Investments: As of December 31, 2014 and 2013, NU had an equity ownership interest in an energy investment fund of $17.8 million and $9.8 million, respectively.
Equity investments are included in Other Long-Term Assets on the balance sheets and net earnings related to these equity investments are included in Other Income, Net on the statements of income.
K.       Revenues
Regulated Companies: The Regulated companies' retail revenues are based on rates approved by their respective state regulatory commissions. In general, rates can only be changed through formal proceedings with the state regulatory commissions. The Regulated companies' rates are designed to recover the costs to provide service to their customers, including a return on investment. The Regulated companies also utilize regulatory commission-approved tracking mechanisms to recover certain costs on a fully-reconciling basis. These tracking mechanisms require rates to be changed periodically to ensure recovery of actual costs incurred. WMECO and CL&P (effective December 1, 2014), each have a revenue decoupling mechanism to recover a pre-established level of baseline distribution delivery service revenues per year, independent of actual customer usage. Decoupling mechanisms effectively break the relationship between sales volumes and revenues recognized.
A significant portion of the Regulated companies' retail revenues relate to the recovery of costs incurred for the sale of electricity and natural gas purchased on behalf of customers. These energy supply costs are recovered from customers in rates through cost tracking mechanisms. Energy purchases are recorded in Purchased Power, Fuel and Transmission, and the sale of energy associated with these purchases are recorded in Operating Revenues.
Regulated Companies' Unbilled Revenues: Because customers are billed throughout the month based on pre-determined cycles rather than on a calendar month basis, an estimate of electricity or natural gas delivered to customers for which the customers have not yet been billed is calculated as of the balance sheet date. Unbilled revenues are included in Operating Revenues on the statements of income and are assets on the balance sheets. Actual amounts billed to customers when meter readings become available may vary from the estimated amount.
The Regulated companies estimate unbilled sales monthly using the daily load cycle method. The daily load cycle method allocates billed sales to the current calendar month based on the daily load for each billing cycle. The billed sales are subtracted from total month load, net of delivery losses, to estimate unbilled sales. Unbilled revenues are estimated by first allocating unbilled sales to the respective customer classes, then applying an estimated rate by customer class to those sales.
Regulated Companies' Transmission Revenues - Wholesale Rates: Wholesale transmission revenues are recovered through FERC approved formula rates. Wholesale transmission revenues for CL&P, NSTAR Electric, PSNH, and WMECO are collected under the ISO New England Inc. Transmission, Markets and Services Tariff (ISO-NE Tariff). The ISO-NE Tariff includes Regional Network Service (RNS), Schedule 21 - NU rate schedules, which recover the costs of transmission and other transmission-related services for CL&P, PSNH and WMECO, and Schedule 21 - NSTAR rate schedules, which recover costs of transmission and other transmission-related services for NSTAR Electric. The RNS rate, administered by ISO-NE and billed to all New England transmission load, including CL&P, NSTAR Electric, PSNH and WMECO's distribution businesses, is reset on June 1st of each year and recovers the revenue requirements associated with transmission facilities that benefit the entire New England region. Schedule 21 - NU and Schedule 21 - NSTAR rates, administered by NU, recovers the remainder of the transmission revenue requirements. The Schedule 21 - NU rate is reset on January 1st and June 1st of each year, while the Schedule 21 - NSTAR rate is reset on June 1st of each year. The Schedule 21 - NU and Schedule 21 - NSTAR rate calculations recover total transmission revenue requirements net of revenues received from other sources (i.e., RNS, rentals, etc.), thereby ensuring that NU recovers all of CL&P's, NSTAR Electric's, PSNH's and WMECO's regional and local transmission revenue requirements in accordance with the ISO-NE Tariff. RNS, Schedule 21 - NU and Schedule 21 - NSTAR rates provide for the annual reconciliation and recovery or refund of estimated costs to actual costs. The financial impacts of differences between actual and estimated costs are deferred for future recovery from, or refunded to, transmission customers. See Note 11E, "Commitments and Contingencies – FERC Base ROE Complaints," for complaints filed at FERC relating to NU's base ROE.
Regulated Companies' Transmission Revenues - Retail Rates: A significant portion of the NU transmission segment revenue comes from ISO-NE charges to the distribution businesses of CL&P, NSTAR Electric, PSNH and WMECO, each of which recovers these costs through rates charged to their retail customers. CL&P, NSTAR Electric, PSNH and WMECO each have a retail transmission cost tracking mechanism as part of their rates, which allows the electric distribution companies to charge their retail customers for transmission costs on a timely basis.
L.       Operating Expenses
Costs related to fuel and natural gas included in Purchased Power, Fuel and Transmission on the statements of income were as follows:
	For the Years Ended December 31,
(Millions of Dollars)	2014			2013			2012
NU - Natural Gas and Fuel (1)	$	599.4		$	466.5		$	346.8
PSNH - Fuel		113.4			104.8			103.4
(1) NSTAR Gas natural gas costs were included in NU beginning April 10, 2012.
M.       Allowance for Funds Used During Construction
AFUDC represents the cost of borrowed and equity funds used to finance construction and is included in the cost of the Regulated companies' utility plant. The portion of AFUDC attributable to borrowed funds is recorded as a reduction of Other Interest Expense, and the AFUDC related to equity funds is recorded as Other Income, Net on the statements of income. AFUDC costs are recovered from customers over the service life of the related plant in the form of increased revenue collected as a result of higher depreciation expense.
The Regulated companies' average AFUDC rate is based on a FERC-prescribed formula using the cost of a company's short-term financings and capitalization (preferred stock, long-term debt and common equity), as appropriate. The average rate is applied to average eligible CWIP amounts to calculate AFUDC.
NU		For the Years Ended December 31,
(Millions of Dollars, except percentages)		2014			2013			2012 (1)
Borrowed Funds		$	5.8		$	4.1		$	5.3
Equity Funds			13.7			7.1			6.8
Total AFUDC		$	19.5		$	11.2		$	12.1
Average AFUDC Rate			3.40%			2.70%			3.70%
(1) NSTAR amounts were included in NU beginning April 10, 2012.
		For the Years Ended December 31,
		2014											2013								2012
(Millions of Dollars,					NSTAR										NSTAR								NSTAR
except percentages)		CL&P			Electric			PSNH			WMECO		CL&P		Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
Borrowed Funds		$	1.9		$	2		$	0.6		$	0.9	$	2.2	$	0.5	$	0.5	$	0.5	$	2.5	$	0.3	$	1.6	$	0.5
Equity Funds			2.9			3.8			0.6			1.7		2.9		-		0.2		1		1.9		-		1.9		1
Total AFUDC		$	4.8		$	5.8		$	1.2		$	2.6	$	5.1	$	0.5	$	0.7	$	1.5	$	4.4	$	0.3	$	3.5	$	1.5
Average AFUDC Rate			3.40%			2.50%			1.80%			5.60%		3.70%		0.50%		1.10%		6.10%		3.60%		0.40%		5.90%		6.80%
N.       Other Income, Net
Items included within Other Income, Net on the statements of income primarily consist of investment income/(loss), interest income, AFUDC related to equity funds, and equity in earnings. Investment income/(loss) primarily relates to debt and equity securities held in trust. For further information, see Note 5, "Marketable Securities," to the financial statements. For further information on AFUDC related to equity funds, see Note 1M, "Summary of Significant Accounting Policies – Allowance for Funds Used During Construction," to the financial statements.
O.       Other Taxes
Gross receipts taxes levied by the state of Connecticut are collected by CL&P and Yankee Gas from their respective customers. These gross receipts taxes are shown on a gross basis with collections in Operating Revenues and payments in Taxes Other Than Income Taxes on the statements of income as follows:
	For the Years Ended December 31,
(Millions of Dollars)	2014			2013			2012
NU	$	148.2		$	144.1		$	135
CL&P		127.9			128.2			120.7
Certain sales taxes are collected by NU's companies that serve customers in Connecticut and Massachusetts as agents for state and local governments and are recorded on a net basis with no impact on the statements of income.
P.			Supplemental Cash Flow Information
NU				As of and For the Years Ended December 31,
(Millions of Dollars)				2014			2013			2012 (1)
Cash Paid/(Received) During the Year for:
	Interest, Net of Amounts Capitalized			$	349.6		$	343.3		$	356.5
	Income Taxes				334.2			50			-12.8
Non-Cash Investing Activities:
	Plant Additions Included in Accounts Payable (As of)				181.9			193.1			160.6
(1) NSTAR amounts were included in NU beginning April 10, 2012.
		As of and For the Years Ended December 31,
		2014											2013								2012
					NSTAR										NSTAR								NSTAR
(Millions of Dollars)		CL&P			Electric			PSNH			WMECO		CL&P		Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
Cash Paid/(Received) During the Year for:
	Interest, Net of Amounts Capitalized	$	144.1		$	75.3		$	41.1		$	25.9	$	131.6	$	75.8	$	43.3	$	25.8	$	129.4	$	94.6	$	49.8	$	25.8
	Income Taxes		135.4			217.1			2.3			25.1		55		163.4		-30.1		-69		-42		88.1		14.7		-8.4
Non-Cash Investing Activities:
	Plant Additions Included in Accounts Payable (As of)		63.5			34.6			39.3			14.2		51.4		57		34.9		19.5		42.8		50		16.8		30
In 2014, as a result of damages awarded to the Yankee Companies for spent nuclear fuel lawsuits against the DOE described in Note 11C, "Commitments and Contingencies - Contractual Obligations - Yankee Companies," NU received total proceeds of $132.1 million, which were net of $80.6 million in proceeds CYAPC and YAEC returned to non-affiliated member companies.
The merger of NU with NSTAR on April 10, 2012 represented a significant non-cash transaction. Refer to Note 21, "Merger of NU and NSTAR," for further information.
Q.       Related Parties
NUSCO, NU's service company, provides centralized accounting, administrative, engineering, financial, information technology, legal, operational, planning, purchasing, and other services to NU's companies. The Rocky River Realty Company, Renewable Properties, Inc. and Properties, Inc., three other NU subsidiaries, construct, acquire or lease some of the property and facilities used by NU's companies.
As of both December 31, 2014 and 2013, CL&P, PSNH and WMECO had long-term receivables from NUSCO in the amounts of $25 million, $3.8 million and $5.5 million, respectively, which were included in Other Long-Term Assets on the balance sheets. These amounts related to the funding of investments held in trust by NUSCO in connection with certain postretirement benefits for CL&P, PSNH and WMECO employees and have been eliminated in consolidation on the NU financial statements.
Included in the CL&P, NSTAR Electric, PSNH and WMECO balance sheets as of December 31, 2014 and 2013 were Accounts Receivable from Affiliated Companies and Accounts Payable to Affiliated Companies relating to transactions between CL&P, NSTAR Electric, PSNH and WMECO and other subsidiaries that are wholly-owned by NU. These amounts have been eliminated in consolidation on the NU financial statements.
R.       Severance Benefits
For the years ended December 31, 2014 and 2013, NU recorded severance benefit expenses of $15 million and $9.7 million, respectively, in connection with the partial outsourcing of information technology functions and facilities closures, as well as ongoing post-merger integration. As of December 31, 2014 and 2013, the severance accrual totaled $10.4 million and $14.7 million, respectively, and was included in Other Current Liabilities on the balance sheets.

REGULATORY_ACCOUNTING

REGULATORY ACCOUNTING	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Public Utilities Disclosure [Text Block]	2.       REGULATORY ACCOUNTING
	The rates charged to the customers of NU's Regulated companies are designed to collect each company's costs to provide service, including a return on investment. Therefore, the accounting policies of the Regulated companies follow the application of accounting guidance for entities with rate-regulated operations and reflect the effects of the rate-making process.
	Management believes it is probable that each of the Regulated companies will recover their respective investments in long-lived assets, including regulatory assets. If management were to determine that it could no longer apply the accounting guidance applicable to rate-regulated enterprises to any of the Regulated companies' operations, or that management could not conclude it is probable that costs would be recovered from customers in future rates, the costs would be charged to net income in the period in which the determination is made.
	Regulatory Assets: The components of regulatory assets are as follows:
	NU	As of December 31,
	(Millions of Dollars)	2014			2013
	Benefit Costs	$	2,016.00		$	1,240.20
	Derivative Liabilities		425.5			638
	Income Taxes, Net		635.3			626.2
	Storm Restoration Costs		502.8			589.6
	Goodwill-related		505.4			525.9
	Regulatory Tracker Mechanisms		350.5			323.4
	Contractual Obligations - Yankee Companies		123.8			154.2
	Buy Out Agreements for Power Contracts		42.6			70.2
	Other Regulatory Assets		124.7			126.8
	Total Regulatory Assets		4,726.60			4,294.50
	Less: Current Portion		672.5			535.8
	Total Long-Term Regulatory Assets	$	4,054.10		$	3,758.70
			As of December 31,
			2014									2013
						NSTAR								NSTAR
	(Millions of Dollars)		CL&P			Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Benefit Costs		$	445.4		$	515.9	$	174.3	$	85	$	297.7	$	496.7	$	100.6	$	57.3
	Derivative Liabilities			410.9			4.5		-		-		630.4		7.7		-		-
	Income Taxes, Net			437.7			83.7		38		35.5		415.5		84		40.3		43.7
	Storm Restoration Costs			319.6			103.7		47.7		31.8		397.8		109.3		43.7		38.8
	Goodwill-related			-			433.9		-		-		-		451.5		-		-
	Regulatory Tracker Mechanisms			16.1			141.4		103.5		33		8		169.5		83.3		32.6
	Buy Out Agreements for Power Contracts			-			38.6		4		-		-		64.7		5.5		-
	Other Regulatory Assets			66.1			56.1		37.3		12.9		64.6		55.9		38.1		16.7
	Total Regulatory Assets			1,695.80			1,377.80		404.8		198.2		1,814.00		1,439.30		311.5		189.1
	Less: Current Portion			220.3			198.7		111.7		51.9		150.9		204.1		92.2		43
	Total Long-Term Regulatory Assets		$	1,475.50		$	1,179.10	$	293.1	$	146.3	$	1,663.10	$	1,235.20	$	219.3	$	146.1
	Regulatory Costs in Other Long-Term Assets: The Regulated companies had $60.5 million ($1.3 million for CL&P, $33.2 million for NSTAR Electric, $0.9 million for PSNH, and $11 million for WMECO) and $65.1 million ($7.3 million for CL&P, $33.4 million for NSTAR Electric, and $10.1 million for WMECO) of additional regulatory costs as of December 31, 2014 and 2013, respectively, that were included in Other Long-Term Assets on the balance sheets. These amounts represent incurred costs for which recovery has not yet been specifically approved by the applicable regulatory agency. However, based on regulatory policies or past precedent on similar costs, management believes it is probable that these costs will ultimately be approved and recovered from customers in rates. The NSTAR Electric balance as of December 31, 2014 and 2013 primarily related to costs deferred in connection with the basic service bad debt adder. See Note 11G, "Commitments and Contingencies – Basic Service Bad Debt Adder," for further information.
	Equity Return on Regulatory Assets: For rate-making purposes, the Regulated companies recover the carrying costs related to their regulatory assets. For certain regulatory assets, the carrying cost recovered includes an equity return component. This equity return, which is not recorded on the balance sheets, totaled $1.7 million and $1.9 million for CL&P and $43.3 million and $33.1 million for PSNH as of December 31, 2014 and 2013, respectively. These carrying costs will be recovered from customers in future rates.
	Regulatory Assets - The following provides further information about regulatory assets:
	Benefit Costs: NU's Pension, SERP and PBOP Plans are accounted for in accordance with accounting guidance on defined benefit pension and other PBOP plans. The liability recorded by the Regulated companies to recognize the funded status of their retiree benefit plans are offset by regulatory assets in lieu of a charge to Accumulated Other Comprehensive Income/(Loss), reflecting ultimate recovery from customers through rates. All amounts are remeasured annually. Regulatory accounting was also applied to the portions of NU's service company costs that support the Regulated companies, as these amounts are also recoverable. As these regulatory assets do not represent a cash outlay for the Regulated companies, no carrying charge is recovered from customers.
	The increase in the funded status liability of the retiree benefit plans and the corresponding regulatory assets was primarily driven by a change in mortality assumptions, which increased the estimate of benefits to be provided to plan participants, and a decrease in the discount rate assumption. For further information on the funded status liability and related regulatory assets of the Pension, SERP and PBOP plans, see Note 9A, "Employee Benefits – Pension Benefits and Postretirement Benefits Other Than Pensions."
	CL&P, NSTAR Electric, PSNH and WMECO recover benefit costs related to their distribution and transmission operations from customers in rates as allowed by their applicable regulatory commissions. NSTAR Electric and WMECO each recover their qualified pension and PBOP expenses related to distribution operations through rate reconciling mechanisms that fully track the change in net pension and PBOP expenses each year.
	Derivative Liabilities: Regulatory assets recorded as an offset to derivative liabilities relate to the fair value of contracts used to purchase energy and energy-related products that will be recovered from customers in future rates. See Note 4, "Derivative Instruments," to the financial statements for further information.  These assets are excluded from rate base and are being recovered as the actual settlements occur over the duration of the contracts.
	Income Taxes, Net: The tax effect of temporary book-tax differences (differences between the periods in which transactions affect income in the financial statements and the periods in which they affect the determination of taxable income, including those differences relating to uncertain tax positions) is accounted for in accordance with the rate-making treatment of the applicable regulatory commissions and accounting guidance for income taxes. Differences in income taxes between the accounting guidance and the rate-making treatment of the applicable regulatory commissions are recorded as regulatory assets. As these assets are offset by deferred income tax liabilities, no carrying charge is collected. For further information regarding income taxes, see Note 10, "Income Taxes," to the financial statements.
	Storm Restoration Costs: The storm restoration cost deferrals relate to costs incurred at CL&P, NSTAR Electric, PSNH and WMECO that each company expects to recover from customers. A storm must meet certain criteria to be declared a major storm with the criteria specific to each state jurisdiction and utility company. Once a storm is declared major, all qualifying expenses incurred during storm restoration efforts are deferred and recovered from customers. In addition to storm restoration costs, CL&P and PSNH are each allowed to recover storm pre-staging costs in accordance with applicable regulation.
	CL&P, NSTAR Electric, PSNH and WMECO experienced several significant storm events, including Tropical Storm Irene in 2011, the October 2011 snowstorm, Storm Sandy in 2012 and the February 2013 blizzard. As a result of these storm events, each company suffered extensive damage to its distribution and transmission systems resulting in customer outages. Each company incurred significant costs to repair damage and restore customers' service. In addition, on November 26, 2014, a snowstorm caused damage to the electric delivery systems of PSNH and WMECO. This snowstorm resulted in estimated deferred storm restoration costs of approximately $23 million at PSNH and approximately $3 million at WMECO. The storm restoration cost regulatory asset balance at CL&P, NSTAR Electric, PSNH and WMECO reflects deferrable costs incurred for major storm events. Management believes the storm restoration costs were prudent and meet the criteria for specific cost recovery in Connecticut, Massachusetts and New Hampshire, and that recovery from customers is probable through the applicable regulatory recovery process.
	Storm Filings: Each electric utility has sought, or is seeking, recovery of its deferred storm restoration costs through its applicable regulatory recovery process.
	CL&P: As of December 31, 2014, all CL&P deferred storm costs have been reviewed and approved for recovery in distribution rates.
	On March 12, 2014, the PURA approved recovery of $365 million of deferred storm restoration costs (with carrying charges) associated with five major storms that occurred in 2011 and 2012 and ordered CL&P to capitalize approximately $18 million of the deferred storm restoration costs as utility plant, which will be recovered through depreciation expense in future rate proceedings. CL&P will recover the $365 million in its distribution rates over a six-year period that commenced on December 1, 2014. The remaining costs were either disallowed or are probable of recovery from other sources. These costs did not have a material impact on CL&P's financial position, results of operations or cash flows.
	Effective June 1, 2014, CL&P received $65.4 million of DOE Phase II Damages proceeds. On June 17, 2014, the PURA ordered CL&P to refund these proceeds to customers by offsetting the deferred storm restoration costs regulatory asset. For further information on the DOE Phase II Damages proceeds received from the Yankee Companies, see Note 11C, "Commitments and Contingencies - Contractual Obligations - Yankee Companies," to the financial statements.
	On December 17, 2014, as part of the distribution rate case decision, CL&P was also allowed recovery of the 2013 storm costs and residual 2012 Storm Sandy costs over a seven-year period that commenced on December 1, 2014.
	NSTAR Electric: On December 30, 2013, the DPU approved recovery of $34.2 million of NSTAR Electric's storm restoration costs, plus carrying costs, related to Tropical Storm Irene in 2011 and the October 2011 snowstorm. NSTAR Electric is recovering these costs, plus carrying costs, in its distribution rates over a five-year period that commenced on January 1, 2014.
	PSNH: On June 27, 2013, the NHPUC approved an increase to PSNH's distribution rates effective July 1, 2013, which included a $5 million increase to the level of funding for the major storm cost reserve. The major storm cost reserve is used to offset the storm restoration cost regulatory asset. On April 7, 2014, PSNH received an audit report from the NHPUC approving storm costs from 2011 through March 2013.
	WMECO: On December 20, 2013, the DPU approved WMECO's 2013 Annual Storm Reserve Recovery Cost Adjustment filing to begin recovering the October 2011 snowstorm and 2012 Storm Sandy restoration costs, which commenced on January 1, 2014, subject to further review and reconciliation. On December 5, 2014, the DPU approved the majority of deferred storm costs through 2011.
	Goodwill-related: The goodwill regulatory asset originated from the transaction that created NSTAR in 1999. This regulatory asset is currently being amortized and recovered from customers in rates without a carrying charge over a 40-year period (as of December 31, 2014, there were 25 years of amortization remaining).
	Regulatory Tracker Mechanisms: The Regulated companies' approved rates are designed to recover their incurred costs to provide service to customers. The Regulated companies recover certain of their costs on a fully-reconciling basis through regulatory commission-approved tracking mechanisms. The difference between the costs incurred (or the rate recovery allowed) and the actual revenues is recorded as regulatory assets (for undercollections) or regulatory liabilities (for overcollections) to be included in future customer rates each year. Carrying charges are recorded on all material regulatory tracker mechanisms.
	CL&P, NSTAR Electric, PSNH and WMECO each recover the costs associated with the procurement of energy, transmission related costs from FERC-approved transmission tariffs, energy efficiency programs, low income assistance programs, and restructuring and stranded costs as a result of deregulation, on a fully reconciling basis. Energy procurement costs at PSNH include the costs related to its generating stations.
	WMECO's and CL&P's (effective December 1, 2014) distribution revenue is decoupled from their customer sales volume. CL&P and WMECO reconcile their annual base distribution rate recovery to a pre-established level of baseline distribution delivery service revenue. Any difference between the allowed level of distribution revenue and the actual amount incurred during a 12-month period is adjusted through rates in the following period.
	Contractual Obligations - Yankee Companies: CL&P, NSTAR Electric, PSNH and WMECO are responsible for their proportionate share of the remaining costs of the CYAPC, YAEC and MYAPC nuclear facilities, including nuclear fuel storage. A portion of these amounts was recorded as a regulatory asset. Amounts for CL&P are earning a return and are being recovered through the CTA. Amounts for NSTAR Electric and WMECO are being recovered without a return through the transition charge. Amounts for PSNH were fully recovered in 2006. As a result of NU's consolidation of CYAPC and YAEC, NU's regulatory asset balance also includes the regulatory assets of CYAPC and YAEC, which totaled $97.8 million and $129.8 million as of December 31, 2014 and 2013, respectively. Intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation of the NU financial statements.
	Buy Out Agreements for Power Contracts: NSTAR Electric's balance represents the contract termination liability related to certain purchase power contract buy out agreements that were executed in 2004. The contracts' termination payments occur through September 2016 and are collected from customers through NSTAR Electric's transition charge over the same period. NSTAR Electric does not earn a return on this regulatory asset. PSNH's balance represents payments associated with the termination of various power purchase contracts that were recorded as regulatory assets and are amortized over the remaining life of the contracts.
	Other Regulatory Assets: Other Regulatory Assets primarily include asset retirement obligations, environmental remediation costs, losses associated with the reacquisition or redemption of long-term debt and various other items, partially offset by purchase price adjustments recorded as Regulatory Assets in connection with the merger with NSTAR.
	Regulatory Liabilities: The components of regulatory liabilities are as follows:
	NU	As of December 31,
	(Millions of Dollars)	2014			2013
	Cost of Removal	$	439.9		$	435.1
	Regulatory Tracker Mechanisms		192.3			151.2
	AFUDC - Transmission		67.1			68.1
Other Regulatory Liabilities		50.8			52.9
Total Regulatory Liabilities		750.1			707.3
Less: Current Portion		235			204.3
Total Long-Term Regulatory Liabilities	$	515.1		$	503
		As of December 31,
		2014									2013
					NSTAR								NSTAR
(Millions of Dollars)		CL&P			Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
Cost of Removal		$	19.7		$	258.3	$	50.3	$	1.1	$	29.1	$	250	$	49.7	$	-
Regulatory Tracker Mechanisms			122.6			20.7		14.2		22.3		95.6		21.9		21.6		21.1
AFUDC - Transmission			53.6			4.4		-		9.1		54.7		4.1		-		9.3
Other Regulatory Liabilities			10.1			28.9		2.9		0.8		8.4		31.1		1		3.4
Total Regulatory Liabilities			206			312.3		67.4		33.3		187.8		307.1		72.3		33.8
Less: Current Portion			124.7			49.6		16		22.5		94		54		20.6		19.9
Total Long-Term Regulatory Liabilities		$	81.3		$	262.7	$	51.4	$	10.8	$	93.8	$	253.1	$	51.7	$	13.9
Cost of Removal: NU's Regulated companies currently recover amounts in rates for future costs of removal of plant assets over the lives of the assets. The estimated cost to remove utility assets from service is recognized as a component of depreciation expense and the cumulative amounts collected from customers but not yet expended is recognized as a regulatory liability. Expended costs that exceed amounts collected from customers are recognized as regulatory assets, as they are probable of recovery in future rates.
AFUDC - Transmission: AFUDC was recorded by CL&P and WMECO for their NEEWS projects through May 31, 2011, all of which was reserved as a regulatory liability to reflect rate base recovery for 100 percent of the CWIP as a result of FERC-approved transmission incentives. Effective June 1, 2011, FERC approved changes to the ISO-NE Tariff in order to include 100 percent of the NEEWS CWIP in regional rate base. As a result, CL&P and WMECO no longer record AFUDC on NEEWS CWIP. NSTAR Electric recorded AFUDC on reliability-related projects over $5 million through December 31, 2014, 50 percent of which was recorded as a regulatory liability to reflect rate base recovery for 50 percent of the CWIP as a result of FERC-approved transmission incentives.
Other Regulatory Liabilities: Other Regulatory Liabilities primarily includes amounts that are subject to various rate reconciling mechanisms that, as of each period end date, would result in refunds to customers.
2014 Regulatory Developments: As a result of actions taken by the FERC and other developments in the pending base ROE complaint proceedings described in Note 11E, "Commitments and Contingencies – FERC Base ROE Complaints," in 2014 the Company recorded reserves at its electric subsidiaries to recognize the potential financial impact of the first and second complaints. As of December 31, 2014, the cumulative pre-tax reserves (excluding interest), which exclude refunds for the first complaint refund period, totaled $60.7 million at NU, $33.5 million at CL&P, $13.6 million at NSTAR Electric, $5.1 million at PSNH and $8.5 million at WMECO. As of December 31, 2013, as a result of the 2013 FERC ALJ initial decision, the Company had an aggregate pre-tax reserve (excluding interest) of $23.7 million at NU, $12.8 million at CL&P, $5.7 million at NSTAR Electric, $2.3 million at PSNH and $2.9 million at WMECO. These reserves were recorded as a regulatory liability in Regulatory Tracker Mechanisms and as a reduction of Operating Revenues.
Effective June 1, 2014, as a result of damages awarded to the Yankee Companies for spent nuclear fuel lawsuits against the DOE described in Note 11C, "Commitments and Contingencies - Contractual Obligations - Yankee Companies," the Yankee Companies returned the DOE Phase II Damages proceeds to the member companies, including CL&P, NSTAR Electric, PSNH, and WMECO, for the benefit of their respective customers. CL&P's refund obligation to customers of $65.4 million was recorded as an offset to the deferred storm restoration costs regulatory asset, as directed by PURA. NSTAR Electric's, PSNH's and WMECO's refund obligation to customers of $29.1 million, $13.1 million and $18.1 million, respectively, was recorded as a regulatory liability in Regulatory Tracker Mechanisms. Refunds to customers for these DOE proceeds began in 2014.
On December 31, 2014, NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General filed a comprehensive settlement agreement with the DPU. The comprehensive settlement agreement included resolution of the outstanding NSTAR Electric CPSL program filings for the periods 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the NSTAR Electric energy efficiency program filings regarding LBR for the periods 2008 through 2011. If approved by the DPU, NSTAR Electric and NSTAR Gas will be required to refund a total of $44.7 million to their respective customers, which was included in Regulatory Tracker Mechanisms and Other Regulatory Liabilities as of December 31, 2014. For further information, see Note 11F, "Commitments and Contingencies – 2014 Comprehensive Settlement Agreement."

PROPERTY_PLANT_AND_ACCUMULATED

PROPERTY, PLANT AND ACCUMULATED DEPRECIATION	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Property Plant And Equipment Disclosure [Text Block]	3.       PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION
	Utility property, plant and equipment is recorded at original cost. Original cost includes materials, labor, construction overhead and AFUDC for regulated property. The cost of repairs and maintenance, including planned major maintenance activities, is charged to Operating Expenses as incurred.
	The following tables summarize the investments in utility property, plant and equipment by asset category:
	NU			As of December 31,
	(Millions of Dollars)			2014			2013
	Distribution - Electric			$	12,495.20		$	11,950.20
	Distribution - Natural Gas				2,595.40			2,425.90
	Transmission				6,930.70			6,412.50
	Generation				1,170.90			1,152.30
	Electric and Natural Gas Utility				23,192.20			21,940.90
	Other (1)				551.3			508.7
	Property, Plant and Equipment, Gross				23,743.50			22,449.60
	Less: Accumulated Depreciation
		Electric and Natural Gas Utility			-5,777.80			-5,387.00
		Other			-231.8			-196.2
	Total Accumulated Depreciation				-6,009.60			-5,583.20
	Property, Plant and Equipment, Net				17,733.90			16,866.40
	Construction Work in Progress				913.1			709.8
	Total Property, Plant and Equipment, Net			$	18,647.00		$	17,576.20
	(1)       These assets are primarily comprised of building improvements, computer software, hardware and equipment and telecommunications assets at NU's service company and unregulated companies.
		As of December 31,
		2014												2013
					NSTAR												NSTAR
	(Millions of Dollars)	CL&P			Electric			PSNH			WMECO			CL&P			Electric		PSNH		WMECO
	Distribution	$	5,158.80		$	4,895.50		$	1,696.70		$	784.2		$	4,930.70		$	4,694.70	$	1,608.20	$	756.6
	Transmission		3,274.00			1,928.50			789.7			891			3,071.90			1,772.30		695.7		826.4
	Generation		-			-			1,136.50			34.4			-			-		1,131.20		21.1
	Property, Plant and Equipment, Gross		8,432.80			6,824.00			3,622.90			1,709.60			8,002.60			6,467.00		3,435.10		1,604.10
	Less: Accumulated Depreciation		-1,928.00			-1,761.40			-1,090.00			-297.4			-1,804.10			-1,631.30		-1,021.80		-271.5
	Property, Plant and Equipment, Net		6,504.80			5,062.60			2,532.90			1,412.20			6,198.50			4,835.70		2,413.30		1,332.60
	Construction Work in Progress		304.9			272.8			102.9			49.1			252.8			208.2		54.3		48.5
	Total Property, Plant and Equipment, Net	$	6,809.70		$	5,335.40		$	2,635.80		$	1,461.30		$	6,451.30		$	5,043.90	$	2,467.60	$	1,381.10
	Depreciation of utility assets is calculated on a straight-line basis using composite rates based on the estimated remaining useful lives of the various classes of property (estimated useful life for PSNH distribution). The composite rates are subject to approval by the appropriate state regulatory agency. The composite rates include a cost of removal component, which is collected from customers over the lives of the plant assets and is recognized as a regulatory liability. Depreciation rates are applied to property from the time it is placed in service.
	Upon retirement from service, the cost of the utility asset is charged to the accumulated provision for depreciation. The actual incurred removal costs are applied against the related regulatory liability.
	The depreciation rates for the various classes of utility property, plant and equipment aggregate to composite rates as follows:
	(Percent)		2014			2013			2012
	NU		3	%		2.8	%		2.5	%
	CL&P		2.7			2.5			2.5
	NSTAR Electric		3			2.9			2.8
	PSNH		3			3			3
	WMECO		3.3			2.9			3.3
	The following table summarizes average useful lives of depreciable assets:
			Average Depreciable Life
	(Years)		NU			CL&P			NSTAR Electric			PSNH			WMECO
	Distribution			34.9			37.5			32.3			32.3			30.9
	Transmission			42.5			39.8			44			43.7			49.9
	Generation			31.9			-			-			32.1			25
	Other			14.2			-			-			-			-

DERIVATIVE_INSTRUMENTS

DERIVATIVE INSTRUMENTS	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Derivative Instruments and Hedging Activities Disclosure [Text Block]	4.       DERIVATIVE INSTRUMENTS
	The Regulated companies purchase and procure energy and energy-related products, which are subject to price volatility, for their customers. The costs associated with supplying energy to customers are recoverable through customer rates. The Regulated companies manage the risks associated with the price volatility of energy and energy-related products through the use of derivative and nonderivative contracts.
	Many of the derivative contracts meet the definition of, and are designated as, normal and qualify for accrual accounting under the applicable accounting guidance. The costs and benefits of derivative contracts that meet the definition of normal are recognized in Operating Expenses or Operating Revenues on the statements of income, as applicable, as electricity or natural gas is delivered.
	Derivative contracts that are not designated as normal are recorded at fair value as current or long-term Derivative Assets or Derivative Liabilities on the balance sheets. For the Regulated companies, regulatory assets or regulatory liabilities are recorded to offset the fair values of derivatives, as contract settlements are recovered from, or refunded to, customers in their respective energy supply rates. For NU's unregulated wholesale marketing contracts that expired on December 31, 2013, changes in fair values of derivatives were included in Net Income.
	The gross fair values of derivative assets and liabilities with the same counterparty are offset and reported as net Derivative Assets or Derivative Liabilities, with current and long-term portions, on the balance sheets. The following table presents the gross fair values of contracts categorized by risk type and the net amount recorded as current or long-term derivative asset or liability:
				As of December 31,
				2014									2013
				Commodity Supply						Net Amount			Commodity Supply						Net Amount
				and Price Risk						Recorded as			and Price Risk							Recorded as
	(Millions of Dollars)			Management			Netting (1)			a Derivative			Management			Netting (1)			a Derivative
	Current Derivative Assets:
	Level 2:
		NU		$	-		$	-		$	-		$	1.9		$	-0.3		$	1.6
	Level 3:
		NU			16.2			-6.6			9.6			18.4			-9.8			8.6
		CL&P			16.1			-6.6			9.5			17.1			-9.8			7.3
		NSTAR Electric			0.1			-			0.1			1.2			-			1.2
	Long-Term Derivative Assets:
	Level 2:
		NU		$	-		$	-		$	-		$	0.2		$	-		$	0.2
	Level 3:
		NU			93.5			-19.2			74.3			116.2			-42.2			74
		CL&P			93.5			-19.2			74.3			113.6			-42.2			71.4
	Current Derivative Liabilities:
	Level 2:
		NU		$	-9.8		$	-		$	-9.8		$	-		$	-		$	-
	Level 3:
		NU			-90			-			-90			-93.7			-			-93.7
		CL&P			-88.5			-			-88.5			-92.2			-			-92.2
		NSTAR Electric			-1.5			-			-1.5			-1.5			-			-1.5
	Long-Term Derivative Liabilities:
	Level 2:
		NU		$	-0.3		$	-		$	-0.3		$	-		$	-		$	-
	Level 3:
		NU			-409.3			-			-409.3			-624.1			-			-624.1
		CL&P			-406.2			-			-406.2			-617.1			-			-617.1
		NSTAR Electric			-3.1			-			-3.1			-7			-			-7
	(1)       Amounts represent derivative assets and liabilities that NU elected to record net on the balance sheets. These amounts are subject to master netting agreements or similar agreements for which the right of offset exists.
	The business activities that result in the recognition of derivative assets also create exposure to various counterparties. As of December 31, 2014, NU and CL&P's derivative assets were exposed to counterparty credit risk. Of NU's and CL&P's derivative assets, $64 million was contracted with investment grade entities.
	For further information on the fair value of derivative contracts, see Note 1H, "Summary of Significant Accounting Policies - Fair Value Measurements," and Note 1I, "Summary of Significant Accounting Policies - Derivative Accounting," to the financial statements.
	Derivative Contracts At Fair Value with Offsetting Regulatory Amounts
	Commodity Supply and Price Risk Management: As required by regulation, CL&P, along with UI, has capacity-related contracts with generation facilities. CL&P has a sharing agreement with UI, with 80 percent of each contract allocated to CL&P and 20 percent allocated to UI. The combined capacity of these contracts is 787 MW. The capacity contracts extend through 2026 and obligate both CL&P and UI to make or receive payments on a monthly basis to or from the generation facilities based on the difference between a set capacity price and the capacity market price received in the ISO-NE capacity markets. In addition, CL&P has a contract to purchase 0.1 million MWh of energy per year through 2020.
	NSTAR Electric has a renewable energy contract to purchase 0.1 million MWh of energy per year through 2018 and a capacity-related contract to purchase up to 35 MW per year through 2019.
	As of December 31, 2014 and 2013, NU had NYMEX future contracts in order to reduce variability associated with the purchase price of approximately 8.8 million and 9.1 million MMBtu of natural gas, respectively.
	The following table presents the current change in fair value, primarily recovered through rates from customers, associated with NU's derivative contracts:
					Gain/(Loss) Recognized on Derivatives
	(Millions of Dollars)				For the Years Ended December 31,
	NU				2014			2013			2012
	Balance Sheets:
		Regulatory Assets and Liabilities			$	134.4		$	160.6		$	-29
	Statements of Income:
		Purchased Power, Fuel and Transmission				-			1			-0.7
	Credit Risk
	Certain of NU's derivative contracts contain credit risk contingent features. These features require NU to maintain investment grade credit ratings from the major rating agencies and to post collateral for contracts in a net liability position over specified credit limits. As of December 31, 2014, NU had approximately $10 million of derivative contracts in a net liability position that were subject to credit risk contingent features and would have been required to post additional collateral of approximately $10 million if NU parent's unsecured debt credit ratings had been downgraded to below investment grade. As of December 31, 2013, there were no derivative contracts in a net liability position that were subject to credit risk contingent features.
	Fair Value Measurements of Derivative Instruments
	Derivative contracts classified as Level 2 in the fair value hierarchy relate to the financial contracts for natural gas futures. Prices are obtained from broker quotes and are based on actual market activity. The contracts are valued using NYMEX natural gas prices. Valuations of these contracts also incorporate discount rates using the yield curve approach.
	The fair value of derivative contracts classified as Level 3 utilizes significant unobservable inputs. The fair value is modeled using income techniques, such as discounted cash flow valuations adjusted for assumptions relating to exit price. Significant observable inputs for valuations of these contracts include energy and energy-related product prices in future years for which quoted prices in an active market exist. Fair value measurements categorized in Level 3 of the fair value hierarchy are prepared by individuals with expertise in valuation techniques, pricing of energy and energy-related products, and accounting requirements. The future power and capacity prices for periods that are not quoted in an active market or established at auction are based on available market data and are escalated based on estimates of inflation to address the full time period of the contract.
	Valuations of derivative contracts using a discounted cash flow methodology include assumptions regarding the timing and likelihood of scheduled payments and also reflect non-performance risk, including credit, using the default probability approach based on the counterparty's credit rating for assets and the Company's credit rating for liabilities. Valuations incorporate estimates of premiums or discounts that would be required by a market participant to arrive at an exit price, using historical market transactions adjusted for the terms of the contract.
	The following is a summary of NU's, including CL&P's and NSTAR Electric's, Level 3 derivative contracts and the range of the significant unobservable inputs utilized in the valuations over the duration of the contracts:
		As of December 31,
		2014									2013
			Range						Period Covered			Range						Period Covered
	Energy Prices:
	NU	$	52			per MWh			2020		$	49	-	77	per MWh			2018 - 2029
CL&P	$	52			per MWh			2020		$	56	-	58	per MWh			2018 - 2029
Capacity Prices:
NU	$	5.3	-	12.98		per kW-Month		2016 - 2026		$	5.07	-	11.82		per kW-Month		2017 - 2029
CL&P	$	11.08	-	12.98		per kW-Month		2018 - 2026		$	5.07	-	10.42		per kW-Month		2017 - 2026
NSTAR Electric	$	5.3	-	11.1		per kW-Month		2016 - 2019		$	5.07	-	7.38		per kW-Month		2017 - 2019
Forward Reserve:
NU, CL&P	$	5.8	-	9.5		per kW-Month		2015 - 2024		$	3.3				per kW-Month		2014 - 2024
REC Prices:
NU	$	38	-	56	per REC			2015 - 2018		$	36	-	87		per REC		2014 - 2029
NSTAR Electric	$	38	-	56	per REC			2015 - 2018		$	36	-	70		per REC		2014 - 2018
Exit price premiums of 7 percent through 24 percent are also applied on these contracts and reflect the most recent market activity available for similar type contracts.
Significant increases or decreases in future energy or capacity prices in isolation would decrease or increase, respectively, the fair value of the derivative liability. Any increases in the risk premiums would increase the fair value of the derivative liabilities. Changes in these fair values are recorded as a regulatory asset or liability and would not impact net income.
Valuations using significant unobservable inputs: The following tables present changes in the Level 3 category of derivative assets and derivative liabilities measured at fair value on a recurring basis. The derivative assets and liabilities are presented on a net basis.
(Millions of Dollars)		NU			CL&P			NSTAR Electric
Derivatives, Net:
Fair Value as of January 1, 2013		$	-878.6		$	-866.2		$	-14.9
Net Realized/Unrealized Gains Included in:
	Net Income (1)		10.9			-			-
	Regulatory Assets and Liabilities		158.3			148.9			3.5
Settlements			74.2			86.7			4.1
Fair Value as of December 31, 2013		$	-635.2		$	-630.6		$	-7.3
Net Realized/Unrealized Gains Included in Regulatory Assets and Liabilities			141.3			139.7			4.3
Settlements			78.5			80			-1.5
Fair Value as of December 31, 2014		$	-415.4		$	-410.9		$	-4.5
The Net Income impact for the year ended December 31, 2013 related to the unregulated wholesale marketing sales contract that was offset by the losses on the unregulated sourcing contracts classified as Level 2 in the fair value hierarchy, resulting in a total net gain of $1 million.

MARKETABLE_SECURITIES

MARKETABLE SECURITIES	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Marketable Securities [Text Block]	5.       MARKETABLE SECURITIES
	NU maintains trusts to fund certain non-qualified executive benefits and WMECO maintains a spent nuclear fuel trust to fund WMECO's prior period spent nuclear fuel liability. These trusts hold marketable securities. These trusts are not subject to regulatory oversight by state or federal agencies. In addition, CYAPC and YAEC maintain legally restricted trusts, each of which holds marketable securities, for settling the decommissioning obligations of their nuclear power plants.
	The Company elected to record mutual funds at fair value and certain other equity investments as trading securities, with the changes in fair values recorded in Other Income, Net on the statements of income. As of December 31, 2014 and 2013, the mutual funds and equity investments were classified as Level 1 in the fair value hierarchy and totaled $85.1 million and $57.2 million, respectively. For the years ended December 31, 2014, 2013 and 2012, net gains on these securities of $1.9 million, $10.2 million and $5.9 million, respectively, were recorded in Other Income, Net on the statements of income. Dividend income is recorded in Other Income, Net when dividends are declared. All other marketable securities are accounted for as available-for-sale.
	Available-for-Sale Securities: The following is a summary of NU's and WMECO's available-for-sale securities. These securities are recorded at fair value and are included in current and long-term Marketable Securities on the balance sheets.
			As of December 31,
			2014												2013
						Pre-Tax			Pre-Tax								Pre-Tax		Pre-Tax
			Amortized			Unrealized			Unrealized						Amortized		Unrealized		Unrealized
	(Millions of Dollars)		Cost			Gains			Losses			Fair Value			Cost		Gains		Losses		Fair Value
	NU
		Debt Securities (1)	$	313		$	7.5		$	-0.3		$	320.2		$	299.2	$	2.5	$	-2.1	$	299.6
		Equity Securities (1)		160.6			73.3			-			233.9			163.6		60.5		-		224.1
	WMECO
		Debt Securities (2)		58.2			-			-0.1			58.1			57.9		-		-		57.9
	NU's amounts include CYAPC's and YAEC's marketable securities held in nuclear decommissioning trusts of $450.8 million and $424 million as of December 31, 2014 and 2013, respectively, which are legally restricted and can only be used for the costs of decommissioning of the nuclear power plants owned by these companies. Unrealized gains and losses for the nuclear decommissioning trusts are recorded in Marketable Securities with the corresponding offset to Other Long-Term Liabilities on the balance sheets, with no impact on the statements of income. All of the equity securities accounted for as available-for-sale securities are held in the CYAPC and YAEC trusts.
	Unrealized gains and losses on debt securities held by WMECO are recorded in Marketable Securities with the corresponding offset to Other Long-Term Assets on the balance sheets.
	Unrealized Losses and Other-than-Temporary Impairment: There have been no significant unrealized losses, other-than-temporary impairments or credit losses for NU or WMECO. Factors considered in determining whether a credit loss exists include the duration and severity of the impairment, adverse conditions specifically affecting the issuer, and the payment history, ratings and rating changes of the security. For asset-backed debt securities, underlying collateral and expected future cash flows are also evaluated.
	Realized Gains and Losses: Realized gains and losses on available-for-sale securities are recorded in Other Income, Net for NU's benefit trust, Other Long-Term Assets for WMECO, and offset in Other Long-Term Liabilities for CYAPC and YAEC. NU utilizes the specific identification basis method for the NU benefit trust and the average cost basis method for the WMECO trust and the CYAPC and YAEC nuclear decommissioning trusts to compute the realized gains and losses on the sale of available-for-sale securities.
	Contractual Maturities: As of December 31, 2014, the contractual maturities of available-for-sale debt securities are as follows:
			NU						WMECO
			Amortized						Amortized
	(Millions of Dollars)		Cost			Fair Value			Cost			Fair Value
	Less than one year (1)		$	55		$	55		$	28.7		$	28.7
	One to five years			88.8			89.1			25.8			25.8
	Six to ten years			66			67.7			0.7			0.7
	Greater than ten years			103.2			108.4			3			2.9
	Total Debt Securities		$	313		$	320.2		$	58.2		$	58.1
	(1)       Amounts in the Less than one year NU category include securities in the CYAPC and YAEC nuclear decommissioning trusts, which are restricted and are classified in long-term Marketable Securities on the balance sheets.
	Fair Value Measurements: The following table presents the marketable securities recorded at fair value on a recurring basis by the level in which they are classified within the fair value hierarchy:
					NU						WMECO
					As of December 31,						As of December 31,
	(Millions of Dollars)				2014			2013			2014			2013
	Level 1:
		Mutual Funds and Equities			$	319		$	281.3		$	-		$	-
		Money Market Funds				24.9			32.9			4.3			10.9
	Total Level 1				$	343.9		$	314.2		$	4.3		$	10.9
	Level 2:
		U.S. Government Issued Debt Securities (Agency and Treasury)			$	51.3		$	61.4		$	-		$	6.8
		Corporate Debt Securities				49.1			53.6			14.7			15.1
		Asset-Backed Debt Securities				54.1			30.4			14.5			9
		Municipal Bonds				116.3			105.5			13			11.2
		Other Fixed Income Securities				24.5			15.8			11.6			4.9
	Total Level 2				$	295.3		$	266.7		$	53.8		$	47
	Total Marketable Securities				$	639.2		$	580.9		$	58.1		$	57.9
	U.S. government issued debt securities are valued using market approaches that incorporate transactions for the same or similar bonds and adjustments for yields and maturity dates. Corporate debt securities are valued using a market approach, utilizing recent trades of the same or similar instrument and also incorporating yield curves, credit spreads and specific bond terms and conditions. Asset-backed debt securities include collateralized mortgage obligations, commercial mortgage backed securities, and securities collateralized by auto loans, credit card loans or receivables. Asset-backed debt securities are valued using recent trades of similar instruments, prepayment assumptions, yield curves, issuance and maturity dates, and tranche information. Municipal bonds are valued using a market approach that incorporates reported trades and benchmark yields. Other fixed income securities are valued using pricing models, quoted prices of securities with similar characteristics, and discounted cash flows.

ASSET_RETIREMENT_OBLIGATIONS

ASSET RETIREMENT OBLIGATIONS	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Asset Retirement Obligation Disclosure [Text Block]	6.       ASSET RETIREMENT OBLIGATIONS
	NU, including CL&P, NSTAR Electric, PSNH and WMECO, recognizes a liability for the fair value of an ARO on the obligation date if the liability's fair value can be reasonably estimated and is conditional on a future event. Settlement dates and future costs are reasonably estimated when sufficient information becomes available. Management has identified various categories of AROs, primarily certain assets containing asbestos and hazardous contamination and has performed fair value calculations, reflecting expected probabilities for settlement scenarios.
	The fair value of an ARO is recorded as a liability in Other Long-Term Liabilities with a corresponding amount included in Property, Plant and Equipment, Net on the balance sheets. The ARO assets are depreciated, and the ARO liabilities are accreted over the estimated life of the obligation with corresponding credits recorded as accumulated depreciation and ARO liabilities, respectively. As the Regulated companies are rate-regulated on a cost-of-service basis, these companies apply regulatory accounting guidance and both the depreciation and accretion costs associated with the Regulated companies' AROs are recorded as increases to Regulatory Assets on the balance sheets.
	A reconciliation of the beginning and ending carrying amounts of ARO liabilities are as follows:
	NU			As of December 31,
	(Millions of Dollars)			2014		2013
	Balance as of Beginning of Year			$	424.9	$	412.2
	Liabilities Incurred During the Year				1.3		0.1
	Liabilities Settled During the Year				-19.5		-13.8
	Accretion				25.1		23.8
	Revisions in Estimated Cash Flows				-5.5		2.6
	Balance as of End of Year			$	426.3	$	424.9
		As of December 31,
		2014								2013
				NSTAR								NSTAR
	(Millions of Dollars)	CL&P		Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Balance as of Beginning of Year	$	35	$	32.8	$	19.5	$	4.5	$	33.6	$	31.4	$	18.4	$	4.3
	Liabilities Incurred During the Year		-		-		-		1.1		-		-		-		-
	Liabilities Settled During the Year		-1.1		-		-		-		-0.7		-0.1		-		-
	Accretion		1.9		1.5		1.1		0.3		2.2		1.5		1.2		0.3
	Revisions in Estimated Cash Flows		-0.5		-		-		-		-0.1		-		-0.1		-0.1
	Balance as of End of Year	$	35.3	$	34.3	$	20.6	$	5.9	$	35	$	32.8	$	19.5	$	4.5
	NU's amounts include CYAPC and YAEC's AROs of $317.3 million and $318.8 million as of December 31, 2014 and 2013, respectively. The fair value of the ARO for CYAPC and YAEC includes uncertainties of the fuel off-load dates related to the DOE's timing of performance regarding its obligation to dispose of the spent nuclear fuel and high level waste. The incremental asset recorded as an offset to the ARO liability was fully depreciated since the plants have no remaining useful life. Any changes in the assumptions used to calculate the fair value of the ARO liability are recorded with a corresponding offset to the related regulatory asset. The assets held in the CYAPC and YAEC nuclear decommissioning trusts are restricted for settling the ARO and all other decommissioning obligations. For further information on the assets held in the nuclear decommissioning trusts, see Note 5, "Marketable Securities," to the financial statements.

SHORT_TERM_DEBT

SHORT TERM DEBT	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Short Term Debt Text Block	7.       SHORT-TERM DEBT
	Short-Term Borrowing Limits:  The amount of short-term borrowings that may be incurred by CL&P, NSTAR Electric and WMECO is subject to periodic approval by the FERC. As a result of the NHPUC having jurisdiction over PSNH's short-term debt, PSNH is not currently required to obtain FERC approval for its short-term borrowings. On July 31, 2013, the FERC granted authorization to allow CL&P and WMECO to incur total short-term borrowings up to a maximum of $600 million and $300 million, respectively, effective January 1, 2014 through December 31, 2015. On June 11, 2014, the FERC granted authorization to allow NSTAR Electric to issue total short-term debt securities in an aggregate principal amount not to exceed $655 million outstanding at any one time, effective October 24, 2014 through October 23, 2016.
	PSNH is authorized by regulation of the NHPUC to incur short-term borrowings up to 10 percent of net fixed plant plus an additional $60 million until further ordered by the NHPUC. As of December 31, 2014, PSNH's short-term debt authorization under the 10 percent of net fixed plant test plus $60 million totaled approximately $306 million.
	CL&P's certificate of incorporation contains preferred stock provisions restricting the amount of unsecured debt that CL&P may incur, including limiting unsecured indebtedness with a maturity of less than 10 years to 10 percent of total capitalization. As of December 31, 2014, CL&P had $432.1 million of unsecured debt capacity available under this authorization.
	Yankee Gas and NSTAR Gas are not required to obtain approval from any state or federal authority to incur short-term debt.
	Credit Agreements and Commercial Paper Programs: NU parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas are parties to a five-year $1.45 billion revolving credit facility. The revolving credit facility is to be used primarily to backstop NU parent's $1.45 billion commercial paper program. The commercial paper program allows NU parent to issue commercial paper as a form of short-term debt. Effective July 23, 2014, NU parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas extended the expiration date of their joint revolving credit facility for one additional year to September 6, 2019. CL&P has a borrowing sublimit of $600 million and PSNH and WMECO each have borrowing sublimits of $300 million. As of December 31, 2014 and 2013, NU had approximately $1.1 billion and $1.01 billion, respectively, in short-term borrowings outstanding under the NU parent commercial paper program, leaving $348.9 million and $435.5 million of available borrowing capacity as of December 31, 2014 and 2013, respectively. The weighted-average interest rate on these borrowings as of December 31, 2014 and 2013 was 0.43 percent and 0.24 percent, respectively, which is generally based on A2/P2 rated commercial paper. As of December 31, 2014, there were intercompany loans from NU of $133.4 million to CL&P, $90.5 million to PSNH and $21.4 million to WMECO. As of December 31, 2013, there were intercompany loans from NU of $287.3 million to CL&P and $86.5 million to PSNH.
	NSTAR Electric has a five-year $450 million revolving credit facility. This facility serves to backstop NSTAR Electric's existing $450 million commercial paper program. Effective July 23, 2014, NSTAR Electric extended the expiration date of its revolving credit facility for one additional year to September 6, 2019. As of December 31, 2014 and 2013, NSTAR Electric had $302 million and $103.5 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $148 million and $346.5 million of available borrowing capacity as of December 31, 2014 and 2013, respectively. The weighted-average interest rate on these borrowings as of December 31, 2014 and 2013 was 0.27 percent and 0.13 percent, respectively, which is generally based on A2/P1 rated commercial paper.
	Except as described below, amounts outstanding under the commercial paper programs are included in Notes Payable for NU and NSTAR Electric and classified in current liabilities on the balance sheets as all borrowings are outstanding for no more than 364 days at one time. Intercompany loans from NU to CL&P, PSNH and WMECO are included in Notes Payable to NU Parent and classified in current liabilities on the balance sheets. Intercompany loans from NU to CL&P, PSNH and WMECO are eliminated in consolidation in NU's balance sheets.
	On January 15, 2015, NU parent issued $150 million of 1.60 percent Series G Senior Notes due to mature in 2018 and $300 million of 3.15 percent Series H Senior Notes, due to mature in 2025. The proceeds, net of issuance costs, were used to repay short-term borrowings outstanding under the NU commercial paper program. As the debt issuances refinanced short-term debt, the short-term debt was classified as Long-Term Debt as of December 31, 2014. On January 2, 2014, Yankee Gas issued $100 million of Series L First Mortgage Bonds and $25 million of the proceeds was used to repay short-term borrowings outstanding under the NU commercial paper program. As the debt issuance refinanced short-term debt, these amounts were classified as Long-Term Debt on NU's balance sheet as of December 31, 2013. See Note 8, "Long-Term Debt" for further information on these debt issuances.
	Under the credit facilities, NU and its subsidiaries must comply with certain financial and non-financial covenants, including a consolidated debt to total capitalization ratio. As of December 31, 2014 and 2013, NU and its subsidiaries were in compliance with these covenants. If NU or its subsidiaries were not in compliance with these covenants, an event of default would occur requiring all outstanding borrowings by such borrower to be repaid and additional borrowings by such borrower would not be permitted under its respective credit facility.

LONGTERM_DEBT

LONG-TERM DEBT	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Long-term Debt [Text Block]	8.       LONG-TERM DEBT
	Details of long-term debt outstanding are as follows:
	CL&P		As of December 31,
	(Millions of Dollars)		2014			2013
	First Mortgage Bonds:
		7.875% 1994 Series D due 2024	$	139.8		$	139.8
		4.800% 2004 Series A due 2014 (1)		-			150
		5.750% 2004 Series B due 2034		130			130
		5.000% 2005 Series A due 2015		100			100
		5.625% 2005 Series B due 2035		100			100
		6.350% 2006 Series A due 2036		250			250
		5.375% 2007 Series A due 2017		150			150
		5.750% 2007 Series B due 2037		150			150
		5.750% 2007 Series C due 2017		100			100
		6.375% 2007 Series D due 2037		100			100
		5.650% 2008 Series A due 2018		300			300
		5.500% 2009 Series A due 2019		250			250
		2.500% 2013 Series A due 2023		400			400
		4.300% 2014 Series A due 2044 (2)		250			-
	Total First Mortgage Bonds			2,419.80			2,319.80
	Pollution Control Revenue Bonds:
		4.375% Fixed Rate Tax Exempt due 2028		120.5			120.5
		1.550% Fixed Rate Tax Exempt due 2031 (3)		62			62
	Total Pollution Control Revenue Bonds			182.5			182.5
	Spent Nuclear Fuel Obligation			244.5			244.4
	Less Amounts due Within One Year			-162			-150
	Unamortized Premiums and Discounts, Net			-4.8			-5.5
	CL&P Long-Term Debt		$	2,680.00		$	2,591.20
	NSTAR Electric		As of December 31,
	(Millions of Dollars)		2014			2013
	Debentures:
		4.875% due 2014 (4)	$	-		$	300
		5.750% due 2036		200			200
		5.625% due 2017		400			400
		5.500% due 2040		300			300
		2.375% due 2022		400			400
		Variable Rate due 2016 (5)		200			200
		4.400% due 2044 (4)		300			-
	Total Debentures			1,800.00			1,800.00
	Bonds:
		7.375% Tax Exempt Sewage Facility Revenue Bonds, due 2015		4.7			6.4
	Total Bonds			4.7			6.4
	Less Amounts due Within One Year			-4.7			-301.7
	Unamortized Premiums and Discounts, Net			-7.3			-5.3
	NSTAR Electric Long-Term Debt		$	1,792.70		$	1,499.40
	PSNH		As of December 31,
	(Millions of Dollars)		2014			2013
	First Mortgage Bonds:
		5.25% Series L due 2014 (6)	$	-		$	50
		5.60% Series M due 2035		50			50
		6.15% Series N due 2017		70			70
		6.00% Series O due 2018		110			110
		4.50% Series P due 2019		150			150
		4.05% Series Q due 2021		122			122
		3.20% Series R due 2021		160			160
		3.50% Series S due 2023 (7)		325			250
	Total First Mortgage Bonds			987			962
	Pollution Control Revenue Bonds:
		Adjustable Rate Tax Exempt Series A due 2021		89.3			89.3
	Total Pollution Control Revenue Bonds			89.3			89.3
	Less Amounts due Within One Year			-			-50
	Unamortized Premiums and Discounts, Net			-			-2.3
	PSNH Long-Term Debt		$	1,076.30		$	999
	WMECO		As of December 31,
(Millions of Dollars)		2014			2013
Notes:
	5.90% Senior Notes Series B, due 2034	$	50		$	50
	5.24% Senior Notes Series C, due 2015		50			50
	6.70% Senior Notes Series D, due 2037		40			40
	5.10% Senior Notes Series E, due 2020		95			95
	3.50% Senior Notes Series F, due 2021		250			250
	3.88% Senior Notes Series G, due 2023		80			80
Total Notes			565			565
Spent Nuclear Fuel Obligation			57.4			57.3
Less Amounts due Within One Year			-50			-
Unamortized Premiums and Discounts, Net			6.1			7.1
WMECO Long-Term Debt		$	578.5		$	629.4
OTHER		As of December 31,
(Millions of Dollars)		2014				2013
Yankee Gas - First Mortgage Bonds:
	8.48% Series B due 2022	$	20		$	20
	4.80% Series G due 2014 (8)		-			75
	5.26% Series H due 2019		50			50
	5.35% Series I due 2035		50			50
	6.90% Series J due 2018		100			100
	4.87% Series K due 2020		50			50
	4.82% Series L due 2044 (8)		100			-
Total First Mortgage Bonds			370			345
Unamortized Premium			0.6			0.7
Yankee Gas Long-Term Debt			370.6			345.7
NSTAR Gas - First Mortgage Bonds:
	9.95% Series J due 2020		25			25
	7.11% Series K due 2033		35			35
	7.04% Series M due 2017		25			25
	4.46% Series N due 2020		125			125
NSTAR Gas Long-Term Debt			210			210
Other - Notes and Debentures:
	1.45% Senior Notes Series E due 2018 (NU Parent)		300			300
	2.80% Senior Notes Series F due 2023 (NU Parent)		450			450
	4.50% Debentures due 2019 (NU Parent)		350			350
	NU Commercial Paper Borrowings(8) (9)		446.3			25
	Spent Nuclear Fuel Obligation (CYAPC)		179.4			179.4
Total Other Notes and Debentures			1,725.70			1,304.40
Fair Value Adjustment (10)			202.3			230.7
Less Amounts due Within One Year			-			-
Less Fair Value Adjustment - Current Portion (10)			-28.9			-31.7
Unamortized Premiums and Discounts, Net			-1.2			-1.3
Total Other Long-Term Debt		$	2,478.50		$	2,057.80
Total NU Long-Term Debt		$	8,606.00		$	7,776.80
On September 15, 2014, CL&P repaid at maturity the $150 million of 4.80 percent 2004 Series A First Mortgage Bonds, using short-term borrowings.
On April 24, 2014, CL&P issued $250 million of 4.30 percent 2014 Series A First Mortgage Bonds, due to mature in 2044. The proceeds, net of issuance costs, were used to repay short-term borrowings.
On February 12, 2015, CL&P notified the trustee that it intends to purchase and cancel the bonds on April 1, 2015, after they have been tendered by the bondholders.
On March 7, 2014, NSTAR Electric issued $300 million of 4.40 percent debentures, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $300 million of 4.875 percent debentures that matured on April 15, 2014.
As of December 31, 2014 and 2013, the interest rate was 0.4721 percent and 0.478 percent, respectively.
On July 15, 2014, PSNH repaid at maturity the $50 million of 5.25 percent Series L First Mortgage Bonds using short-term borrowings.
On October 14, 2014, PSNH issued $75 million of first mortgage bonds at a yield of 3.144 percent due to mature in 2023. The first mortgage bonds are part of the same series of PSNH's existing 3.50 percent Series S First Mortgage Bonds that were initially issued in November 2013. The proceeds, net of issuance costs, were used to repay short-term borrowings.
On January 2, 2014, Yankee Gas issued $100 million of 4.82 percent Series L First Mortgage Bonds, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $75 million 4.80 percent Series G First Mortgage Bonds that matured on January 1, 2014 and to pay $25 million in short-term borrowings. As the debt issuance refinanced short-term debt, these amounts were classified as Long-Term Debt on NU's balance sheet as of December 31, 2013.
On January 15, 2015, NU parent issued $150 million of 1.60 percent Series G Senior Notes due to mature in 2018 and $300 million of 3.15 percent Series H Notes, due to mature in 2025. The proceeds, net of issuance costs, were used to repay short-term borrowings outstanding under the NU commercial paper program. As the debt issuances refinanced short-term debt, the short-term debt was classified as Long-Term Debt as of December 31, 2014.
Amount relates to the purchase price adjustment required to record the NSTAR long-term debt at fair value on the date of the merger.
Long-term debt maturities, mandatory tender payments and cash sinking fund requirements on debt outstanding for the years 2015 through 2019 and thereafter are shown below. These amounts exclude the spent nuclear fuel obligation, net unamortized premiums and discounts, and other fair value adjustments as of December 31, 2014:
(Millions of Dollars)	NU			CL&P			NSTAR Electric		PSNH		WMECO
2015	$	216.7		$	162		$	4.7	$	-	$	50
2016		200			-			200		-		-
2017		745			250			400		70		-
2018		810			300			-		110		-
2019		800			250			-		150		-
Thereafter		4,956.60			1,640.30			1,200.00		746.3		515
Total	$	7,728.30		$	2,602.30		$	1,804.70	$	1,076.30	$	565
The utility plant of CL&P, PSNH, Yankee Gas and NSTAR Gas is subject to the lien of each company's respective first mortgage bond indenture. The NSTAR Electric, WMECO and NU parent debt is unsecured.
CL&P's obligation to repay each series of PCRBs is secured by first mortgage bonds. Each such series of first mortgage bonds contains similar terms and provisions as the applicable series of PCRBs. If CL&P fails to meet its obligations under the first mortgage bonds, then the holder of the first mortgage bonds (the issuer of the PCRBs) would have rights under the first mortgage bonds. CL&P's $62 million tax-exempt PCRBs, which are subject to mandatory tender for purchase on April 1, 2015 and carry a coupon rate of 1.55 percent during the current three-year fixed rate period, cannot be redeemed prior to its tender date. CL&P's $120.5 million tax-exempt PCRBs will be subject to redemption at par on or after September 1, 2021. All other long-term debt securities are subject to make-whole provisions.
PSNH's obligation to repay the PCRBs is secured by first mortgage bonds and bond insurance. The first mortgage bonds contain similar terms and provisions as the PCRBs. If PSNH fails to meet its obligations under the first mortgage bonds, then the holder of the first mortgage bonds (the issuer of the PCRBs) would have rights under the first mortgage bonds. The PSNH Series A tax-exempt PCRBs are currently callable at 100 percent of par. The PCRBs bear interest at a rate that is periodically set pursuant to auctions. PSNH is not obligated to purchase these PCRBs, which mature in 2021, from the remarketing agent. As of December 31, 2014 and 2013, the interest rate was 0.175 percent and 0.088 percent, respectively.
The long-term debt agreements provide that NU and certain of its subsidiaries must comply with certain covenants as are customarily included in such agreements, including a minimum equity requirement for NSTAR Gas. Under the minimum equity requirement, the outstanding long-term debt of NSTAR Gas must not exceed equity.
Yankee Gas has certain long-term debt agreements that contain cross-default provisions. No other debt issuances contain cross-default provisions as of December 31, 2014.
On August 27, 2014, PURA approved CL&P's request to extend the authorization period for issuance of up to $366.4 million in long-term debt from December 31, 2014 to December 31, 2015.
On October 3, 2014, FERC granted authorization to allow NPT to issue short-term and long-term debt securities in an aggregate amount not to exceed $500 million outstanding at any one time, effective December 31, 2014 through December 31, 2016.
On November 26, 2014, PURA approved Yankee Gas' request to extend the authorization period for issuance of up to $200 million in long-term debt from December 31, 2014 to December 31, 2015.
On January 12, 2015, NSTAR Gas filed an application with the DPU requesting authorization to issue up to $100 million in long-term debt for the period ending December 31, 2015.
Spent Nuclear Fuel Obligation: Under the Nuclear Waste Policy Act of 1982, CL&P and WMECO must pay the DOE for the costs of disposal of spent nuclear fuel and high-level radioactive waste for the period prior to the sale of their ownership shares in the Millstone nuclear power stations. The Millstone Nuclear Generating station was made up of Millstone 1, Millstone 2, and Millstone 3 and all three units were sold in March 2001.
The DOE is responsible for the selection and development of repositories for, and the disposal of, spent nuclear fuel and high-level radioactive waste. For nuclear fuel used to generate electricity prior to April 7, 1983 (Prior Period Spent Nuclear Fuel) for CL&P and WMECO, an accrual has been recorded for the full liability, and payment must be made by CL&P and WMECO to the DOE prior to the first delivery of spent fuel to the DOE. After the sale of Millstone, CL&P and WMECO remained responsible for their share of the disposal costs associated with the Prior Period Spent Nuclear Fuel. Until such payment to the DOE is made, the outstanding liability will continue to accrue interest at the 3-month Treasury bill yield rate. In addition, as a result of consolidating CYAPC, NU has consolidated $179.4 million in additional spent nuclear fuel obligations, including interest, as of both December 31, 2014 and 2013. The obligation due to the DOE for the disposal of CL&P's and WMECO's Prior Period Spent Nuclear Fuel and CYAPC's spent nuclear fuel obligation include accumulated interest costs of $350.4 million and $350.3 million ($178 million and $177.9 million for CL&P and $41.8 million and $41.7 million for WMECO) as of December 31, 2014 and 2013, respectively.
WMECO and CYAPC maintain trusts to fund amounts due to the DOE for the disposal of spent nuclear fuel. For further information on these trusts, see Note 5, "Marketable Securities," to the financial statements.

PENSION_BENEFITS_AND_POSTRETIR

PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Pension and Other Postretirement Benefits Disclosure [Text Block]	9.       EMPLOYEE BENEFITS
	A.       Pension Benefits and Postretirement Benefits Other Than Pensions
	As of December 31, 2014, NUSCO sponsored two defined benefit retirement plans that covered eligible employees, including employees of CL&P, NSTAR Electric, PSNH and WMECO (NUSCO Pension Plan and NSTAR Pension Plan). Effective January 1, 2015, the two plans were merged into one plan, sponsored by NUSCO. The NUSCO and NSTAR Pension Plans are subject to the provisions of ERISA, as amended by the PPA of 2006. NU's policy is to annually fund the Pension Plans in an amount at least equal to an amount that will satisfy federal requirements. In addition, NU maintains non-qualified defined benefit retirement plans sponsored by NUSCO (herein collectively referred to as the SERP Plans), which provide benefits in excess of Internal Revenue Code limitations to eligible current and retired participants.
	As of December 31, 2014, NUSCO also sponsored defined benefit postretirement plans that provide certain retiree benefits, primarily medical, dental and life insurance, to retiring employees that meet certain age and service eligibility requirements (NUSCO PBOP Plans and NSTAR PBOP Plan). Effective January 1, 2015, the plans were merged into one plan, sponsored by NUSCO. Under certain circumstances, eligible retirees are required to contribute to the costs of postretirement benefits. The benefits provided under the PBOP Plans are not vested and the Company has the right to modify any benefit provision subject to applicable laws at that time.
	Because the Regulated companies recover the retiree benefit costs from customers through rates, regulatory assets are recorded in lieu of an adjustment to Accumulated Other Comprehensive Income/(Loss) to record the funded status of the Pension, SERP and PBOP Plans. Regulatory accounting was also applied to the portions of the NUSCO costs that support the Regulated companies, as these costs are also recovered from customers. Adjustments to the Pension and PBOP funded status for the unregulated companies are recorded on an after-tax basis to Accumulated Other Comprehensive Income/(Loss). For further information, see Note 2, "Regulatory Accounting," and Note 14, "Accumulated Other Comprehensive Income/(Loss)," to the financial statements. The SERP Plans do not have plan assets.
	For the years ended December 31, 2014 and 2013, the expected return on plan assets for the NUSCO Pension and PBOP Plans was calculated by applying the assumed rate of return to a four-year rolling average of plan asset fair values. This calculation recognized investment gains or losses over a four-year period from the years in which they occurred. Investment gains or losses for this purpose are the difference between the calculated expected return and the actual return. As investment gains and losses are reflected in the average plan asset fair values, they are subject to amortization with other unrecognized actuarial gains or losses. For the NSTAR Pension and PBOP Plans, the entire difference between the actual return and calculated expected return on plan assets is reflected as a component of unrecognized actuarial gain or loss. Unrecognized actuarial gains or losses are amortized as a component of Pension and PBOP expense over the estimated average future employee service period.
	Pension and SERP Plans: As of December 31, 2013, the funded status of the NSTAR Pension Plan was recorded on NSTAR Electric's balance sheet, while the total SERP obligation was recorded on NSTAR Electric & Gas' balance sheet. As of December 31, 2013, all NSTAR employees were employed by NSTAR Electric & Gas. On January 1, 2014, NSTAR Electric & Gas was merged into NUSCO (service company merger) and, concurrently, all employees were transferred to the company they predominantly provide services for: NUSCO, NSTAR Electric or NSTAR Gas. As a result of the employee transfers, the pension and SERP assets and liabilities were attributed by participant and transferred to the applicable company's balance sheets. This change had no impact on the income statement or net assets of NSTAR Electric or NU. For the year ended December 31, 2014, the NUSCO and NSTAR pension and SERP plans are accounted for under the multiple-employer approach, with each company's balance sheet reflecting its share of the funded status of the plans. The following tables provide information on the Pension and SERP Plan benefit obligations, fair values of Pension Plan assets, and funded status:
			Pension and SERP
	NU		As of December 31,
	(Millions of Dollars)		2014			2013
	Change in Benefit Obligation
	Benefit Obligation as of Beginning of Year		$	-4,676.50		$	-5,022.80
	Service Cost			-79.9			-102.3
	Interest Cost			-225.7			-206.7
	Actuarial Gain/(Loss)			-739.6			433.6
	Benefits Paid - Pension			230.3			216.6
	Benefits Paid - SERP			5.2			5.1
	Benefit Obligation as of End of Year		$	-5,486.20		$	-4,676.50
	Change in Pension Plan Assets
	Fair Value of Plan Assets as of Beginning of Year		$	3,985.90		$	3,411.30
	Employer Contributions			171.6			284.7
	Actual Return on Plan Assets			199.3			506.5
	Benefits Paid			-230.3			-216.6
	Fair Value of Plan Assets as of End of Year		$	4,126.50		$	3,985.90
	Funded Status as of December 31st		$	-1,359.70		$	-690.6
			Pension and SERP
			As of December 31, 2014												As of December 31, 2013
						NSTAR												NSTAR
	(Millions of Dollars)		CL&P			Electric			PSNH			WMECO			CL&P			Electric (1)			PSNH			WMECO
	Change in Benefit Obligation
	Benefit Obligation as of Beginning of Year		$	-1,083.40		$	-1,353.30		$	-529		$	-223.9		$	-1,178.00		$	-1,430.00		$	-576		$	-243.1
	Decrease due to transfer of employees			26.4			479.9			32.2			6.2			-			-			-			-
	Service Cost			-20.2			-13.6			-9.7			-3.5			-24.9			-33.1			-13.1			-4.7
	Interest Cost			-50.5			-41.3			-23.8			-10.3			-48.3			-58			-23.6			-10
	Actuarial Gain/(Loss)			-161			-107			-73.3			-29.8			110.7			96.6			62.4			22.4
	Benefits Paid - Pension			58.3			52.4			22.8			11.9			56.6			71.2			21.1			11.5
	Benefits Paid - SERP			0.3			0.3			0.1			-			0.5			-			0.2			-
	Benefit Obligation as of End of Year		$	-1,230.10		$	-982.6		$	-580.7		$	-249.4		$	-1,083.40		$	-1,353.30		$	-529		$	-223.9
	Change in Pension Plan Assets
	Fair Value of Plan Assets as of Beginning of Year		$	1,016.30		$	1,235.30		$	528.6		$	240.4		$	937.6		$	1,069.10		$	386.6		$	218.5
	Decrease due to transfer of employees			-26.4			-441.4			-32.2			-6.2			-			-			-			-
	Employer Contributions			-			101			-			-			-			82			108.3			-
	Actual Return on Plan Assets			49.2			36.5			24.8			11.7			135.3			155.4			54.8			33.4
	Benefits Paid			-58.3			-52.4			-22.8			-11.9			-56.6			-71.2			-21.1			-11.5
	Fair Value of Plan Assets as of End of Year		$	980.8		$	879		$	498.4		$	234		$	1,016.30		$	1,235.30		$	528.6		$	240.4
	Funded Status as of December 31st		$	-249.3		$	-103.6		$	-82.3		$	-15.4		$	-67.1		$	-118		$	-0.4		$	16.5
	NSTAR Electric amounts do not include benefit obligations of the NSTAR SERP Plan as of December 31, 2013.
	During 2014, the Society of Actuaries released a series of updated mortality tables resulting from recent studies that measured mortality rates for various groups of individuals. The updated mortality tables released in 2014 increased life expectancy of plan participants by 3 to 5 years and have the effect of increasing the estimate of benefits to be provided to plan participants. The impact of this adoption on NU's funded status liability for the year ended December 31, 2014 was an increase of approximately $340 million. In addition, the decreases in the discount rates resulted in an increase on NU's funded status liability of approximately $530 million. Partially offsetting these increases are the impact of other actuarial assumptions.
	As of December 31, 2013, prepaid pension assets for PSNH and WMECO were included in Other Long-Term Assets on their accompanying balance sheets. The pension and SERP benefits funded status includes the current portion of the SERP liability, which is included in Other Current Liabilities on the accompanying balance sheets. Although NU maintains marketable securities in a supplemental benefit trust, the SERP plan itself does not contain any assets. See Note 5, "Marketable Securities," to the financial statements.
	The accumulated benefit obligation for the Pension and SERP Plans is as follows:
	(Millions of Dollars)		NU			CL&P			NSTAR Electric (1)			PSNH			WMECO
	2014		$	5,000.10		$	1,101.40		$	910.4		$	524.5		$	226.4
	2013			4,538.80			1,058.00			1,280.60			520.1			220.6
	-1	NSTAR Electric amounts do not include the accumulated benefit obligation for the SERP Plan as of December 31, 2013.
	The following actuarial assumptions were used in calculating the Pension and SERP Plans' year end funded status:
			Pension and SERP
			As of December 31,
			2014			2013
	Discount Rate		4.2	%		4.85	%	-	5.03	%
	Compensation/Progression Rate		3.5	%		3.5	%	-	4	%
	Pension and SERP Expense: NU charges net periodic pension expense to its subsidiaries based on the actual participant demographic data for each subsidiary's participants. Benefit payments to participants and contributions are also tracked for each subsidiary. The actual investment return in the trust is allocated to each of the subsidiaries annually in proportion to the investment return expected to be earned during the year. For the years ended December 31, 2013 and 2012 (prior to the service company merger), the net periodic pension expense recorded at NSTAR Electric represented the full cost of the plan with a portion of the costs allocated to affiliated companies based on participant demographic data.
	The components of net periodic benefit expense for the Pension and SERP Plans are shown below. The net periodic benefit expense and the intercompany allocations less the capitalized portion of pension is included in Operations and Maintenance on the statements of income. Capitalized pension amounts relate to employees working on capital projects and are included in Property, Plant and Equipment, Net. Intercompany allocations are not included in the CL&P, NSTAR Electric, PSNH and WMECO net periodic benefit expense amounts. Pension and SERP expense reflected in the statements of cash flows for CL&P, NSTAR Electric, PSNH and WMECO does not include the intercompany allocations and the corresponding capitalized portion, as these amounts are cash settled on a short-term basis.
			Pension and SERP
			For the Year Ended December 31, 2014
								NSTAR
(Millions of Dollars)		NU			CL&P			Electric			PSNH			WMECO
Service Cost		$	79.9		$	20.2		$	13.6		$	9.7		$	3.5
Interest Cost			225.7			50.5			41.3			23.8			10.3
Expected Return on Plan Assets			-310.8			-75.4			-63			-38.1			-17.9
Actuarial Loss			128.4			33.7			23.5			11.6			6.9
Prior Service Cost			4.4			1.8			-			0.7			0.4
Total Net Periodic Benefit Expense		$	127.6		$	30.8		$	15.4		$	7.7		$	3.2
Intercompany Allocations			N/A		$	26.7		$	10.4		$	7.6		$	5.1
Capitalized Pension Expense		$	35.2		$	17.6		$	7.9		$	3		$	2.4
		Pension and SERP
		For the Year Ended December 31, 2013
								NSTAR
(Millions of Dollars)		NU			CL&P			Electric (1)			PSNH			WMECO
Service Cost		$	102.3		$	24.9		$	33.1		$	13.1		$	4.7
Interest Cost			206.7			48.3			58			23.6			10
Expected Return on Plan Assets			-278.1			-73.8			-84.4			-35.4			-17.4
Actuarial Loss			210.5			55.9			58.1			21.6			11.8
Prior Service Cost/(Credit)			4			1.8			-0.3			0.7			0.4
Total Net Periodic Benefit Expense		$	245.4		$	57.1		$	64.5		$	23.6		$	9.5
Intercompany Allocations			N/A		$	44.9		$	-8.4		$	10.5		$	8
Capitalized Pension Expense		$	73.2		$	28		$	28.9		$	7.3		$	5.2
		Pension and SERP
		For the Year Ended December 31, 2012
								NSTAR
(Millions of Dollars)		NU (2)			CL&P			Electric (1)			PSNH			WMECO
Service Cost		$	84.3		$	21.8		$	30.3		$	11.8		$	4.1
Interest Cost			198.3			51.2			58.9			24.4			10.5
Expected Return on Plan Assets			-220.9			-70.6			-65.6			-28.2			-16.4
Actuarial Loss			172.4			49.6			63.1			16.2			10.7
Prior Service Cost/(Credit)			7.9			3.6			-0.6			1.5			0.8
Total Net Periodic Benefit Expense		$	242		$	55.6		$	86.1		$	25.7		$	9.7
Curtailments and Settlements		$	2.2		$	-		$	-		$	-		$	-
Intercompany Allocations			N/A		$	42.8		$	-12.3		$	10.1		$	8.1
Capitalized Pension Expense		$	70.6		$	26.8		$	30.7		$	7.9		$	5.1
NSTAR Electric's allocated expense associated with the NSTAR SERP was $3.2 million and $3.6 million for the years ended December 31, 2013 and 2012, respectively, and were not included in the NSTAR Electric amounts in the tables above. For the year ended December 31, 2014, the SERP amounts are now allocated to NSTAR Electric due to the service company merger.
NSTAR amounts were included in NU beginning April 10, 2012.
The following actuarial assumptions were used to calculate Pension and SERP expense amounts:
		Pension and SERP
		For the Years Ended December 31,
		2014						2013						2012
Discount Rate		4.85	%	-	5.03	%		4.13	%	-	4.24	%		4.52	%	-	5.03	%
Expected Long-Term Rate of Return		8.25%						8.25%						7.3	%	-	8.25	%
Compensation/Progression Rate		3.5	%	-	4	%		3.5	%	-	4	%		3.5	%	-	4	%
The following is a summary of the changes in plan assets and benefit obligations recognized in Regulatory Assets and Other Comprehensive Income (OCI) as well as amounts in Regulatory Assets and OCI reclassified as net periodic benefit expense during the years presented:
		Amounts Reclassified To/From
		Regulatory Assets						OCI
		For the Years Ended December 31,
(Millions of Dollars)		2014			2013			2014			2013
Actuarial (Gains)/Losses Arising During the Year		$	797.3		$	-635.2		$	55.9		$	-28.9
Actuarial Losses Reclassified as Net Periodic Benefit Expense			-122.8			-201.2			-5.6			-9.4
Prior Service Cost Reclassified as Net Periodic Benefit Expense			-4.2			-3.8			-0.2			-0.2
The following is a summary of the remaining Regulatory Assets and Accumulated Other Comprehensive Loss amounts that have not been recognized as components of net periodic benefit expense as of December 31, 2014 and 2013, and the amounts that are expected to be recognized as components in 2015:
			Regulatory Assets as of						Expected			AOCI as of					Expected
		December 31,							2015		December 31,						2015
(Millions of Dollars)		2014			2013			Expense			2014			2013			Expense
Actuarial Loss		$	1,811.90		$	1,137.40		$	149.1		$	93.5		$	43.2		$	6.4
Prior Service Cost			13.2			17.4			3.5			0.8			1			0.2
PBOP Plans: As of December 31, 2013, the funded status of the NSTAR PBOP Plan was recorded on the NSTAR Electric & Gas balance sheet. As of December 31, 2013, all NSTAR employees were employed by NSTAR Electric & Gas. On January 1, 2014, concurrent with the service company merger, the PBOP assets and liabilities were attributed by participant and transferred to the applicable company's balance sheets. This change had no impact on the income statement or net assets of NSTAR Electric or NU. For the year ended December 31, 2014, the NUSCO and NSTAR PBOP Plans are accounted for under the multiple-employer approach, with each company's balance sheet reflecting its share of the funded status of the plans.
NU annually funds postretirement costs through tax deductible contributions to external trusts.
The following tables provide information on PBOP Plan benefit obligations, fair values of plan assets, and funded status:
		PBOP
NU		As of December 31,
(Millions of Dollars)		2014			2013
Change in Benefit Obligation
Benefit Obligation as of Beginning of Year		$	-1,038.00		$	-1,233.30
Service Cost			-12.5			-16.9
Interest Cost			-49.5			-47.2
Actuarial Gain/(Loss)			-95.5			200.9
Benefits Paid			47.6			58.5
Benefit Obligation as of End of Year		$	-1,147.90		$	-1,038.00
Change in PBOP Plan Assets
Fair Value of Plan Assets as of Beginning of Year		$	826.5		$	709.1
Actual Return on Plan Assets			43.7			118.3
Employer Contributions			40			57.6
Benefits Paid			-47.6			-58.5
Fair Value of Plan Assets as of End of Year		$	862.6		$	826.5
Funded Status as of December 31st		$	-285.3		$	-211.5
		PBOP
		As of December 31,
		2014												2013
					NSTAR
(Millions of Dollars)		CL&P			Electric			PSNH			WMECO				CL&P			PSNH			WMECO
Change in Benefit Obligation
Benefit Obligation as of Beginning of Year		$	-180.4		$	-		$	-93.5		$	-38.7			$	-196.8		$	-100.2		$	-42.5
Decrease/(Increase) due to transfer of employees			3.7			-395.5			4.3			1				-			-			-
Service Cost			-2.2			-3.1			-1.3			-0.4				-3.4			-2.3			-0.7
Interest Cost			-8.1			-19.4			-4.3			-1.7				-7.9			-4			-1.7
Actuarial Gain/(Loss)			3.5			-68.6			-1.1			1.3				13.3			7.2			3.3
Benefits Paid			9.6			17.9			4.1			1.9				14.4			5.8			2.9
Benefit Obligation as of End of Year		$	-173.9		$	-468.7		$	-91.8		$	-36.6			$	-180.4		$	-93.5		$	-38.7
Change in PBOP Plan Assets
Fair Value of Plan Assets as of Beginning of Year		$	151.3		$	-		$	81.8		$	35.3			$	132.2		$	69.5		$	31
(Decrease)/Increase due to transfer of employees			-3.2			316.7			-3.1			-1				-			-			-
Actual Return on Plan Assets			6.3			18.4			3.8			1.6				24.8			13.4			6
Employer Contributions			4.2			19.3			2.5			0.4				8.7			4.7			1.2
Benefits Paid			-9.6			-17.9			-4.1			-1.9				-14.4			-5.8			-2.9
Fair Value of Plan Assets as of End of Year		$	149		$	336.5		$	80.9		$	34.4			$	151.3		$	81.8		$	35.3
Funded Status as of December 31st		$	-24.9		$	-132.2		$	-10.9		$	-2.2			$	-29.1		$	-11.7		$	-3.4
During 2014, the Society of Actuaries released a series of updated mortality tables resulting from recent studies that measured mortality rates for various groups of individuals. The updated mortality tables released in 2014 increased life expectancy of plan participants by 3 to 5 years and have the effect of increasing the estimate of benefits to be provided to plan participants. The impact of this adoption on NU's funded status liability for the year ended December 31, 2014 was an increase of approximately $82 million.
In addition, the decreases in the discount rates resulted in an increase on NU's funded status liability of approximately $110 million. Partially offsetting these increases are the impact of other actuarial assumptions.
The following actuarial assumptions were used in calculating the PBOP Plans' year end funded status:
		PBOP
		As of December 31,
		2014			2013
Discount Rate		4.22	%		4.78	%	-	5.1	%
Health Care Cost Trend Rate		6.5	%		7.00%
PBOP Expense: NU charges net periodic postretirement benefits expense to its subsidiaries based on the actual participant demographic data for each subsidiary's participants. Benefit payments to participants and contributions are also tracked for each subsidiary. The actual investment return in the trust each year is allocated to each of the subsidiaries annually in proportion to the investment return expected to be earned during the year. For the years ended December 31, 2013 and 2012 (prior to the service company merger), the net periodic postretirement expense of the NSTAR PBOP Plan allocated to NSTAR Electric was $4.6 million and $34.1 million, respectively.
The components of net periodic benefit expense for the PBOP Plans are shown below. The net periodic benefit expense and the intercompany allocations less the capitalized portion of PBOP is included in Operations and Maintenance on the statements of income. Capitalized PBOP amounts relate to employees working on capital projects and are included in Property, Plant and Equipment, Net. Intercompany allocations are not included in the CL&P, NSTAR Electric, PSNH and WMECO net periodic benefit expense amounts. PBOP expense reflected in the statements of cash flows for CL&P, NSTAR Electric, PSNH and WMECO does not include the intercompany allocations and the corresponding capitalized portion, as these amounts are cash settled on a short-term basis.
			PBOP
		For the Year Ended December 31, 2014
								NSTAR
(Millions of Dollars)		NU			CL&P			Electric			PSNH			WMECO
Service Cost		$	12.5		$	2.2		$	3.1		$	1.3		$	0.4
Interest Cost			49.5			8.1			19.4			4.3			1.7
Expected Return on Plan Assets			-63.3			-10.5			-25.9			-5.4			-2.3
Actuarial Loss/(Gain)			12.2			4.2			-0.5			2.2			0.5
Prior Service Credit			-2.8			-			-1.9			-			-
Total Net Periodic Benefit Expense/(Income)		$	8.1		$	4		$	-5.8		$	2.4		$	0.3
Intercompany Allocations			N/A		$	3.8		$	0.8		$	1		$	0.7
Capitalized PBOP Expense/(Income)		$	1.4		$	1.8		$	-2.3		$	0.8		$	0.2
		PBOP
		For the Year Ended December 31, 2013
(Millions of Dollars)		NU			CL&P			PSNH			WMECO
Service Cost		$	16.9		$	3.4		$	2.3		$	0.7
Interest Cost			47.2			7.9			4			1.7
Expected Return on Plan Assets			-55.4			-10.1			-5.2			-2.3
Actuarial Loss			26			7.4			3.6			1.1
Prior Service Credit			-2.1			-			-			-
Total Net Periodic Benefit Expense		$	32.6		$	8.6		$	4.7		$	1.2
Intercompany Allocations			N/A		$	7.1		$	1.6		$	1.3
Capitalized PBOP Expense		$	8.8		$	3.9		$	1.3		$	0.6
		PBOP
		For the Year Ended December 31, 2012
(Millions of Dollars)		NU (1)			CL&P			PSNH			WMECO
Service Cost		$	15.7		$	3		$	2		$	0.6
Interest Cost			49			9.2			4.6			2
Expected Return on Plan Assets			-39.2			-9.1			-4.6			-2.1
Actuarial Loss			36			7.5			3.6			1.2
Prior Service Credit			-1.4			-			-			-
Net Transition Obligation Cost			12.2			6.1			2.5			1.3
Total Net Periodic Benefit Expense		$	72.3		$	16.7		$	8.1		$	3
Intercompany Allocations			N/A		$	7.9		$	2		$	1.5
Capitalized PBOP Expense		$	26.6		$	8.2		$	2.3		$	1.6
-1	NSTAR amounts were included in NU beginning April 10, 2012.
The following actuarial assumptions were used to calculate PBOP expense amounts:
		PBOP
		For the Years Ended December 31,
		2014						2013						2012
Discount Rate		4.78	%	-	5.1	%		4.04	%	-	4.35	%		4.58	%	-	4.84	%
Expected Long-Term Rate of Return		8.25%						8.25%						7.3	%	-	8.25	%
As of December 31, 2014 and 2013, the health care cost trend rate assumption used to determine the PBOP Plans' funded status was 6.5 percent and 7 percent, respectively, subsequently decreasing to an ultimate rate of 4.5 percent in 2023. The health care cost trend rate assumption used to calculate the PBOP expense amounts was 7 percent for the year ended December 31, 2014.
Assumed health care cost trend rates have a significant effect on the amounts reported for the health care plans. The effect of changing the assumed health care cost trend rate by one percentage point for the year ended December 31, 2014 would have the following effects:
	One Percentage			One Percentage
(Millions of Dollars)	Point Increase			Point Decrease
Effect on PBOP Obligation	$	111.2		$	-88.4
Effect on Total Service and Interest Cost Components		5.3			-4.4
The following is a summary of the changes in plan assets and benefit obligations recognized in Regulatory Assets and OCI as well as amounts in Regulatory Assets and OCI reclassified as net periodic benefit (expense)/income during the years presented:
		Amounts Reclassified To/From
	Regulatory Assets						OCI
	For the Years Ended December 31,
(Millions of Dollars)	2014			2013			2014			2013
Actuarial Losses/(Gains) Arising During the Year	$	115.1		$	-262		$	0.4		$	-1.9
Actuarial Losses Reclassified as Net Periodic Benefit Expense		-11.6			-24.9			-0.6			-1.1
Prior Service Credit Reclassified as Net Periodic Benefit Income		2.8			2.1			-			-
The following is a summary of the remaining Regulatory Assets and Accumulated Other Comprehensive Loss amounts that have not been recognized as components of net periodic benefit expense as of December 31, 2014 and 2013, and the amounts that are expected to be recognized as components in 2015:
		Regulatory Assets as of					Expected				AOCI as of					Expected
	December 31,						2015				December 31,					2015
(Millions of Dollars)	2014			2013			Expense			2014			2013			Expense
Actuarial Loss	$	192.7		$	89.2		$	6.9		$	6		$	6.2		$	0.3
Prior Service Credit		-1.8			-4.6			-0.5			-			-			-
Estimated Future Benefit Payments: The following benefit payments, which reflect expected future service, are expected to be paid by the Pension, SERP and PBOP Plans:
(Millions of Dollars)	2015			2016				2017		2018			2019			2020-2024
Pension and SERP	$	244.5		$	253.6		$	268.9		$	273.4		$	285.4		$	1,591.10
PBOP		58.7			59.7			60.6			61.3			62			318.8
Contributions: NU contributed $171.6 million to the Pension Plans in 2014, of which $101 million was contributed by NSTAR Electric. Based on the current status of the Pension Plans, NU expects to make contributions of approximately $155 million in 2015, of which $5 million will be contributed by NSTAR Electric and $1 million will be contributed by PSNH. The remaining $149 million is expected to be contributed by other NU subsidiaries, primarily NUSCO.
NU contributed $40 million to the PBOP Plans in 2014 and expects to make approximately $27 million in contributions in 2015. This amount will be funded into the 401(h) account and VEBAs up to the maximum tax-deductible level permitted.
Fair Value of Pension and PBOP Plan Assets: Pension and PBOP funds are held in external trusts. Trust assets, including accumulated earnings, must be used exclusively for Pension and PBOP payments. NU's investment strategy for its Pension and PBOP Plans is to maximize the long-term rates of return on these plans' assets within an acceptable level of risk. The investment strategy for each asset category includes a diversification of asset types, fund strategies and fund managers and establishes target asset allocations that are routinely reviewed and periodically rebalanced. PBOP assets are comprised of assets held in the PBOP Plans as well as specific assets within the defined benefit pension plan trust (401(h) assets). The investment policy and strategy of the 401(h) assets is consistent with those of the defined benefit pension plans. NU's expected long-term rates of return on Pension and PBOP Plan assets are based on target asset allocation assumptions and related expected long-term rates of return. In developing its expected long-term rate of return assumptions for the Pension and PBOP Plans, NU evaluated input from consultants, as well as long-term inflation assumptions and historical returns. For the year ended December 31, 2014, management has assumed long-term rates of return of 8.25 percent for the Pension and PBOP Plan assets. These long-term rates of return are based on the assumed rates of return for the target asset allocations as follows:
		As of December 31, 2014 and 2013
		Pension and Tax-Exempt PBOP Plans
		Target Asset Allocation		Assumed Rate of Return
Equity Securities:
	United States	24%		9%
	International	10%		9%
	Emerging Markets	6%		10%
	Private Equity	10%		13%
Debt Securities:
	Fixed Income	15%		5%
	High Yield Fixed Income	9%		7.50%
	Emerging Markets Debt	6%		7.50%
Real Estate and Other Assets		9%		7.50%
Hedge Funds		11%		7%
The Taxable PBOP Plans have a target asset allocation of 70 percent equity securities and 30 percent fixed income securities.
The following table presents, by asset category, the Pension and PBOP Plan assets recorded at fair value on a recurring basis by the level in which they are classified within the fair value hierarchy:
		NU Pension Plans
		Fair Value Measurements as of December 31,
(Millions of Dollars)		2014												2013
Asset Category:		Level 1			Level 2			Level 3			Total			Level 1			Level 2			Level 3			Total
Equity Securities (1)		$	414.7		$	1,035.00		$	292.2		$	1,741.90		$	326.8		$	1,172.10		$	255.5		$	1,754.40
Private Equity			18.8			-			367.9			386.7			96.4			-			300.3			396.7
Fixed Income (2)			10.2			561.4			722			1,293.60			11.6			605.1			589.5			1,206.20
Real Estate and Other Assets			-			132			265.8			397.8			-			88.2			288.5			376.7
Hedge Funds			-			20			475			495			-			-			416.9			416.9
Total Master Trust Assets		$	443.7		$	1,748.40		$	2,122.90		$	4,315.00		$	434.8		$	1,865.40		$	1,850.70		$	4,150.90
	Less: 401(h) PBOP Assets (3)											-188.5												-165
Total Pension Assets											$	4,126.50											$	3,985.90
		NU PBOP Plans
		Fair Value Measurements as of December 31,
(Millions of Dollars)		2014												2013
Asset Category:		Level 1			Level 2			Level 3			Total			Level 1			Level 2			Level 3			Total
Cash and Cash Equivalents		$	-		$	-		$	-		$	-		$	11.1		$	-		$	-		$	11.1
Equity Securities (1)			104.1			172.8			75.1			352			110.3			176.8			69.1			356.2
Private Equity			-			-			24.9			24.9			-			-			17.9			17.9
Fixed Income (2)			16.1			110			78.3			204.4			-			119.7			51.5			171.2
Real Estate and Other Assets						19.4			15			34.4			-			14.2			33.9			48.1
Hedge Funds						-			58.4			58.4			-			-			57			57
Total		$	120.2		$	302.2		$	251.7		$	674.1		$	121.4		$	310.7		$	229.4		$	661.5
	Add: 401(h) PBOP Assets (3)											188.5												165
Total PBOP Assets											$	862.6											$	826.5
United States, International and Emerging Markets equity securities classified as Level 2 include investments in commingled funds. Level 3 investments include hedge funds that are overlayed with equity index swaps and futures contracts and funds invested in equities that have redemption restrictions.
Fixed Income investments classified as Level 3 investments include fixed income funds that invest in a variety of opportunistic fixed income strategies, and hedge funds that are overlayed with fixed income futures.
The assets of the Pension Plans include a 401(h) account that has been allocated to provide health and welfare postretirement benefits under the PBOP Plans.
Effective January 1, 2013, the NSTAR Pension Plan assets were transferred into the NUSCO Pension Plan master trust. The NUSCO Pension Plan is entitled to approximately 66 percent of each asset category in the master trust, the NSTAR Pension Plan is entitled to approximately 30 percent of each asset category in the master trust and the 401(h) plans are entitled to approximately four percent of each asset category in the master trust. For the years ended December 31, 2014 and 2013 the NUSCO Pension Plan was entitled to $2,803.6 million and $2,750.4 million respectively and the NSTAR Pension Plan was entitled to $1,322.9 million and $1,235.3 million, respectively. Also effective January 1, 2013, the NSTAR PBOP Plan assets were transferred into a master trust with the NUSCO PBOP Plan assets and assets were allocated to each plan. For the years ended December 31, 2014 and 2013, the NUSCO PBOP Plan was entitled to $399 million and $391 million, respectively, and the NSTAR PBOP Plan was entitled to $463.6 million and $435.5 million, respectively. CL&P, PSNH and WMECO are allocated a portion of the NUSCO Pension and PBOP Plan assets. NSTAR Electric is entitled to a portion of the NSTAR Pension and PBOP Plan assets.
The Company values assets based on observable inputs when available. Equity securities, exchange traded funds and futures contracts classified as Level 1 in the fair value hierarchy are priced based on the closing price on the primary exchange as of the balance sheet date. Commingled funds included in Level 2 equity securities are recorded at the net asset value provided by the asset manager, which is based on the market prices of the underlying equity securities. Swaps are valued using pricing models that incorporate interest rates and equity and fixed income index closing prices to determine a net present value of the cash flows. Fixed income securities, such as government issued securities, corporate bonds and high yield bond funds, are included in Level 2 and are valued using pricing models, quoted prices of securities with similar characteristics or discounted cash flows. The pricing models utilize observable inputs such as recent trades for the same or similar instruments, yield curves, discount margins and bond structures. Hedge funds and investments in opportunistic fixed income funds are recorded at net asset value based on the values of the underlying assets. The assets in the hedge funds and opportunistic fixed income funds are valued using observable inputs and are classified as Level 3 within the fair value hierarchy due to redemption restrictions. Private Equity investments and Real Estate and Other Assets are valued using the net asset value provided by the partnerships, which are based on discounted cash flows of the underlying investments, real estate appraisals or public market comparables of the underlying investments. These investments are classified as Level 3 due to redemption restrictions.
Fair Value Measurements Using Significant Unobservable Inputs (Level 3): The following tables present changes in the Level 3 category of Pension and PBOP Plan assets for the years ended December 31, 2014 and 2013:
		NU Pension Plans
		Equity			Private			Fixed			Real Estate and			Hedge
(Millions of Dollars)		Securities			Equity			Income			Other Assets			Funds			Total
Balance as of January 1, 2013		$	322.7		$	267.9		$	315.1		$	235.4		$	418.9		$	1,560.00
Transfer Between Categories			-			-			32.5			-			-32.5			-
Actual Return/(Loss) on Plan Assets:
	Relating to Assets Still Held as of Year End		20.6			15.4			55.3			12.9			33.4			137.6
	Relating to Assets Distributed During the Year		12.2			13.7			-1			6.2			-			31.1
Purchases, Sales and Settlements			-100			3.3			187.6			34			-2.9			122
Balance as of December 31, 2013		$	255.5		$	300.3		$	589.5		$	288.5		$	416.9		$	1,850.70
Actual Return/(Loss) on Plan Assets:
	Relating to Assets Still Held as of Year End		-2.3			14			45.2			-3.6			23.5			76.8
	Relating to Assets Distributed During the Year		-			13.9			-6.2			28.3			-15.2			20.8
Purchases, Sales and Settlements			39			39.7			93.5			-47.4			49.8			174.6
Balance as of December 31, 2014		$	292.2		$	367.9		$	722		$	265.8		$	475		$	2,122.90
		NU PBOP Plans
		Equity			Private			Fixed			Real Estate and			Hedge
(Millions of Dollars)		Securities			Equity			Income			Other Assets			Funds			Total
Balance as of January 1, 2013		$	36.3		$	11.3		$	32.1		$	26.7		$	39.6		$	146
Actual Return/(Loss) on Plan Assets:
	Relating to Assets Still Held as of Year End		20.8			1.5			4.1			3.9			5.4			35.7
	Relating to Assets Distributed During the Year		-			0.2			-			-0.1			-			0.1
Purchases, Sales and Settlements			12			4.9			15.3			3.4			12			47.6
Balance as of December 31, 2013		$	69.1		$	17.9		$	51.5		$	33.9		$	57		$	229.4
Actual Return/(Loss) on Plan Assets:
	Relating to Assets Still Held as of Year End		6			1.3			1.9			-2.8			1.4			7.8
	Relating to Assets Distributed During the Year		-			0.1			-			-2.2			-			-2.1
Purchases, Sales and Settlements			-			5.6			24.9			-13.9			-			16.6
Balance as of December 31, 2014		$	75.1		$	24.9		$	78.3		$	15		$	58.4		$	251.7

DEFINED_CONTRIBUTION_PLANS

DEFINED CONTRIBUTION PLANS	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Compensation and Employee Benefit Plans [Text Block]	B.       Defined Contribution Plans
	Effective January 1, 2014, NU maintains one defined contribution plan on behalf of eligible participants, the NUSCO 401k Plan. The NUSCO 401k Plan provides for employee and employer contributions up to statutory limits. For eligible employees, the NUSCO 401k Plan provides employer matching contributions of either 100 percent up to a maximum of three percent of eligible compensation or 50 percent up to a maximum of eight percent of eligible compensation. For newly hired employees beginning in 2014, the NUSCO 401k Plan provides employer matching contributions of 100 percent up to a maximum of three percent of eligible compensation.
	The NUSCO 401k Plan also contains a K-Vantage feature on behalf of eligible participants, which provides an additional employer contribution based on age and years of service. K-Vantage participants are not eligible to actively participate in the NU defined benefit plans.
	The total defined contribution plan employer matching contributions, including the K-Vantage program contributions, were as follows:
						NSTAR
	(Millions of Dollars)	NU (1)		CL&P		Electric		PSNH		WMECO
	2014	$	29.7	$	5	$	6.3	$	3.2	$	1
	2013		37		5.1		8.5		3.3		1
	2012		25.7		4.8		9		3.3		0.9
	NSTAR amounts were included in NU beginning April 10, 2012.
	Allocations of NU common shares were made from NU treasury shares to satisfy the NUSCO 401k Plan obligation to provide 100 percent of the matching contribution in NU common shares. For treasury shares used to satisfy the NUSCO 401k Plan employer matching contributions, compensation expense is recognized equal to the fair value of shares that have been allocated to participants. Any difference between the fair value and the average cost of the allocated treasury shares is charged or credited to Capital Surplus, Paid In. For the years ended December 31, 2014, 2013 and 2012, NU recognized $22 million, $9.1 million and $8.9 million, respectively, of compensation expense related to treasury shares used to satisfy the matching contribution.

SHAREBASED_PAYMENTS

SHARE-BASED PAYMENTS	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Shareholders' Equity and Share-based Payments [Text Block]	C.       Share-Based Payments
	Share-based compensation awards are recorded using a fair-value-based method at the date of grant. NU, CL&P, NSTAR Electric, PSNH and WMECO record compensation expense related to these awards, as applicable, for shares issued or sold to their respective employees and officers, as well as the allocation of costs associated with shares issued or sold to NU's service company employees and officers that support CL&P, NSTAR Electric, PSNH and WMECO.
	Upon consummation of the merger with NSTAR, the NSTAR 1997 Share Incentive Plan and the NSTAR 2007 Long-Term Incentive Plan were assumed by NU. Share-based awards granted under the NSTAR Plans and held by NSTAR employees and officers were generally converted into outstanding NU share-based compensation awards with an estimated fair value of $53.2 million. Refer to Note 21, "Merger of NU and NSTAR," for further information regarding the merger transaction. Specifically, as of the merger closing, and as adjusted by the exchange ratio, NU converted (1) outstanding NSTAR stock options into 2,664,894 NU stock options valued at $30.5 million, (2) NSTAR deferred shares and NSTAR performance shares into 421,775 NU RSU's valued at $15.5 million, and (3) NSTAR RSU retention awards into 195,619 NU RSU retention awards valued at $7.2 million.
	NU Incentive Plans: NU maintains long-term equity-based incentive plans in which NU, CL&P, NSTAR Electric, PSNH and WMECO employees, officers and board members are eligible to participate. The incentive plans authorize NU to grant up to 8,000,000 new shares for various types of awards, including RSUs and performance shares, to eligible employees, officers, and board members. As of December 31, 2014 and 2013, NU had 3,112,020 and 3,440,590 common shares, respectively, available for issuance under these plans. NU also maintains an ESPP for eligible employees.
	NU accounts for its various share-based plans as follows:
	RSUs - NU records compensation expense, net of estimated forfeitures, on a straight-line basis over the requisite service period based upon the fair value of NU's common shares at the date of grant. The par value of RSUs is reclassified to Common Stock from APIC as RSUs become issued as common shares.
	Performance Shares - NU records compensation expense, net of estimated forfeitures, on a straight-line basis over the requisite service period. Performance shares vest based upon the extent to which Company goals are achieved. Vesting of outstanding performance shares is based upon both the Company's EPS growth over the requisite service period and the total shareholder return as compared to the Edison Electric Institute (EEI) Index during the requisite service period. The fair value of performance shares is determined at the date of grant using a lattice model.
	Stock Options - Stock options issued under the NSTAR Incentive Plan that were outstanding immediately prior to the completion of the merger with NSTAR converted into fully vested options to acquire NU common shares, as adjusted by the exchange ratio. The fair value of these awards on the merger date was included in the purchase price as it represented consideration transferred in the merger. Accordingly, no compensation expense was recorded for these stock options.
	ESPP Shares - For shares sold under the ESPP, no compensation expense was recorded as the ESPP qualifies as a non-compensatory plan.
	RSUs: NU granted RSUs under the annual long-term incentive programs that are subject to three-year graded vesting schedules for employees, and one-year graded vesting schedules, or immediate vesting, for board members. RSUs are paid in shares, reduced by amounts sufficient to satisfy withholdings for income taxes, subsequent to vesting. A summary of RSU transactions is as follows:
					Weighted Average
		RSUs			Grant-Date
		(Units)			Fair Value
	Outstanding as of January 1, 2012		959,920		$	26.36
	Granted		614,930		$	33.04
	Converted NSTAR Awards upon Merger		617,394		$	36.79
	Converted from NU Performance Shares upon Merger		451,358		$	34.32
	Shares issued		-363,779		$	29.05
	Forfeited		-96,504		$	34.97
	Outstanding as of December 31, 2012		2,183,319		$	31.99
	Granted		373,939		$	39.56
	Shares issued		-891,129		$	32.15
	Forfeited		-29,689		$	33.75
	Outstanding as of December 31, 2013		1,636,440		$	33.61
	Granted		338,576		$	42.27
	Shares issued		-567,209		$	33.48
	Forfeited		-27,060		$	39.62
	Outstanding as of December 31, 2014		1,380,747		$	35.67
	As of December 31, 2014 and 2013, the number and weighted average grant-date fair value of unvested RSUs was 1,024,729 and $38.14 per share, and 1,162,216 and $36.58 per share, respectively. The number and weighted average grant-date fair value of RSUs vested and either paid or deferred during 2014 was 437,887 and $37.36 per share, respectively. As of December 31, 2014, 356,018 RSUs were fully vested and deferred and an additional 973,493 are expected to vest.
	Performance Shares: NU granted performance shares under the annual long-term incentive programs that vest based upon the extent to which Company goals are achieved at the end of three-year performance measurement periods. Performance shares are paid in shares, after the performance measurement period. A summary of performance share transactions is as follows:
		Performance			Weighted Average
		Shares			Grant-Date
		(Units)			Fair Value
	Outstanding as of January 1, 2012		483,133		$	29.18
	Granted		225,935		$	35.09
	Converted to RSUs upon Merger		-451,358		$	34.32
	Shares issued		-106,773		$	24.52
	Outstanding as of December 31, 2012		150,937		$	25.04
	Granted		191,961		$	40.96
	Shares issued		-150,944		$	25.04
	Forfeited		-1,526		$	40.93
	Outstanding as of December 31, 2013		190,428		$	40.96
	Granted		193,396		$	43.4
	Shares issued		-2,009		$	41.46
	Forfeited		-6,171		$	42.02
	Outstanding as of December 31, 2014		375,644		$	42.2
	Upon closing of the merger with NSTAR, 451,358 performance shares under the NU 2011 and 2012 Long-Term Incentive Programs converted to RSUs according to the terms of these programs. Performance shares under the NU 2010 Incentive Program were measured based upon a modified performance period through the date of the merger, in accordance with the terms of the program, and were fully distributed in 2013.
	The total compensation expense and associated future income tax benefit recognized by NU, CL&P, NSTAR Electric, PSNH and WMECO for share-based compensation awards are as follows:
	NU			For the Years Ended December 31,
	(Millions of Dollars)			2014			2013			2012 (1)
	Compensation Expense			$	24.6		$	27		$	25.8
	Future Income Tax Benefit				10.3			10.7			10.2
		For the Years Ended December 31,
		2014											2013								2012
					NSTAR										NSTAR								NSTAR
	(Millions of Dollars)	CL&P			Electric			PSNH			WMECO		CL&P		Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Compensation Expense	$	8.1		$	7.4		$	3		$	1.3	$	6.8	$	7.5	$	2.3	$	1.3	$	4.8	$	7.4	$	1.8	$	1
	Future Income Tax Benefit		3.4			3.1			1.3			0.5		2.7		3		0.9		0.5		1.9		2.9		0.7		0.4
	NSTAR amounts were included in NU beginning April 10, 2012.
	As of December 31, 2014, there was $15.7 million of total unrecognized compensation expense related to nonvested share-based awards for NU, $6.1 million for CL&P, $4.3 million for NSTAR Electric, $2 million for PSNH and $1 million for WMECO. This cost is expected to be recognized ratably over a weighted-average period of 1.65 years for NU, 1.68 years for CL&P, 1.69 years for NSTAR Electric, 1.71 years for PSNH and 1.68 years for WMECO.
	For the years ended December 31, 2014 and 2012, additional tax benefits totaling $9.5 million and $8.5 million increased cash flows from financing activities. For the year ended December 31, 2013, additional tax benefits totaling $5.5 million decreased cash flows from financing activities.
	Stock Options: Stock options were granted under the NU and NSTAR incentive plans. Options currently outstanding expire ten years from the date of grant and are fully vested. The weighted average remaining contractual lives for the options outstanding as of December 31, 2014 is 4.3 years. A summary of stock option transactions is as follows:
					Weighted Average			Intrinsic Value
			Options		Exercise Price			(Millions)
Outstanding and Exercisable - January 1, 2012		47,374		$	18.78
Converted NSTAR Options upon Merger		2,664,894		$	23.99
Exercised		-1,166,511		$	22.53		$	18.7
Outstanding and Exercisable - December 31, 2012		1,545,757		$	24.92
Exercised		-324,382		$	20.97		$	6.7
Outstanding and Exercisable - December 31, 2013		1,221,375		$	25.97
Exercised		-869,759		$	25.68		$	16.4
Outstanding and Exercisable - December 31, 2014		351,616		$	26.69		$	9.4
Cash received for options exercised during the year ended December 31, 2014 totaled $22.3 million. The tax benefit realized from stock options exercised totaled $6.6 million for the year ended December 31, 2014.
Employee Share Purchase Plan: NU maintains an ESPP for eligible employees, which allows for NU common shares to be purchased by employees at the end of successive six-month offering periods at 95 percent of the closing market price on the last day of each six-month period. Employees are permitted to purchase shares having a value not exceeding 25 percent of their compensation as of the beginning of the offering period up to a specified limit. The ESPP qualifies as a non-compensatory plan under accounting guidance for share-based payments, and no compensation expense is recorded for ESPP purchases.
During 2014, employees purchased 40,779 shares at discounted prices of $41.61 and $41.71. Employees purchased 39,526 shares in 2013 at discounted prices of $38.69 and $42.19. As of December 31, 2014 and 2013, 776,975 and 817,754 shares, respectively, remained available for future issuance under the ESPP.
An income tax rate of 40 percent is used to estimate the tax effect on total share-based payments determined under the fair value-based method for all awards. The Company generally settles stock option exercises and fully vested RSUs and performance shares with either the issuance of new common shares or the issuance of common shares purchased in the open market.

OTHER_RETIREMENT_BENEFITS

OTHER RETIREMENT BENEFITS	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Compensation Related Costs, General [Text Block]	D.       Other Retirement Benefits
	NU provides benefits for retirement and other benefits for certain current and past company officers of NU, including CL&P, PSNH and WMECO. These benefits are accounted for on an accrual basis and expensed over the service lives of the employees. The actuarially-determined liability for these benefits, which is included in Other Long-Term Liabilities on the balance sheets, as well as the related expense, are as follows:
	NU		For the Years Ended December 31,
	(Millions of Dollars)		2014			2013			2012
	Actuarially-Determined Liability		$	57.5		$	51.3		$	54.6
	Other Retirement Benefits Expense			4.5			4.4			4.7
		For the Years Ended December 31,
		2014											2013						2012
	(Millions of Dollars)	CL&P			NSTAR Electric			PSNH			WMECO		CL&P		PSNH		WMECO		CL&P		PSNH		WMECO
	Actuarially-Determined Liability	$	0.4		$	-		$	2.6		$	0.2	$	0.4	$	2.3	$	0.1	$	0.4	$	2.5	$	0.2
	Other Retirement Benefits Expense		2.1			0.3			0.9			0.4		2.5		1		0.5		2.6		1		0.5

INCOME_TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Income Tax Disclosure [Text Block]	10.       INCOME TAXES
	The components of income tax expense are as follows:
	NU	For the Years Ended December 31,
	(Millions of Dollars)	2014			2013			2012 (1)
	Current Income Taxes:
	Federal	$	4.4		$	8.8		$	-30.9
	State		24.5			-9.4			17.6
	Total Current		28.9			-0.6			-13.3
	Deferred Income Taxes, Net:
	Federal		406.8			386.2			291.3
	State		36.5			45.4			0.8
	Total Deferred		443.3			431.6			292.1
	Investment Tax Credits, Net		-3.9			-4.1			-3.9
	Income Tax Expense	$	468.3		$	426.9		$	274.9
	(1) NSTAR amounts were included in NU beginning April 10, 2012.
			For the Years Ended December 31,
			2014												2013												2012
						NSTAR												NSTAR											NSTAR
	(Millions of Dollars)		CL&P			Electric			PSNH			WMECO			CL&P			Electric			PSNH			WMECO			CL&P		Electric		PSNH		WMECO
	Current Income Taxes:
		Federal	$	-0.2		$	75		$	-22.6		$	1.9		$	20.1		$	95.8		$	-8.2		$	-53.4		$	-47.8	$	93.5	$	-0.9	$	-24.7
		State		4.3			20.2			-0.1			1.8			-6.7			29.6			3.6			4.2			3.1		27.6		3.4		3.4
	Total Current			4.1			95.2			-22.7			3.7			13.4			125.4			-4.6			-49.2			-44.7		121.1		2.5		-21.3
	Deferred Income Taxes, Net:
		Federal		138			88			79.6			28.1			114.9			49.8			64.5			84.7			141.5		11.4		46.5		51.2
		State		-7.1			20.1			15.2			6			15.1			-1			11.2			2.3			-0.5		-7.1		12		2.7
	Total Deferred			130.9			108.1			94.8			34.1			130			48.8			75.7			87			141		4.3		58.5		53.9
	Investment Tax Credits, Net			-1.5			-1.3			-			-0.5			-1.7			-1.3			-			-0.4			-1.9		-1.4		-		-0.5
	Income Tax Expense		$	133.5		$	202		$	72.1		$	37.3		$	141.7		$	172.9		$	71.1		$	37.4		$	94.4	$	124	$	61	$	32.1
	A reconciliation between income tax expense and the expected tax expense at the statutory rate is as follows:
	NU		For the Years Ended December 31,
	(Millions of Dollars, except percentages)		2014				2013				2012 (1)
	Income Before Income Tax Expense		$	1,295.40			$	1,220.60			$	808
	Statutory Federal Income Tax Expense at 35%			453.4				427.2				282.8
	Tax Effect of Differences:
		Depreciation		-5.6				-7.4				-10.8
		Investment Tax Credit Amortization		-3.9				-4.1				-3.9
		Other Federal Tax Credits		-3.5				-3.7				-3.8
		State Income Taxes, Net of Federal Impact		42.5				27.6				4.4
		Dividends on ESOP		-8				-8				-6.4
		Tax Asset Valuation Allowance/Reserve Adjustments		-2.9				-4.3				7.6
		Other, Net		-3.7				-0.4				5
	Income Tax Expense		$	468.3			$	426.9			$	274.9
	Effective Tax Rate			36.20%				35.00%				34.00%
	(1) NSTAR amounts were included in NU beginning April 10, 2012.
			For the Years Ended December 31,
			2014												2013												2012
	(Millions of Dollars,					NSTAR												NSTAR											NSTAR
		except percentages)	CL&P			Electric			PSNH			WMECO			CL&P			Electric			PSNH			WMECO			CL&P		Electric		PSNH		WMECO
	Income Before Income Tax Expense		$	421.2		$	505.1		$	186.1		$	95.1		$	421.1		$	441.4		$	182.5		$	97.8		$	304.2	$	314.2	$	157.9	$	86.6
	Statutory Federal Income Tax Expense at 35%			147.4			176.8			65.1			33.3			147.4			154.5			63.9			34.2			106.5		110		55.3		30.3
	Tax Effect of Differences:
		Depreciation		-3.6			-1.3			0.3			-0.2			-7			0.1			0.6			-			-9		-		-0.3		0.2
		Investment Tax Credit Amortization		-1.5			-1.3			-			-0.5			-1.7			-1.3			-			-0.4			-1.9		-1.4		-		-0.5
		Other Federal Tax Credits		-			-			-3.5			-			-			-			-3.7			-			-		-		-3.8		-
		State Income Taxes, Net of Federal Impact		4.4			26.2			9.8			5			5			18.6			9.6			4.2			0.1		13.4		10		4
		Tax Asset Valuation Allowance/ Reserve Adjustments		-6.3			-			-			-			0.4			-			-			-			1.6		-		-		-
		Other, Net		-6.9			1.6			0.4			-0.3			-2.4			1			0.7			-0.6			-2.9		2		-0.2		-1.9
	Income Tax Expense		$	133.5		$	202		$	72.1		$	37.3		$	141.7		$	172.9		$	71.1		$	37.4		$	94.4	$	124	$	61	$	32.1
	Effective Tax Rate			31.70%			40.00%			38.70%			39.20%			33.60%			39.20%			39.00%			38.20%			31.00%		39.50%		38.60%		37.10%
	NU, CL&P, NSTAR Electric, PSNH and WMECO file a consolidated federal income tax return and unitary, combined and separate state income tax returns. These entities are also parties to a tax allocation agreement under which taxable subsidiaries do not pay any more taxes than they would have otherwise paid had they filed a separate company tax return, and subsidiaries generating tax losses, if any, are paid for their losses when utilized.
	Deferred tax assets and liabilities are recognized for the future tax effects of temporary differences between the carrying amounts and the tax basis of assets and liabilities. The tax effect of temporary differences is accounted for in accordance with the rate-making treatment of the applicable regulatory commissions and relevant accounting authoritative literature. The tax effects of temporary differences that give rise to the net accumulated deferred income tax obligations are as follows:
	NU			As of December 31,
	(Millions of Dollars)			2014			2013
Deferred Tax Assets:
	Employee Benefits		$	632.2		$	435.2
	Derivative Liabilities			199.6			272.9
	Regulatory Deferrals - Liabilities			366.7			272.7
	Allowance for Uncollectible Accounts			60.5			65
	Tax Effect - Tax Regulatory Liabilities			10			16.2
	Federal Net Operating Loss Carryforwards			59.1			158
	Purchase Accounting Adjustment			126.2			132.8
	Other			198.7			230.6
Total Deferred Tax Assets				1,653.00			1,583.40
	Less: Valuation Allowance			5.1			24.3
Net Deferred Tax Assets			$	1,647.90		$	1,559.10
Deferred Tax Liabilities:
	Accelerated Depreciation and Other Plant-Related Differences		$	4,215.90		$	3,806.50
	Property Tax Accruals			109.6			95.1
	Regulatory Amounts:
		Regulatory Deferrals - Assets		1,277.90			1,146.70
		Tax Effect - Tax Regulatory Assets		240.2			248.2
		Goodwill Regulatory Asset - 1999 Merger		203.2			211.5
		Derivative Assets		32.6			30.1
		Other		196.3			157.1
Total Deferred Tax Liabilities			$	6,275.70		$	5,695.20
			As of December 31,
			2014												2013
						NSTAR												NSTAR
(Millions of Dollars)			CL&P			Electric			PSNH			WMECO			CL&P			Electric			PSNH			WMECO
Deferred Tax Assets:
	Employee Benefits		$	129		$	39.9		$	46.8		$	9.2		$	56		$	38.3		$	15.5		$	-1.8
	Derivative Liabilities			193			1.8			-			-			272.4			3.3			-			-2.9
	Regulatory Deferrals - Liabilities			73.9			181.3			46.5			11.4			61.5			114.7			40.9			1
	Allowance for Uncollectible Accounts			32.3			13.8			3.2			3.8			31.2			15.4			3.1			3.3
	Tax Effect - Tax Regulatory Liabilities			3.1			1.8			2.1			2.5			4.7			5.4			2.1			1.6
	Federal Net Operating Loss Carryforwards			-			-			32.1			4.5			51			-			56.6			18.6
	Other			53.8			19.9			48.9			4.9			75.3			31.3			40.3			8.3
Total Deferred Tax Assets				485.1			258.5			179.6			36.3			552.1			208.4			158.5			28.1
	Less: Valuation Allowance			4			-			-			-			23.1			-			-			-
Net Deferred Tax Assets			$	481.1		$	258.5		$	179.6		$	36.3		$	529		$	208.4		$	158.5		$	28.1
Deferred Tax Liabilities:
	Accelerated Depreciation and Other Plant-Related Differences		$	1,378.60		$	1,296.90		$	596.6		$	385.8		$	1,238.10		$	1,179.40		$	526.6		$	361.1
	Property Tax Accruals			58.1			25			7.4			12.8			49.3			25.3			7.1			5.9
	Regulatory Amounts:
		Regulatory Deferrals - Assets		502.3			276			147.6			60.4			550.4			276.2			109.3			49.3
		Tax Effect - Tax Regulatory Assets		166.9			35.5			15.9			9.3			160.1			36			16.3			18.2
		Goodwill Regulatory Asset - 1999 Merger		-			174.4			-			-			-			181.6			-			-
		Derivative Assets		32.6			-			-			-			29			0.5			-			-
		Other		19.4			33.5			35.6			2.8			20.6			26.4			28			3.6
Total Deferred Tax Liabilities			$	2,157.90		$	1,841.30		$	803.1		$	471.1		$	2,047.50		$	1,725.40		$	687.3		$	438.1
Carryforwards: The following tables provide the amounts and expiration dates of state tax credit and loss carryforwards and federal tax credit and net operating loss carryforwards:
	As of December 31, 2014
							NSTAR
(Millions of Dollars)	NU			CL&P			Electric			PSNH			WMECO			Expiration Range
Federal Net Operating Loss	$	168.8		$	-		$	-		$	91.8		$	12.7		2031 - 2032
Federal Tax Credit		16.3			0.1			0.2			11.1			-		2031 - 2034
Federal Charitable Contribution		19.4			-			-			-			-		2016 - 2018
State Tax Credit		99.7			71			-			-			-		2014 - 2019
State Loss Carryforwards		40.6			-			-			-			-		2014 - 2034
State Charitable Contribution		2.1			-			-			-			-		2015 - 2018
	As of December 31, 2013
							NSTAR
(Millions of Dollars)	NU			CL&P			Electric			PSNH			WMECO			Expiration Range
Federal Net Operating Loss	$	451.3		$	145.8		$	-		$	161.8		$	53.3		2031 - 2032
Federal Tax Credit		8			-			-			7.6			-		2031 - 2033
Federal Charitable Contribution		33.7			-			-			-			-		2015 - 2017
State Tax Credit		104.7			86.8			-			-			-		2013 - 2018
State Loss Carryforwards		12.1			-			-			-			-		2013 - 2015
State Charitable Contribution		1			-			-			-			-		2015
In 2014, the Company recorded a reduction to its state credit carryforwards of $11 million (CL&P $10.1 million), net of tax, as a result of an update to reflect the amounts expired. Further, the Company decreased its valuation allowance reserve for state credits by $19.2 million at CL&P, net of tax, to reflect an update for expired state credits and latest estimate of usage.
For 2014, state credit and state loss carryforwards have been partially reserved by a valuation allowance of $4.4 million (net of federal income tax). For 2013, state credit and state loss carryforwards have been partially reserved by a valuation allowance of $23.7 million (net of federal income tax).
Unrecognized Tax Benefits: A reconciliation of the activity in unrecognized tax benefits, all of which would impact the effective tax rate if recognized, is as follows:
(Millions of Dollars)		NU			CL&P
Balance as of January 1, 2012		$	73.5		$	46.5
	Gross Increases - Current Year		10.3			2.5
	Gross Increases - Prior Year		0.1			-
	Gross Decreases - Prior Year		-0.8			-
Balance as of December 31, 2012			83.1			49
	Gross Increases - Current Year		8.2			2.1
	Gross Decreases - Prior Year		-1.1			-0.3
	Settlements		-49.8			-39.4
	Lapse of Statute of Limitations		-2.2			-
Balance as of December 31, 2013			38.2			11.4
	Gross Increases - Current Year		9.3			2.7
	Gross Increases - Prior Year		0.3			0.2
	Lapse of Statute of Limitations		-1.6			-
Balance as of December 31, 2014		$	46.2		$	14.3
Interest and Penalties: Interest on uncertain tax positions is recorded and generally classified as a component of Other Interest Expense on the statements of income. However, when resolution of uncertainties results in the Company receiving interest income, any related interest benefit is recorded in Other Income, Net on the statements of income. No penalties have been recorded. The amount of interest expense/(income) on uncertain tax positions recognized and the related accrued interest payable/(receivable) are as follows:
Other Interest		For the Years Ended December 31,									Accrued Interest		As of December 31,
Expense/(Income)		2014			2013			2012			Expense		2014			2013
(Millions of Dollars)											(Millions of Dollars)
NU (1)		$	0.4		$	-8.6		$	3.1		NU		$	1.9		$	1.5
CL&P			-			-4			1.3		CL&P			-			-
NSTAR amounts were included in NU beginning April 10, 2012.
Tax Positions: During 2014, NU did not resolve any of its uncertain tax positions.
During 2013, NU received a Final Determination from the Connecticut Department of Revenue Services (DRS) that concluded its audit of NU's Connecticut income tax returns for the years 2005 through 2008. The DRS Determination resulted in total NU and CL&P after-tax benefits of $13.6 million and $6.9 million, respectively, that included a reduction in NU and CL&P pre-tax interest expense of $8.7 million and $4 million, or $5.2 million and $2.4 million after-tax, respectively. Further, the income tax expense impact resulted in a tax benefit to NU and CL&P of $8.4 million and $4.5 million after-tax, respectively.
Open Tax Years: The following table summarizes NU, CL&P, NSTAR Electric, PSNH and WMECO's tax years that remain subject to examination by major tax jurisdictions as of December 31, 2014:
Description		Tax Years
Federal		2014
Connecticut		2011 - 2014
Massachusetts		2011 - 2014
New Hampshire		2011 - 2014
NU estimates that during the next twelve months, differences of a non-timing nature could be resolved, resulting in a zero to $2 million decrease in unrecognized tax benefits by NU. These estimated changes are not expected to have a material impact on the earnings of NU. Other companies' impacts are not expected to be material.
2014 Federal Legislation: On December 19, 2014, the "Tax Increase Prevention Act of 2014" became law, which extended the accelerated deduction of depreciation to businesses through 2014. This extended stimulus provides NU with cash flow benefits of approximately $200 million (approximately $70 million at CL&P, $50 million at NSTAR Electric, $35 million at PSNH, and $15 million at WMECO) in 2015.
2013 Federal Legislation: On January 2, 2013, the "American Taxpayer Relief Act of 2012" became law, which extended the accelerated deduction of depreciation to businesses through 2013. This extended stimulus provided NU with cash flow benefits of approximately $300 million (approximately $95 million at CL&P, $85 million at NSTAR Electric, $35 million at PSNH, and $50 million at WMECO).
On September 13, 2013, the Internal Revenue Service issued final Tangible Property regulations that are meant to simplify, clarify and make more administrable previously issued guidance. NU is in compliance with the new regulations, but continues to evaluate several new potential elections.
2013 Massachusetts: On July 24, 2013, Massachusetts enacted a law that changed the income tax rate applicable to utility companies effective January 1, 2014, from 6.5 percent to 8 percent. The tax law change required NU to remeasure its accumulated deferred income taxes and resulted in NU increasing its deferred tax liability with an offsetting regulatory asset of approximately $61 million at its utility companies ($46.3 million at NSTAR Electric and $9.8 million at WMECO).

COMMITMENTS_AND_CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Commitments and Contingencies Disclosure [Text Block]	11.       COMMITMENTS AND CONTINGENCIES
	A.       Environmental Matters
	General: NU, CL&P, NSTAR Electric, PSNH and WMECO are subject to environmental laws and regulations intended to mitigate or remove the effect of past operations and improve or maintain the quality of the environment. These laws and regulations require the removal or the remedy of the effect on the environment of the disposal or release of certain specified hazardous substances at current and former operating sites. NU, CL&P, NSTAR Electric, PSNH and WMECO have an active environmental auditing and training program and believe that they are substantially in compliance with all enacted laws and regulations.
	Environmental reserves are accrued when assessments indicate it is probable that a liability has been incurred and an amount can be reasonably estimated. The approach used estimates the liability based on the most likely action plan from a variety of available remediation options, including no action required or several different remedies ranging from establishing institutional controls to full site remediation and monitoring.
	These estimates are subjective in nature as they take into consideration several different remediation options at each specific site. The reliability and precision of these estimates can be affected by several factors, including new information concerning either the level of contamination at the site, the extent of NU, CL&P, NSTAR Electric, PSNH and WMECO's responsibility or the extent of remediation required, recently enacted laws and regulations or a change in cost estimates due to certain economic factors.
	The amounts recorded as environmental liabilities included in Other Current Liabilities and Other Long-Term Liabilities on the balance sheets represent management's best estimate of the liability for environmental costs, and take into consideration site assessment, remediation and long-term monitoring costs. The environmental liability also takes into account recurring costs of managing hazardous substances and pollutants, mandated expenditures to remediate previously contaminated sites and any other infrequent and non-recurring clean-up costs. A reconciliation of the activity in the environmental reserves is as follows:
	(Millions of Dollars)	NU			CL&P			NSTAR Electric			PSNH			WMECO
	Balance as of January 1, 2013	$	39.4		$	3.7		$	1.7		$	4.9		$	0.6
	Additions		3.5			0.2			0.2			1			-
	Payments/Reductions		-7.5			-0.5			-0.7			-0.5			-0.2
	Balance as of December 31, 2013		35.4			3.4			1.2			5.4			0.4
	Additions		12.7			1			-			0.1			0.2
	Payments/Reductions		-4.8			-0.6			-0.1			-0.3			-0.1
	Balance as of December 31, 2014	$	43.3		$	3.8		$	1.1		$	5.2		$	0.5
	These liabilities are estimated on an undiscounted basis and do not assume that any amounts are recoverable from insurance companies or other third parties. The environmental reserves include sites at different stages of discovery and remediation and do not include any unasserted claims.
	It is possible that new information or future developments could require a reassessment of the potential exposure to related environmental matters. As this information becomes available, management will continue to assess the potential exposure and adjust the reserves accordingly.
	The number of environmental sites and reserves related to these sites for which remediation or long-term monitoring, preliminary site work or site assessment are being performed are as follows:
		As of December 31, 2014						As of December 31, 2013
					Reserve						Reserve
		Number of Sites			(in millions)			Number of Sites			(in millions)
	NU		65		$	43.3			68		$	35.4
	CL&P		16			3.8			18			3.4
	NSTAR Electric		13			1.1			12			1.2
	PSNH		13			5.2			15			5.4
	WMECO		4			0.5			5			0.4
	Included in the NU number of sites and reserve amounts above are former MGP sites that were operated several decades ago and manufactured gas from coal and other processes, which resulted in certain by-products remaining in the environment that may pose a potential risk to human health and the environment. The reserve balance related to these former MGP sites was $38.8 million and $31.4 million as of December 31, 2014 and 2013, respectively, and relates primarily to the natural gas business segment. The increase in the reserve balance for the MGP sites was due to the completion of the site assessment at three sites. The assessments provided new information related to the extent and nature of the contamination and the costs of required remediation.
	As of December 31, 2014, for 5 environmental sites (1 for CL&P, and 1 for WMECO) that are included in the Company's reserve for environmental costs, the information known and nature of the remediation options at those sites allow for the Company to estimate the range of losses for environmental costs. As of December 31, 2014, $17.7 million ($1 million for CL&P and $0.3 million for WMECO) had been accrued as a liability for these sites, which represent management's best estimates of the liabilities for environmental costs. These amounts are the best estimates with estimated ranges of additional losses from zero to $24 million.
	As of December 31, 2014, for 15 environmental sites (3 for CL&P, 3 for NSTAR Electric and 2 for PSNH) that are included in the Company's reserve for environmental costs, management cannot reasonably estimate the exposure to loss in excess of the reserve, or range of loss, as these sites are under investigation and/or there is significant uncertainty as to what remedial actions, if any, the Company may be required to undertake. As of December 31, 2014, $13.4 million ($1.4 million for CL&P, $0.2 million for PSNH) had been accrued as a liability for these sites. As of December 31, 2014, for the remaining 45 environmental sites (12 for CL&P, 10 for NSTAR Electric, 11 for PSNH, and 3 for WMECO) that are included in the Company's reserve for environmental costs, the $12.2 million accrual ($1.4 million for CL&P, $1.1 million for NSTAR Electric, $5 million for PSNH, and $0.2 million for WMECO) represents management's best estimate of the liability and no additional loss is anticipated.
	CERCLA: Of the total environmental sites, 9 sites (1 for CL&P, 3 for NSTAR Electric and 3 for PSNH) are superfund sites under the federal Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA) and its amendments or state equivalents for which the Company has been notified that it is a potentially responsible party but for which the site assessment and remediation are not being managed by the Company.  As of December 31, 2014, a liability of $0.7 million accrued on these sites represents management's best estimate of its potential remediation costs with respect to these superfund sites.
	Environmental Rate Recovery: PSNH, NSTAR Gas and Yankee Gas have rate recovery mechanisms for MGP related environmental costs. CL&P recovers a certain level of environmental costs currently in rates but does not have an environmental cost recovery tracking mechanism. Accordingly, changes in CL&P's environmental reserves impact CL&P's Net Income. NSTAR Electric and WMECO do not have a separate regulatory mechanism to recover environmental costs from its customers, and changes in NSTAR Electric's and WMECO's environmental reserves impact Net Income.
	B.			Long-Term Contractual Arrangements
	Estimated Future Annual Costs: The estimated future annual costs of significant long-term contractual arrangements as of December 31, 2014 are as follows:
	NU
	(Millions of Dollars)				2015			2016			2017			2018			2019			Thereafter			Total
	Supply and Stranded Cost				$	196.6		$	169.2		$	101.9		$	65.3		$	38.1		$	82.4		$	653.5
	Renewable Energy					204.3			240.5			239.6			204.2			202.9			1,994.60			3,086.10
	Peaker CfDs					26.3			25.4			10.5			-			-			-			62.2
	Natural Gas Procurement					133.7			116.3			45.6			31.9			25.8			85.1			438.4
	Coal, Wood and Other					99			25.2			5			5			1.9			15			151.1
	Transmission Support Commitments					25.9			21.6			19			21.2			21.3			21.3			130.3
	Total				$	685.8		$	598.2		$	421.6		$	327.6		$	290		$	2,198.40		$	4,521.60
	CL&P
	(Millions of Dollars)				2015			2016			2017			2018			2019			Thereafter			Total
	Supply and Stranded Cost				$	134.3		$	136.8		$	79		$	42		$	25		$	43.6		$	460.7
	Renewable Energy					61.2			70.3			71.3			72.1			72.1			715.9			1,062.90
	Peaker CfDs					26.3			25.4			10.5			-			-			-			62.2
	Transmission Support Commitments					10.2			8.5			7.5			8.4			8.4			8.4			51.4
	Yankee Billings					1.4			0.8			0.8			0.9			0.9			11.8			16.6
	Total				$	233.4		$	241.8		$	169.1		$	123.4		$	106.4		$	779.7		$	1,653.80
	NSTAR Electric
	(Millions of Dollars)				2015			2016			2017			2018			2019			Thereafter			Total
	Supply and Stranded Cost				$	34.3		$	14.1		$	4.8		$	5.5		$	5.5		$	31.4		$	95.6
	Renewable Energy					85.4			99.9			96.9			59.6			57.7			319.8			719.3
	Transmission Support Commitments					8.1			6.7			5.9			6.6			6.6			6.6			40.5
	Yankee Billings					0.5			0.3			0.3			0.3			0.3			4			5.7
	Total				$	128.3		$	121		$	107.9		$	72		$	70.1		$	361.8		$	861.1
	PSNH
	(Millions of Dollars)				2015			2016			2017			2018			2019			Thereafter			Total
	Supply and Stranded Cost				$	28		$	18.3		$	18.1		$	17.8		$	7.6		$	7.4		$	97.2
	Renewable Energy					57.7			67.9			69			70.1			70.7			932.4			1,267.80
	Coal, Wood and Other					99			25.2			5			5			1.9			15			151.1
	Transmission Support Commitments					5.5			4.6			4			4.5			4.5			4.5			27.6
	Yankee Billings					0.4			0.3			0.3			0.3			0.3			4.7			6.3
	Total				$	190.6		$	116.3		$	96.4		$	97.7		$	85		$	964		$	1,550.00
WMECO
(Millions of Dollars)				2015			2016			2017			2018			2019			Thereafter			Total
Renewable Energy				$	-		$	2.4		$	2.4		$	2.4		$	2.4		$	26.5		$	36.1
Transmission Support Commitments					2.1			1.8			1.6			1.7			1.8			1.8			10.8
Yankee Billings					0.3			0.2			0.2			0.2			0.2			3			4.1
Total				$	2.4		$	4.4		$	4.2		$	4.3		$	4.4		$	31.3		$	51
Supply and Stranded Cost: CL&P, NSTAR Electric and PSNH have various IPP contracts or purchase obligations for electricity, including payment obligations resulting from the buydown of electricity purchase contracts. Such contracts extend through 2024 for CL&P, 2030 for NSTAR Electric and 2023 for PSNH.
In addition, CL&P, along with UI, has four capacity CfDs for a total of approximately 787 MW of capacity consisting of three generation projects and one demand response project. The capacity CfDs extend through 2026 and obligate both CL&P and UI to make or receive payments on a monthly basis to or from the generation facilities based on the difference between a set capacity price and the capacity market prices received by the generation facilities in the ISO-NE capacity markets. CL&P has a sharing agreement with UI, whereby UI will share 20 percent of the costs and benefits of these contracts. CL&P's portion of the costs and benefits of these contracts will be paid by or refunded to CL&P's customers.
The contractual obligations table does not include CL&P's SS or LRS, or NSTAR Electric's or WMECO's default service contracts, the amounts of which vary with customers' energy needs. The contractual obligations table also does not include PSNH's short-term power supply management.
Renewable Energy: Renewable energy contracts include non-cancellable commitments under contracts of CL&P, NSTAR Electric, PSNH, and WMECO for the purchase of energy and capacity from renewable energy facilities. Such contracts extend through 2035 for CL&P, 2030 for NSTAR Electric, 2033 for PSNH and 2030 for WMECO.
The contractual obligations table does not include long-term commitments signed by CL&P, NSTAR Electric and WMECO, as required by the PURA and DPU, for the purchase of renewable energy and related products that are contingent on the future construction of energy facilities.
Peaker CfDs: In 2008, CL&P entered into three CfDs with developers of peaking generation units approved by the PURA (Peaker CfDs). These units have a total of approximately 500 MW of peaking capacity. As directed by the PURA, CL&P and UI have entered into a sharing agreement, whereby CL&P is responsible for 80 percent and UI for 20 percent of the net costs or benefits of these CfDs. The Peaker CfDs pay the generation facility owner the difference between capacity, forward reserve and energy market revenues and a cost-of-service payment stream for 30 years. The ultimate cost or benefit to CL&P under these contracts will depend on the costs of plant operation and the prices that the projects receive for capacity and other products in the ISO-NE markets. CL&P's portion of the amounts paid or received under the Peaker CfDs will be recoverable from or refunded to CL&P's customers.
Natural Gas Procurement: NU's natural gas distribution businesses have long-term contracts for the purchase, transportation and storage of natural gas in the normal course of business as part of its portfolio of supplies. These contracts extend through 2029.
Coal, Wood and Other: PSNH has entered into various arrangements for the purchase of coal, wood and the transportation services for fuel supply for its electric generating assets. Also included in the table above is a contract for capacity on the Portland Natural Gas Transmission System (PNGTS) pipeline that extends through 2018. The costs on this contract of $11.4 million are not recoverable from customers.
Transmission Support Commitments: Along with other New England utilities, CL&P, NSTAR Electric, PSNH and WMECO entered into agreements in 1985 to support transmission and terminal facilities that were built to import electricity from the Hydro-Québec system in Canada. CL&P, NSTAR Electric, PSNH and WMECO are obligated to pay, over a 30-year period ending in 2020, their proportionate shares of the annual operation and maintenance expenses and capital costs of those facilities.
The total costs incurred under these agreements were as follows:
NU		For the Years Ended December 31,
(Millions of Dollars)		2014			2013			2012 (1)
Supply and Stranded Cost		$	99.2		$	141		$	216.8
Renewable Energy			114.4			91.3			48.7
Peaker CfDs			18.1			51.9			59.3
Natural Gas Procurement			482.5			349.8			243.1
Coal, Wood and Other			120.5			112.6			105.2
Transmission Support Commitments			25			24.9			24.8
		For the Years Ended December 31,
		2014												2013											2012
					NSTAR												NSTAR										NSTAR
(Millions of Dollars)		CL&P			Electric			PSNH			WMECO			CL&P			Electric			PSNH			WMECO		CL&P		Electric		PSNH		WMECO
Supply and Stranded Cost		$	63		$	7		$	26		$	3.2		$	77.6		$	32.4		$	29		$	2	$	158.2	$	36.3	$	30.5	$	0.9
Renewable Energy			0.7			87.4			26.3			-			-			84.9			6.4			-		-		60.2		4.1		-
Peaker CfDs			18.1			-			-			-			51.9			-			-			-		59.3		-		-		-
Coal, Wood and Other			-			-			120.5			-			-			-			112.6			-		-		-		105.2		-
Transmission Support Commitments			9.9			7.7			5.3			2.1			9.8			7.7			5.3			2.1		9.6		7.6		5.2		2
(1)       NSTAR amounts were included in NU beginning April 10, 2012.
C.       Contractual Obligations - Yankee Companies
CL&P, NSTAR Electric, PSNH and WMECO have decommissioning and plant closure cost obligations to the Yankee Companies, which have each completed the physical decommissioning of their respective nuclear facilities and are now engaged in the long-term storage of their spent fuel. The Yankee Companies collect decommissioning and closure costs through wholesale, FERC-approved rates charged under power purchase agreements with several New England utilities, including CL&P, NSTAR Electric, PSNH and WMECO. These companies in turn recover these costs from their customers through state regulatory commission-approved retail rates.
CL&P, NSTAR Electric, PSNH and WMECO's percentage share of the obligations to support the Yankee Companies under FERC-approved rate tariffs is the same as their respective ownership percentages in the Yankee Companies. For further information on the ownership percentages, see Note 1J, "Summary of Significant Accounting Policies - Equity Method Investments," to the financial statements.
The Yankee Companies have collected or are currently collecting amounts that management believes are adequate to recover the remaining decommissioning and closure cost estimates for the respective plants. Management believes CL&P, NSTAR Electric and WMECO will recover their shares of these decommissioning and closure obligations from their customers. PSNH has already recovered its share of these costs from its customers.
Spent Nuclear Fuel Litigation:
DOE Phase I Damages – In 1998, the Yankee Companies filed separate complaints against the DOE in the Court of Federal Claims seeking monetary damages resulting from the DOE's failure to begin accepting spent nuclear fuel for disposal by January 31, 1998 pursuant to the terms of the 1983 spent fuel and high level waste disposal contracts between the Yankee Companies and the DOE (DOE Phase I Damages). Phase I covered damages for the period 1998 through 2002. Following multiple appeals and cross-appeals, in December 2012, the judgment awarding CYAPC $39.6 million, YAEC $38.3 million and MYAPC $81.7 million became final.
In January 2013, the proceeds from the DOE Phase I Damages Claim were received by the Yankee Companies and transferred to each Yankee Company's respective decommissioning trust. As a result of NU's consolidation of CYAPC and YAEC, the financial statements reflected an increase of $77.9 million in marketable securities for CYAPC and YAEC's Phase I Damage awards that were invested in the nuclear decommissioning trusts in 2013.
In June 2013, FERC approved CYAPC, YAEC and MYAPC to reduce rates in their wholesale power contracts through the application of the DOE proceeds for the benefit of customers. Changes to the terms of the wholesale power contracts became effective on July 1, 2013. In accordance with the FERC order, CL&P, NSTAR Electric, PSNH and WMECO began receiving the benefit of the DOE proceeds, and the benefits have been passed on to customers.
On September 17, 2014, in accordance with the MYAPC refund plan, MYAPC returned a portion of the DOE Phase I Damages proceeds to the member companies, including CL&P, NSTAR Electric, PSNH, and WMECO, in the amount of $3.2 million, $1.1 million, $1.4 million and $0.8 million, respectively. These amounts reduced receivables at CL&P, NSTAR Electric, PSNH and WMECO.
DOE Phase II Damages - In December 2007, the Yankee Companies each filed subsequent lawsuits against the DOE seeking recovery of actual damages incurred related to the alleged failure of the DOE to provide for a permanent facility to store spent nuclear fuel generated in years 2001 through 2008 for CYAPC and YAEC and from 2002 through 2008 for MYAPC (DOE Phase II Damages). In November 2013, the court issued a final judgment awarding CYAPC $126.3 million, YAEC $73.3 million, and MYAPC $35.8 million. On January 14, 2014, the Yankee Companies received a letter from the U.S. Department of Justice stating that the DOE will not appeal the court's final judgment.
In March and April 2014, CYAPC, YAEC and MYAPC received payment of $126.3 million, $73.3 million and $35.8 million, respectively, of the DOE Phase II Damages proceeds and made the required informational filing with FERC in accordance with the process and methodology outlined in the 2013 FERC order. The Yankee Companies returned the DOE Phase II Damages proceeds to the member companies, including CL&P, NSTAR Electric, PSNH, and WMECO, for the benefit of their respective customers, on June 1, 2014.
As of December 31, 2014, CL&P's refund obligation to customers of $65.4 million was recorded as an offset to the deferred storm restoration costs regulatory asset, as directed by PURA. NSTAR Electric's, PSNH's and WMECO's refund obligation to customers of $29.1 million, $13.1 million and $18.1 million, respectively, was recorded as a regulatory liability in each company's respective regulatory tracker mechanisms. Refunds to customers for these DOE proceeds began in the third quarter of 2014. For further information, see Note 2, "Regulatory Accounting," to the financial statements.
DOE Phase III Damages – In August 2013, the Yankee Companies each filed subsequent lawsuits against the DOE seeking recovery of actual damages incurred in the years 2009 through 2012. The presiding judge issued a Pre-Trial Scheduling Order on September 3, 2014 that set the case for trial from June 30 to July 2, 2015.
D.       Guarantees and Indemnifications
NU parent provides credit assurances on behalf of its subsidiaries, including CL&P, NSTAR Electric, PSNH and WMECO, in the form of guarantees in the normal course of business.
NU provided guarantees and various indemnifications on behalf of external parties as a result of the sales of former subsidiaries of NU Enterprises and the termination of an unregulated business, with maximum exposures either not specified or not material.
NU also issued a guaranty under which, beginning at the time the Northern Pass Transmission line goes into commercial operation, NU will guarantee the financial obligations of NPT under the TSA in an amount not to exceed $25 million. NU's obligations under the guaranty expire upon the full, final and indefeasible payment of the guaranteed obligations.
Management does not anticipate a material impact to Net Income as a result of these various guarantees and indemnifications.
The following table summarizes NU's guarantees of its subsidiaries, including CL&P, NSTAR Electric, PSNH and WMECO, as of December 31, 2014:
				Maximum Exposure
Subsidiary		Description		(in millions)				Expiration Dates
Various		Surety Bonds (1)		$	60			2015 - 2016
NUSCO and Rocky River Realty Company		Lease Payments for Vehicles and Real Estate		$	14.4			2019 and 2024
(1)       Surety bond expiration dates reflect termination dates, the majority of which will be renewed or extended. Certain surety bonds contain credit ratings triggers that would require NU parent to post collateral in the event that the unsecured debt credit ratings of NU are downgraded.
E.        FERC Base ROE Complaints
Beginning in 2011, several New England state attorneys general, state regulatory commissions, consumer advocates, consumer groups, municipal parties and other parties (the "Complainants") jointly filed three separate complaints at FERC. In the first complaint, filed in 2011, the Complainants alleged that the NETOs' base ROE of 11.14 percent that was utilized since 2006 was unjust and unreasonable, asserted that the rate was excessive due to changes in the capital markets, and sought an order to reduce it prospectively from the date of the final FERC order and for the 15-month period beginning October 1, 2011 to December 31, 2012 (the "first complaint refund period"). In the pursuant second and third complaints, filed in 2012 and 2014, respectively, the Complainants challenged the NETOs' base ROE and sought refunds for the 15-month periods beginning December 27, 2012 and July 31, 2014, respectively.
In 2014, the FERC determined that the base ROE should be set at 10.57 percent for the first complaint refund period and that a utility's total or maximum ROE should not exceed the top of the new zone of reasonableness (7.03 percent to 11.74 percent). The FERC ordered the NETOs to provide refunds to customers for the first complaint refund period and set the new base ROE of 10.57 percent prospectively from October 16, 2014. In late 2014, the NETOs made a compliance filing, and began refunding amounts from the first complaint period, inclusive of incentive ROE adders that exceeded the 11.74 percent as compared to the total company transmission ROE. Complainants have challenged the compliance filing.
As a result of the actions taken by the FERC and other developments in this matter, NU recorded reserves in 2013 and 2014 to recognize the potential financial impacts of the first and second complaints. The Company is unable to determine any amount related to the third complaint. The following is a summary of the cumulative pre-tax reserves (excluding interest) established by the Company in 2013 and 2014:
		NU
		For the Years Ended December 31,
(Millions of Dollars)		2013			2014			Total
1st Complaint - Base ROE		$	23.7		$	1.2		$	24.9
2nd Complaint - Base ROE			-			27.4			27.4
Incentive ROE (1st and 2nd Complaint)			-			8.4			8.4
Cumulative Reserve		$	23.7		$	37		$	60.7
		CL&P									NSTAR Electric
		For the Years Ended December 31,									For the Years Ended December 31,
(Millions of Dollars)		2013			2014			Total			2013			2014			Total
1st Complaint - Base ROE		$	12.8		$	0.5		$	13.3		$	5.7		$	0.4		$	6.1
2nd Complaint - Base ROE			-			13.5			13.5			-			7.5			7.5
Incentive ROE (1st and 2nd Complaint)			-			6.7			6.7			-			-			-
Cumulative Reserve		$	12.8		$	20.7		$	33.5		$	5.7		$	7.9		$	13.6
		PSNH									WMECO
		For the Years Ended December 31,									For the Years Ended December 31,
(Millions of Dollars)		2013			2014			Total			2013			2014			Total
1st Complaint - Base ROE		$	2.3		$	0.1		$	2.4		$	2.9		$	0.2		$	3.1
2nd Complaint - Base ROE			-			2.7			2.7			-			3.7			3.7
Incentive ROE (1st and 2nd Complaint)			-			-			-			-			1.7			1.7
Cumulative Reserve		$	2.3		$	2.8		$	5.1		$	2.9		$	5.6		$	8.5
As of December 31, 2014, the cumulative reserves above do not reflect refunds totaling $4.8 million at NU, $2.7 million at CL&P, $1 million at NSTAR Electric, $0.5 million at PSNH and $0.6 million at WMECO for the first complaint refund period.
The aggregate after-tax net charge to 2014 earnings resulting from the 2014 FERC orders totaled $22.4 million at NU, $12.4 million at CL&P, $4.9 million at NSTAR Electric, $1.7 million at PSNH and $3.4 million at WMECO. In 2013, the aggregate after-tax charge to earnings totaled $14.3 million at NU, $7.7 million at CL&P, $3.4 million at NSTAR Electric, $1.4 million at PSNH and $1.8 million at WMECO.
Although management is uncertain on the final outcome on the second and third complaints regarding the base ROE and the incentive ROE adder, management believes the current reserves established are appropriate to reflect probable and reasonably estimable refunds.
F.       2014 Comprehensive Settlement Agreement
On December 31, 2014, NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General filed a comprehensive settlement agreement with the DPU. The comprehensive settlement agreement included resolution of the outstanding NSTAR Electric CPSL program filings for the periods 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the NSTAR Electric energy efficiency program filings regarding LBR for the periods 2008 through 2011. If approved by the DPU, NSTAR Electric and NSTAR Gas will be required to refund a total of $44.7 million to their respective customers, which was included in regulatory liabilities as of December 31, 2014. Upon the DPU's approval, NSTAR Electric will adjust its regulatory liabilities, which it expects will result in a benefit of $23 million in the first quarter of 2015. Management expects a response from the DPU in the first quarter of 2015.
G.       Basic Service Bad Debt Adder
In accordance with a generic 2005 DPU order, electric utilities in Massachusetts recover the energy-related portion of bad debt costs in their Basic Service rates. In 2007, NSTAR Electric filed its 2006 Basic Service reconciliation with the DPU proposing an adjustment related to the increase of its Basic Service bad debt charge-offs. The DPU issued an order approving the implementation of a revised Basic Service rate but instructed NSTAR Electric to reduce distribution rates by an amount equal to the increase in its Basic Service bad debt charge-offs. This adjustment to NSTAR Electric's distribution rates would eliminate the fully reconciling nature of the Basic Service bad debt adder.
In 2010, NSTAR Electric filed an appeal of the DPU's order with the SJC. NSTAR Electric's position was that it had fully removed the collection of energy-related bad debt costs from its distribution rates effective January 1, 2006. Therefore, no further adjustment to distribution rates was warranted. In 2012, the SJC vacated the DPU order and remanded the matter to the DPU for further review.
As of December 31, 2014, NSTAR Electric has a total deferred regulatory asset of approximately $33 million of costs associated with energy-related bad debt.
On January 7, 2015, the DPU issued an order on remand stating that NSTAR Electric had, in fact, removed energy-related bad debt costs from distribution rates effective January 1, 2006. The DPU order approved NSTAR Electric's 2005 and 2006 reconciliation filings and ordered NSTAR Electric and the Massachusetts Attorney General to collaborate on the submission of a proposal for the reconciliation of energy-related bad debt costs for the open years of 2007 through 2014 by April 7, 2015. Management expects to present a proposal to the Attorney General in the first quarter of 2015 with a decision from the DPU later in 2015.
H.       Litigation and Legal Proceedings
NU, including CL&P, NSTAR Electric, PSNH and WMECO, are involved in legal, tax and regulatory proceedings regarding matters arising in the ordinary course of business, which involve management's assessment to determine the probability of whether a loss will occur and, if probable, its best estimate of probable loss. The Company records and discloses losses when these losses are probable and reasonably estimable, and discloses matters when losses are probable but not estimable or when losses are reasonably possible. Legal costs related to the defense of loss contingencies are expensed as incurred.

LEASES

LEASES	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Leases of Lessee Disclosure [Text Block]	LEASES
	NU, including CL&P, NSTAR Electric, PSNH and WMECO, has entered into lease agreements, some of which are capital leases, for the use of data processing and office equipment, vehicles, service centers, and office space. In addition, CL&P, NSTAR Electric, PSNH and WMECO incur costs associated with leases entered into by NUSCO and Rocky River Realty Company, which are included below in their respective operating lease rental expenses and future minimum rental payments. These intercompany lease amounts are eliminated on an NU consolidated basis. The provisions of the NU, CL&P, NSTAR Electric, PSNH, and WMECO lease agreements generally contain renewal options. Certain lease agreements contain payments impacted by the commercial paper rate plus a credit spread or the consumer price index.
	Operating lease rental payments charged to expense are as follows:
									NSTAR
	(Millions of Dollars)		NU (1)			CL&P			Electric			PSNH			WMECO
	2014		$	14.3		$	6		$	7.8		$	1.5		$	1.2
	2013			16.3			8.1			6.7			1.7			2.9
	2012			14.8			8.2			6.2			2.5			3
	NSTAR amounts were included in NU beginning April 10, 2012.
	Future minimum rental payments, excluding executory costs, such as property taxes, state use taxes, insurance, and maintenance, under long-term noncancelable leases, as of December 31, 2014 are as follows:
	Operating Leases							NSTAR
	(Millions of Dollars)	NU			CL&P			Electric			PSNH			WMECO
	2015	$	20.1		$	4.3		$	10		$	1.1		$	1.2
	2016		17.6			3.8			8.8			1			1
	2017		14.6			2.6			7.7			0.8			0.8
	2018		10.5			1.5			5.8			0.6			0.6
	2019		8.6			1.1			4.7			0.5			0.6
	Thereafter		22.5			4			10.4			1.5			2.5
	Future minimum lease payments	$	93.9		$	17.3		$	47.4		$	5.5		$	6.7
	Capital Leases
	(Millions of Dollars)	NU			CL&P			PSNH
	2015	$	2.4		$	2		$	0.4
	2016		2.2			1.9			0.3
	2017		2.1			2			0.1
	2018		2.1			2			0.1
	2019		2			2			0
	Thereafter		3.5			3.5			0
	Future minimum lease payments		14.3			13.4			0.9
	Less amount representing interest		4.9			5			0
	Present value of future minimum lease payments	$	9.4		$	8.4		$	0.9
	CL&P entered into certain contracts for the purchase of energy that qualify as leases. These contracts do not have minimum lease payments and therefore are not included in the tables above. However, such contracts have been included in the contractual obligations table in Note 11B, "Commitments and Contingencies - Long-Term Contractual Arrangements," to the financial statements.

FAIR_VALUE_OF_FINANCIAL_INSTRU

FAIR VALUE OF FINANCIAL INSTRUMENTS	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Fair Value Disclosures [Text Block]	13.       FAIR VALUE OF FINANCIAL INSTRUMENTS
	The following methods and assumptions were used to estimate the fair value of each of the following financial instruments:
	Preferred Stock and Long-Term Debt: The fair value of CL&P's and NSTAR Electric's preferred stock is based upon pricing models that incorporate interest rates and other market factors, valuations or trades of similar securities and cash flow projections. The fair value of long-term debt securities is based upon pricing models that incorporate quoted market prices for those issues or similar issues adjusted for market conditions, credit ratings of the respective companies and treasury benchmark yields. The fair values provided in the tables below are classified as Level 2 within the fair value hierarchy. Carrying amounts and estimated fair values are as follows:
		As of December 31,
		2014				2013
	NU	Carrying		Fair		Carrying		Fair
	(Millions of Dollars)	Amount		Value		Amount		Value
	Preferred Stock Not Subject to Mandatory Redemption	$	155.6	$	153.6	$	155.6	$	152.7
	Long-Term Debt		8,851.60		9,451.20		8,310.20		8,443.10
		As of December 31, 2014
		CL&P				NSTAR Electric				PSNH				WMECO
		Carrying		Fair		Carrying		Fair		Carrying		Fair		Carrying		Fair
	(Millions of Dollars)	Amount		Value		Amount		Value		Amount		Value		Amount		Value
	Preferred Stock Not Subject to Mandatory Redemption	$	116.2	$	112	$	43	$	41.6	$	-	$	-	$	-	$	-
	Long-Term Debt		2,842.00		3,214.50		1,797.40		1,993.50		1,076.30		1,137.90		628.5		689.4
		As of December 31, 2013
		CL&P				NSTAR Electric				PSNH				WMECO
		Carrying		Fair		Carrying		Fair		Carrying		Fair		Carrying		Fair
	(Millions of Dollars)	Amount		Value		Amount		Value		Amount		Value		Amount		Value
	Preferred Stock Not Subject to Mandatory Redemption	$	116.2	$	110.5	$	43	$	42.2	$	-	$	-	$	-	$	-
	Long-Term Debt		2,741.20		2,952.80		1,801.10		1,888.00		1,049.00		1,073.90		629.4		640.1
	Derivative Instruments: Derivative instruments are carried at fair value. For further information, see Note 4, "Derivative Instruments," to the financial statements.
	Other Financial Instruments: Investments in marketable securities are carried at fair value. For further information, see Note 5, "Marketable Securities," to the financial statements. The carrying value of other financial instruments included in current assets and current liabilities, including cash and cash equivalents and special deposits, approximates their fair value due to the short-term nature of these instruments.
	See Note 1H, "Summary of Significant Accounting Policies - Fair Value Measurements," for the fair value measurement policy and the fair value hierarchy.

ACCUMULATED_OTHER_COMPREHENSIV

ACCUMULATED OTHER COMPREHENSIVE INCOME	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Comprehensive Income Note [Text Block]	14.       ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS)
	The changes in accumulated other comprehensive income/(loss) by component, net of tax effect, is as follows:
			For the Year Ended December 31, 2014											For the Year Ended December 31, 2013
			Qualified			Unrealized								Qualified		Unrealized
			Cash Flow			Gains/(Losses)			Defined					Cash Flow		Gains/(Losses)		Defined
			Hedging			on Marketable			Benefit					Hedging		on Marketable		Benefit
	(Millions of Dollars)		Instruments			Securities			Plans			Total		Instruments		Securities		Plans		Total
	AOCI as of January 1st		$	-14.4		$	0.4		$	-32		$	-46	$	-16.4	$	1.3	$	-57.8	$	-72.9
	OCI Before Reclassifications			-			0.3			-34.2			-33.9		-		-0.9		19.4		18.5
	Amounts Reclassified from AOCI			2			-			3.9			5.9		2		-		6.4		8.4
	Net OCI			2			0.3			-30.3			-28		2		-0.9		25.8		26.9
	AOCI as of December 31st		$	-12.4		$	0.7		$	-62.3		$	-74	$	-14.4	$	0.4	$	-32	$	-46
	NU's qualified cash flow hedging instruments represent interest rate swap agreements on debt issuances that were settled in prior years. The settlement amount was recorded in AOCI and is being amortized into Net Income over the term of the underlying debt instrument. CL&P, PSNH and WMECO continue to amortize interest rate swaps settled in prior years from AOCI into Interest Expense over the remaining life of the associated long-term debt, which are not material to their respective financial statements.
	The tax effects of Defined Benefit Plan OCI amounts before reclassifications, which relate to actuarial gains and losses that arose during 2014, 2013 and 2012 were recognized in AOCI as net deferred tax assets of $22.3 million and $6.2 million in 2014 and 2012, respectively, and net deferred tax liabilities of $11.4 million in 2013.
	The following table sets forth the amounts reclassified from AOCI by component and the impacted line item on the statements of income:
		For the Years Ended December 31,
		2014			2013			2012
		Amounts Reclassified			Amounts Reclassified			Amounts Reclassified			Statements of Income
	(Millions of Dollars)	from AOCI			from AOCI			from AOCI			Line Item Impacted
	Qualified Cash Flow Hedging Instruments	$	-3.4		$	-3.4		$	-3.3		Interest Expense
	Tax Effect		1.4			1.4			1.3		Income Tax Expense
	Qualified Cash Flow Hedging Instruments, Net of Tax	$	-2		$	-2		$	-2
	Defined Benefit Plan Costs:
	Amortization of Actuarial Losses	$	-6.2		$	-10.5		$	-8.9		Operations and Maintenance (1)
	Amortization of Prior Service Cost		-0.2			-0.2			-0.2		Operations and Maintenance (1)
	Amortization of Transition Obligation		0			0			-0.2		Operations and Maintenance (1)
	Total Defined Benefit Plan Costs		-6.4			-10.7			-9.3
	Tax Effect		2.5			4.3			3.5		Income Tax Expense
	Defined Benefit Plan Costs, Net of Tax	$	-3.9		$	-6.4		$	-5.8
	Total Amounts Reclassified from AOCI, Net of Tax	$	-5.9		$	-8.4		$	-7.8
	(1)       These amounts are included in the computation of net periodic Pension, SERP and PBOP costs. See Note 9A, "Employee Benefits - Pension Benefits and Postretirement Benefits Other Than Pensions," for further information.
	As of December 31, 2014, it is estimated that a pre-tax amount of $3.5 million ($0.7 million for CL&P, $2 million for PSNH and $0.6 million for WMECO) will be reclassified from AOCI as a decrease to Net Income over the next 12 months as a result of the amortization of the interest rate swap agreements, which have been settled. In addition, it is estimated that a pre-tax amount of $6.9 million will be reclassified from AOCI as a decrease to Net Income over the next 12 months as a result of the amortization of Pension, SERP and PBOP costs.

DIVIDEND_RESTRICTIONS

DIVIDEND RESTRICTIONS	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Dividend Payment Restrictions [Table Text Block]	15.       DIVIDEND RESTRICTIONS
	NU parent's ability to pay dividends may be affected by certain state statutes, the ability of its subsidiaries to pay common dividends and the leverage restriction tied to its consolidated total debt to total capitalization ratio requirement in its revolving credit agreement.
	CL&P, NSTAR Electric, PSNH and WMECO are subject to Section 305 of the Federal Power Act that makes it unlawful for a public utility to make or pay a dividend from any funds "properly included in its capital account." Management believes that this Federal Power Act restriction, as applied to CL&P, NSTAR Electric, PSNH and WMECO, would not be construed or applied by the FERC to prohibit the payment of dividends for lawful and legitimate business purposes from retained earnings. In addition, certain state statutes may impose additional limitations on such companies and on Yankee Gas and NSTAR Gas. Such state law restrictions do not restrict payment of dividends from retained earnings or net income. Pursuant to the joint revolving credit agreement of NU, CL&P, PSNH, WMECO, Yankee Gas and NSTAR Gas, and the NSTAR Electric revolving credit agreement, each company is required to maintain consolidated total debt to total capitalization ratio of no greater than 65 percent at all times. As of December 31, 2014, all companies were in compliance with such covenant. The Retained Earnings balances subject to these restrictions were $2.4 billion for NU, $1.1 billion for CL&P, $1.5 billion for NSTAR Electric, $486.5 million for PSNH and $178.8 million for WMECO as of December 31, 2014. As of December 31, 2014, NU, CL&P, NSTAR Electric, PSNH, WMECO, Yankee Gas and NSTAR Gas were in compliance with all such provisions of the revolving credit agreements that may restrict the payment of dividends. PSNH is further required to reserve an additional amount under its FERC hydroelectric license conditions. As of December 31, 2014, $13 million of PSNH's Retained Earnings was subject to restriction under its FERC hydroelectric license conditions and PSNH was in compliance with this provision.

COMMON_SHARES

COMMON SHARES	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Stock by Class [Text Block]	16.       COMMON SHARES
	The following table sets forth the NU common shares and the shares of common stock of CL&P, NSTAR Electric, PSNH and WMECO that were authorized and issued and the respective per share par values:
				Shares
				Authorized	Issued
		Per Share		as of December 31,	as of December 31,
		Par Value		2014 and 2013	2014		2013
	NU	$	5	380,000,000		333,359,172		333,113,492
	CL&P	$	10	24,500,000		6,035,205		6,035,205
	NSTAR Electric	$	1	100,000,000		100		100
	PSNH	$	1	100,000,000		301		301
	WMECO	$	25	1,072,471		434,653		434,653
	As of December 31, 2014 and 2013, there were 16,375,835 and 17,796,672 NU common shares held as treasury shares, respectively. As of December 31, 2014 and 2013, NU common shares outstanding were 316,983,337 and 315,273,559, respectively.

PREFFERED_STOCK_NOT_SUBJECT_TO

PREFFERED STOCK NOT SUBJECT TO MANDATORY REDEMPTION	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Preferred Stock Text Block	17.       PREFERRED STOCK NOT SUBJECT TO MANDATORY REDEMPTION
	The CL&P and NSTAR Electric preferred stock is not subject to mandatory redemption and is presented as a noncontrolling interest of a subsidiary in NU's financial statements.
	CL&P Preferred Stock: CL&P's charter authorizes it to issue up to 9 million shares of preferred stock ($50 par value per share). The issuance of additional preferred shares would be subject to PURA approval. Preferred stockholders have liquidation rights equal to the par value of the preferred stock, which they would receive in preference to any distributions to any junior stock. Were there to be a shortfall, all preferred stockholders would share ratably in available liquidation assets.
	NSTAR Electric Preferred Stock: NSTAR Electric is authorized to issue 2,890,000 shares ($100 par value per share). NSTAR Electric has two outstanding series of cumulative preferred stock. Upon liquidation, holders of cumulative preferred stock are entitled to receive a liquidation preference before any distribution to holders of common stock. The liquidation preference for each outstanding series of cumulative preferred stock is equal to the par value, plus accrued and unpaid dividends. Were there to be a shortfall, holders of cumulative preferred stock would share ratably in available liquidation assets.
	Details of preferred stock not subject to mandatory redemption are as follows (in millions except in redemption price and shares):
					Redemption Price		Shares Outstanding as of	As of December 31,
	Series				Per Share		December 31, 2014 and 2013	2014		2013
	CL&P
	$	1.9		Series of 1947	$	52.5	163,912	$	8.2	$	8.2
	$	2		Series of 1947	$	54	336,088		16.8		16.8
	$	2.04		Series of 1949	$	52	100,000		5		5
	$	2.2		Series of 1949	$	52.5	200,000		10		10
		3.9	%	Series of 1949	$	50.5	160,000		8		8
	$	2.06		Series E of 1954	$	51	200,000		10		10
	$	2.09		Series F of 1955	$	51	100,000		5		5
		4.5	%	Series of 1956	$	50.75	104,000		5.2		5.2
		4.96	%	Series of 1958	$	50.5	100,000		5		5
		4.5	%	Series of 1963	$	50.5	160,000		8		8
		5.28	%	Series of 1967	$	51.43	200,000		10		10
	$	3.24		Series G of 1968	$	51.84	300,000		15		15
		6.56	%	Series of 1968	$	51.44	200,000		10		10
	Total CL&P						2,324,000	$	116.2	$	116.2
	NSTAR Electric
		4.25	%	Series	$	103.625	180,000	$	18	$	18
		4.78	%	Series	$	102.8	250,000		25		25
	Total NSTAR Electric						430,000	$	43	$	43
	Fair Value Adjustment due to Merger with NSTAR								-3.6		-3.6
	Total NU - Preferred Stock of Subsidiaries							$	155.6	$	155.6

COMMON_SHARESHOLDERS_EQUITY_AN

COMMON SHARESHOLDERS' EQUITY AND NONCONTROLLING INTERESTS (NU)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Stockholders' Equity Note Disclosure [Text Block]	18	COMMON SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS
	A summary of the changes in Common Shareholders' Equity and Noncontrolling Interests of NU is as follows:
									Noncontrolling
			Common						Interest -
			Shareholders'		Noncontrolling		Total		Preferred Stock
	(Millions of Dollars)		Equity		Interest		Equity		of Subsidiaries
	Balance as of January 1, 2012		$	4,012.70	$	3	$	4,015.70	$	116.2
	Net Income			533.1		-		533.1		-
	Purchase Price of NSTAR (1)			5,038.30		-		5,038.30		-
	Other Equity Impacts of Merger with NSTAR (2)			3.4		-3.4		-		39.4
	Dividends on Common Shares			-375.5		-		-375.5		-
	Dividends on Preferred Stock			-7		-		-7		-7
	Issuance of Common Shares			13.3		-		13.3		-
	Contributions to NPT			-		0.3		0.3		-
	Other Transactions, Net			21.1		-		21.1		-
	Net Income Attributable to Noncontrolling Interests			-0.1		0.1		-		7
	Other Comprehensive Loss			-2.2		-		-2.2		-
	Balance as of December 31, 2012		$	9,237.10	$	-	$	9,237.10	$	155.6
	Net Income			793.7		-		793.7		-
	Dividends on Common Shares			-462.7		-		-462.7		-
	Dividends on Preferred Stock			-7.7		-		-7.7		-7.7
	Issuance of Common Shares			11.1		-		11.1		-
	Other Transactions, Net			13.2		-		13.2		-
	Net Income Attributable to Noncontrolling Interests			-		-		-		7.7
	Other Comprehensive Income			26.8		-		26.8		-
	Balance as of December 31, 2013		$	9,611.50	$	-	$	9,611.50	$	155.6
	Net Income			827.1		-		827.1		-
	Dividends on Common Shares			-496.5		-		-496.5		-
	Dividends on Preferred Stock			-7.5		-		-7.5		-7.5
	Issuance of Common Shares			6.6		-		6.6		-
	Other Transactions, Net			63.6		-		63.6		-
	Net Income Attributable to Noncontrolling Interests			-		-		-		7.5
	Other Comprehensive Loss			-28		-		-28		-
	Balance as of December 31, 2014		$	9,976.80	$	-	$	9,976.80	$	155.6
	(1)       On April 10, 2012, NU issued approximately 136 million common shares to the NSTAR shareholders in connection with the merger. See Note 21, "Merger of NU and NSTAR," for further information.
	(2)       The preferred stock of NSTAR Electric is not subject to mandatory redemption and has been presented as a noncontrolling interest in NSTAR Electric in NU's financial statements. In addition, upon completion of the merger, an NSTAR subsidiary that held 25 percent of NPT was merged into EETV, resulting in EETV owning 100 percent of NPT. Accordingly, the noncontrolling interest balance was eliminated and 100 percent ownership of NPT was reflected in Common Shareholders' Equity.
	For the years ended December 31, 2014, 2013 and 2012, there was no change in ownership of the common equity of CL&P and NSTAR Electric.

EARNINGS_PER_SHARE

EARNINGS PER SHARE	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Earnings Per Share [Text Block]	19.       EARNINGS PER SHARE
	Basic EPS is computed based upon the weighted average number of common shares outstanding during each period. Diluted EPS is computed on the basis of the weighted average number of common shares outstanding plus the potential dilutive effect of certain share-based compensation awards as if they were converted into common shares. For the years ended December 31, 2014, 2013 and 2012, there were 3,643, 1,575 and 4,266, respectively, antidilutive share awards excluded from the computation.
	The following table sets forth the components of basic and diluted EPS:
			For the Years Ended December 31,
	(Millions of Dollars, except share information)		2014		2013		2012
	Net Income Attributable to Controlling Interest		$	819.5	$	786	$	525.9
	Weighted Average Common Shares Outstanding:
		Basic		316,136,748		315,311,387		277,209,819
		Dilutive Effect		1,280,666		899,773		783,812
		Diluted		317,417,414		316,211,160		277,993,631
	Basic EPS		$	2.59	$	2.49	$	1.9
	Diluted EPS		$	2.58	$	2.49	$	1.89
	On April 10, 2012, NU issued approximately 136 million common shares as a result of the merger with NSTAR, which are reflected in the weighted average common shares outstanding.
	RSUs and performance shares are included in basic weighted average common shares outstanding as of the date that all necessary vesting conditions have been satisfied. The dilutive effect of unvested RSUs and performance shares is calculated using the treasury stock method. Assumed proceeds of these units under the treasury stock method consist of the remaining compensation cost to be recognized and a theoretical tax benefit. The theoretical tax benefit is calculated as the tax impact of the intrinsic value of the units (the difference between the market value of the average units outstanding for the period, using the average market price during the period, and the grant date market value).
	The dilutive effect of stock options to purchase common shares is also calculated using the treasury stock method. Assumed proceeds for stock options consist of cash proceeds that would be received upon exercise, and a theoretical tax benefit. The theoretical tax benefit is calculated as the tax impact of the intrinsic value of the stock options (the difference between the market value of the average stock options outstanding for the period, using the average market price during the period, and the exercise price).

SEGMENT_INFORMATION

SEGMENT INFORMATION	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Segment Reporting Disclosure [Text Block]	20.       SEGMENT INFORMATION
	Presentation: NU is organized between the Electric Distribution, Electric Transmission and Natural Gas Distribution reportable segments and Other based on a combination of factors, including the characteristics of each segments' products and services, the sources of operating revenues and expenses and the regulatory environment in which each segment operates. These reportable segments represented substantially all of NU's total consolidated revenues for the years ended December 31, 2014, 2013 and 2012. Revenues from the sale of electricity and natural gas primarily are derived from residential, commercial and industrial customers and are not dependent on any single customer. The Electric Distribution reportable segment includes the generation activities of PSNH and WMECO.
	The remainder of NU's operations is presented as Other in the tables below and primarily consists of 1) the equity in earnings of NU parent from its subsidiaries and intercompany interest income, both of which are eliminated in consolidation, and interest expense related to the debt of NU parent, 2) the revenues and expenses of NU's service company, most of which are eliminated in consolidation, 3) the operations of CYAPC and YAEC, and 4) the results of other non-regulated subsidiaries, which are not part of its core business.
	Cash flows used for investments in plant included in the segment information below are cash capital expenditures that do not include amounts incurred but not paid, cost of removal, AFUDC related to equity funds, and the capitalized portions of pension expense.
	NU's reportable segments are determined based upon the level at which NU's chief operating decision maker assesses performance and makes decisions about the allocation of company resources. Each of NU's subsidiaries, including CL&P, NSTAR Electric, PSNH and WMECO, has one reportable segment. NU's operating segments and reporting units are consistent with its reportable business segments.
	NSTAR amounts were included in NU beginning April 10, 2012.
	NU's segment information is as follows:
		For the Year Ended December 31, 2014
		Electric		Natural Gas
	(Millions of Dollars)	Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues	$	5,663.40	$	1,007.30	$	1,018.20	$	790.9	$	-737.9	$	7,741.90
	Depreciation and Amortization		-384.6		-68.1		-150.5		-42.1		19.9		-625.4
	Other Operating Expenses		-4,366.20		-786.7		-302.1		-748		719.3		-5,483.70
	Operating Income		912.6		152.5		565.6		0.8		1.3		1,632.80
	Interest Expense		-191.6		-34		-104.1		-36.6		4.2		-362.1
	Interest Income		5.1		-		0.9		3.6		-3.6		6
	Other Income, Net		10.7		0.2		10.3		916		-918.6		18.6
	Income Tax (Expense)/Benefit		-269.7		-46.4		-174.5		22.3		-		-468.3
	Net Income		467.1		72.3		298.2		906.1		-916.7		827
	Net Income Attributable to Noncontrolling Interests		-4.7		-		-2.8		-		-		-7.5
	Net Income Attributable to Controlling Interest	$	462.4	$	72.3	$	295.4	$	906.1	$	-916.7	$	819.5
	Total Assets (as of)	$	17,563.40	$	3,030.90	$	7,625.60	$	12,682.50	$	-11,124.40	$	29,778.00
	Cash Flows Used for Investments in Plant	$	645.2	$	176.7	$	731.6	$	50.2	$	-	$	1,603.70
		For the Year Ended December 31, 2013
		Electric		Natural Gas
	(Millions of Dollars)	Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues	$	5,362.30	$	855.8	$	978.7	$	777.5	$	-673.1	$	7,301.20
	Depreciation and Amortization		-604.8		-66.7		-136.2		-62.2		10.2		-859.7
	Other Operating Expenses		-3,927.70		-659.4		-281.8		-715		671.8		-4,912.10
	Operating Income		829.8		129.7		560.7		0.3		8.9		1,529.40
	Interest Expense		-175		-33.1		-100.3		-35.5		5.2		-338.7
	Interest Income		4.1		-		0.7		5.4		-5.6		4.6
	Other Income, Net		12.9		0.8		10.9		858.9		-858.2		25.3
	Income Tax (Expense)/Benefit		-240		-36.5		-182.1		31.9		-0.2		-426.9
	Net Income		431.8		60.9		289.9		861		-849.9		793.7
	Net Income Attributable to Noncontrolling Interests		-4.8		-		-2.9		-		-		-7.7
	Net Income Attributable to Controlling Interest	$	427	$	60.9	$	287	$	861	$	-849.9	$	786
	Total Assets (as of)	$	17,260.00	$	2,759.70	$	6,745.80	$	11,842.40	$	-10,812.40	$	27,795.50
	Cash Flows Used for Investments in Plant	$	639	$	168.1	$	618.5	$	31.2	$	-	$	1,456.80
		For the Year Ended December 31, 2012
		Electric		Natural Gas
	(Millions of Dollars)	Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues	$	4,716.50	$	572.9	$	861.5	$	803.8	$	-680.9	$	6,273.80
	Depreciation and Amortization		-530.3		-49.1		-109.2		-56.4		4.2		-740.8
	Other Operating Expenses		-3,585.40		-445.2		-251.6		-817		684.4		-4,414.80
	Operating Income/(Loss)		600.8		78.6		500.7		-69.6		7.7		1,118.20
	Interest Expense		-165.6		-31.3		-96.7		-43.6		7.3		-329.9
	Interest Income		2.8		-		0.4		7.1		-7.1		3.2
	Other Income, Net		8.9		0.4		7.3		795		-795.1		16.5
	Income Tax (Expense)/Benefit		-150.2		-16.9		-159.2		55.5		-4.1		-274.9
	Net Income		296.7		30.8		252.5		744.4		-791.3		533.1
	Net Income Attributable to Noncontrolling Interests		-4.4		-		-2.8		-		-		-7.2
	Net Income Attributable to Controlling Interest	$	292.3	$	30.8	$	249.7	$	744.4	$	-791.3	$	525.9
	Cash Flows Used for Investments in Plant	$	611.7	$	148.7	$	663.6	$	48.3	$	-	$	1,472.30

MERGER_OF_NU_AND_NSTAR

MERGER OF NU AND NSTAR	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Business Combination Disclosure	21.       MERGER OF NU AND NSTAR
	On April 10, 2012, NU acquired 100 percent of the outstanding common shares of NSTAR. Pursuant to the terms and conditions of the Agreement and Plan of Merger, as amended, (the "Merger Agreement,") NSTAR and its subsidiaries became wholly-owned subsidiaries of NU.
	NSTAR was a holding company engaged through its subsidiaries in the energy delivery business serving electric and natural gas distribution customers in Massachusetts. As part of the merger, NSTAR shareholders received 1.312 NU common shares for each NSTAR common share owned (the "exchange ratio") as of the acquisition date. NU issued approximately 136 million common shares to the NSTAR shareholders as a result of the merger.
	Purchase Price: Pursuant to the merger, all of the NSTAR common shares were exchanged at the fixed exchange ratio of 1.312 NU common shares for each NSTAR common share. The total consideration transferred in the merger was based on the closing price of NU common shares on April 9, 2012, the day prior to the date the merger was completed, and was calculated as follows:
	NSTAR common shares outstanding as of April 9, 2012 (in thousands)*			103,696
	Exchange ratio			1.312
	NU common shares issued for NSTAR common shares outstanding (in thousands)			136,049
	Closing price of NU common shares on April 9, 2012		$	36.79
	Value of common shares issued (in millions)		$	5,005
	Fair value of NU replacement stock-based compensation awards related to pre-merger service (in millions)			33
	Total purchase price (in millions)		$	5,038
	*       Included 109 thousand shares related to NSTAR stock-based compensation awards that vested immediately prior to the merger.
	Certain of NSTAR's stock-based compensation awards, including deferred shares, performance shares and all outstanding stock options, were replaced with NU awards using the exchange ratio upon consummation of the merger. In accordance with accounting guidance for business combinations, the portion of the fair value of these awards attributable to service provided prior to the merger was included in the purchase price as it represented consideration transferred in the merger. See Note 9C, "Employee Benefits – Share-Based Payments," for further information.
	Purchase Price Allocation: The allocation of the total purchase price to the estimated fair values of the assets acquired and liabilities assumed was determined based on the accounting guidance for fair value measurements. The allocation of the total purchase price included adjustments to record the fair value of NSTAR's unregulated telecommunications business, regulatory assets not earning a return, lease agreements, long-term debt and the preferred stock of NSTAR Electric. The fair values of NSTAR's assets and liabilities were determined based on significant estimates and assumptions, including Level 3 inputs, that were judgmental in nature. These estimates and assumptions included the timing and amounts of projected future cash flows and discount rates reflecting risk inherent in future cash flows.
	In accordance with accounting guidance for business combinations, the excess of the purchase price over the estimated fair values of the assets acquired and liabilities assumed was recognized as goodwill.
	The goodwill from the merger with NSTAR totaled $3.2 billion and was allocated to NU's reporting units based on their estimated fair values. See Note 22, "Goodwill," for the allocation of goodwill to each reporting unit.
	Pro Forma Financial Information: The following unaudited pro forma financial information reflects the pro forma combined results of operations of NU and NSTAR and reflects the amortization of purchase price adjustments assuming the merger had taken place on January 1, 2011. The unaudited pro forma financial information has been presented for illustrative purposes only and is not necessarily indicative of the consolidated results of operations that would have been achieved or the future consolidated results of operations of NU.
	(Pro forma amounts in millions, except per share amounts)	For the Year Ended December 31, 2012
	Operating Revenues	$	7,004
	Net Income Attributable to Controlling Interest		630
	Basic EPS		2
	Diluted EPS		1.99
	Pro forma net income does not include potential cost savings associated with the merger. Pro forma net income also excludes certain non-recurring merger costs and costs related to the Connecticut and Massachusetts merger settlement agreements described below, with the following aggregate after-tax impacts:
	(Millions of Dollars)		For the Year Ended December 31, 2012
	Transaction and Other Costs		$	32
	Settlement Agreement Impacts			60
	Total After-Tax Non-Recurring Costs Excluded from Pro Forma Net Income Attributable to Controlling Interest		$	92
	Regulatory Approvals: On February 15, 2012, NU and NSTAR reached comprehensive merger settlement agreements with the Massachusetts Attorney General and the DOER. The Attorney General settlement agreement covered a variety of rate-making and rate design issues, including a base distribution rate freeze through 2015 for NSTAR Electric, NSTAR Gas and WMECO and $15 million, $3 million and $3 million in the form of rate credits to their respective customers. On April 4, 2012, the DPU approved the settlement agreements and the merger of NU and NSTAR.
	On March 13, 2012, NU and NSTAR reached a comprehensive merger settlement agreement with both the Connecticut Attorney General and the Connecticut Office of Consumer Counsel. The settlement agreement covered a variety of matters, including a $25 million rate credit to CL&P customers, a CL&P base distribution rate freeze until December 1, 2014, and the establishment of a $15 million fund for energy efficiency and other initiatives to be disbursed at the direction of the DEEP. In the agreement, CL&P agreed to forego rate recovery of $40 million of the deferred storm restoration costs associated with restoration activities following Tropical Storm Irene and the October 2011 snowstorm. On April 2, 2012, the PURA approved the settlement agreement and the merger of NU and NSTAR.
	The pre-tax financial impacts of the Connecticut and Massachusetts merger settlement agreements that were recognized in 2012 by NU, CL&P, NSTAR Electric, and WMECO are summarized as follows:
	(Millions of Dollars)	NU			CL&P		NSTAR Electric		WMECO
	Customer Rate Credits	$	46		$	25	$	15	$	3
	Storm Costs Deferral Reduction		40			40		-		-
	Establishment of Energy Efficiency Fund		15			-		-		-
	Total Pre-Tax Settlement Agreement Impacts	$	101		$	65	$	15	$	3

GOODWILL

GOODWILL	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Goodwill Disclosure [Text Block]	22.       GOODWILL
	In accordance with the accounting standards, goodwill is not subject to amortization. However, goodwill is subject to fair value-based rules for measuring impairment, and resulting write-downs, if any, are charged to Operating Expenses. These accounting standards require that goodwill be reviewed at least annually for impairment and whenever facts or circumstances indicate that there may be an impairment. NU uses October 1st as the annual goodwill impairment testing date.
	On April 10, 2012, upon consummation of the merger with NSTAR, NU recorded approximately $3.2 billion of goodwill. With the completion of the merger, NU reviewed its management structure and determined that the reporting units for the purpose of testing goodwill for impairment are Electric Distribution, Electric Transmission and Natural Gas Distribution. NU's reporting units are consistent with the operating segments underlying the reportable segments identified in Note 20, "Segment Information," to the financial statements. Accordingly, the goodwill resulting from the merger was allocated to the Electric Distribution, Electric Transmission and Natural Gas Distribution reporting units based on the estimated fair values of the reporting units as of the merger date.
	Prior to the merger with NSTAR, the only reporting unit that maintained goodwill was the Natural Gas Distribution reportable segment related to the acquisition of the parent of Yankee Gas in 2000. This goodwill was recorded at Yankee Gas. The goodwill balance at Yankee Gas as of December 31, 2014 and 2013 was $0.3 billion.
	NU completed its annual goodwill impairment test for each of its reporting units as of October 1, 2014 and determined that no impairment exists. There were no events subsequent to October 1, 2014 that indicated impairment of goodwill.
	There were no changes to the goodwill balance or the allocation of goodwill as of December 31, 2014 or 2013. The allocation of goodwill to NU's reporting units as of both December 31, 2014 and 2013 was as follows:
		Electric		Electric		Natural Gas
	(Billions of Dollars)	Distribution		Transmission		Distribution		Total
	Goodwill Allocation	$	2.5	$	0.6	$	0.4	$	3.5

VARIABLE_INTEREST_ENTITIES

VARIABLE INTEREST ENTITIES	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Variable Interest Entities Text Block	23.       VARIABLE INTEREST ENTITIES
	The Company's variable interests outside of the consolidated group are not material and consist of contracts that are required by regulation and provide for regulatory recovery of contract costs and benefits through customer rates. NU, CL&P and NSTAR Electric hold variable interests in variable interest entities (VIEs) through agreements with certain entities that own single renewable energy or peaking generation power plants and with other independent power producers. NU, CL&P and NSTAR Electric do not control the activities that are economically significant to these VIEs or provide financial or other support to these VIEs. Therefore, NU, CL&P and NSTAR Electric do not consolidate any power plant VIEs.

QUARTERLY_FINANCIAL_DATA_Unaud

QUARTERLY FINANCIAL DATA (Unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Quarterly Financial Information [Text Block]	QUARTERLY FINANCIAL DATA (UNAUDITED)
	24.
	NU Consolidated Statements of Quarterly Financial Data	Quarter Ended
	(Millions of Dollars, except per share information)	March 31,		June 30,		September 30,		December 31,
	2014
	Operating Revenues	$	2,290.60	$	1,677.60	$	1,892.50	$	1,881.20
	Operating Income		467.7		294		440.9		430.2
	Net Income		237.8		129.2		236.5		223.6
	Net Income Attributable to Controlling Interest		236		127.4		234.6		221.5
	Basic EPS (a)	$	0.75	$	0.4	$	0.74	$	0.69
	Diluted EPS (a)	$	0.74	$	0.4	$	0.74	$	0.69
	2013
	Operating Revenues	$	1,995.00	$	1,635.90	$	1,892.60	$	1,777.70
	Operating Income		418.9		350.6		399.3		360.6
	Net Income		230		173.1		211.4		179.2
	Net Income Attributable to Controlling Interest		228.1		171		209.5		177.4
	Basic and Diluted EPS (a)	$	0.72	$	0.54	$	0.66	$	0.56
	(a) The summation of quarterly EPS data may not equal annual data due to rounding.
	Statements of Quarterly Financial Data	Quarter Ended
	(Millions of Dollars)	March 31,		June 30,		September 30,		December 31,
	CL&P
	2014
	Operating Revenues	$	734.6	$	587.3	$	695.6	$	675.1
	Operating Income		158		92.1		146.2		159
	Net Income		79.3		37.4		83.9		87.2
	2013
	Operating Revenues	$	624.1	$	569.3	$	648.4	$	600.5
	Operating Income		149.7		136.8		133.9		119.2
	Net Income		85		67.9		66.3		60.2
	NSTAR Electric
	2014
	Operating Revenues	$	666.2	$	561.5	$	727.9	$	581.1
	Operating Income		118.4		121.5		206.6		132
	Net Income		58.1		60.1		115.6		69.3
	2013
	Operating Revenues	$	592.3	$	570.4	$	753.9	$	576.9
	Operating Income		94.5		112.5		192		109.2
	Net Income		48.1		58		107		55.4
	PSNH
	2014
	Operating Revenues	$	299.8	$	211.6	$	223.7	$	224.4
	Operating Income		64		49		56.4		60
	Net Income		32.6		24.1		28.2		29
	2013
	Operating Revenues	$	273.8	$	216.1	$	218.6	$	226.9
	Operating Income		58.1		54.3		56.6		56.2
	Net Income		29		27.2		28.4		26.8
	WMECO
	2014
	Operating Revenues	$	137.4	$	108.3	$	118.1	$	129.6
	Operating Income		34.7		17.7		31.2		34
	Net Income		18.1		7		14.7		18
	2013
	Operating Revenues	$	125	$	115	$	121.8	$	110.9
	Operating Income		35.6		32.4		28.9		22.4
	Net Income		18.6		16.4		15		10.4

SIGNIFCANT_ACCOUNTING_POLICIES

SIGNIFCANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Organization Consolidation And Presentation Of Financial Statements Disclosure Text Block	The consolidated financial statements of NU, NSTAR Electric and PSNH include the accounts of each of their respective subsidiaries. Intercompany transactions have been eliminated in consolidation. The accompanying consolidated financial statements of NU, NSTAR Electric and PSNH and the financial statements of CL&P and WMECO are herein collectively referred to as the "financial statements."
	The preparation of the financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
	NU's consolidated financial information includes NSTAR and its subsidiaries' results of operations beginning April 10, 2012. The information disclosed for the year ended December 31, 2012 for NSTAR Electric is presented on a comparable basis.
	NU consolidates CYAPC and YAEC because CL&P's, NSTAR Electric's, PSNH's and WMECO's combined ownership interest in each of these entities is greater than 50 percent. Intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation of the NU financial statements.
	NU's utility subsidiaries are subject to the application of accounting guidance for entities with rate-regulated operations that considers the effect of regulation resulting from differences in the timing of the recognition of certain revenues and expenses from those of other businesses and industries. NU's utility subsidiaries' distribution, generation and transmission businesses and NPT are subject to rate-regulation that is based on cost recovery and meets the criteria for application of rate-regulated accounting. See Note 2, "Regulatory Accounting," for further information.
	Certain reclassifications of prior year data were made in the accompanying balance sheets for NU, CL&P, NSTAR Electric and PSNH. These reclassifications were made to conform to the current year presentation.
	In accordance with accounting guidance on noncontrolling interests in consolidated financial statements, the Preferred Stock of CL&P and the Preferred Stock of NSTAR Electric, which are not owned by NU or its consolidated subsidiaries and are not subject to mandatory redemption, have been presented as noncontrolling interests in the financial statements of NU.  The Preferred Stock of CL&P and the Preferred Stock of NSTAR Electric are considered to be temporary equity and have been classified between liabilities and permanent shareholders' equity on the balance sheets of NU, CL&P and NSTAR Electric due to a provision in the preferred stock agreements of both CL&P and NSTAR Electric that grant preferred stockholders the right to elect a majority of the CL&P and NSTAR Electric Board of Directors, respectively, should certain conditions exist, such as if preferred dividends are in arrears for a specified amount of time.  The Net Income reported in the statements of income and cash flows represents net income prior to apportionment to noncontrolling interests, which is represented by dividends on preferred stock of CL&P and NSTAR Electric.
	As of December 31, 2014 and 2013, NU's carrying amount of goodwill was approximately $3.5 billion. NU performs an assessment for possible impairment of its goodwill at least annually. NU completed its annual goodwill impairment test for each of its reporting units as of October 1, 2014 and determined that no impairment exists. See Note 22, "Goodwill," for further information.
Derivatives Offsetting Fair Value Amounts Policy	The gross fair values of derivative assets and liabilities with the same counterparty are offset and reported as net Derivative Assets or Derivative Liabilities, with current and long-term portions, on the balance sheets.
Fair Value Of Financial Instruments Policy	H.       Fair Value Measurements
	Fair value measurement guidance is applied to derivative contracts that are not elected or designated as "normal purchases or normal sales" (normal) and to the marketable securities held in trusts. Fair value measurement guidance is also applied to valuations of the investments used to calculate the funded status of pension and PBOP plans and nonrecurring fair value measurements of nonfinancial assets such as goodwill and AROs, and is also used to estimate the fair value of preferred stock and long-term debt.
	Fair Value Hierarchy: In measuring fair value, NU uses observable market data when available and minimizes the use of unobservable inputs. Inputs used in fair value measurements are categorized into three fair value hierarchy levels for disclosure purposes. The entire fair value measurement is categorized based on the lowest level of input that is significant to the fair value measurement. NU evaluates the classification of assets and liabilities measured at fair value on a quarterly basis, and NU's policy is to recognize transfers between levels of the fair value hierarchy as of the end of the reporting period. The three levels of the fair value hierarchy are described below:
	Level 1 - Inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities as of the reporting date.  Active markets are those in which transactions for the asset or liability occur in sufficient frequency and volume to provide pricing information on an ongoing basis.
	Level 2 - Inputs are quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-derived valuations in which all significant inputs are observable.
	Level 3 - Quoted market prices are not available. Fair value is derived from valuation techniques in which one or more significant inputs or assumptions are unobservable. Where possible, valuation techniques incorporate observable market inputs that can be validated to external sources such as industry exchanges, including prices of energy and energy-related products.
	Determination of Fair Value: The valuation techniques and inputs used in NU's fair value measurements are described in Note 4, "Derivative Instruments," Note 5, "Marketable Securities," Note 6, "Asset Retirement Obligations," Note 9A, "Employee Benefits – Pension Benefits and Postretirement Benefits Other Than Pensions," Note 13, "Fair Value of Financial Instruments," and Note 21, "Merger of NU and NSTAR," to the financial statements.
	Derivative contracts classified as Level 2 in the fair value hierarchy relate to the financial contracts for natural gas futures. Prices are obtained from broker quotes and are based on actual market activity. The contracts are valued using NYMEX natural gas prices. Valuations of these contracts also incorporate discount rates using the yield curve approach.
	The fair value of derivative contracts classified as Level 3 utilizes significant unobservable inputs. The fair value is modeled using income techniques, such as discounted cash flow valuations adjusted for assumptions relating to exit price. Significant observable inputs for valuations of these contracts include energy and energy-related product prices in future years for which quoted prices in an active market exist. Fair value measurements categorized in Level 3 of the fair value hierarchy are prepared by individuals with expertise in valuation techniques, pricing of energy and energy-related products, and accounting requirements. The future power and capacity prices for periods that are not quoted in an active market or established at auction are based on available market data and are escalated based on estimates of inflation to address the full time period of the contract.
	Valuations of derivative contracts using a discounted cash flow methodology include assumptions regarding the timing and likelihood of scheduled payments and also reflect non-performance risk, including credit, using the default probability approach based on the counterparty's credit rating for assets and the Company's credit rating for liabilities. Valuations incorporate estimates of premiums or discounts that would be required by a market participant to arrive at an exit price, using historical market transactions adjusted for the terms of the contract.
	NU, including CL&P, NSTAR Electric, PSNH and WMECO, recognizes a liability for the fair value of an ARO on the obligation date if the liability's fair value can be reasonably estimated and is conditional on a future event. Settlement dates and future costs are reasonably estimated when sufficient information becomes available. Management has identified various categories of AROs, primarily certain assets containing asbestos and hazardous contamination and has performed fair value calculations, reflecting expected probabilities for settlement scenarios.
	The fair value of an ARO is recorded as a liability in Other Long-Term Liabilities with a corresponding amount included in Property, Plant and Equipment, Net on the balance sheets. The ARO assets are depreciated, and the ARO liabilities are accreted over the estimated life of the obligation with corresponding credits recorded as accumulated depreciation and ARO liabilities, respectively. As the Regulated companies are rate-regulated on a cost-of-service basis, these companies apply regulatory accounting guidance and both the depreciation and accretion costs associated with the Regulated companies' AROs are recorded as increases to Regulatory Assets on the balance sheets.
	The Company values assets based on observable inputs when available. Equity securities, exchange traded funds and futures contracts classified as Level 1 in the fair value hierarchy are priced based on the closing price on the primary exchange as of the balance sheet date. Commingled funds included in Level 2 equity securities are recorded at the net asset value provided by the asset manager, which is based on the market prices of the underlying equity securities. Swaps are valued using pricing models that incorporate interest rates and equity and fixed income index closing prices to determine a net present value of the cash flows. Fixed income securities, such as government issued securities, corporate bonds and high yield bond funds, are included in Level 2 and are valued using pricing models, quoted prices of securities with similar characteristics or discounted cash flows. The pricing models utilize observable inputs such as recent trades for the same or similar instruments, yield curves, discount margins and bond structures. Hedge funds and investments in opportunistic fixed income funds are recorded at net asset value based on the values of the underlying assets. The assets in the hedge funds and opportunistic fixed income funds are valued using observable inputs and are classified as Level 3 within the fair value hierarchy due to redemption restrictions. Private Equity investments and Real Estate and Other Assets are valued using the net asset value provided by the partnerships, which are based on discounted cash flows of the underlying investments, real estate appraisals or public market comparables of the underlying investments. These investments are classified as Level 3 due to redemption restrictions.
	The following methods and assumptions were used to estimate the fair value of each of the following financial instruments:
	Certain of NSTAR's stock-based compensation awards, including deferred shares, performance shares and all outstanding stock options, were replaced with NU awards using the exchange ratio upon consummation of the merger. In accordance with accounting guidance for business combinations, the portion of the fair value of these awards attributable to service provided prior to the merger was included in the purchase price as it represented consideration transferred in the merger. See Note 9C, "Employee Benefits – Share-Based Payments," for further information.
	Purchase Price Allocation: The allocation of the total purchase price to the estimated fair values of the assets acquired and liabilities assumed was determined based on the accounting guidance for fair value measurements. The allocation of the total purchase price included adjustments to record the fair value of NSTAR's unregulated telecommunications business, regulatory assets not earning a return, lease agreements, long-term debt and the preferred stock of NSTAR Electric. The fair values of NSTAR's assets and liabilities were determined based on significant estimates and assumptions, including Level 3 inputs, that were judgmental in nature. These estimates and assumptions included the timing and amounts of projected future cash flows and discount rates reflecting risk inherent in future cash flows.
Allowance For Funds Used During Construction Policy	M.       Allowance for Funds Used During Construction
	AFUDC represents the cost of borrowed and equity funds used to finance construction and is included in the cost of the Regulated companies' utility plant. The portion of AFUDC attributable to borrowed funds is recorded as a reduction of Other Interest Expense, and the AFUDC related to equity funds is recorded as Other Income, Net on the statements of income. AFUDC costs are recovered from customers over the service life of the related plant in the form of increased revenue collected as a result of higher depreciation expense.
	The Regulated companies' average AFUDC rate is based on a FERC-prescribed formula using the cost of a company's short-term financings and capitalization (preferred stock, long-term debt and common equity), as appropriate. The average rate is applied to average eligible CWIP amounts to calculate AFUDC.
Other Taxes Policy	Certain sales taxes are collected by NU's companies that serve customers in Connecticut and Massachusetts as agents for state and local governments and are recorded on a net basis with no impact on the statements of income.
Public Utilities Policy Text Block	The rates charged to the customers of NU's Regulated companies are designed to collect each company's costs to provide service, including a return on investment. Therefore, the accounting policies of the Regulated companies follow the application of accounting guidance for entities with rate-regulated operations and reflect the effects of the rate-making process.
	Management believes it is probable that each of the Regulated companies will recover their respective investments in long-lived assets, including regulatory assets. If management were to determine that it could no longer apply the accounting guidance applicable to rate-regulated enterprises to any of the Regulated companies' operations, or that management could not conclude it is probable that costs would be recovered from customers in future rates, the costs would be charged to net income in the period in which the determination is made.
Derivatives Policy Text Block	I.       Derivative Accounting
	Many of the Regulated companies' contracts for the purchase and sale of energy or energy-related products are derivatives. The accounting treatment for energy contracts entered into varies and depends on the intended use of the particular contract and on whether or not the contract is a derivative. For the Regulated companies, regulatory assets or regulatory liabilities are recorded to offset the fair values of derivative contracts, as contract settlements are recovered from, or refunded to, customers in future rates.
	The application of derivative accounting is complex and requires management judgment in the following respects: identification of derivatives and embedded derivatives, election and designation of the normal exception, and determination of the fair value of derivative contracts. All of these judgments can have a significant impact on the financial statements.
	The judgment applied in the election of the normal exception (and resulting accrual accounting) includes the conclusion that it is probable at the inception of the contract and throughout its term that it will result in physical delivery of the underlying product and that the quantities will be used or sold by the business in the normal course of business. If facts and circumstances change and management can no longer support this conclusion, then the normal exception and accrual accounting is terminated, and fair value accounting is applied prospectively.
	The fair value of derivative contracts is based upon the contract terms and conditions and the underlying market price or fair value per unit. When quantities are not specified in the contract, the Company determines whether the contract has a determinable quantity by using amounts referenced in default provisions and other relevant sections of the contract. The fair value of derivative assets and liabilities with the same counterparty are offset and recorded as a net derivative asset or liability on the balance sheets. Changes in the fair value of derivative contracts are recorded as regulatory assets or liabilities and do not impact net income.
	For further information regarding derivative contracts, see Note 4, "Derivative Instruments," to the financial statements.
	Many of the derivative contracts meet the definition of, and are designated as, normal and qualify for accrual accounting under the applicable accounting guidance. The costs and benefits of derivative contracts that meet the definition of normal are recognized in Operating Expenses or Operating Revenues on the statements of income, as applicable, as electricity or natural gas is delivered.
	Derivative contracts that are not designated as normal are recorded at fair value as current or long-term Derivative Assets or Derivative Liabilities on the balance sheets. For the Regulated companies, regulatory assets or regulatory liabilities are recorded to offset the fair values of derivatives, as contract settlements are recovered from, or refunded to, customers in their respective energy supply rates. For NU's unregulated wholesale marketing contracts that expired on December 31, 2013, changes in fair values of derivatives were included in Net Income.
Marketable Securities Policy	NU maintains trusts to fund certain non-qualified executive benefits and WMECO maintains a spent nuclear fuel trust to fund WMECO's prior period spent nuclear fuel liability. These trusts hold marketable securities. These trusts are not subject to regulatory oversight by state or federal agencies. In addition, CYAPC and YAEC maintain legally restricted trusts, each of which holds marketable securities, for settling the decommissioning obligations of their nuclear power plants.
	The Company elected to record mutual funds at fair value and certain other equity investments as trading securities, with the changes in fair values recorded in Other Income, Net on the statements of income. As of December 31, 2014 and 2013, the mutual funds and equity investments were classified as Level 1 in the fair value hierarchy and totaled $85.1 million and $57.2 million, respectively. For the years ended December 31, 2014, 2013 and 2012, net gains on these securities of $1.9 million, $10.2 million and $5.9 million, respectively, were recorded in Other Income, Net on the statements of income. Dividend income is recorded in Other Income, Net when dividends are declared. All other marketable securities are accounted for as available-for-sale.
	U.S. government issued debt securities are valued using market approaches that incorporate transactions for the same or similar bonds and adjustments for yields and maturity dates. Corporate debt securities are valued using a market approach, utilizing recent trades of the same or similar instrument and also incorporating yield curves, credit spreads and specific bond terms and conditions. Asset-backed debt securities include collateralized mortgage obligations, commercial mortgage backed securities, and securities collateralized by auto loans, credit card loans or receivables. Asset-backed debt securities are valued using recent trades of similar instruments, prepayment assumptions, yield curves, issuance and maturity dates, and tranche information. Municipal bonds are valued using a market approach that incorporates reported trades and benchmark yields. Other fixed income securities are valued using pricing models, quoted prices of securities with similar characteristics, and discounted cash flows.
Environmental Costs Policy	Environmental Matters
	General: NU, CL&P, NSTAR Electric, PSNH and WMECO are subject to environmental laws and regulations intended to mitigate or remove the effect of past operations and improve or maintain the quality of the environment. These laws and regulations require the removal or the remedy of the effect on the environment of the disposal or release of certain specified hazardous substances at current and former operating sites. NU, CL&P, NSTAR Electric, PSNH and WMECO have an active environmental auditing and training program and believe that they are substantially in compliance with all enacted laws and regulations.
	These liabilities are estimated on an undiscounted basis and do not assume that any amounts are recoverable from insurance companies or other third parties. The environmental reserves include sites at different stages of discovery and remediation and do not include any unasserted claims.
	It is possible that new information or future developments could require a reassessment of the potential exposure to related environmental matters. As this information becomes available, management will continue to assess the potential exposure and adjust the reserves accordingly.
Earnings Per Share Policy Text Block	RSUs and performance shares are included in basic weighted average common shares outstanding as of the date that all necessary vesting conditions have been satisfied. The dilutive effect of unvested RSUs and performance shares is calculated using the treasury stock method. Assumed proceeds of these units under the treasury stock method consist of the remaining compensation cost to be recognized and a theoretical tax benefit. The theoretical tax benefit is calculated as the tax impact of the intrinsic value of the units (the difference between the market value of the average units outstanding for the period, using the average market price during the period, and the grant date market value).
	The dilutive effect of stock options to purchase common shares is also calculated using the treasury stock method. Assumed proceeds for stock options consist of cash proceeds that would be received upon exercise, and a theoretical tax benefit. The theoretical tax benefit is calculated as the tax impact of the intrinsic value of the stock options (the difference between the market value of the average stock options outstanding for the period, using the average market price during the period, and the exercise price).
Segment Reporting Policy Policy Text Block	SEGMENT INFORMATION
	Presentation: NU is organized between the Electric Distribution, Electric Transmission and Natural Gas Distribution reportable segments and Other based on a combination of factors, including the characteristics of each segments' products and services, the sources of operating revenues and expenses and the regulatory environment in which each segment operates. These reportable segments represented substantially all of NU's total consolidated revenues for the years ended December 31, 2014, 2013 and 2012. Revenues from the sale of electricity and natural gas primarily are derived from residential, commercial and industrial customers and are not dependent on any single customer. The Electric Distribution reportable segment includes the generation activities of PSNH and WMECO.
	The remainder of NU's operations is presented as Other in the tables below and primarily consists of 1) the equity in earnings of NU parent from its subsidiaries and intercompany interest income, both of which are eliminated in consolidation, and interest expense related to the debt of NU parent, 2) the revenues and expenses of NU's service company, most of which are eliminated in consolidation, 3) the operations of CYAPC and YAEC, and 4) the results of other non-regulated subsidiaries, which are not part of its core business.
	Cash flows used for investments in plant included in the segment information below are cash capital expenditures that do not include amounts incurred but not paid, cost of removal, AFUDC related to equity funds, and the capitalized portions of pension expense.
Consolidation Variable Interest Entity Policy	VARIABLE INTEREST ENTITIES
	The Company's variable interests outside of the consolidated group are not material and consist of contracts that are required by regulation and provide for regulatory recovery of contract costs and benefits through customer rates. NU, CL&P and NSTAR Electric hold variable interests in variable interest entities (VIEs) through agreements with certain entities that own single renewable energy or peaking generation power plants and with other independent power producers. NU, CL&P and NSTAR Electric do not control the activities that are economically significant to these VIEs or provide financial or other support to these VIEs. Therefore, NU, CL&P and NSTAR Electric do not consolidate any power plant VIEs.
Cash and Cash Equivalents, Policy [Policy Text Block]	Cash and cash equivalents include cash on hand and short-term cash investments that are highly liquid in nature and have original maturities of three months or less. At the end of each reporting period, any overdraft amounts are reclassified from Cash and Cash Equivalents to Accounts Payable on the balance sheets.
Asset Retirement Obligations, Policy [Policy Text Block]	NU's amounts include CYAPC and YAEC's AROs of $317.3 million and $318.8 million as of December 31, 2014 and 2013, respectively. The fair value of the ARO for CYAPC and YAEC includes uncertainties of the fuel off-load dates related to the DOE's timing of performance regarding its obligation to dispose of the spent nuclear fuel and high level waste. The incremental asset recorded as an offset to the ARO liability was fully depreciated since the plants have no remaining useful life. Any changes in the assumptions used to calculate the fair value of the ARO liability are recorded with a corresponding offset to the related regulatory asset. The assets held in the CYAPC and YAEC nuclear decommissioning trusts are restricted for settling the ARO and all other decommissioning obligations. For further information on the assets held in the nuclear decommissioning trusts, see Note 5, "Marketable Securities," to the financial statements.
Goodwill and Intangible Assets, Policy [Policy Text Block]	The goodwill from the merger with NSTAR totaled $3.2 billion and was allocated to NU's reporting units based on their estimated fair values. See Note 22, "Goodwill," for the allocation of goodwill to each reporting unit.
	In accordance with the accounting standards, goodwill is not subject to amortization. However, goodwill is subject to fair value-based rules for measuring impairment, and resulting write-downs, if any, are charged to Operating Expenses. These accounting standards require that goodwill be reviewed at least annually for impairment and whenever facts or circumstances indicate that there may be an impairment. NU uses October 1st as the annual goodwill impairment testing date.
	On April 10, 2012, upon consummation of the merger with NSTAR, NU recorded approximately $3.2 billion of goodwill. With the completion of the merger, NU reviewed its management structure and determined that the reporting units for the purpose of testing goodwill for impairment are Electric Distribution, Electric Transmission and Natural Gas Distribution. NU's reporting units are consistent with the operating segments underlying the reportable segments identified in Note 20, "Segment Information," to the financial statements. Accordingly, the goodwill resulting from the merger was allocated to the Electric Distribution, Electric Transmission and Natural Gas Distribution reporting units based on the estimated fair values of the reporting units as of the merger date.
	Prior to the merger with NSTAR, the only reporting unit that maintained goodwill was the Natural Gas Distribution reportable segment related to the acquisition of the parent of Yankee Gas in 2000. This goodwill was recorded at Yankee Gas. The goodwill balance at Yankee Gas as of December 31, 2014 and 2013 was $0.3 billion.
	NU completed its annual goodwill impairment test for each of its reporting units as of October 1, 2014 and determined that no impairment exists. There were no events subsequent to October 1, 2014 that indicated impairment of goodwill.
Pension And Other Postretirement Plans Policy	A.       Pension Benefits and Postretirement Benefits Other Than Pensions
	As of December 31, 2014, NUSCO sponsored two defined benefit retirement plans that covered eligible employees, including employees of CL&P, NSTAR Electric, PSNH and WMECO (NUSCO Pension Plan and NSTAR Pension Plan). Effective January 1, 2015, the two plans were merged into one plan, sponsored by NUSCO. The NUSCO and NSTAR Pension Plans are subject to the provisions of ERISA, as amended by the PPA of 2006. NU's policy is to annually fund the Pension Plans in an amount at least equal to an amount that will satisfy federal requirements. In addition, NU maintains non-qualified defined benefit retirement plans sponsored by NUSCO (herein collectively referred to as the SERP Plans), which provide benefits in excess of Internal Revenue Code limitations to eligible current and retired participants.
	As of December 31, 2014, NUSCO also sponsored defined benefit postretirement plans that provide certain retiree benefits, primarily medical, dental and life insurance, to retiring employees that meet certain age and service eligibility requirements (NUSCO PBOP Plans and NSTAR PBOP Plan). Effective January 1, 2015, the plans were merged into one plan, sponsored by NUSCO. Under certain circumstances, eligible retirees are required to contribute to the costs of postretirement benefits. The benefits provided under the PBOP Plans are not vested and the Company has the right to modify any benefit provision subject to applicable laws at that time.
	Because the Regulated companies recover the retiree benefit costs from customers through rates, regulatory assets are recorded in lieu of an adjustment to Accumulated Other Comprehensive Income/(Loss) to record the funded status of the Pension, SERP and PBOP Plans. Regulatory accounting was also applied to the portions of the NUSCO costs that support the Regulated companies, as these costs are also recovered from customers. Adjustments to the Pension and PBOP funded status for the unregulated companies are recorded on an after-tax basis to Accumulated Other Comprehensive Income/(Loss). For further information, see Note 2, "Regulatory Accounting," and Note 14, "Accumulated Other Comprehensive Income/(Loss)," to the financial statements. The SERP Plans do not have plan assets.
	For the years ended December 31, 2014 and 2013, the expected return on plan assets for the NUSCO Pension and PBOP Plans was calculated by applying the assumed rate of return to a four-year rolling average of plan asset fair values. This calculation recognized investment gains or losses over a four-year period from the years in which they occurred. Investment gains or losses for this purpose are the difference between the calculated expected return and the actual return. As investment gains and losses are reflected in the average plan asset fair values, they are subject to amortization with other unrecognized actuarial gains or losses. For the NSTAR Pension and PBOP Plans, the entire difference between the actual return and calculated expected return on plan assets is reflected as a component of unrecognized actuarial gain or loss. Unrecognized actuarial gains or losses are amortized as a component of Pension and PBOP expense over the estimated average future employee service period.
	Pension and SERP Plans: As of December 31, 2013, the funded status of the NSTAR Pension Plan was recorded on NSTAR Electric's balance sheet, while the total SERP obligation was recorded on NSTAR Electric & Gas' balance sheet. As of December 31, 2013, all NSTAR employees were employed by NSTAR Electric & Gas. On January 1, 2014, NSTAR Electric & Gas was merged into NUSCO (service company merger) and, concurrently, all employees were transferred to the company they predominantly provide services for: NUSCO, NSTAR Electric or NSTAR Gas. As a result of the employee transfers, the pension and SERP assets and liabilities were attributed by participant and transferred to the applicable company's balance sheets. This change had no impact on the income statement or net assets of NSTAR Electric or NU.
	Pension and SERP Expense: NU charges net periodic pension expense to its subsidiaries based on the actual participant demographic data for each subsidiary's participants. Benefit payments to participants and contributions are also tracked for each subsidiary. The actual investment return in the trust is allocated to each of the subsidiaries annually in proportion to the investment return expected to be earned during the year. For the years ended December 31, 2013 and 2012 (prior to the service company merger), the net periodic pension expense recorded at NSTAR Electric represented the full cost of the plan with a portion of the costs allocated to affiliated companies based on participant demographic data.
	PBOP Expense: NU charges net periodic postretirement benefits expense to its subsidiaries based on the actual participant demographic data for each subsidiary's participants. Benefit payments to participants and contributions are also tracked for each subsidiary. The actual investment return in the trust each year is allocated to each of the subsidiaries annually in proportion to the investment return expected to be earned during the year. For the years ended December 31, 2013 and 2012 (prior to the service company merger),
Property, Plant and Equipment, Depreciation Methods	Utility property, plant and equipment is recorded at original cost. Original cost includes materials, labor, construction overhead and AFUDC for regulated property. The cost of repairs and maintenance, including planned major maintenance activities, is charged to Operating Expenses as incurred. Depreciation of utility assets is calculated on a straight-line basis using composite rates based on the estimated remaining useful lives of the various classes of property (estimated useful life for PSNH distribution). The composite rates are subject to approval by the appropriate state regulatory agency. The composite rates include a cost of removal component, which is collected from customers over the lives of the plant assets and is recognized as a regulatory liability. Depreciation rates are applied to property from the time it is placed in service. Upon retirement from service, the cost of the utility asset is charged to the accumulated provision for depreciation. The actual incurred removal costs are applied against the related regulatory liability.
Income Tax, Policy [Policy Text Block]	10.       INCOME TAXES
	Deferred tax assets and liabilities are recognized for the future tax effects of temporary differences between the carrying amounts and the tax basis of assets and liabilities. The tax effect of temporary differences is accounted for in accordance with the rate-making treatment of the applicable regulatory commissions and relevant accounting authoritative literature.
	Interest and Penalties: Interest on uncertain tax positions is recorded and generally classified as a component of Other Interest Expense on the statements of income. However, when resolution of uncertainties results in the Company receiving interest income, any related interest benefit is recorded in Other Income, Net on the statements of income. No penalties have been recorded.
Member of Group Filing Consolidated Return, Policy [Policy Text Block]	NU, CL&P, NSTAR Electric, PSNH and WMECO file a consolidated federal income tax return and unitary, combined and separate state income tax returns. These entities are also parties to a tax allocation agreement under which taxable subsidiaries do not pay any more taxes than they would have otherwise paid had they filed a separate company tax return, and subsidiaries generating tax losses, if any, are paid for their losses when utilized.
Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block]	C.       Share-Based Payments
	Share-based compensation awards are recorded using a fair-value-based method at the date of grant. NU, CL&P, NSTAR Electric, PSNH and WMECO record compensation expense related to these awards, as applicable, for shares issued or sold to their respective employees and officers, as well as the allocation of costs associated with shares issued or sold to NU's service company employees and officers that support CL&P, NSTAR Electric, PSNH and WMECO.
	NU Incentive Plans: NU maintains long-term equity-based incentive plans in which NU, CL&P, NSTAR Electric, PSNH and WMECO employees, officers and board members are eligible to participate. The incentive plans authorize NU to grant up to 8,000,000 new shares for various types of awards, including RSUs and performance shares, to eligible employees, officers, and board members.
	NU accounts for its various share-based plans as follows:
	RSUs - NU records compensation expense, net of estimated forfeitures, on a straight-line basis over the requisite service period based upon the fair value of NU's common shares at the date of grant. The par value of RSUs is reclassified to Common Stock from APIC as RSUs become issued as common shares.
	Performance Shares - NU records compensation expense, net of estimated forfeitures, on a straight-line basis over the requisite service period. Performance shares vest based upon the extent to which Company goals are achieved. Vesting of outstanding performance shares is based upon both the Company's EPS growth over the requisite service period and the total shareholder return as compared to the Edison Electric Institute (EEI) Index during the requisite service period. The fair value of performance shares is determined at the date of grant using a lattice model.
	Stock Options - Stock options issued under the NSTAR Incentive Plan that were outstanding immediately prior to the completion of the merger with NSTAR converted into fully vested options to acquire NU common shares, as adjusted by the exchange ratio. The fair value of these awards on the merger date was included in the purchase price as it represented consideration transferred in the merger. Accordingly, no compensation expense was recorded for these stock options.
	ESPP Shares - For shares sold under the ESPP, no compensation expense was recorded as the ESPP qualifies as a non-compensatory plan.
	RSUs: NU granted RSUs under the annual long-term incentive programs that are subject to three-year graded vesting schedules for employees, and one-year graded vesting schedules, or immediate vesting, for board members. RSUs are paid in shares, reduced by amounts sufficient to satisfy withholdings for income taxes, subsequent to vesting.
	As of December 31, 2014 and 2013, the number and weighted average grant-date fair value of unvested RSUs was 1,024,729 and $38.14 per share, and 1,162,216 and $36.58 per share, respectively. The number and weighted average grant-date fair value of RSUs vested and either paid or deferred during 2014 was 437,887 and $37.36 per share, respectively. As of December 31, 2014, 356,018 RSUs were fully vested and deferred and an additional 973,493 are expected to vest.
	Performance Shares: NU granted performance shares under the annual long-term incentive programs that vest based upon the extent to which Company goals are achieved at the end of three-year performance measurement periods. Performance shares are paid in shares, after the performance measurement period. A summary of performance share transactions is as follows:
	Upon closing of the merger with NSTAR, 451,358 performance shares under the NU 2011 and 2012 Long-Term Incentive Programs converted to RSUs according to the terms of these programs. Performance shares under the NU 2010 Incentive Program were measured based upon a modified performance period through the date of the merger, in accordance with the terms of the program, and were fully distributed in 2013.
	Stock Options: Stock options were granted under the NU and NSTAR incentive plans. Options currently outstanding expire ten years from the date of grant and are fully vested. The weighted average remaining contractual lives for the options outstanding as of December 31, 2014 is 4.3 years. A summary of stock option transactions is as follows:
	Employee Share Purchase Plan: NU maintains an ESPP for eligible employees, which allows for NU common shares to be purchased by employees at the end of successive six-month offering periods at 95 percent of the closing market price on the last day of each six-month period. Employees are permitted to purchase shares having a value not exceeding 25 percent of their compensation as of the beginning of the offering period up to a specified limit. The ESPP qualifies as a non-compensatory plan under accounting guidance for share-based payments, and no compensation expense is recorded for ESPP purchases.
	D.       Other Retirement Benefits
	NU provides benefits for retirement and other benefits for certain current and past company officers of NU, including CL&P, PSNH and WMECO. These benefits are accounted for on an accrual basis and expensed over the service lives of the employees. The actuarially-determined liability for these benefits, which is included in Other Long-Term Liabilities on the balance sheets, as well as the related expense, are as follows:
Employee Stock Ownership Plan (ESOP), Policy [Policy Text Block]	B.       Defined Contribution Plans
	Effective January 1, 2014, NU maintains one defined contribution plan on behalf of eligible participants, the NUSCO 401k Plan. The NUSCO 401k Plan provides for employee and employer contributions up to statutory limits. For eligible employees, the NUSCO 401k Plan provides employer matching contributions of either 100 percent up to a maximum of three percent of eligible compensation or 50 percent up to a maximum of eight percent of eligible compensation. For newly hired employees beginning in 2014, the NUSCO 401k Plan provides employer matching contributions of 100 percent up to a maximum of three percent of eligible compensation.
	The NUSCO 401k Plan also contains a K-Vantage feature on behalf of eligible participants, which provides an additional employer contribution based on age and years of service. K-Vantage participants are not eligible to actively participate in the NU defined benefit plans.
	The total defined contribution plan employer matching contributions, including the K-Vantage program contributions, were as follows:

Provision_for_Uncollectible_Ac

Provision for Uncollectible Accounts (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Accounts Notes Loans And Financing Receivable Text Block		Total Provision for Uncollectible Accounts				Uncollectible Hardship
		As of December 31,				As of December 31,
	(Millions of Dollars)	2014		2013		2014		2013
	NU	$	175.3	$	171.3	$	91.5	$	81.2
	CL&P		84.3		82		74		67.3
	NSTAR Electric		40.7		41.7		-		-
	PSNH		7.7		7.4		-		-
	WMECO		9.9		10		6.2		5.5

Equity_Method_Investments_Tabl

Equity Method Investments (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Equity Method Investments [Table Text Block]	(Percent)	CYAPC		YAEC		MYAPC
	CL&P	34.5	%	24.5	%	12	%
	NSTAR Electric	14		14		4
	PSNH	5		7		5
	WMECO	9.5		7		3
		As of December 31,
	(Millions of Dollars)	2014		2013
	CL&P	$	1.2	$	1.2
	NSTAR Electric		0.5		0.5
	PSNH		0.3		0.3
	WMECO		0.3		0.3

Operating_Expenses_Tables

Operating Expenses (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Fuel Costs [Table Text Block]		For the Years Ended December 31,
	(Millions of Dollars)	2014		2013		2012
	NU - Natural Gas and Fuel (1)	$	599.4	$	466.5	$	346.8
	PSNH - Fuel		113.4		104.8		103.4
	(1) NSTAR Gas natural gas costs were included in NU beginning April 10, 2012.

Allowance_for_Funds_Used_Durin

Allowance for Funds Used During Construction (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Allowance For Funds Used During Construction [Table Text Block]	NU	For the Years Ended December 31,
	(Millions of Dollars, except percentages)	2014		2013		2012 (1)
	Borrowed Funds	$	5.8	$	4.1	$	5.3
	Equity Funds		13.7		7.1		6.8
	Total AFUDC	$	19.5	$	11.2	$	12.1
	Average AFUDC Rate		3.40%		2.70%		3.70%
	(1) NSTAR amounts were included in NU beginning April 10, 2012.
		For the Years Ended December 31,
		2014								2013								2012
	(Millions of Dollars,			NSTAR								NSTAR								NSTAR
	except percentages)	CL&P		Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Borrowed Funds	$	1.9	$	2	$	0.6	$	0.9	$	2.2	$	0.5	$	0.5	$	0.5	$	2.5	$	0.3	$	1.6	$	0.5
	Equity Funds		2.9		3.8		0.6		1.7		2.9		-		0.2		1		1.9		-		1.9		1
	Total AFUDC	$	4.8	$	5.8	$	1.2	$	2.6	$	5.1	$	0.5	$	0.7	$	1.5	$	4.4	$	0.3	$	3.5	$	1.5
	Average AFUDC Rate		3.40%		2.50%		1.80%		5.60%		3.70%		0.50%		1.10%		6.10%		3.60%		0.40%		5.90%		6.80%

Other_Taxes_Tables

Other Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
State Of Connecticut Gross Earnings Taxes [Table Text Block]		For the Years Ended December 31,
	(Millions of Dollars)	2014		2013		2012
	NU	$	148.2	$	144.1	$	135
	CL&P		127.9		128.2		120.7

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]	P.			Supplemental Cash Flow Information
	NU				As of and For the Years Ended December 31,
	(Millions of Dollars)				2014			2013			2012 (1)
	Cash Paid/(Received) During the Year for:
		Interest, Net of Amounts Capitalized			$	349.6		$	343.3		$	356.5
		Income Taxes				334.2			50			-12.8
	Non-Cash Investing Activities:
		Plant Additions Included in Accounts Payable (As of)				181.9			193.1			160.6
	(1) NSTAR amounts were included in NU beginning April 10, 2012.
			As of and For the Years Ended December 31,
			2014											2013								2012
						NSTAR										NSTAR								NSTAR
	(Millions of Dollars)		CL&P			Electric			PSNH			WMECO		CL&P		Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Cash Paid/(Received) During the Year for:
		Interest, Net of Amounts Capitalized	$	144.1		$	75.3		$	41.1		$	25.9	$	131.6	$	75.8	$	43.3	$	25.8	$	129.4	$	94.6	$	49.8	$	25.8
		Income Taxes		135.4			217.1			2.3			25.1		55		163.4		-30.1		-69		-42		88.1		14.7		-8.4
	Non-Cash Investing Activities:
		Plant Additions Included in Accounts Payable (As of)		63.5			34.6			39.3			14.2		51.4		57		34.9		19.5		42.8		50		16.8		30

REGULATORY_ACCOUNTING_Tables

REGULATORY ACCOUNTING (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Regulatory Assets Text Block	NU	As of December 31,
	(Millions of Dollars)	2014			2013
	Benefit Costs	$	2,016.00		$	1,240.20
	Derivative Liabilities		425.5			638
	Income Taxes, Net		635.3			626.2
	Storm Restoration Costs		502.8			589.6
	Goodwill-related		505.4			525.9
	Regulatory Tracker Mechanisms		350.5			323.4
	Contractual Obligations - Yankee Companies		123.8			154.2
	Buy Out Agreements for Power Contracts		42.6			70.2
	Other Regulatory Assets		124.7			126.8
	Total Regulatory Assets		4,726.60			4,294.50
	Less: Current Portion		672.5			535.8
	Total Long-Term Regulatory Assets	$	4,054.10		$	3,758.70
			As of December 31,
			2014									2013
						NSTAR								NSTAR
	(Millions of Dollars)		CL&P			Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Benefit Costs		$	445.4		$	515.9	$	174.3	$	85	$	297.7	$	496.7	$	100.6	$	57.3
	Derivative Liabilities			410.9			4.5		-		-		630.4		7.7		-		-
	Income Taxes, Net			437.7			83.7		38		35.5		415.5		84		40.3		43.7
	Storm Restoration Costs			319.6			103.7		47.7		31.8		397.8		109.3		43.7		38.8
	Goodwill-related			-			433.9		-		-		-		451.5		-		-
	Regulatory Tracker Mechanisms			16.1			141.4		103.5		33		8		169.5		83.3		32.6
	Buy Out Agreements for Power Contracts			-			38.6		4		-		-		64.7		5.5		-
	Other Regulatory Assets			66.1			56.1		37.3		12.9		64.6		55.9		38.1		16.7
	Total Regulatory Assets			1,695.80			1,377.80		404.8		198.2		1,814.00		1,439.30		311.5		189.1
	Less: Current Portion			220.3			198.7		111.7		51.9		150.9		204.1		92.2		43
	Total Long-Term Regulatory Assets		$	1,475.50		$	1,179.10	$	293.1	$	146.3	$	1,663.10	$	1,235.20	$	219.3	$	146.1
Schedule Of Regulatory Liabilities Text Block	NU	As of December 31,
	(Millions of Dollars)	2014			2013
	Cost of Removal	$	439.9		$	435.1
	Regulatory Tracker Mechanisms		192.3			151.2
	AFUDC - Transmission		67.1			68.1
	Other Regulatory Liabilities		50.8			52.9
	Total Regulatory Liabilities		750.1			707.3
	Less: Current Portion		235			204.3
	Total Long-Term Regulatory Liabilities	$	515.1		$	503
			As of December 31,
			2014									2013
						NSTAR								NSTAR
	(Millions of Dollars)		CL&P			Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Cost of Removal		$	19.7		$	258.3	$	50.3	$	1.1	$	29.1	$	250	$	49.7	$	-
	Regulatory Tracker Mechanisms			122.6			20.7		14.2		22.3		95.6		21.9		21.6		21.1
	AFUDC - Transmission			53.6			4.4		-		9.1		54.7		4.1		-		9.3
	Other Regulatory Liabilities			10.1			28.9		2.9		0.8		8.4		31.1		1		3.4
	Total Regulatory Liabilities			206			312.3		67.4		33.3		187.8		307.1		72.3		33.8
	Less: Current Portion			124.7			49.6		16		22.5		94		54		20.6		19.9
	Total Long-Term Regulatory Liabilities		$	81.3		$	262.7	$	51.4	$	10.8	$	93.8	$	253.1	$	51.7	$	13.9

PROPERTY_PLANT_AND_ACCUMULATED1

PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Public Utility Property, Plant, and Equipment [Text Block]	NU			As of December 31,
	(Millions of Dollars)			2014			2013
	Distribution - Electric			$	12,495.20		$	11,950.20
	Distribution - Natural Gas				2,595.40			2,425.90
	Transmission				6,930.70			6,412.50
	Generation				1,170.90			1,152.30
	Electric and Natural Gas Utility				23,192.20			21,940.90
	Other (1)				551.3			508.7
	Property, Plant and Equipment, Gross				23,743.50			22,449.60
	Less: Accumulated Depreciation
		Electric and Natural Gas Utility			-5,777.80			-5,387.00
		Other			-231.8			-196.2
	Total Accumulated Depreciation				-6,009.60			-5,583.20
	Property, Plant and Equipment, Net				17,733.90			16,866.40
	Construction Work in Progress				913.1			709.8
	Total Property, Plant and Equipment, Net			$	18,647.00		$	17,576.20
		As of December 31,
		2014												2013
					NSTAR												NSTAR
	(Millions of Dollars)	CL&P			Electric			PSNH			WMECO			CL&P			Electric		PSNH		WMECO
	Distribution	$	5,158.80		$	4,895.50		$	1,696.70		$	784.2		$	4,930.70		$	4,694.70	$	1,608.20	$	756.6
	Transmission		3,274.00			1,928.50			789.7			891			3,071.90			1,772.30		695.7		826.4
	Generation		-			-			1,136.50			34.4			-			-		1,131.20		21.1
	Property, Plant and Equipment, Gross		8,432.80			6,824.00			3,622.90			1,709.60			8,002.60			6,467.00		3,435.10		1,604.10
	Less: Accumulated Depreciation		-1,928.00			-1,761.40			-1,090.00			-297.4			-1,804.10			-1,631.30		-1,021.80		-271.5
	Property, Plant and Equipment, Net		6,504.80			5,062.60			2,532.90			1,412.20			6,198.50			4,835.70		2,413.30		1,332.60
	Construction Work in Progress		304.9			272.8			102.9			49.1			252.8			208.2		54.3		48.5
	Total Property, Plant and Equipment, Net	$	6,809.70		$	5,335.40		$	2,635.80		$	1,461.30		$	6,451.30		$	5,043.90	$	2,467.60	$	1,381.10
	(Percent)		2014			2013			2012
	NU		3	%		2.8	%		2.5	%
	CL&P		2.7			2.5			2.5
	NSTAR Electric		3			2.9			2.8
	PSNH		3			3			3
	WMECO		3.3			2.9			3.3
	The following table summarizes average useful lives of depreciable assets:
			Average Depreciable Life
	(Years)		NU			CL&P			NSTAR Electric			PSNH			WMECO
	Distribution			34.9			37.5			32.3			32.3			30.9
	Transmission			42.5			39.8			44			43.7			49.9
	Generation			31.9			-			-			32.1			25
	Other			14.2			-			-			-			-

DERIVATIVE_INSTRUMENTS_Tables

DERIVATIVE INSTRUMENTS (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Derivative Instruments In Statement Of Financial Position Fair Value Text Block
				As of December 31,
				2014									2013
				Commodity Supply						Net Amount			Commodity Supply						Net Amount
				and Price Risk						Recorded as			and Price Risk							Recorded as
	(Millions of Dollars)			Management			Netting (1)			a Derivative			Management			Netting (1)			a Derivative
	Current Derivative Assets:
	Level 2:
		NU		$	-		$	-		$	-		$	1.9		$	-0.3		$	1.6
	Level 3:
		NU			16.2			-6.6			9.6			18.4			-9.8			8.6
		CL&P			16.1			-6.6			9.5			17.1			-9.8			7.3
		NSTAR Electric			0.1			-			0.1			1.2			-			1.2
	Long-Term Derivative Assets:
	Level 2:
		NU		$	-		$	-		$	-		$	0.2		$	-		$	0.2
	Level 3:
		NU			93.5			-19.2			74.3			116.2			-42.2			74
		CL&P			93.5			-19.2			74.3			113.6			-42.2			71.4
	Current Derivative Liabilities:
	Level 2:
		NU		$	-9.8		$	-		$	-9.8		$	-		$	-		$	-
	Level 3:
		NU			-90			-			-90			-93.7			-			-93.7
		CL&P			-88.5			-			-88.5			-92.2			-			-92.2
		NSTAR Electric			-1.5			-			-1.5			-1.5			-			-1.5
	Long-Term Derivative Liabilities:
	Level 2:
		NU		$	-0.3		$	-		$	-0.3		$	-		$	-		$	-
	Level 3:
		NU			-409.3			-			-409.3			-624.1			-			-624.1
		CL&P			-406.2			-			-406.2			-617.1			-			-617.1
		NSTAR Electric			-3.1			-			-3.1			-7			-			-7
Schedule Of Derivative Instruments Gain Loss In Statement Of Financial Performance Text Block					Gain/(Loss) Recognized on Derivatives
	(Millions of Dollars)				For the Years Ended December 31,
	NU				2014			2013			2012
	Balance Sheets:
		Regulatory Assets and Liabilities			$	134.4		$	160.6		$	-29
	Statements of Income:
		Purchased Power, Fuel and Transmission				-			1			-0.7
Fairvalueinputsliabilitiesquantitativeinformationtabletextblock		As of December 31,
		2014									2013
			Range						Period Covered			Range						Period Covered
	Energy Prices:
	NU	$	52			per MWh			2020		$	49	-	77	per MWh			2018 - 2029
	CL&P	$	52			per MWh			2020		$	56	-	58	per MWh			2018 - 2029
	Capacity Prices:
	NU	$	5.3	-	12.98		per kW-Month		2016 - 2026		$	5.07	-	11.82		per kW-Month		2017 - 2029
	CL&P	$	11.08	-	12.98		per kW-Month		2018 - 2026		$	5.07	-	10.42		per kW-Month		2017 - 2026
	NSTAR Electric	$	5.3	-	11.1		per kW-Month		2016 - 2019		$	5.07	-	7.38		per kW-Month		2017 - 2019
	Forward Reserve:
	NU, CL&P	$	5.8	-	9.5		per kW-Month		2015 - 2024		$	3.3				per kW-Month		2014 - 2024
	REC Prices:
	NU	$	38	-	56	per REC			2015 - 2018		$	36	-	87		per REC		2014 - 2029
	NSTAR Electric	$	38	-	56	per REC			2015 - 2018		$	36	-	70		per REC		2014 - 2018
FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputReconciliationTableTextBlock	(Millions of Dollars)		NU			CL&P			NSTAR Electric
	Derivatives, Net:
	Fair Value as of January 1, 2013		$	-878.6		$	-866.2		$	-14.9
	Net Realized/Unrealized Gains Included in:
		Net Income (1)		10.9			-			-
		Regulatory Assets and Liabilities		158.3			148.9			3.5
	Settlements			74.2			86.7			4.1
	Fair Value as of December 31, 2013		$	-635.2		$	-630.6		$	-7.3
	Net Realized/Unrealized Gains Included in Regulatory Assets and Liabilities			141.3			139.7			4.3
	Settlements			78.5			80			-1.5
	Fair Value as of December 31, 2014		$	-415.4		$	-410.9		$	-4.5

MARKETABLE_SECURITIES_Tables

MARKETABLE SECURITIES (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Available For Sale Securities Reconciliation [Table TextBlock]			As of December 31,
			2014												2013
						Pre-Tax			Pre-Tax								Pre-Tax		Pre-Tax
			Amortized			Unrealized			Unrealized						Amortized		Unrealized		Unrealized
	(Millions of Dollars)		Cost			Gains			Losses			Fair Value			Cost		Gains		Losses		Fair Value
	NU
		Debt Securities (1)	$	313		$	7.5		$	-0.3		$	320.2		$	299.2	$	2.5	$	-2.1	$	299.6
		Equity Securities (1)		160.6			73.3			-			233.9			163.6		60.5		-		224.1
	WMECO
		Debt Securities (2)		58.2			-			-0.1			58.1			57.9		-		-		57.9
Investments Classified by Contractual Maturity Date [Table Text Block]			NU						WMECO
			Amortized						Amortized
	(Millions of Dollars)		Cost			Fair Value			Cost			Fair Value
	Less than one year (1)		$	55		$	55		$	28.7		$	28.7
	One to five years			88.8			89.1			25.8			25.8
	Six to ten years			66			67.7			0.7			0.7
	Greater than ten years			103.2			108.4			3			2.9
	Total Debt Securities		$	313		$	320.2		$	58.2		$	58.1
Fair Value Heirarchy [Table Text Block]					NU						WMECO
					As of December 31,						As of December 31,
	(Millions of Dollars)				2014			2013			2014			2013
	Level 1:
		Mutual Funds and Equities			$	319		$	281.3		$	-		$	-
		Money Market Funds				24.9			32.9			4.3			10.9
	Total Level 1				$	343.9		$	314.2		$	4.3		$	10.9
	Level 2:
		U.S. Government Issued Debt Securities (Agency and Treasury)			$	51.3		$	61.4		$	-		$	6.8
		Corporate Debt Securities				49.1			53.6			14.7			15.1
		Asset-Backed Debt Securities				54.1			30.4			14.5			9
		Municipal Bonds				116.3			105.5			13			11.2
		Other Fixed Income Securities				24.5			15.8			11.6			4.9
	Total Level 2				$	295.3		$	266.7		$	53.8		$	47
	Total Marketable Securities				$	639.2		$	580.9		$	58.1		$	57.9

ASSET_RETIREMENT_OBLIGATIONS_T

ASSET RETIREMENT OBLIGATIONS (TABLE)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Change in Asset Retirement Obligation [Table Text Block]	NU			As of December 31,
	(Millions of Dollars)			2014		2013
	Balance as of Beginning of Year			$	424.9	$	412.2
	Liabilities Incurred During the Year				1.3		0.1
	Liabilities Settled During the Year				-19.5		-13.8
	Accretion				25.1		23.8
	Revisions in Estimated Cash Flows				-5.5		2.6
	Balance as of End of Year			$	426.3	$	424.9
		As of December 31,
		2014								2013
				NSTAR								NSTAR
	(Millions of Dollars)	CL&P		Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Balance as of Beginning of Year	$	35	$	32.8	$	19.5	$	4.5	$	33.6	$	31.4	$	18.4	$	4.3
	Liabilities Incurred During the Year		-		-		-		1.1		-		-		-		-
	Liabilities Settled During the Year		-1.1		-		-		-		-0.7		-0.1		-		-
	Accretion		1.9		1.5		1.1		0.3		2.2		1.5		1.2		0.3
	Revisions in Estimated Cash Flows		-0.5		-		-		-		-0.1		-		-0.1		-0.1
	Balance as of End of Year	$	35.3	$	34.3	$	20.6	$	5.9	$	35	$	32.8	$	19.5	$	4.5

LONGTERM_DEBT_TABLES

LONG-TERM DEBT (TABLES)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Maturities of Long-term Debt [Table Text Block]	(Millions of Dollars)	NU			CL&P			NSTAR Electric		PSNH		WMECO
	2015	$	216.7		$	162		$	4.7	$	-	$	50
	2016		200			-			200		-		-
	2017		745			250			400		70		-
	2018		810			300			-		110		-
	2019		800			250			-		150		-
	Thereafter		4,956.60			1,640.30			1,200.00		746.3		515
	Total	$	7,728.30		$	2,602.30		$	1,804.70	$	1,076.30	$	565
Schedule of Long-term Debt Instruments [Table Text Block]	CL&P		As of December 31,
	(Millions of Dollars)		2014			2013
	First Mortgage Bonds:
		7.875% 1994 Series D due 2024	$	139.8		$	139.8
		4.800% 2004 Series A due 2014 (1)		-			150
		5.750% 2004 Series B due 2034		130			130
		5.000% 2005 Series A due 2015		100			100
		5.625% 2005 Series B due 2035		100			100
		6.350% 2006 Series A due 2036		250			250
		5.375% 2007 Series A due 2017		150			150
		5.750% 2007 Series B due 2037		150			150
		5.750% 2007 Series C due 2017		100			100
		6.375% 2007 Series D due 2037		100			100
		5.650% 2008 Series A due 2018		300			300
		5.500% 2009 Series A due 2019		250			250
		2.500% 2013 Series A due 2023		400			400
		4.300% 2014 Series A due 2044 (2)		250			-
	Total First Mortgage Bonds			2,419.80			2,319.80
	Pollution Control Revenue Bonds:
		4.375% Fixed Rate Tax Exempt due 2028		120.5			120.5
		1.550% Fixed Rate Tax Exempt due 2031 (3)		62			62
	Total Pollution Control Revenue Bonds			182.5			182.5
	Spent Nuclear Fuel Obligation			244.5			244.4
	Less Amounts due Within One Year			-162			-150
	Unamortized Premiums and Discounts, Net			-4.8			-5.5
	CL&P Long-Term Debt		$	2,680.00		$	2,591.20
	NSTAR Electric		As of December 31,
	(Millions of Dollars)		2014			2013
	Debentures:
		4.875% due 2014 (4)	$	-		$	300
		5.750% due 2036		200			200
		5.625% due 2017		400			400
		5.500% due 2040		300			300
		2.375% due 2022		400			400
		Variable Rate due 2016 (5)		200			200
		4.400% due 2044 (4)		300			-
	Total Debentures			1,800.00			1,800.00
	Bonds:
		7.375% Tax Exempt Sewage Facility Revenue Bonds, due 2015		4.7			6.4
	Total Bonds			4.7			6.4
	Less Amounts due Within One Year			-4.7			-301.7
	Unamortized Premiums and Discounts, Net			-7.3			-5.3
	NSTAR Electric Long-Term Debt		$	1,792.70		$	1,499.40
	PSNH		As of December 31,
	(Millions of Dollars)		2014			2013
	First Mortgage Bonds:
		5.25% Series L due 2014 (6)	$	-		$	50
		5.60% Series M due 2035		50			50
		6.15% Series N due 2017		70			70
		6.00% Series O due 2018		110			110
		4.50% Series P due 2019		150			150
		4.05% Series Q due 2021		122			122
		3.20% Series R due 2021		160			160
		3.50% Series S due 2023 (7)		325			250
	Total First Mortgage Bonds			987			962
	Pollution Control Revenue Bonds:
		Adjustable Rate Tax Exempt Series A due 2021		89.3			89.3
	Total Pollution Control Revenue Bonds			89.3			89.3
	Less Amounts due Within One Year			-			-50
	Unamortized Premiums and Discounts, Net			-			-2.3
	PSNH Long-Term Debt		$	1,076.30		$	999
	WMECO		As of December 31,
	(Millions of Dollars)		2014			2013
	Notes:
	5.90% Senior Notes Series B, due 2034	$	50		$	50
	5.24% Senior Notes Series C, due 2015		50			50
	6.70% Senior Notes Series D, due 2037		40			40
	5.10% Senior Notes Series E, due 2020		95			95
	3.50% Senior Notes Series F, due 2021		250			250
	3.88% Senior Notes Series G, due 2023		80			80
Total Notes			565			565
Spent Nuclear Fuel Obligation			57.4			57.3
Less Amounts due Within One Year			-50			-
Unamortized Premiums and Discounts, Net			6.1			7.1
WMECO Long-Term Debt		$	578.5		$	629.4
OTHER		As of December 31,
(Millions of Dollars)		2014				2013
Yankee Gas - First Mortgage Bonds:
	8.48% Series B due 2022	$	20		$	20
	4.80% Series G due 2014 (8)		-			75
	5.26% Series H due 2019		50			50
	5.35% Series I due 2035		50			50
	6.90% Series J due 2018		100			100
	4.87% Series K due 2020		50			50
	4.82% Series L due 2044 (8)		100			-
Total First Mortgage Bonds			370			345
Unamortized Premium			0.6			0.7
Yankee Gas Long-Term Debt			370.6			345.7
NSTAR Gas - First Mortgage Bonds:
	9.95% Series J due 2020		25			25
	7.11% Series K due 2033		35			35
	7.04% Series M due 2017		25			25
	4.46% Series N due 2020		125			125
NSTAR Gas Long-Term Debt			210			210
Other - Notes and Debentures:
	1.45% Senior Notes Series E due 2018 (NU Parent)		300			300
	2.80% Senior Notes Series F due 2023 (NU Parent)		450			450
	4.50% Debentures due 2019 (NU Parent)		350			350
	NU Commercial Paper Borrowings(8) (9)		446.3			25
	Spent Nuclear Fuel Obligation (CYAPC)		179.4			179.4
Total Other Notes and Debentures			1,725.70			1,304.40
Fair Value Adjustment (10)			202.3			230.7
Less Amounts due Within One Year			-			-
Less Fair Value Adjustment - Current Portion (10)			-28.9			-31.7
Unamortized Premiums and Discounts, Net			-1.2			-1.3
Total Other Long-Term Debt		$	2,478.50		$	2,057.80
Total NU Long-Term Debt		$	8,606.00		$	7,776.80

Recovered_Sheet1

Pension Benefits and Postretirement Benefits Other Than Pensions (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Net Benefit Costs [Table Text Block]			Pension and SERP
			For the Year Ended December 31, 2014
									NSTAR
	(Millions of Dollars)		NU			CL&P			Electric			PSNH			WMECO
	Service Cost		$	79.9		$	20.2		$	13.6		$	9.7		$	3.5
	Interest Cost			225.7			50.5			41.3			23.8			10.3
	Expected Return on Plan Assets			-310.8			-75.4			-63			-38.1			-17.9
	Actuarial Loss			128.4			33.7			23.5			11.6			6.9
	Prior Service Cost			4.4			1.8			-			0.7			0.4
	Total Net Periodic Benefit Expense		$	127.6		$	30.8		$	15.4		$	7.7		$	3.2
	Intercompany Allocations			N/A		$	26.7		$	10.4		$	7.6		$	5.1
	Capitalized Pension Expense		$	35.2		$	17.6		$	7.9		$	3		$	2.4
			Pension and SERP
			For the Year Ended December 31, 2013
									NSTAR
	(Millions of Dollars)		NU			CL&P			Electric (1)			PSNH			WMECO
	Service Cost		$	102.3		$	24.9		$	33.1		$	13.1		$	4.7
	Interest Cost			206.7			48.3			58			23.6			10
	Expected Return on Plan Assets			-278.1			-73.8			-84.4			-35.4			-17.4
	Actuarial Loss			210.5			55.9			58.1			21.6			11.8
	Prior Service Cost/(Credit)			4			1.8			-0.3			0.7			0.4
	Total Net Periodic Benefit Expense		$	245.4		$	57.1		$	64.5		$	23.6		$	9.5
	Intercompany Allocations			N/A		$	44.9		$	-8.4		$	10.5		$	8
	Capitalized Pension Expense		$	73.2		$	28		$	28.9		$	7.3		$	5.2
			Pension and SERP
			For the Year Ended December 31, 2012
									NSTAR
	(Millions of Dollars)		NU (2)			CL&P			Electric (1)			PSNH			WMECO
	Service Cost		$	84.3		$	21.8		$	30.3		$	11.8		$	4.1
	Interest Cost			198.3			51.2			58.9			24.4			10.5
	Expected Return on Plan Assets			-220.9			-70.6			-65.6			-28.2			-16.4
	Actuarial Loss			172.4			49.6			63.1			16.2			10.7
	Prior Service Cost/(Credit)			7.9			3.6			-0.6			1.5			0.8
	Total Net Periodic Benefit Expense		$	242		$	55.6		$	86.1		$	25.7		$	9.7
	Curtailments and Settlements		$	2.2		$	-		$	-		$	-		$	-
	Intercompany Allocations			N/A		$	42.8		$	-12.3		$	10.1		$	8.1
	Capitalized Pension Expense		$	70.6		$	26.8		$	30.7		$	7.9		$	5.1
				PBOP
			For the Year Ended December 31, 2014
									NSTAR
	(Millions of Dollars)		NU			CL&P			Electric			PSNH			WMECO
	Service Cost		$	12.5		$	2.2		$	3.1		$	1.3		$	0.4
	Interest Cost			49.5			8.1			19.4			4.3			1.7
	Expected Return on Plan Assets			-63.3			-10.5			-25.9			-5.4			-2.3
	Actuarial Loss/(Gain)			12.2			4.2			-0.5			2.2			0.5
	Prior Service Credit			-2.8			-			-1.9			-			-
	Total Net Periodic Benefit Expense/(Income)		$	8.1		$	4		$	-5.8		$	2.4		$	0.3
	Intercompany Allocations			N/A		$	3.8		$	0.8		$	1		$	0.7
	Capitalized PBOP Expense/(Income)		$	1.4		$	1.8		$	-2.3		$	0.8		$	0.2
			PBOP
			For the Year Ended December 31, 2013
	(Millions of Dollars)		NU			CL&P			PSNH			WMECO
	Service Cost		$	16.9		$	3.4		$	2.3		$	0.7
	Interest Cost			47.2			7.9			4			1.7
	Expected Return on Plan Assets			-55.4			-10.1			-5.2			-2.3
	Actuarial Loss			26			7.4			3.6			1.1
	Prior Service Credit			-2.1			-			-			-
	Total Net Periodic Benefit Expense		$	32.6		$	8.6		$	4.7		$	1.2
	Intercompany Allocations			N/A		$	7.1		$	1.6		$	1.3
	Capitalized PBOP Expense		$	8.8		$	3.9		$	1.3		$	0.6
			PBOP
			For the Year Ended December 31, 2012
	(Millions of Dollars)		NU (1)			CL&P			PSNH			WMECO
	Service Cost		$	15.7		$	3		$	2		$	0.6
Interest Cost			49			9.2			4.6			2
Expected Return on Plan Assets			-39.2			-9.1			-4.6			-2.1
Actuarial Loss			36			7.5			3.6			1.2
Prior Service Credit			-1.4			-			-			-
Net Transition Obligation Cost			12.2			6.1			2.5			1.3
Total Net Periodic Benefit Expense		$	72.3		$	16.7		$	8.1		$	3
Intercompany Allocations			N/A		$	7.9		$	2		$	1.5
Capitalized PBOP Expense		$	26.6		$	8.2		$	2.3		$	1.6
-1	NSTAR amounts were included in NU beginning April 10, 2012.
Schedule of Defined Benefit Plans Disclosures [Table Text Block]	The accumulated benefit obligation for the Pension and SERP Plans is as follows:
	(Millions of Dollars)		NU			CL&P			NSTAR Electric (1)			PSNH			WMECO
	2014		$	5,000.10		$	1,101.40		$	910.4		$	524.5		$	226.4
	2013			4,538.80			1,058.00			1,280.60			520.1			220.6
	-1	NSTAR Electric amounts do not include the accumulated benefit obligation for the SERP Plan as of December 31, 2013.
	The following actuarial assumptions were used in calculating the Pension and SERP Plans' year end funded status:
Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Table Text Block]	The following is a summary of the changes in plan assets and benefit obligations recognized in Regulatory Assets and Other Comprehensive Income (OCI) as well as amounts in Regulatory Assets and OCI reclassified as net periodic benefit expense during the years presented:
			Amounts Reclassified To/From
			Regulatory Assets						OCI
			For the Years Ended December 31,
	(Millions of Dollars)		2014			2013			2014			2013
	Actuarial (Gains)/Losses Arising During the Year		$	797.3		$	-635.2		$	55.9		$	-28.9
	Actuarial Losses Reclassified as Net Periodic Benefit Expense			-122.8			-201.2			-5.6			-9.4
	Prior Service Cost Reclassified as Net Periodic Benefit Expense			-4.2			-3.8			-0.2			-0.2
	The following is a summary of the remaining Regulatory Assets and Accumulated Other Comprehensive Loss amounts that have not been recognized as components of net periodic benefit expense as of December 31, 2014 and 2013, and the amounts that are expected to be recognized as components in 2015:
				Regulatory Assets as of						Expected			AOCI as of					Expected
			December 31,							2015		December 31,						2015
	(Millions of Dollars)		2014			2013			Expense			2014			2013			Expense
	Actuarial Loss		$	1,811.90		$	1,137.40		$	149.1		$	93.5		$	43.2		$	6.4
	Prior Service Cost			13.2			17.4			3.5			0.8			1			0.2
	The following is a summary of the changes in plan assets and benefit obligations recognized in Regulatory Assets and OCI as well as amounts in Regulatory Assets and OCI reclassified as net periodic benefit (expense)/income during the years presented:
			Amounts Reclassified To/From
		Regulatory Assets						OCI
		For the Years Ended December 31,
	(Millions of Dollars)	2014			2013			2014			2013
	Actuarial Losses/(Gains) Arising During the Year	$	115.1		$	-262		$	0.4		$	-1.9
	Actuarial Losses Reclassified as Net Periodic Benefit Expense		-11.6			-24.9			-0.6			-1.1
	Prior Service Credit Reclassified as Net Periodic Benefit Income		2.8			2.1			-			-
	The following is a summary of the remaining Regulatory Assets and Accumulated Other Comprehensive Loss amounts that have not been recognized as components of net periodic benefit expense as of December 31, 2014 and 2013, and the amounts that are expected to be recognized as components in 2015:
			Regulatory Assets as of					Expected				AOCI as of					Expected
		December 31,						2015				December 31,					2015
	(Millions of Dollars)	2014			2013			Expense			2014			2013			Expense
	Actuarial Loss	$	192.7		$	89.2		$	6.9		$	6		$	6.2		$	0.3
	Prior Service Credit		-1.8			-4.6			-0.5			-			-			-
Schedule of Assumptions Used [Table Text Block]			Pension and SERP
			As of December 31,
			2014			2013
	Discount Rate		4.2	%		4.85	%	-	5.03	%
	Compensation/Progression Rate		3.5	%		3.5	%	-	4	%
	The following actuarial assumptions were used to calculate Pension and SERP expense amounts:
			Pension and SERP
			For the Years Ended December 31,
			2014						2013						2012
	Discount Rate		4.85	%	-	5.03	%		4.13	%	-	4.24	%		4.52	%	-	5.03	%
	Expected Long-Term Rate of Return		8.25%						8.25%						7.3	%	-	8.25	%
	Compensation/Progression Rate		3.5	%	-	4	%		3.5	%	-	4	%		3.5	%	-	4	%
	The following actuarial assumptions were used in calculating the PBOP Plans' year end funded status:
			PBOP
			As of December 31,
			2014			2013
	Discount Rate		4.22	%		4.78	%	-	5.1	%
	Health Care Cost Trend Rate		6.5	%		7.00%
	The following actuarial assumptions were used to calculate PBOP expense amounts:
			PBOP
			For the Years Ended December 31,
			2014						2013						2012
	Discount Rate		4.78	%	-	5.1	%		4.04	%	-	4.35	%		4.58	%	-	4.84	%
	Expected Long-Term Rate of Return		8.25%						8.25%						7.3	%	-	8.25	%
Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates [Table Text Block]		One Percentage			One Percentage
	(Millions of Dollars)	Point Increase			Point Decrease
	Effect on PBOP Obligation	$	111.2		$	-88.4
	Effect on Total Service and Interest Cost Components		5.3			-4.4
Schedule of Expected Benefit Payments [Table Text Block]	Estimated Future Benefit Payments: The following benefit payments, which reflect expected future service, are expected to be paid by the Pension, SERP and PBOP Plans:
	(Millions of Dollars)	2015			2016				2017		2018			2019			2020-2024
	Pension and SERP	$	244.5		$	253.6		$	268.9		$	273.4		$	285.4		$	1,591.10
	PBOP		58.7			59.7			60.6			61.3			62			318.8
Schedule of Benefit Obligations in Excess of Fair Value of Plan Assets [Table Text Block]			Pension and SERP
	NU		As of December 31,
	(Millions of Dollars)		2014			2013
	Change in Benefit Obligation
	Benefit Obligation as of Beginning of Year		$	-4,676.50		$	-5,022.80
	Service Cost			-79.9			-102.3
	Interest Cost			-225.7			-206.7
	Actuarial Gain/(Loss)			-739.6			433.6
	Benefits Paid - Pension			230.3			216.6
	Benefits Paid - SERP			5.2			5.1
	Benefit Obligation as of End of Year		$	-5,486.20		$	-4,676.50
	Change in Pension Plan Assets
	Fair Value of Plan Assets as of Beginning of Year		$	3,985.90		$	3,411.30
	Employer Contributions			171.6			284.7
	Actual Return on Plan Assets			199.3			506.5
	Benefits Paid			-230.3			-216.6
	Fair Value of Plan Assets as of End of Year		$	4,126.50		$	3,985.90
	Funded Status as of December 31st		$	-1,359.70		$	-690.6
			Pension and SERP
			As of December 31, 2014												As of December 31, 2013
						NSTAR												NSTAR
	(Millions of Dollars)		CL&P			Electric			PSNH			WMECO			CL&P			Electric (1)			PSNH			WMECO
	Change in Benefit Obligation
	Benefit Obligation as of Beginning of Year		$	-1,083.40		$	-1,353.30		$	-529		$	-223.9		$	-1,178.00		$	-1,430.00		$	-576		$	-243.1
	Decrease due to transfer of employees			26.4			479.9			32.2			6.2			-			-			-			-
	Service Cost			-20.2			-13.6			-9.7			-3.5			-24.9			-33.1			-13.1			-4.7
	Interest Cost			-50.5			-41.3			-23.8			-10.3			-48.3			-58			-23.6			-10
	Actuarial Gain/(Loss)			-161			-107			-73.3			-29.8			110.7			96.6			62.4			22.4
	Benefits Paid - Pension			58.3			52.4			22.8			11.9			56.6			71.2			21.1			11.5
	Benefits Paid - SERP			0.3			0.3			0.1			-			0.5			-			0.2			-
	Benefit Obligation as of End of Year		$	-1,230.10		$	-982.6		$	-580.7		$	-249.4		$	-1,083.40		$	-1,353.30		$	-529		$	-223.9
	Change in Pension Plan Assets
	Fair Value of Plan Assets as of Beginning of Year		$	1,016.30		$	1,235.30		$	528.6		$	240.4		$	937.6		$	1,069.10		$	386.6		$	218.5
	Decrease due to transfer of employees			-26.4			-441.4			-32.2			-6.2			-			-			-			-
	Employer Contributions			-			101			-			-			-			82			108.3			-
	Actual Return on Plan Assets			49.2			36.5			24.8			11.7			135.3			155.4			54.8			33.4
	Benefits Paid			-58.3			-52.4			-22.8			-11.9			-56.6			-71.2			-21.1			-11.5
	Fair Value of Plan Assets as of End of Year		$	980.8		$	879		$	498.4		$	234		$	1,016.30		$	1,235.30		$	528.6		$	240.4
	Funded Status as of December 31st		$	-249.3		$	-103.6		$	-82.3		$	-15.4		$	-67.1		$	-118		$	-0.4		$	16.5
			PBOP
	NU		As of December 31,
	(Millions of Dollars)		2014			2013
	Change in Benefit Obligation
	Benefit Obligation as of Beginning of Year		$	-1,038.00		$	-1,233.30
	Service Cost			-12.5			-16.9
	Interest Cost			-49.5			-47.2
	Actuarial Gain/(Loss)			-95.5			200.9
	Benefits Paid			47.6			58.5
	Benefit Obligation as of End of Year		$	-1,147.90		$	-1,038.00
	Change in PBOP Plan Assets
	Fair Value of Plan Assets as of Beginning of Year		$	826.5		$	709.1
	Actual Return on Plan Assets			43.7			118.3
	Employer Contributions			40			57.6
	Benefits Paid			-47.6			-58.5
	Fair Value of Plan Assets as of End of Year		$	862.6		$	826.5
	Funded Status as of December 31st		$	-285.3		$	-211.5
			PBOP
			As of December 31,
			2014												2013
						NSTAR
	(Millions of Dollars)		CL&P			Electric			PSNH			WMECO				CL&P			PSNH			WMECO
	Change in Benefit Obligation
	Benefit Obligation as of Beginning of Year		$	-180.4		$	-		$	-93.5		$	-38.7			$	-196.8		$	-100.2		$	-42.5
	Decrease/(Increase) due to transfer of employees			3.7			-395.5			4.3			1				-			-			-
Service Cost			-2.2			-3.1			-1.3			-0.4				-3.4			-2.3			-0.7
Interest Cost			-8.1			-19.4			-4.3			-1.7				-7.9			-4			-1.7
Actuarial Gain/(Loss)			3.5			-68.6			-1.1			1.3				13.3			7.2			3.3
Benefits Paid			9.6			17.9			4.1			1.9				14.4			5.8			2.9
Benefit Obligation as of End of Year		$	-173.9		$	-468.7		$	-91.8		$	-36.6			$	-180.4		$	-93.5		$	-38.7
Change in PBOP Plan Assets
Fair Value of Plan Assets as of Beginning of Year		$	151.3		$	-		$	81.8		$	35.3			$	132.2		$	69.5		$	31
(Decrease)/Increase due to transfer of employees			-3.2			316.7			-3.1			-1				-			-			-
Actual Return on Plan Assets			6.3			18.4			3.8			1.6				24.8			13.4			6
Employer Contributions			4.2			19.3			2.5			0.4				8.7			4.7			1.2
Benefits Paid			-9.6			-17.9			-4.1			-1.9				-14.4			-5.8			-2.9
Fair Value of Plan Assets as of End of Year		$	149		$	336.5		$	80.9		$	34.4			$	151.3		$	81.8		$	35.3
Funded Status as of December 31st		$	-24.9		$	-132.2		$	-10.9		$	-2.2			$	-29.1		$	-11.7		$	-3.4
Schedule of Effect of Significant Unobservable Inputs, Changes in Plan Assets [Table Text Block]			NU Pension Plans
			Equity			Private			Fixed			Real Estate and			Hedge
	(Millions of Dollars)		Securities			Equity			Income			Other Assets			Funds			Total
	Balance as of January 1, 2013		$	322.7		$	267.9		$	315.1		$	235.4		$	418.9		$	1,560.00
	Transfer Between Categories			-			-			32.5			-			-32.5			-
	Actual Return/(Loss) on Plan Assets:
		Relating to Assets Still Held as of Year End		20.6			15.4			55.3			12.9			33.4			137.6
		Relating to Assets Distributed During the Year		12.2			13.7			-1			6.2			-			31.1
	Purchases, Sales and Settlements			-100			3.3			187.6			34			-2.9			122
	Balance as of December 31, 2013		$	255.5		$	300.3		$	589.5		$	288.5		$	416.9		$	1,850.70
	Actual Return/(Loss) on Plan Assets:
		Relating to Assets Still Held as of Year End		-2.3			14			45.2			-3.6			23.5			76.8
		Relating to Assets Distributed During the Year		-			13.9			-6.2			28.3			-15.2			20.8
	Purchases, Sales and Settlements			39			39.7			93.5			-47.4			49.8			174.6
	Balance as of December 31, 2014		$	292.2		$	367.9		$	722		$	265.8		$	475		$	2,122.90
			NU PBOP Plans
			Equity			Private			Fixed			Real Estate and			Hedge
	(Millions of Dollars)		Securities			Equity			Income			Other Assets			Funds			Total
	Balance as of January 1, 2013		$	36.3		$	11.3		$	32.1		$	26.7		$	39.6		$	146
	Actual Return/(Loss) on Plan Assets:
		Relating to Assets Still Held as of Year End		20.8			1.5			4.1			3.9			5.4			35.7
		Relating to Assets Distributed During the Year		-			0.2			-			-0.1			-			0.1
	Purchases, Sales and Settlements			12			4.9			15.3			3.4			12			47.6
	Balance as of December 31, 2013		$	69.1		$	17.9		$	51.5		$	33.9		$	57		$	229.4
	Actual Return/(Loss) on Plan Assets:
		Relating to Assets Still Held as of Year End		6			1.3			1.9			-2.8			1.4			7.8
		Relating to Assets Distributed During the Year		-			0.1			-			-2.2			-			-2.1
	Purchases, Sales and Settlements			-			5.6			24.9			-13.9			-			16.6
	Balance as of December 31, 2014		$	75.1		$	24.9		$	78.3		$	15		$	58.4		$	251.7
Schedule of Allocation of Plan Assets [Table Text Block]			As of December 31, 2014 and 2013
			Pension and Tax-Exempt PBOP Plans
			Target Asset Allocation		Assumed Rate of Return
	Equity Securities:
		United States	24%		9%
		International	10%		9%
		Emerging Markets	6%		10%
		Private Equity	10%		13%
	Debt Securities:
		Fixed Income	15%		5%
		High Yield Fixed Income	9%		7.50%
		Emerging Markets Debt	6%		7.50%
	Real Estate and Other Assets		9%		7.50%
	Hedge Funds		11%		7%
			NU Pension Plans
			Fair Value Measurements as of December 31,
	(Millions of Dollars)		2014												2013
	Asset Category:		Level 1			Level 2			Level 3			Total			Level 1			Level 2			Level 3			Total
	Equity Securities (1)		$	414.7		$	1,035.00		$	292.2		$	1,741.90		$	326.8		$	1,172.10		$	255.5		$	1,754.40
	Private Equity			18.8			-			367.9			386.7			96.4			-			300.3			396.7
	Fixed Income (2)			10.2			561.4			722			1,293.60			11.6			605.1			589.5			1,206.20
	Real Estate and Other Assets			-			132			265.8			397.8			-			88.2			288.5			376.7
	Hedge Funds			-			20			475			495			-			-			416.9			416.9
	Total Master Trust Assets		$	443.7		$	1,748.40		$	2,122.90		$	4,315.00		$	434.8		$	1,865.40		$	1,850.70		$	4,150.90
		Less: 401(h) PBOP Assets (3)											-188.5												-165
	Total Pension Assets											$	4,126.50											$	3,985.90
			NU PBOP Plans
			Fair Value Measurements as of December 31,
	(Millions of Dollars)		2014												2013
	Asset Category:		Level 1			Level 2			Level 3			Total			Level 1			Level 2			Level 3			Total
	Cash and Cash Equivalents		$	-		$	-		$	-		$	-		$	11.1		$	-		$	-		$	11.1
	Equity Securities (1)			104.1			172.8			75.1			352			110.3			176.8			69.1			356.2
	Private Equity			-			-			24.9			24.9			-			-			17.9			17.9
	Fixed Income (2)			16.1			110			78.3			204.4			-			119.7			51.5			171.2
	Real Estate and Other Assets						19.4			15			34.4			-			14.2			33.9			48.1
	Hedge Funds						-			58.4			58.4			-			-			57			57
	Total		$	120.2		$	302.2		$	251.7		$	674.1		$	121.4		$	310.7		$	229.4		$	661.5
		Add: 401(h) PBOP Assets (3)											188.5												165
	Total PBOP Assets											$	862.6											$	826.5

DEFINED_CONTRIBUTION_PLANS_TAB

DEFINED CONTRIBUTION PLANS (TABLES)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Defined Contribution Plans 401k [Table Text Block]						NSTAR
	(Millions of Dollars)	NU (1)		CL&P		Electric		PSNH		WMECO
	2014	$	29.7	$	5	$	6.3	$	3.2	$	1
	2013		37		5.1		8.5		3.3		1
	2012		25.7		4.8		9		3.3		0.9

ShareBased_Payments_Tables

Share-Based Payments (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block]					Weighted Average
		RSUs			Grant-Date
		(Units)			Fair Value
	Outstanding as of January 1, 2012		959,920		$	26.36
	Granted		614,930		$	33.04
	Converted NSTAR Awards upon Merger		617,394		$	36.79
	Converted from NU Performance Shares upon Merger		451,358		$	34.32
	Shares issued		-363,779		$	29.05
	Forfeited		-96,504		$	34.97
	Outstanding as of December 31, 2012		2,183,319		$	31.99
	Granted		373,939		$	39.56
	Shares issued		-891,129		$	32.15
	Forfeited		-29,689		$	33.75
	Outstanding as of December 31, 2013		1,636,440		$	33.61
	Granted		338,576		$	42.27
	Shares issued		-567,209		$	33.48
	Forfeited		-27,060		$	39.62
	Outstanding as of December 31, 2014		1,380,747		$	35.67
		Performance			Weighted Average
		Shares			Grant-Date
		(Units)			Fair Value
	Outstanding as of January 1, 2012		483,133		$	29.18
	Granted		225,935		$	35.09
	Converted to RSUs upon Merger		-451,358		$	34.32
	Shares issued		-106,773		$	24.52
	Outstanding as of December 31, 2012		150,937		$	25.04
	Granted		191,961		$	40.96
	Shares issued		-150,944		$	25.04
	Forfeited		-1,526		$	40.93
	Outstanding as of December 31, 2013		190,428		$	40.96
	Granted		193,396		$	43.4
	Shares issued		-2,009		$	41.46
	Forfeited		-6,171		$	42.02
	Outstanding as of December 31, 2014		375,644		$	42.2
	NU			For the Years Ended December 31,
	(Millions of Dollars)			2014			2013			2012 (1)
	Compensation Expense			$	24.6		$	27		$	25.8
	Future Income Tax Benefit				10.3			10.7			10.2
		For the Years Ended December 31,
		2014											2013								2012
					NSTAR										NSTAR								NSTAR
	(Millions of Dollars)	CL&P			Electric			PSNH			WMECO		CL&P		Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Compensation Expense	$	8.1		$	7.4		$	3		$	1.3	$	6.8	$	7.5	$	2.3	$	1.3	$	4.8	$	7.4	$	1.8	$	1
	Future Income Tax Benefit		3.4			3.1			1.3			0.5		2.7		3		0.9		0.5		1.9		2.9		0.7		0.4
					Weighted Average			Intrinsic Value
			Options		Exercise Price			(Millions)
	Outstanding and Exercisable - January 1, 2012		47,374		$	18.78
	Converted NSTAR Options upon Merger		2,664,894		$	23.99
	Exercised		-1,166,511		$	22.53		$	18.7
	Outstanding and Exercisable - December 31, 2012		1,545,757		$	24.92
	Exercised		-324,382		$	20.97		$	6.7
	Outstanding and Exercisable - December 31, 2013		1,221,375		$	25.97
	Exercised		-869,759		$	25.68		$	16.4
	Outstanding and Exercisable - December 31, 2014		351,616		$	26.69		$	9.4

Other_Retirement_Benefits_Tabl

Other Retirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits by Title of Individual and Type of Deferred Compensation [Table Text Block]
	NU		For the Years Ended December 31,
	(Millions of Dollars)		2014			2013			2012
	Actuarially-Determined Liability		$	57.5		$	51.3		$	54.6
	Other Retirement Benefits Expense			4.5			4.4			4.7
		For the Years Ended December 31,
		2014											2013						2012
	(Millions of Dollars)	CL&P			NSTAR Electric			PSNH			WMECO		CL&P		PSNH		WMECO		CL&P		PSNH		WMECO
	Actuarially-Determined Liability	$	0.4		$	-		$	2.6		$	0.2	$	0.4	$	2.3	$	0.1	$	0.4	$	2.5	$	0.2
	Other Retirement Benefits Expense		2.1			0.3			0.9			0.4		2.5		1		0.5		2.6		1		0.5

INCOME_TAXES_Tables

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Components of Income Tax Expense (Benefit) [Table Text Block]	NU	For the Years Ended December 31,
	(Millions of Dollars)	2014			2013			2012 (1)
	Current Income Taxes:
	Federal	$	4.4		$	8.8		$	-30.9
	State		24.5			-9.4			17.6
	Total Current		28.9			-0.6			-13.3
	Deferred Income Taxes, Net:
	Federal		406.8			386.2			291.3
	State		36.5			45.4			0.8
	Total Deferred		443.3			431.6			292.1
	Investment Tax Credits, Net		-3.9			-4.1			-3.9
	Income Tax Expense	$	468.3		$	426.9		$	274.9
	(1) NSTAR amounts were included in NU beginning April 10, 2012.
			For the Years Ended December 31,
			2014												2013												2012
						NSTAR												NSTAR											NSTAR
	(Millions of Dollars)		CL&P			Electric			PSNH			WMECO			CL&P			Electric			PSNH			WMECO			CL&P		Electric		PSNH		WMECO
	Current Income Taxes:
		Federal	$	-0.2		$	75		$	-22.6		$	1.9		$	20.1		$	95.8		$	-8.2		$	-53.4		$	-47.8	$	93.5	$	-0.9	$	-24.7
		State		4.3			20.2			-0.1			1.8			-6.7			29.6			3.6			4.2			3.1		27.6		3.4		3.4
	Total Current			4.1			95.2			-22.7			3.7			13.4			125.4			-4.6			-49.2			-44.7		121.1		2.5		-21.3
	Deferred Income Taxes, Net:
		Federal		138			88			79.6			28.1			114.9			49.8			64.5			84.7			141.5		11.4		46.5		51.2
		State		-7.1			20.1			15.2			6			15.1			-1			11.2			2.3			-0.5		-7.1		12		2.7
	Total Deferred			130.9			108.1			94.8			34.1			130			48.8			75.7			87			141		4.3		58.5		53.9
	Investment Tax Credits, Net			-1.5			-1.3			-			-0.5			-1.7			-1.3			-			-0.4			-1.9		-1.4		-		-0.5
	Income Tax Expense		$	133.5		$	202		$	72.1		$	37.3		$	141.7		$	172.9		$	71.1		$	37.4		$	94.4	$	124	$	61	$	32.1
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	A reconciliation between income tax expense and the expected tax expense at the statutory rate is as follows:
	NU		For the Years Ended December 31,
	(Millions of Dollars, except percentages)		2014				2013				2012 (1)
	Income Before Income Tax Expense		$	1,295.40			$	1,220.60			$	808
	Statutory Federal Income Tax Expense at 35%			453.4				427.2				282.8
	Tax Effect of Differences:
		Depreciation		-5.6				-7.4				-10.8
		Investment Tax Credit Amortization		-3.9				-4.1				-3.9
		Other Federal Tax Credits		-3.5				-3.7				-3.8
		State Income Taxes, Net of Federal Impact		42.5				27.6				4.4
		Dividends on ESOP		-8				-8				-6.4
		Tax Asset Valuation Allowance/Reserve Adjustments		-2.9				-4.3				7.6
		Other, Net		-3.7				-0.4				5
	Income Tax Expense		$	468.3			$	426.9			$	274.9
	Effective Tax Rate			36.20%				35.00%				34.00%
	(1) NSTAR amounts were included in NU beginning April 10, 2012.
			For the Years Ended December 31,
			2014												2013												2012
	(Millions of Dollars,					NSTAR												NSTAR											NSTAR
		except percentages)	CL&P			Electric			PSNH			WMECO			CL&P			Electric			PSNH			WMECO			CL&P		Electric		PSNH		WMECO
	Income Before Income Tax Expense		$	421.2		$	505.1		$	186.1		$	95.1		$	421.1		$	441.4		$	182.5		$	97.8		$	304.2	$	314.2	$	157.9	$	86.6
	Statutory Federal Income Tax Expense at 35%			147.4			176.8			65.1			33.3			147.4			154.5			63.9			34.2			106.5		110		55.3		30.3
	Tax Effect of Differences:
		Depreciation		-3.6			-1.3			0.3			-0.2			-7			0.1			0.6			-			-9		-		-0.3		0.2
		Investment Tax Credit Amortization		-1.5			-1.3			-			-0.5			-1.7			-1.3			-			-0.4			-1.9		-1.4		-		-0.5
		Other Federal Tax Credits		-			-			-3.5			-			-			-			-3.7			-			-		-		-3.8		-
		State Income Taxes, Net of Federal Impact		4.4			26.2			9.8			5			5			18.6			9.6			4.2			0.1		13.4		10		4
		Tax Asset Valuation Allowance/ Reserve Adjustments		-6.3			-			-			-			0.4			-			-			-			1.6		-		-		-
		Other, Net		-6.9			1.6			0.4			-0.3			-2.4			1			0.7			-0.6			-2.9		2		-0.2		-1.9
	Income Tax Expense		$	133.5		$	202		$	72.1		$	37.3		$	141.7		$	172.9		$	71.1		$	37.4		$	94.4	$	124	$	61	$	32.1
	Effective Tax Rate			31.70%			40.00%			38.70%			39.20%			33.60%			39.20%			39.00%			38.20%			31.00%		39.50%		38.60%		37.10%
Schedule of Deferred Tax Assets and Liabilities [Table Text Block]	NU			As of December 31,
	(Millions of Dollars)			2014			2013
	Deferred Tax Assets:
		Employee Benefits		$	632.2		$	435.2
		Derivative Liabilities			199.6			272.9
		Regulatory Deferrals - Liabilities			366.7			272.7
		Allowance for Uncollectible Accounts			60.5			65
		Tax Effect - Tax Regulatory Liabilities			10			16.2
		Federal Net Operating Loss Carryforwards			59.1			158
		Purchase Accounting Adjustment			126.2			132.8
		Other			198.7			230.6
	Total Deferred Tax Assets				1,653.00			1,583.40
		Less: Valuation Allowance			5.1			24.3
	Net Deferred Tax Assets			$	1,647.90		$	1,559.10
	Deferred Tax Liabilities:
		Accelerated Depreciation and Other Plant-Related Differences		$	4,215.90		$	3,806.50
		Property Tax Accruals			109.6			95.1
		Regulatory Amounts:
			Regulatory Deferrals - Assets		1,277.90			1,146.70
			Tax Effect - Tax Regulatory Assets		240.2			248.2
			Goodwill Regulatory Asset - 1999 Merger		203.2			211.5
			Derivative Assets		32.6			30.1
			Other		196.3			157.1
	Total Deferred Tax Liabilities			$	6,275.70		$	5,695.20
				As of December 31,
				2014												2013
							NSTAR												NSTAR
	(Millions of Dollars)			CL&P			Electric			PSNH			WMECO			CL&P			Electric			PSNH			WMECO
	Deferred Tax Assets:
		Employee Benefits		$	129		$	39.9		$	46.8		$	9.2		$	56		$	38.3		$	15.5		$	-1.8
		Derivative Liabilities			193			1.8			-			-			272.4			3.3			-			-2.9
		Regulatory Deferrals - Liabilities			73.9			181.3			46.5			11.4			61.5			114.7			40.9			1
		Allowance for Uncollectible Accounts			32.3			13.8			3.2			3.8			31.2			15.4			3.1			3.3
		Tax Effect - Tax Regulatory Liabilities			3.1			1.8			2.1			2.5			4.7			5.4			2.1			1.6
		Federal Net Operating Loss Carryforwards			-			-			32.1			4.5			51			-			56.6			18.6
		Other			53.8			19.9			48.9			4.9			75.3			31.3			40.3			8.3
	Total Deferred Tax Assets				485.1			258.5			179.6			36.3			552.1			208.4			158.5			28.1
		Less: Valuation Allowance			4			-			-			-			23.1			-			-			-
	Net Deferred Tax Assets			$	481.1		$	258.5		$	179.6		$	36.3		$	529		$	208.4		$	158.5		$	28.1
	Deferred Tax Liabilities:
		Accelerated Depreciation and Other Plant-Related Differences		$	1,378.60		$	1,296.90		$	596.6		$	385.8		$	1,238.10		$	1,179.40		$	526.6		$	361.1
		Property Tax Accruals			58.1			25			7.4			12.8			49.3			25.3			7.1			5.9
		Regulatory Amounts:
			Regulatory Deferrals - Assets		502.3			276			147.6			60.4			550.4			276.2			109.3			49.3
			Tax Effect - Tax Regulatory Assets		166.9			35.5			15.9			9.3			160.1			36			16.3			18.2
			Goodwill Regulatory Asset - 1999 Merger		-			174.4			-			-			-			181.6			-			-
			Derivative Assets		32.6			-			-			-			29			0.5			-			-
			Other		19.4			33.5			35.6			2.8			20.6			26.4			28			3.6
	Total Deferred Tax Liabilities			$	2,157.90		$	1,841.30		$	803.1		$	471.1		$	2,047.50		$	1,725.40		$	687.3		$	438.1
Summary of Operating Loss Carryforwards [Table Text Block]	Carryforwards: The following tables provide the amounts and expiration dates of state tax credit and loss carryforwards and federal tax credit and net operating loss carryforwards:
		As of December 31, 2014
								NSTAR
	(Millions of Dollars)	NU			CL&P			Electric			PSNH			WMECO			Expiration Range
	Federal Net Operating Loss	$	168.8		$	-		$	-		$	91.8		$	12.7		2031 - 2032
	Federal Tax Credit		16.3			0.1			0.2			11.1			-		2031 - 2034
	Federal Charitable Contribution		19.4			-			-			-			-		2016 - 2018
	State Tax Credit		99.7			71			-			-			-		2014 - 2019
	State Loss Carryforwards		40.6			-			-			-			-		2014 - 2034
	State Charitable Contribution		2.1			-			-			-			-		2015 - 2018
		As of December 31, 2013
								NSTAR
	(Millions of Dollars)	NU			CL&P			Electric			PSNH			WMECO			Expiration Range
	Federal Net Operating Loss	$	451.3		$	145.8		$	-		$	161.8		$	53.3		2031 - 2032
	Federal Tax Credit		8			-			-			7.6			-		2031 - 2033
	Federal Charitable Contribution		33.7			-			-			-			-		2015 - 2017
	State Tax Credit		104.7			86.8			-			-			-		2013 - 2018
	State Loss Carryforwards		12.1			-			-			-			-		2013 - 2015
	State Charitable Contribution		1			-			-			-			-		2015
Summary of Income Tax Contingencies [Table Text Block]	(Millions of Dollars)		NU			CL&P
	Balance as of January 1, 2012		$	73.5		$	46.5
		Gross Increases - Current Year		10.3			2.5
		Gross Increases - Prior Year		0.1			-
		Gross Decreases - Prior Year		-0.8			-
	Balance as of December 31, 2012			83.1			49
		Gross Increases - Current Year		8.2			2.1
		Gross Decreases - Prior Year		-1.1			-0.3
		Settlements		-49.8			-39.4
		Lapse of Statute of Limitations		-2.2			-
	Balance as of December 31, 2013			38.2			11.4
		Gross Increases - Current Year		9.3			2.7
		Gross Increases - Prior Year		0.3			0.2
		Lapse of Statute of Limitations		-1.6			-
	Balance as of December 31, 2014		$	46.2		$	14.3
	Other Interest		For the Years Ended December 31,									Accrued Interest		As of December 31,
	Expense/(Income)		2014			2013			2012			Expense		2014			2013
	(Millions of Dollars)											(Millions of Dollars)
	NU (1)		$	0.4		$	-8.6		$	3.1		NU		$	1.9		$	1.5
	CL&P			-			-4			1.3		CL&P			-			-

Environmental_Matters_Tables

Environmental Matters (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Environmental Loss Contingencies By Site Text Block	(Millions of Dollars)	NU		CL&P		NSTAR Electric		PSNH		WMECO
	Balance as of January 1, 2013	$	39.4	$	3.7	$	1.7	$	4.9	$	0.6
	Additions		3.5		0.2		0.2		1		-
	Payments/Reductions		-7.5		-0.5		-0.7		-0.5		-0.2
	Balance as of December 31, 2013		35.4		3.4		1.2		5.4		0.4
	Additions		12.7		1		-		0.1		0.2
	Payments/Reductions		-4.8		-0.6		-0.1		-0.3		-0.1
	Balance as of December 31, 2014	$	43.3	$	3.8	$	1.1	$	5.2	$	0.5
		As of December 31, 2014				As of December 31, 2013
				Reserve				Reserve
		Number of Sites		(in millions)		Number of Sites		(in millions)
	NU		65	$	43.3		68	$	35.4
	CL&P		16		3.8		18		3.4
	NSTAR Electric		13		1.1		12		1.2
	PSNH		13		5.2		15		5.4
	WMECO		4		0.5		5		0.4

LongTerm_Contractual_Arrangeme

Long-Term Contractual Arrangements (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Long-term Purchase Commitment [Table Text Block]	B.		Long-Term Contractual Arrangements
	Estimated Future Annual Costs: The estimated future annual costs of significant long-term contractual arrangements as of December 31, 2014 are as follows:
	NU
	(Millions of Dollars)			2015			2016			2017			2018			2019			Thereafter			Total
	Supply and Stranded Cost			$	196.6		$	169.2		$	101.9		$	65.3		$	38.1		$	82.4		$	653.5
	Renewable Energy				204.3			240.5			239.6			204.2			202.9			1,994.60			3,086.10
	Peaker CfDs				26.3			25.4			10.5			-			-			-			62.2
	Natural Gas Procurement				133.7			116.3			45.6			31.9			25.8			85.1			438.4
	Coal, Wood and Other				99			25.2			5			5			1.9			15			151.1
	Transmission Support Commitments				25.9			21.6			19			21.2			21.3			21.3			130.3
	Total			$	685.8		$	598.2		$	421.6		$	327.6		$	290		$	2,198.40		$	4,521.60
	CL&P
	(Millions of Dollars)			2015			2016			2017			2018			2019			Thereafter			Total
	Supply and Stranded Cost			$	134.3		$	136.8		$	79		$	42		$	25		$	43.6		$	460.7
	Renewable Energy				61.2			70.3			71.3			72.1			72.1			715.9			1,062.90
	Peaker CfDs				26.3			25.4			10.5			-			-			-			62.2
	Transmission Support Commitments				10.2			8.5			7.5			8.4			8.4			8.4			51.4
	Yankee Billings				1.4			0.8			0.8			0.9			0.9			11.8			16.6
	Total			$	233.4		$	241.8		$	169.1		$	123.4		$	106.4		$	779.7		$	1,653.80
	NSTAR Electric
	(Millions of Dollars)			2015			2016			2017			2018			2019			Thereafter			Total
	Supply and Stranded Cost			$	34.3		$	14.1		$	4.8		$	5.5		$	5.5		$	31.4		$	95.6
	Renewable Energy				85.4			99.9			96.9			59.6			57.7			319.8			719.3
	Transmission Support Commitments				8.1			6.7			5.9			6.6			6.6			6.6			40.5
	Yankee Billings				0.5			0.3			0.3			0.3			0.3			4			5.7
	Total			$	128.3		$	121		$	107.9		$	72		$	70.1		$	361.8		$	861.1
	PSNH
	(Millions of Dollars)			2015			2016			2017			2018			2019			Thereafter			Total
	Supply and Stranded Cost			$	28		$	18.3		$	18.1		$	17.8		$	7.6		$	7.4		$	97.2
	Renewable Energy				57.7			67.9			69			70.1			70.7			932.4			1,267.80
	Coal, Wood and Other				99			25.2			5			5			1.9			15			151.1
	Transmission Support Commitments				5.5			4.6			4			4.5			4.5			4.5			27.6
	Yankee Billings				0.4			0.3			0.3			0.3			0.3			4.7			6.3
	Total			$	190.6		$	116.3		$	96.4		$	97.7		$	85		$	964		$	1,550.00
	WMECO
	(Millions of Dollars)			2015			2016			2017			2018			2019			Thereafter			Total
	Renewable Energy			$	-		$	2.4		$	2.4		$	2.4		$	2.4		$	26.5		$	36.1
	Transmission Support Commitments				2.1			1.8			1.6			1.7			1.8			1.8			10.8
	Yankee Billings				0.3			0.2			0.2			0.2			0.2			3			4.1
	Total			$	2.4		$	4.4		$	4.2		$	4.3		$	4.4		$	31.3		$	51
	NU	For the Years Ended December 31,
	(Millions of Dollars)	2014			2013			2012 (1)
	Supply and Stranded Cost	$	99.2		$	141		$	216.8
	Renewable Energy		114.4			91.3			48.7
	Peaker CfDs		18.1			51.9			59.3
	Natural Gas Procurement		482.5			349.8			243.1
	Coal, Wood and Other		120.5			112.6			105.2
	Transmission Support Commitments		25			24.9			24.8
		For the Years Ended December 31,
		2014												2013											2012
					NSTAR												NSTAR										NSTAR
	(Millions of Dollars)	CL&P			Electric			PSNH			WMECO			CL&P			Electric			PSNH			WMECO		CL&P		Electric		PSNH		WMECO
	Supply and Stranded Cost	$	63		$	7		$	26		$	3.2		$	77.6		$	32.4		$	29		$	2	$	158.2	$	36.3	$	30.5	$	0.9
	Renewable Energy		0.7			87.4			26.3			-			-			84.9			6.4			-		-		60.2		4.1		-
	Peaker CfDs		18.1			-			-			-			51.9			-			-			-		59.3		-		-		-
	Coal, Wood and Other		-			-			120.5			-			-			-			112.6			-		-		-		105.2		-
	Transmission Support Commitments		9.9			7.7			5.3			2.1			9.8			7.7			5.3			2.1		9.6		7.6		5.2		2

Guarantees_and_Indemnification

Guarantees and Indemnifications (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Guarantee Obligations Text Block			Maximum Exposure
	Subsidiary	Description	(in millions)		Expiration Dates
	Various	Surety Bonds (1)	$	60	2015 - 2016
	NUSCO and Rocky River Realty Company	Lease Payments for Vehicles and Real Estate	$	14.4	2019 and 2024

Other_Contingencies_Tables

Other Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Loss Contingencies By Contingency [Table Text Block]		NU
		For the Years Ended December 31,
	(Millions of Dollars)	2013		2014		Total
	1st Complaint - Base ROE	$	23.7	$	1.2	$	24.9
	2nd Complaint - Base ROE		-		27.4		27.4
	Incentive ROE (1st and 2nd Complaint)		-		8.4		8.4
	Cumulative Reserve	$	23.7	$	37	$	60.7
		CL&P						NSTAR Electric
		For the Years Ended December 31,						For the Years Ended December 31,
	(Millions of Dollars)	2013		2014		Total		2013		2014		Total
	1st Complaint - Base ROE	$	12.8	$	0.5	$	13.3	$	5.7	$	0.4	$	6.1
	2nd Complaint - Base ROE		-		13.5		13.5		-		7.5		7.5
	Incentive ROE (1st and 2nd Complaint)		-		6.7		6.7		-		-		-
	Cumulative Reserve	$	12.8	$	20.7	$	33.5	$	5.7	$	7.9	$	13.6
		PSNH						WMECO
		For the Years Ended December 31,						For the Years Ended December 31,
	(Millions of Dollars)	2013		2014		Total		2013		2014		Total
	1st Complaint - Base ROE	$	2.3	$	0.1	$	2.4	$	2.9	$	0.2	$	3.1
	2nd Complaint - Base ROE		-		2.7		2.7		-		3.7		3.7
	Incentive ROE (1st and 2nd Complaint)		-		-		-		-		1.7		1.7
	Cumulative Reserve	$	2.3	$	2.8	$	5.1	$	2.9	$	5.6	$	8.5

LEASES_Tables

LEASES (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Rent Expense [Table Text Block]									NSTAR
	(Millions of Dollars)		NU (1)			CL&P			Electric			PSNH			WMECO
	2014		$	14.3		$	6		$	7.8		$	1.5		$	1.2
	2013			16.3			8.1			6.7			1.7			2.9
	2012			14.8			8.2			6.2			2.5			3
Schedule of Future Minimum Lease Payments for Capital Leases [Table Text Block]	Capital Leases
	(Millions of Dollars)	NU			CL&P			PSNH
	2015	$	2.4		$	2		$	0.4
	2016		2.2			1.9			0.3
	2017		2.1			2			0.1
	2018		2.1			2			0.1
	2019		2			2			0
	Thereafter		3.5			3.5			0
	Future minimum lease payments		14.3			13.4			0.9
	Less amount representing interest		4.9			5			0
	Present value of future minimum lease payments	$	9.4		$	8.4		$	0.9
Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block]	Operating Leases							NSTAR
	(Millions of Dollars)	NU			CL&P			Electric			PSNH			WMECO
	2015	$	20.1		$	4.3		$	10		$	1.1		$	1.2
	2016		17.6			3.8			8.8			1			1
	2017		14.6			2.6			7.7			0.8			0.8
	2018		10.5			1.5			5.8			0.6			0.6
	2019		8.6			1.1			4.7			0.5			0.6
	Thereafter		22.5			4			10.4			1.5			2.5
	Future minimum lease payments	$	93.9		$	17.3		$	47.4		$	5.5		$	6.7

FAIR_VALUE_OF_FINANCIAL_INSTRU1

FAIR VALUE OF FINANCIAL INSTRUMENTS (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Fair Value By Balance Sheet Grouping Text Block		As of December 31,
		2014				2013
	NU	Carrying		Fair		Carrying		Fair
	(Millions of Dollars)	Amount		Value		Amount		Value
	Preferred Stock Not Subject to Mandatory Redemption	$	155.6	$	153.6	$	155.6	$	152.7
	Long-Term Debt		8,851.60		9,451.20		8,310.20		8,443.10
		As of December 31, 2014
		CL&P				NSTAR Electric				PSNH				WMECO
		Carrying		Fair		Carrying		Fair		Carrying		Fair		Carrying		Fair
	(Millions of Dollars)	Amount		Value		Amount		Value		Amount		Value		Amount		Value
	Preferred Stock Not Subject to Mandatory Redemption	$	116.2	$	112	$	43	$	41.6	$	-	$	-	$	-	$	-
	Long-Term Debt		2,842.00		3,214.50		1,797.40		1,993.50		1,076.30		1,137.90		628.5		689.4
		As of December 31, 2013
		CL&P				NSTAR Electric				PSNH				WMECO
		Carrying		Fair		Carrying		Fair		Carrying		Fair		Carrying		Fair
	(Millions of Dollars)	Amount		Value		Amount		Value		Amount		Value		Amount		Value
	Preferred Stock Not Subject to Mandatory Redemption	$	116.2	$	110.5	$	43	$	42.2	$	-	$	-	$	-	$	-
	Long-Term Debt		2,741.20		2,952.80		1,801.10		1,888.00		1,049.00		1,073.90		629.4		640.1

ACCUMULATED_OTHER_COMPREHENSIV1

ACCUMULATED OTHER COMPREHENSIVE INCOME (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Accumulated Other Comprehensive Income Loss Table Text Block			For the Year Ended December 31, 2014											For the Year Ended December 31, 2013
			Qualified			Unrealized								Qualified		Unrealized
			Cash Flow			Gains/(Losses)			Defined					Cash Flow		Gains/(Losses)		Defined
			Hedging			on Marketable			Benefit					Hedging		on Marketable		Benefit
	(Millions of Dollars)		Instruments			Securities			Plans			Total		Instruments		Securities		Plans		Total
	AOCI as of January 1st		$	-14.4		$	0.4		$	-32		$	-46	$	-16.4	$	1.3	$	-57.8	$	-72.9
	OCI Before Reclassifications			-			0.3			-34.2			-33.9		-		-0.9		19.4		18.5
	Amounts Reclassified from AOCI			2			-			3.9			5.9		2		-		6.4		8.4
	Net OCI			2			0.3			-30.3			-28		2		-0.9		25.8		26.9
	AOCI as of December 31st		$	-12.4		$	0.7		$	-62.3		$	-74	$	-14.4	$	0.4	$	-32	$	-46
Schedule Of Comprehensive Income Loss [Table Text Block]		For the Years Ended December 31,
		2014			2013			2012
		Amounts Reclassified			Amounts Reclassified			Amounts Reclassified			Statements of Income
	(Millions of Dollars)	from AOCI			from AOCI			from AOCI			Line Item Impacted
	Qualified Cash Flow Hedging Instruments	$	-3.4		$	-3.4		$	-3.3		Interest Expense
	Tax Effect		1.4			1.4			1.3		Income Tax Expense
	Qualified Cash Flow Hedging Instruments, Net of Tax	$	-2		$	-2		$	-2
	Defined Benefit Plan Costs:
	Amortization of Actuarial Losses	$	-6.2		$	-10.5		$	-8.9		Operations and Maintenance (1)
	Amortization of Prior Service Cost		-0.2			-0.2			-0.2		Operations and Maintenance (1)
	Amortization of Transition Obligation		0			0			-0.2		Operations and Maintenance (1)
	Total Defined Benefit Plan Costs		-6.4			-10.7			-9.3
	Tax Effect		2.5			4.3			3.5		Income Tax Expense
	Defined Benefit Plan Costs, Net of Tax	$	-3.9		$	-6.4		$	-5.8
	Total Amounts Reclassified from AOCI, Net of Tax	$	-5.9		$	-8.4		$	-7.8

COMMON_SHARES_Tables

COMMON SHARES (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Stock by Class [Text Block]	16.       COMMON SHARES
	The following table sets forth the NU common shares and the shares of common stock of CL&P, NSTAR Electric, PSNH and WMECO that were authorized and issued and the respective per share par values:
				Shares
				Authorized	Issued
		Per Share		as of December 31,	as of December 31,
		Par Value		2014 and 2013	2014		2013
	NU	$	5	380,000,000		333,359,172		333,113,492
	CL&P	$	10	24,500,000		6,035,205		6,035,205
	NSTAR Electric	$	1	100,000,000		100		100
	PSNH	$	1	100,000,000		301		301
	WMECO	$	25	1,072,471		434,653		434,653
	As of December 31, 2014 and 2013, there were 16,375,835 and 17,796,672 NU common shares held as treasury shares, respectively. As of December 31, 2014 and 2013, NU common shares outstanding were 316,983,337 and 315,273,559, respectively.

PREFFERED_STOCK_TABLES

PREFFERED STOCK (TABLES)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Preferred Stock [Text Block]					Redemption Price		Shares Outstanding as of	As of December 31,
	Series				Per Share		December 31, 2014 and 2013	2014		2013
	CL&P
	$	1.9		Series of 1947	$	52.5	163,912	$	8.2	$	8.2
	$	2		Series of 1947	$	54	336,088		16.8		16.8
	$	2.04		Series of 1949	$	52	100,000		5		5
	$	2.2		Series of 1949	$	52.5	200,000		10		10
		3.9	%	Series of 1949	$	50.5	160,000		8		8
	$	2.06		Series E of 1954	$	51	200,000		10		10
	$	2.09		Series F of 1955	$	51	100,000		5		5
		4.5	%	Series of 1956	$	50.75	104,000		5.2		5.2
		4.96	%	Series of 1958	$	50.5	100,000		5		5
		4.5	%	Series of 1963	$	50.5	160,000		8		8
		5.28	%	Series of 1967	$	51.43	200,000		10		10
	$	3.24		Series G of 1968	$	51.84	300,000		15		15
		6.56	%	Series of 1968	$	51.44	200,000		10		10
	Total CL&P						2,324,000	$	116.2	$	116.2
	NSTAR Electric
		4.25	%	Series	$	103.625	180,000	$	18	$	18
		4.78	%	Series	$	102.8	250,000		25		25
	Total NSTAR Electric						430,000	$	43	$	43
	Fair Value Adjustment due to Merger with NSTAR								-3.6		-3.6
	Total NU - Preferred Stock of Subsidiaries							$	155.6	$	155.6

COMMON_SHARESHOLDERS_EQUITY_AN1

COMMON SHARESHOLDERS' EQUITY AND NONCONTROLLING INTERESTS (NU) (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Minority Interest Disclosure Text Block	18	COMMON SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS
	A summary of the changes in Common Shareholders' Equity and Noncontrolling Interests of NU is as follows:
									Noncontrolling
			Common						Interest -
			Shareholders'		Noncontrolling		Total		Preferred Stock
	(Millions of Dollars)		Equity		Interest		Equity		of Subsidiaries
	Balance as of January 1, 2012		$	4,012.70	$	3	$	4,015.70	$	116.2
	Net Income			533.1		-		533.1		-
	Purchase Price of NSTAR (1)			5,038.30		-		5,038.30		-
	Other Equity Impacts of Merger with NSTAR (2)			3.4		-3.4		-		39.4
	Dividends on Common Shares			-375.5		-		-375.5		-
	Dividends on Preferred Stock			-7		-		-7		-7
	Issuance of Common Shares			13.3		-		13.3		-
	Contributions to NPT			-		0.3		0.3		-
	Other Transactions, Net			21.1		-		21.1		-
	Net Income Attributable to Noncontrolling Interests			-0.1		0.1		-		7
	Other Comprehensive Loss			-2.2		-		-2.2		-
	Balance as of December 31, 2012		$	9,237.10	$	-	$	9,237.10	$	155.6
	Net Income			793.7		-		793.7		-
	Dividends on Common Shares			-462.7		-		-462.7		-
	Dividends on Preferred Stock			-7.7		-		-7.7		-7.7
	Issuance of Common Shares			11.1		-		11.1		-
	Other Transactions, Net			13.2		-		13.2		-
	Net Income Attributable to Noncontrolling Interests			-		-		-		7.7
	Other Comprehensive Income			26.8		-		26.8		-
	Balance as of December 31, 2013		$	9,611.50	$	-	$	9,611.50	$	155.6
	Net Income			827.1		-		827.1		-
	Dividends on Common Shares			-496.5		-		-496.5		-
	Dividends on Preferred Stock			-7.5		-		-7.5		-7.5
	Issuance of Common Shares			6.6		-		6.6		-
	Other Transactions, Net			63.6		-		63.6		-
	Net Income Attributable to Noncontrolling Interests			-		-		-		7.5
	Other Comprehensive Loss			-28		-		-28		-
	Balance as of December 31, 2014		$	9,976.80	$	-	$	9,976.80	$	155.6

EARNINGS_PER_SHARE_Tables

EARNINGS PER SHARE (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Earnings Per Share Basic And Diluted [Table Text Block]			For the Years Ended December 31,
	(Millions of Dollars, except share information)		2014		2013		2012
	Net Income Attributable to Controlling Interest		$	819.5	$	786	$	525.9
	Weighted Average Common Shares Outstanding:
		Basic		316,136,748		315,311,387		277,209,819
		Dilutive Effect		1,280,666		899,773		783,812
		Diluted		317,417,414		316,211,160		277,993,631
	Basic EPS		$	2.59	$	2.49	$	1.9
	Diluted EPS		$	2.58	$	2.49	$	1.89

SEGMENT_INFORMATION_Tables

SEGMENT INFORMATION (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting Information [Line Items]
Schedule Of Segment Reporting Information By Segment Text Block		For the Year Ended December 31, 2014
		Electric		Natural Gas
	(Millions of Dollars)	Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues	$	5,663.40	$	1,007.30	$	1,018.20	$	790.9	$	-737.9	$	7,741.90
	Depreciation and Amortization		-384.6		-68.1		-150.5		-42.1		19.9		-625.4
	Other Operating Expenses		-4,366.20		-786.7		-302.1		-748		719.3		-5,483.70
	Operating Income		912.6		152.5		565.6		0.8		1.3		1,632.80
	Interest Expense		-191.6		-34		-104.1		-36.6		4.2		-362.1
	Interest Income		5.1		-		0.9		3.6		-3.6		6
	Other Income, Net		10.7		0.2		10.3		916		-918.6		18.6
	Income Tax (Expense)/Benefit		-269.7		-46.4		-174.5		22.3		-		-468.3
	Net Income		467.1		72.3		298.2		906.1		-916.7		827
	Net Income Attributable to Noncontrolling Interests		-4.7		-		-2.8		-		-		-7.5
	Net Income Attributable to Controlling Interest	$	462.4	$	72.3	$	295.4	$	906.1	$	-916.7	$	819.5
	Total Assets (as of)	$	17,563.40	$	3,030.90	$	7,625.60	$	12,682.50	$	-11,124.40	$	29,778.00
	Cash Flows Used for Investments in Plant	$	645.2	$	176.7	$	731.6	$	50.2	$	-	$	1,603.70
		For the Year Ended December 31, 2013
		Electric		Natural Gas
	(Millions of Dollars)	Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues	$	5,362.30	$	855.8	$	978.7	$	777.5	$	-673.1	$	7,301.20
	Depreciation and Amortization		-604.8		-66.7		-136.2		-62.2		10.2		-859.7
	Other Operating Expenses		-3,927.70		-659.4		-281.8		-715		671.8		-4,912.10
	Operating Income		829.8		129.7		560.7		0.3		8.9		1,529.40
	Interest Expense		-175		-33.1		-100.3		-35.5		5.2		-338.7
	Interest Income		4.1		-		0.7		5.4		-5.6		4.6
	Other Income, Net		12.9		0.8		10.9		858.9		-858.2		25.3
	Income Tax (Expense)/Benefit		-240		-36.5		-182.1		31.9		-0.2		-426.9
	Net Income		431.8		60.9		289.9		861		-849.9		793.7
	Net Income Attributable to Noncontrolling Interests		-4.8		-		-2.9		-		-		-7.7
	Net Income Attributable to Controlling Interest	$	427	$	60.9	$	287	$	861	$	-849.9	$	786
	Total Assets (as of)	$	17,260.00	$	2,759.70	$	6,745.80	$	11,842.40	$	-10,812.40	$	27,795.50
	Cash Flows Used for Investments in Plant	$	639	$	168.1	$	618.5	$	31.2	$	-	$	1,456.80
		For the Year Ended December 31, 2012
		Electric		Natural Gas
	(Millions of Dollars)	Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues	$	4,716.50	$	572.9	$	861.5	$	803.8	$	-680.9	$	6,273.80
	Depreciation and Amortization		-530.3		-49.1		-109.2		-56.4		4.2		-740.8
	Other Operating Expenses		-3,585.40		-445.2		-251.6		-817		684.4		-4,414.80
	Operating Income/(Loss)		600.8		78.6		500.7		-69.6		7.7		1,118.20
	Interest Expense		-165.6		-31.3		-96.7		-43.6		7.3		-329.9
	Interest Income		2.8		-		0.4		7.1		-7.1		3.2
	Other Income, Net		8.9		0.4		7.3		795		-795.1		16.5
	Income Tax (Expense)/Benefit		-150.2		-16.9		-159.2		55.5		-4.1		-274.9
	Net Income		296.7		30.8		252.5		744.4		-791.3		533.1
	Net Income Attributable to Noncontrolling Interests		-4.4		-		-2.8		-		-		-7.2
	Net Income Attributable to Controlling Interest	$	292.3	$	30.8	$	249.7	$	744.4	$	-791.3	$	525.9
	Cash Flows Used for Investments in Plant	$	611.7	$	148.7	$	663.6	$	48.3	$	-	$	1,472.30

MERGER_OF_NU_AND_NSTAR_Tables

MERGER OF NU AND NSTAR (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule of Business Acquisitions by Acquisition Equity Interest Issued or Issuable [Table Text Block]	NSTAR common shares outstanding as of April 9, 2012 (in thousands)*			103,696
	Exchange ratio			1.312
	NU common shares issued for NSTAR common shares outstanding (in thousands)			136,049
	Closing price of NU common shares on April 9, 2012		$	36.79
	Value of common shares issued (in millions)		$	5,005
	Fair value of NU replacement stock-based compensation awards related to pre-merger service (in millions)			33
	Total purchase price (in millions)		$	5,038
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed
Business Acquisition, Pro Forma Information	(Pro forma amounts in millions, except per share amounts)	For the Year Ended December 31, 2012
	Operating Revenues	$	7,004
	Net Income Attributable to Controlling Interest		630
	Basic EPS		2
	Diluted EPS		1.99
Business Acquisition, Integration, Restructuring and Other Related Costs [Text Block]	(Millions of Dollars)		For the Year Ended December 31, 2012
	Transaction and Other Costs		$	32
	Settlement Agreement Impacts			60
	Total After-Tax Non-Recurring Costs Excluded from Pro Forma Net Income Attributable to Controlling Interest		$	92
	(Millions of Dollars)	NU			CL&P		NSTAR Electric		WMECO
	Customer Rate Credits	$	46		$	25	$	15	$	3
	Storm Costs Deferral Reduction		40			40		-		-
	Establishment of Energy Efficiency Fund		15			-		-		-
	Total Pre-Tax Settlement Agreement Impacts	$	101		$	65	$	15	$	3

GOODWILL_Tables

GOODWILL (Tables)	12 Months Ended
	Dec. 31, 2014
Notes To Consolidated Financial Statements [Abstract]
Schedule Of Goodwill Text Block		Electric		Electric		Natural Gas
	(Billions of Dollars)	Distribution		Transmission		Distribution		Total
	Goodwill Allocation	$	2.5	$	0.6	$	0.4	$	3.5

QUARTERLY_FINANCIAL_DATA_Unaud1

QUARTERLY FINANCIAL DATA (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Schedule of Quarterly Financial Information [Table Text Block]	NU Consolidated Statements of Quarterly Financial Data	Quarter Ended
	(Millions of Dollars, except per share information)	March 31,		June 30,		September 30,		December 31,
	2014
	Operating Revenues	$	2,290.60	$	1,677.60	$	1,892.50	$	1,881.20
	Operating Income		467.7		294		440.9		430.2
	Net Income		237.8		129.2		236.5		223.6
	Net Income Attributable to Controlling Interest		236		127.4		234.6		221.5
	Basic EPS (a)	$	0.75	$	0.4	$	0.74	$	0.69
	Diluted EPS (a)	$	0.74	$	0.4	$	0.74	$	0.69
	2013
	Operating Revenues	$	1,995.00	$	1,635.90	$	1,892.60	$	1,777.70
	Operating Income		418.9		350.6		399.3		360.6
	Net Income		230		173.1		211.4		179.2
	Net Income Attributable to Controlling Interest		228.1		171		209.5		177.4
	Basic and Diluted EPS (a)	$	0.72	$	0.54	$	0.66	$	0.56
	(a) The summation of quarterly EPS data may not equal annual data due to rounding.
	Statements of Quarterly Financial Data	Quarter Ended
	(Millions of Dollars)	March 31,		June 30,		September 30,		December 31,
	CL&P
	2014
	Operating Revenues	$	734.6	$	587.3	$	695.6	$	675.1
	Operating Income		158		92.1		146.2		159
	Net Income		79.3		37.4		83.9		87.2
	2013
	Operating Revenues	$	624.1	$	569.3	$	648.4	$	600.5
	Operating Income		149.7		136.8		133.9		119.2
	Net Income		85		67.9		66.3		60.2
	NSTAR Electric
	2014
	Operating Revenues	$	666.2	$	561.5	$	727.9	$	581.1
	Operating Income		118.4		121.5		206.6		132
	Net Income		58.1		60.1		115.6		69.3
	2013
	Operating Revenues	$	592.3	$	570.4	$	753.9	$	576.9
	Operating Income		94.5		112.5		192		109.2
	Net Income		48.1		58		107		55.4
	PSNH
	2014
	Operating Revenues	$	299.8	$	211.6	$	223.7	$	224.4
	Operating Income		64		49		56.4		60
	Net Income		32.6		24.1		28.2		29
	2013
	Operating Revenues	$	273.8	$	216.1	$	218.6	$	226.9
	Operating Income		58.1		54.3		56.6		56.2
	Net Income		29		27.2		28.4		26.8
	WMECO
	2014
	Operating Revenues	$	137.4	$	108.3	$	118.1	$	129.6
	Operating Income		34.7		17.7		31.2		34
	Net Income		18.1		7		14.7		18
	2013
	Operating Revenues	$	125	$	115	$	121.8	$	110.9
	Operating Income		35.6		32.4		28.9		22.4
	Net Income		18.6		16.4		15		10.4

Provision_for_Uncollectible_Ac1

Provision for Uncollectible Accounts (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	$175.30	$171.30
Uncollectible Hardship Accounts Receivable Reserve	91.5	81.2
The Connecticut Light And Power Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	84.3	82
Uncollectible Hardship Accounts Receivable Reserve	74	67.3
NSTAR Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	40.7	41.7
Public Service Company Of New Hampshire [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	7.7	7.4
Western Massachusetts Electric Company [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance For Doubtful Accounts Receivable Current	9.9	10
Uncollectible Hardship Accounts Receivable Reserve	$6.20	$5.50

Fuel_Materials_and_Supplies_an

Fuel, Materials and Supplies and Allowance Inventory (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
fuelcosts	$164.30	$139.50
Public Service Company Of New Hampshire [Member]
fuelcosts	95.1	74.2
Public Service Company Of New Hampshire [Member] | Emission Allowances [Member]
Other Inventory, Noncurrent	$20.10	$19.40

Restricted_Cash_and_Other_Depo

Restricted Cash and Other Deposits (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Special Deposits [Line Items]
Restricted Cash And Cash Equivalents	$3.20	$1.70
Cash Collateral Deposited	9.9	17.9
The Connecticut Light And Power Company [Member]
Special Deposits [Line Items]
Restricted Cash And Cash Equivalents	2.1	1.4
Cash Collateral Deposited	1.2
NSTAR Electric Company [Member]
Special Deposits [Line Items]
Cash Collateral Deposited		9
Public Service Company Of New Hampshire [Member]
Special Deposits [Line Items]
Restricted Cash And Cash Equivalents	1
Cash Collateral Deposited	$2.50

Equity_Method_Investments_Deta

Equity Method Investments (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$1,200	$1,200
NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	500	500
Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	300	300
Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	300	300
CYAPC [Member] | The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	34.50%
CYAPC [Member] | NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	14.00%
CYAPC [Member] | Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	5.00%
CYAPC [Member] | Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	9.50%
YAEC [Member] | The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	24.50%
YAEC [Member] | NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	14.00%
YAEC [Member] | Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	7.00%
YAEC [Member] | Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	7.00%
MYAPC [Member] | The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	12.00%
MYAPC [Member] | NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	4.00%
MYAPC [Member] | Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	5.00%
MYAPC [Member] | Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	3.00%
Energy Investment Fund [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$17,800	$9,800

Operating_Expenses_Details

Operating Expenses (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fuel [Line Items]
Fuel Power Costs	$599.40	$466.50	$346.80
Public Service Company Of New Hampshire [Member]
Fuel [Line Items]
Fuel Power Costs	$113.40	$104.80	$103.40

Allowance_for_Funds_Used_Durin1

Allowance for Funds Used During Construction (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	$5.80	$4.10	$5.30
Public Utilities Allowance For Funds Used During Construction Additions	13.7	7.1	6.8
Public Utilities Allowance For Funds Used During Construction Capitalized Total	19.5	11.2	12.1
Public Utilities Allowance For Funds Used During Construction Rate	3.40%	2.70%	3.70%
The Connecticut Light And Power Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	1.9	2.2	2.5
Public Utilities Allowance For Funds Used During Construction Additions	2.9	2.9	1.9
Public Utilities Allowance For Funds Used During Construction Capitalized Total	4.8	5.1	4.4
Public Utilities Allowance For Funds Used During Construction Rate	3.40%	3.70%	3.60%
NSTAR Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	2	0.5	0.3
Public Utilities Allowance For Funds Used During Construction Additions	3.8
Public Utilities Allowance For Funds Used During Construction Capitalized Total	5.8	0.5	0.3
Public Utilities Allowance For Funds Used During Construction Rate	2.50%	0.50%	0.40%
Public Service Company Of New Hampshire [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.6	0.5	1.6
Public Utilities Allowance For Funds Used During Construction Additions	0.6	0.2	1.9
Public Utilities Allowance For Funds Used During Construction Capitalized Total	1.2	0.7	3.5
Public Utilities Allowance For Funds Used During Construction Rate	1.80%	1.10%	5.90%
Western Massachusetts Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.9	0.5	0.5
Public Utilities Allowance For Funds Used During Construction Additions	1.7	1	1
Public Utilities Allowance For Funds Used During Construction Capitalized Total	$2.60	$1.50	$1.50
Public Utilities Allowance For Funds Used During Construction Rate	5.60%	6.10%	6.80%

Other_Income_Net_Details

Other Income, Net (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	$13.70	$7.10	$6.80
The Connecticut Light And Power Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	2.9	2.9	1.9
Public Service Company Of New Hampshire [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	0.6	0.2	1.9
Western Massachusetts Electric Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	$1.70	$1	$1

Other_Taxes_Details

Other Taxes (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Excise And Sales Taxes	$148.20	$144.10	$135
The Connecticut Light And Power Company [Member]
Excise And Sales Taxes	$127.90	$128.20	$120.70

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Capital Expenditures Incurred But Not Yet Paid	$181,900,000	$193,100,000	$160,600,000
Interest Paid, Net of Amounts Capitalized	349,600,000	343,300,000	356,500,000
Income Taxes Paid	334,200,000	50,000,000	-12,800,000
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	63,500,000	51,400,000	42,800,000
Interest Paid, Net of Amounts Capitalized	144,100,000	131,600,000	129,400,000
Income Taxes Paid	135,400,000	55,000,000	-42,000,000
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	34,600,000	57,000,000	50,000,000
Interest Paid, Net of Amounts Capitalized	75,300,000	75,800,000	94,600,000
Income Taxes Paid	217,100,000	163,400,000	88,100,000
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	39,300,000	34,900,000	16,800,000
Interest Paid, Net of Amounts Capitalized	41,100,000	43,300,000	49,800,000
Income Taxes Paid	2,300,000	-30,100,000	14,700,000
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	14,200,000	19,500,000	30,000,000
Interest Paid, Net of Amounts Capitalized	25,900,000	25,800,000	25,800,000
Income Taxes Paid	$25,100,000	($69,000,000)	($8,400,000)

Severance_Benefits_Details

Severance Benefits (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Notes To Consolidated Financial Statements [Abstract]
Description Of Postemployment Benefits	For the years ended December 31, 2014 and 2013, NU recorded severance benefit expenses of $15 million and $9.7 million, respectively, in connection with the partial outsourcing of information technology functions and facilities closures, as well as ongoing post-merger integration. As of December 31, 2014 and 2013, the severance accrual totaled $10.4 million and $14.7 million, respectively, and was included in Other Current Liabilities on the balance sheets.
Postemployment Benefits Period Expense	$15	$9.70
Postemployment Benefits Liability Current	$10.40	$14.70

Related_Parties_Details

Related Parties (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
The Connecticut Light And Power Company [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco	$25	$25
Public Service Company Of New Hampshire [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco	3.8	3.8
Western Massachusetts Electric Company [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco	$5.50	$5.50

REGULATORY_ACCOUNTING_Details

REGULATORY ACCOUNTING (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Regulatory Asset [Line Items]
Regulatory Assets, Total	$4,726,600,000	$4,294,500,000
Regulatory Assets Current	672,493,000	535,791,000
Regulatory Assets Long Term	4,054,086,000	3,758,694,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	60,500,000	65,100,000
Description of Regulatory Assets and Liabilities	Regulatory Assets - The following provides further information about regulatory assets: Benefit Costs: NU's Pension, SERP and PBOP Plans are accounted for in accordance with accounting guidance on defined benefit pension and other PBOP plans. The liability recorded by the Regulated companies to recognize the funded status of their retiree benefit plans are offset by regulatory assets in lieu of a charge to Accumulated Other Comprehensive Income/(Loss), reflecting ultimate recovery from customers through rates. All amounts are remeasured annually. Regulatory accounting was also applied to the portions of NU's service company costs that support the Regulated companies, as these amounts are also recoverable. As these regulatory assets do not represent a cash outlay for the Regulated companies, no carrying charge is recovered from customers. The increase in the funded status liability of the retiree benefit plans and the corresponding regulatory assets was primarily driven by a change in mortality assumptions, which increased the estimate of benefits to be provided to plan participants, and a decrease in the discount rate assumption. For further information on the funded status liability and related regulatory assets of the Pension, SERP and PBOP plans, see Note 9A, "Employee Benefits – Pension Benefits and Postretirement Benefits Other Than Pensions." CL&P, NSTAR Electric, PSNH and WMECO recover benefit costs related to their distribution and transmission operations from customers in rates as allowed by their applicable regulatory commissions. NSTAR Electric and WMECO each recover their qualified pension and PBOP expenses related to distribution operations through rate reconciling mechanisms that fully track the change in net pension and PBOP expenses each year. Derivative Liabilities: Regulatory assets recorded as an offset to derivative liabilities relate to the fair value of contracts used to purchase energy and energy-related products that will be recovered from customers in future rates. See Note 4, "Derivative Instruments," to the financial statements for further information.  These assets are excluded from rate base and are being recovered as the actual settlements occur over the duration of the contracts. Income Taxes, Net: The tax effect of temporary book-tax differences (differences between the periods in which transactions affect income in the financial statements and the periods in which they affect the determination of taxable income, including those differences relating to uncertain tax positions) is accounted for in accordance with the rate-making treatment of the applicable regulatory commissions and accounting guidance for income taxes. Differences in income taxes between the accounting guidance and the rate-making treatment of the applicable regulatory commissions are recorded as regulatory assets. As these assets are offset by deferred income tax liabilities, no carrying charge is collected. For further information regarding income taxes, see Note 10, "Income Taxes," to the financial statements. Storm Restoration Costs: The storm restoration cost deferrals relate to costs incurred at CL&P, NSTAR Electric, PSNH and WMECO that each company expects to recover from customers. A storm must meet certain criteria to be declared a major storm with the criteria specific to each state jurisdiction and utility company. Once a storm is declared major, all qualifying expenses incurred during storm restoration efforts are deferred and recovered from customers. In addition to storm restoration costs, CL&P and PSNH are each allowed to recover storm pre-staging costs in accordance with applicable regulation. CL&P, NSTAR Electric, PSNH and WMECO experienced several significant storm events, including Tropical Storm Irene in 2011, the October 2011 snowstorm, Storm Sandy in 2012 and the February 2013 blizzard. As a result of these storm events, each company suffered extensive damage to its distribution and transmission systems resulting in customer outages. Each company incurred significant costs to repair damage and restore customers' service. In addition, on November 26, 2014, a snowstorm caused damage to the electric delivery systems of PSNH and WMECO. This snowstorm resulted in estimated deferred storm restoration costs of approximately $23 million at PSNH and approximately $3 million at WMECO. The storm restoration cost regulatory asset balance at CL&P, NSTAR Electric, PSNH and WMECO reflects deferrable costs incurred for major storm events. Management believes the storm restoration costs were prudent and meet the criteria for specific cost recovery in Connecticut, Massachusetts and New Hampshire, and that recovery from customers is probable through the applicable regulatory recovery process. Storm Filings: Each electric utility has sought, or is seeking, recovery of its deferred storm restoration costs through its applicable regulatory recovery process. CL&P: As of December 31, 2014, all CL&P deferred storm costs have been reviewed and approved for recovery in distribution rates. On March 12, 2014, the PURA approved recovery of $365 million of deferred storm restoration costs (with carrying charges) associated with five major storms that occurred in 2011 and 2012 and ordered CL&P to capitalize approximately $18 million of the deferred storm restoration costs as utility plant, which will be recovered through depreciation expense in future rate proceedings. CL&P will recover the $365 million in its distribution rates over a six-year period that commenced on December 1, 2014. The remaining costs were either disallowed or are probable of recovery from other sources. These costs did not have a material impact on CL&P’s financial position, results of operations or cash flows. Effective June 1, 2014, CL&P received $65.4 million of DOE Phase II Damages proceeds. On June 17, 2014, the PURA ordered CL&P to refund these proceeds to customers by offsetting the deferred storm restoration costs regulatory asset. For further information on the DOE Phase II Damages proceeds received from the Yankee Companies, see Note 11C, "Commitments and Contingencies - Contractual Obligations - Yankee Companies," to the financial statements. On December 17, 2014, as part of the distribution rate case decision, CL&P was also allowed recovery of the 2013 storm costs and residual 2012 Storm Sandy costs over a seven-year period that commenced on December 1, 2014. NSTAR Electric: On December 30, 2013, the DPU approved recovery of $34.2 million of NSTAR Electric’s storm restoration costs, plus carrying costs, related to Tropical Storm Irene in 2011 and the October 2011 snowstorm. NSTAR Electric is recovering these costs, plus carrying costs, in its distribution rates over a five-year period that commenced on January 1, 2014. PSNH: On June 27, 2013, the NHPUC approved an increase to PSNH’s distribution rates effective July 1, 2013, which included a $5 million increase to the level of funding for the major storm cost reserve. The major storm cost reserve is used to offset the storm restoration cost regulatory asset. On April 7, 2014, PSNH received an audit report from the NHPUC approving storm costs from 2011 through March 2013. WMECO: On December 20, 2013, the DPU approved WMECO's 2013 Annual Storm Reserve Recovery Cost Adjustment filing to begin recovering the October 2011 snowstorm and 2012 Storm Sandy restoration costs, which commenced on January 1, 2014, subject to further review and reconciliation. On December 5, 2014, the DPU approved the majority of deferred storm costs through 2011. Goodwill-related: The goodwill regulatory asset originated from the transaction that created NSTAR in 1999. This regulatory asset is currently being amortized and recovered from customers in rates without a carrying charge over a 40-year period (as of December 31, 2014, there were 25 years of amortization remaining). Regulatory Tracker Mechanisms: The Regulated companies’ approved rates are designed to recover their incurred costs to provide service to customers. The Regulated companies recover certain of their costs on a fully-reconciling basis through regulatory commission-approved tracking mechanisms. The difference between the costs incurred (or the rate recovery allowed) and the actual revenues is recorded as regulatory assets (for undercollections) or regulatory liabilities (for overcollections) to be included in future customer rates each year. Carrying charges are recorded on all material regulatory tracker mechanisms. CL&P, NSTAR Electric, PSNH and WMECO each recover the costs associated with the procurement of energy, transmission related costs from FERC-approved transmission tariffs, energy efficiency programs, low income assistance programs, and restructuring and stranded costs as a result of deregulation, on a fully reconciling basis. Energy procurement costs at PSNH include the costs related to its generating stations. WMECO's and CL&P's (effective December 1, 2014) distribution revenue is decoupled from their customer sales volume. CL&P and WMECO reconcile their annual base distribution rate recovery to a pre-established level of baseline distribution delivery service revenue. Any difference between the allowed level of distribution revenue and the actual amount incurred during a 12-month period is adjusted through rates in the following period. Contractual Obligations - Yankee Companies: CL&P, NSTAR Electric, PSNH and WMECO are responsible for their proportionate share of the remaining costs of the CYAPC, YAEC and MYAPC nuclear facilities, including nuclear fuel storage. A portion of these amounts was recorded as a regulatory asset. Amounts for CL&P are earning a return and are being recovered through the CTA. Amounts for NSTAR Electric and WMECO are being recovered without a return through the transition charge. Amounts for PSNH were fully recovered in 2006. As a result of NU's consolidation of CYAPC and YAEC, NU's regulatory asset balance also includes the regulatory assets of CYAPC and YAEC, which totaled $97.8 million and $129.8 million as of December 31, 2014 and 2013, respectively. Intercompany transactions between CL&P, NSTAR Electric, PSNH and WMECO and the CYAPC and YAEC companies have been eliminated in consolidation of the NU financial statements. Buy Out Agreements for Power Contracts: NSTAR Electric's balance represents the contract termination liability related to certain purchase power contract buy out agreements that were executed in 2004. The contracts’ termination payments occur through September 2016 and are collected from customers through NSTAR Electric’s transition charge over the same period. NSTAR Electric does not earn a return on this regulatory asset. PSNH's balance represents payments associated with the termination of various power purchase contracts that were recorded as regulatory assets and are amortized over the remaining life of the contracts. Other Regulatory Assets: Other Regulatory Assets primarily include asset retirement obligations, environmental remediation costs, losses associated with the reacquisition or redemption of long-term debt and various other items, partially offset by purchase price adjustments recorded as Regulatory Assets in connection with the merger with NSTAR. Regulatory Liabilities: The components of regulatory liabilities are as follows: Cost of Removal: NU's Regulated companies currently recover amounts in rates for future costs of removal of plant assets over the lives of the assets. The estimated cost to remove utility assets from service is recognized as a component of depreciation expense and the cumulative amounts collected from customers but not yet expended is recognized as a regulatory liability. Expended costs that exceed amounts collected from customers are recognized as regulatory assets, as they are probable of recovery in future rates. AFUDC - Transmission: AFUDC was recorded by CL&P and WMECO for their NEEWS projects through May 31, 2011, all of which was reserved as a regulatory liability to reflect rate base recovery for 100 percent of the CWIP as a result of FERC-approved transmission incentives. Effective June 1, 2011, FERC approved changes to the ISO-NE Tariff in order to include 100 percent of the NEEWS CWIP in regional rate base. As a result, CL&P and WMECO no longer record AFUDC on NEEWS CWIP. NSTAR Electric recorded AFUDC on reliability-related projects over $5 million through December 31, 2014, 50 percent of which was recorded as a regulatory liability to reflect rate base recovery for 50 percent of the CWIP as a result of FERC-approved transmission incentives. Other Regulatory Liabilities: Other Regulatory Liabilities primarily includes amounts that are subject to various rate reconciling mechanisms that, as of each period end date, would result in refunds to customers. 2014 Regulatory Developments: As a result of actions taken by the FERC and other developments in the pending base ROE complaint proceedings described in Note 11E, "Commitments and Contingencies – FERC Base ROE Complaints," in 2014 the Company recorded reserves at its electric subsidiaries to recognize the potential financial impact of the first and second complaints. As of December 31, 2014, the cumulative pre-tax reserves (excluding interest), which exclude refunds for the first complaint refund period, totaled $60.7 million at NU, $33.5 million at CL&P, $13.6 million at NSTAR Electric, $5.1 million at PSNH and $8.5 million at WMECO. As of December 31, 2013, as a result of the 2013 FERC ALJ initial decision, the Company had an aggregate pre-tax reserve (excluding interest) of $23.7 million at NU, $12.8 million at CL&P, $5.7 million at NSTAR Electric, $2.3 million at PSNH and $2.9 million at WMECO. These reserves were recorded as a regulatory liability in Regulatory Tracker Mechanisms and as a reduction of Operating Revenues. Effective June 1, 2014, as a result of damages awarded to the Yankee Companies for spent nuclear fuel lawsuits against the DOE described in Note 11C, "Commitments and Contingencies - Contractual Obligations - Yankee Companies," the Yankee Companies returned the DOE Phase II Damages proceeds to the member companies, including CL&P, NSTAR Electric, PSNH, and WMECO, for the benefit of their respective customers. CL&P's refund obligation to customers of $65.4 million was recorded as an offset to the deferred storm restoration costs regulatory asset, as directed by PURA. NSTAR Electric's, PSNH's and WMECO's refund obligation to customers of $29.1 million, $13.1 million and $18.1 million, respectively, was recorded as a regulatory liability in Regulatory Tracker Mechanisms. Refunds to customers for these DOE proceeds began in 2014. On December 31, 2014, NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General filed a comprehensive settlement agreement with the DPU. The comprehensive settlement agreement included resolution of the outstanding NSTAR Electric CPSL program filings for the periods 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the NSTAR Electric energy efficiency program filings regarding LBR for the periods 2008 through 2011. If approved by the DPU, NSTAR Electric and NSTAR Gas will be required to refund a total of $44.7 million to their respective customers, which was included in Regulatory Tracker Mechanisms and Other Regulatory Liabilities as of December 31, 2014. For further information, see Note 11F, "Commitments and Contingencies – 2014 Comprehensive Settlement Agreement."
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	750,100,000	707,300,000
Regulatory Liability Current	235,022,000	204,278,000
Regulatory Liabilities Long-Term	515,144,000	502,984,000
Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	439,900,000	435,100,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	192,300,000	151,200,000
AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	67,100,000	68,100,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	50,800,000	52,900,000
The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,695,800,000	1,814,000,000
Regulatory Assets Current	220,344,000	150,943,000
Regulatory Assets Long Term	1,475,508,000	1,663,147,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	1,300,000	7,300,000
Public Utilities Amount Of Allowance For Earnings On Equity Capitalized For Rate Making Purposes	1,700,000	1,900,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	206,000,000	187,800,000
Regulatory Liability Current	124,722,000	93,961,000
Regulatory Liabilities Long-Term	81,298,000	93,757,000
The Connecticut Light And Power Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	19,700,000	29,100,000
The Connecticut Light And Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	122,600,000	95,600,000
The Connecticut Light And Power Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	53,600,000	54,700,000
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	10,100,000	8,400,000
NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	1,377,800,000	1,439,300,000
Regulatory Assets Current	198,710,000	204,144,000
Regulatory Assets Long Term	1,179,100,000	1,235,156,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	33,200,000	33,400,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	312,300,000	307,100,000
Regulatory Liability Current	49,611,000	53,958,000
Regulatory Liabilities Long-Term	262,738,000	253,108,000
NSTAR Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	258,300,000	250,000,000
NSTAR Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	20,700,000	21,900,000
NSTAR Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	4,400,000	4,100,000
NSTAR Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	28,900,000	31,100,000
Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	404,800,000	311,500,000
Regulatory Assets Current	111,705,000	92,194,000
Regulatory Assets Long Term	293,115,000	219,346,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	900,000
Public Utilities Amount Of Allowance For Earnings On Equity Capitalized For Rate Making Purposes	43,300,000	33,100,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	67,400,000	72,300,000
Regulatory Liability Current	16,044,000	20,643,000
Regulatory Liabilities Long-Term	51,372,000	51,723,000
Public Service Company Of New Hampshire [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	50,300,000	49,700,000
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	14,200,000	21,600,000
Public Service Company Of New Hampshire [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	0	0
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	2,900,000	1,000,000
Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	198,200,000	189,100,000
Regulatory Assets Current	51,923,000	43,024,000
Regulatory Assets Long Term	146,307,000	146,088,000
Regulatory Assets And Liabilities Other Disclosures Abstract
Amount of Regulatory Costs Not yet Approved	11,000,000	10,100,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	33,300,000	33,800,000
Regulatory Liability Current	22,486,000	19,858,000
Regulatory Liabilities Long-Term	10,835,000	13,873,000
Western Massachusetts Electric Company [Member] | Cost Of Removal [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	1,100,000	0
Western Massachusetts Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	22,300,000	21,100,000
Western Massachusetts Electric Company [Member] | AFUDC Transmission Incentive [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	9,100,000	9,300,000
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities, Total	800,000	3,400,000
CYAPC and YAEC [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	97,800,000	129,800,000
Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	2,016,000,000	1,240,200,000
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	445,400,000	297,700,000
Benefit Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	515,900,000	496,700,000
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	174,300,000	100,600,000
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	85,000,000	57,300,000
Regulatory Assets Offsetting Derivative Liabilities [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	425,500,000	638,000,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	410,900,000	630,400,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	4,500,000	7,700,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Regulatory Assets Offsetting Derivative Liabilities [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	0	0
Deferred Income Tax Charges Member
Regulatory Asset [Line Items]
Regulatory Assets, Total	635,300,000	626,200,000
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	437,700,000	415,500,000
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	83,700,000	84,000,000
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	38,000,000	40,300,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	35,500,000	43,700,000
Storm Restoration Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	502,800,000	589,600,000
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	319,600,000	397,800,000
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	103,700,000	109,300,000
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	47,700,000	43,700,000
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	31,800,000	38,800,000
Goodwill Regulatory Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	505,400,000	525,900,000
Goodwill Regulatory Asset [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	433,900,000	451,500,000
Regulatory Tracker Deferrals Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	350,500,000	323,400,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	16,100,000	8,000,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	141,400,000	169,500,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	103,500,000	83,300,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	33,000,000	32,600,000
Unrecovered Contractual Obligations [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	123,800,000	154,200,000
Power Contracts Buy Out Agreements [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	42,600,000	70,200,000
Power Contracts Buy Out Agreements [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	38,600,000	64,700,000
Power Contracts Buy Out Agreements [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	4,000,000	5,500,000
Other Regulatory Assets [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	124,700,000	126,800,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	66,100,000	64,600,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	56,100,000	55,900,000
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	37,300,000	38,100,000
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]
Regulatory Asset [Line Items]
Regulatory Assets, Total	$12,900,000	$16,700,000

PROPERTY_PLANT_AND_ACCUMULATED2

PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	$12,495,200,000	$11,950,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,595,400,000	2,425,900,000
Public Utilities Property Plant And Equipment Transmission	6,930,700,000	6,412,500,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,170,900,000	1,152,300,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	23,192,200,000	21,940,900,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	551,300,000	508,700,000
Public Utilities Property Plant And Equipment Plant In Service	23,743,500,000	22,449,600,000
Electric And Natural Gas Utility Accumulated Depreciation	-5,777,800,000	-5,387,000,000
Other Accumulated Depreciation	-231,800,000	-196,200,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-6,009,600,000	-5,583,200,000
Property Plant And Equipment Net Excluding Construction Work In Progress	17,733,900,000	16,866,400,000
Public Utilities Property Plant And Equipment Construction Work In Progress	913,100,000	709,800,000
Property, Plant and Equipment, Net	18,647,041,000	17,576,186,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.00%	2.80%	2.50%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	34.9
Transmission Assets Average Useful Life	42.5
Generation Assets Average Useful Life	31.9
Other Assets Average Useful Life	14.2
The Connecticut Light And Power Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	5,158,800,000	4,930,700,000
Public Utilities Property Plant And Equipment Transmission	3,274,000,000	3,071,900,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	8,432,800,000	8,002,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-1,928,000,000	-1,804,100,000
Property Plant And Equipment Net Excluding Construction Work In Progress	6,504,800,000	6,198,500,000
Public Utilities Property Plant And Equipment Construction Work In Progress	304,900,000	252,800,000
Property, Plant and Equipment, Net	6,809,664,000	6,451,259,000
Public Utilities Property Plant and Equipment Depreciation Rates	2.70%	2.50%	2.50%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	37.5
Transmission Assets Average Useful Life	39.8
Generation Assets Average Useful Life	0
Other Assets Average Useful Life	0
NSTAR Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	4,895,500,000	4,694,700,000
Public Utilities Property Plant And Equipment Transmission	1,928,500,000	1,772,300,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,824,000,000	6,467,000,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-1,761,400,000	-1,631,300,000
Property Plant And Equipment Net Excluding Construction Work In Progress	5,062,600,000	4,835,700,000
Public Utilities Property Plant And Equipment Construction Work In Progress	272,800,000	208,200,000
Property, Plant and Equipment, Net	5,335,436,000	5,043,887,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.00%	2.90%	2.80%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	32.3
Transmission Assets Average Useful Life	44
Generation Assets Average Useful Life	0
Other Assets Average Useful Life	0
Public Service Company Of New Hampshire [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	1,696,700,000	1,608,200,000
Public Utilities Property Plant And Equipment Transmission	789,700,000	695,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,136,500,000	1,131,200,000
Public Utilities Property Plant And Equipment Plant In Service	3,622,900,000	3,435,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-1,090,000,000	-1,021,800,000
Property Plant And Equipment Net Excluding Construction Work In Progress	2,532,900,000	2,413,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	102,900,000	54,300,000
Property, Plant and Equipment, Net	2,635,844,000	2,467,556,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.00%	3.00%	3.00%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	32.3
Transmission Assets Average Useful Life	43.7
Generation Assets Average Useful Life	32.1
Other Assets Average Useful Life	0
Western Massachusetts Electric Company [Member]
Public Utility Property Plant And Equipment [Line Items]
Public Utilities Property Plant And Equipment Distribution	784,200,000	756,600,000
Public Utilities Property Plant And Equipment Transmission	891,000,000	826,400,000
Public Utilities Property Plant And Equipment Generation Or Processing	34,400,000	21,100,000
Public Utilities Property Plant And Equipment Plant In Service	1,709,600,000	1,604,100,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-297,400,000	-271,500,000
Property Plant And Equipment Net Excluding Construction Work In Progress	1,412,200,000	1,332,600,000
Public Utilities Property Plant And Equipment Construction Work In Progress	49,100,000	48,500,000
Property, Plant and Equipment, Net	$1,461,321,000	$1,381,060,000
Public Utilities Property Plant and Equipment Depreciation Rates	3.30%	2.90%	3.30%
Useful Lives [Abstract]
Distribution Assets Average Useful Life	30.9
Transmission Assets Average Useful Life	49.9
Generation Assets Average Useful Life	25
Other Assets Average Useful Life	0

DERIVATIVE_INSTURMENTS_Details

DERIVATIVE INSTURMENTS (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instrument Detail Abstract
Derivative Liabilities, Noncurrent	($409,632,000)	($624,050,000)
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract
Description of Derivative Activity Volume	Commodity Supply and Price Risk Management: As required by regulation, CL&P, along with UI, has capacity-related contracts with generation facilities. CL&P has a sharing agreement with UI, with 80 percent of each contract allocated to CL&P and 20 percent allocated to UI. The combined capacity of these contracts is 787 MW. The capacity contracts extend through 2026 and obligate both CL&P and UI to make or receive payments on a monthly basis to or from the generation facilities based on the difference between a set capacity price and the capacity market price received in the ISO-NE capacity markets. In addition, CL&P has a contract to purchase 0.1 million MWh of energy per year through 2020. NSTAR Electric has a renewable energy contract to purchase 0.1 million MWh of energy per year through 2018 and a capacity-related contract to purchase up to 35 MW per year through 2019. As of December 31, 2014 and 2013, NU had NYMEX future contracts in order to reduce variability associated with the purchase price of approximately 8.8 million and 9.1 million MMBtu of natural gas, respectively.
Derivative Credit Risk Related Contingent Features Abstract
Derivative Net Liability Position Aggregate Fair Value	-10,000,000
Additional Collateral Aggregate Fair Value	10,000,000
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsExitPricePremium	Exit price premiums of 7 percent through 24 percent are also applied on these contracts and reflect the most recent market activity available for similar type contracts.
The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Liabilities - Current	-88,459,000	-92,233,000
Derivative Liabilities, Noncurrent	-406,199,000	-617,072,000
Derivative Assets Subject to Credit Risk with investment grade counterparty	64,000,000
Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current		1,600,000
Derivative Liabilities - Current	-9,800,000
Derivative Liabilities, Noncurrent	-300,000
Derivative Assets Noncurrent	0	200,000
Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	9,600,000	8,600,000
Derivative Liabilities - Current		-93,700,000
Derivative Liabilities, Noncurrent	-409,300,000	-624,100,000
Derivative Assets Noncurrent	74,300,000	74,000,000
Consolidated Current Derivative Liabilities	90,000,000
Fair Value Inputs Level 3 [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	5.3	5.07
FairValueInputsEnergyPrices		49
FairValueInputsRenewableEnergyCreditPrices	38	36
FairValueInputsForwardReserve	5.8	3.3
Fair Value Inputs Level 3 [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	12.98	11.82
FairValueInputsEnergyPrices		77
FairValueInputsRenewableEnergyCreditPrices	56	87
FairValueInputsForwardReserve	9.5	3.3
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	9,500,000	7,300,000
Derivative Liabilities - Current	-88,500,000	-92,200,000
Derivative Liabilities, Noncurrent	-406,200,000	-617,100,000
Derivative Assets Noncurrent	74,300,000	71,400,000
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.08	5.07
FairValueInputsEnergyPrices		56
FairValueInputsForwardReserve	5.8	3.3
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	12.98	10.42
FairValueInputsEnergyPrices		58
FairValueInputsForwardReserve	9.5	3.3
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	100,000	1,200,000
Derivative Liabilities - Current	-1,500,000	-1,500,000
Derivative Liabilities, Noncurrent	-3,100,000	-7,000,000
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MinimumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	5.3	5.07
FairValueInputsRenewableEnergyCreditPrices	38	36
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member] | MaximumMember
FairValueInputsQuantitativeInformation[Abstract]
FairValueInputsCapacityPrices	11.1	7.38
FairValueInputsRenewableEnergyCreditPrices	56	70
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current		-300,000
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	-6,600,000	-9,800,000
Derivative Assets Noncurrent	-19,200,000	-42,200,000
Amount Offset Against Derivatives [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	-6,600,000	-9,800,000
Derivative Liabilities, Noncurrent	0
Derivative Assets Noncurrent	-19,200,000	-42,200,000
Commodity [Member]
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract
Derivative Gain Loss On Derivative Net	0	1,000,000	-700,000
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	134,400,000	160,600,000	-29,000,000
Commodity [Member] | Fair Value Inputs Level 2 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current		1,900,000
Derivative Liabilities - Current	-9,800,000
Derivative Liabilities, Noncurrent	-300,000
Derivative Assets Noncurrent	0	200,000
Commodity [Member] | Fair Value Inputs Level 3 [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,200,000	18,400,000
Derivative Liabilities - Current	-90,000,000	-93,700,000
Derivative Liabilities, Noncurrent	-409,300,000	-624,100,000
Derivative Assets Noncurrent	93,500,000	116,200,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	16,100,000	17,100,000
Derivative Liabilities - Current	-88,500,000	-92,200,000
Derivative Liabilities, Noncurrent	-406,200,000	-617,100,000
Derivative Assets Noncurrent	93,500,000	113,600,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]
Derivative Instrument Detail Abstract
Derivative Assets - Current	100,000	1,200,000
Derivative Liabilities - Current	-1,500,000	-1,500,000
Derivative Liabilities, Noncurrent	($3,100,000)	($7,000,000)

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	($635.20)	($878.60)
Net Realized/Unrealized Gains Losses included in Net Income		10.9
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	141.3	158.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	78.5	74.2
Ending Balance	-415.4	-635.2
The Connecticut Light And Power Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	-630.6	-866.2
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	139.7	148.9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	80	86.7
Ending Balance	-410.9	-630.6
NSTAR Electric Company [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	-7.3	-14.9
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	4.3	3.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	-1.5	4.1
Ending Balance	($4.50)	($7.30)

MARKETABLE_SECURITIES_Details

MARKETABLE SECURITIES (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Fair Value Of Securities Under Fair Value Option	$85.10	$57.20
Fair Value, Option, Changes in Fair Value, Gain (Loss)	1.9	10.2	5.9
Investments, Debt and Equity Securities [Abstract]
Marketable Securities, Restricted	450.8	424
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	67.7
Available For Sale Securities Debt Maturities After Ten Years Fair Value	108.4
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	89.1
Available For Sale Securities Debt Maturities Within One Year Fair Value	55
Available For Sale Securities Debt Maturities Fair Value	320.2
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	88.8
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	55
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	103.2
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	66
Available For Sale Securities Debt Maturities Amortized Cost	313
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	639.2	580.9
Western Massachusetts Electric Company [Member]
Available For Sale Securities Debt Maturities Fair Value Abstract
Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value	0.7
Available For Sale Securities Debt Maturities After Ten Years Fair Value	2.9
Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value	25.8
Available For Sale Securities Debt Maturities Within One Year Fair Value	28.7
Available For Sale Securities Debt Maturities Fair Value	58.1
Available For Sale Securities Debt Maturities Amortized Cost Abstract
Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost	25.8
Available For Sale Securities Debt Maturities Within One Year Amortized Cost	28.7
Available For Sale Securities Debt Maturities After Ten Years Amortized Cost	3
Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost	0.7
Available For Sale Securities Debt Maturities Amortized Cost	58.2
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	58.1	57.9
Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	343.9	314.2
Fair Value Inputs Level 1 Member | Western Massachusetts Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	4.3	10.9
Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	295.3	266.7
Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	53.8	47
US Government Debt Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	51.3	61.4
US Government Debt Securities Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	0	6.8
Corporate Bond Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	49.1	53.6
Corporate Bond Securities Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	14.7	15.1
Asset Backed Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	54.1	30.4
Asset Backed Securities Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	14.5	9
Municipal Bonds Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	116.3	105.5
Municipal Bonds Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	13	11.2
Other Debt Securities Member | Fair Value Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	24.5	15.8
Other Debt Securities Member | Fair Value Inputs Level 2 [Member] | Western Massachusetts Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	11.6	4.9
Cash And Cash Equivalents Member | Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	24.9	32.9
Cash And Cash Equivalents Member | Fair Value Inputs Level 1 Member | Western Massachusetts Electric Company [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	4.3	10.9
Mutual Funds [Member] | Fair Value Inputs Level 1 Member
Fair Value Assets And Liabilities Measured On Recurring Basis Abstract
Investments Fair Value Disclosure	319	281.3
Debt Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	320.2	299.6
Available-for-sale Securities, Gross Unrealized Gains	7.5	2.5
AvailableForSaleSecuritiesGrossUnrealizedLoss	0.3	2.1
Available For Sale Securities Amortized Cost	313	299.2
Debt Securities | Western Massachusetts Electric Company [Member]
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	58.1	57.9
Available-for-sale Securities, Gross Unrealized Gains	0	0
AvailableForSaleSecuritiesGrossUnrealizedLoss	0.1	0
Available For Sale Securities Amortized Cost	58.2	57.9
Equity Securities
Investments, Debt and Equity Securities [Abstract]
Available For Sale Securities Fair Value Disclosure	233.9	224.1
Available-for-sale Securities, Gross Unrealized Gains	73.3	60.5
AvailableForSaleSecuritiesGrossUnrealizedLoss	0	0
Available For Sale Securities Amortized Cost	$160.60	$163.60

ASSET_RETIREMENT_OBLIGATIONS_D

ASSET RETIREMENT OBLIGATIONS (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	$424.90	$412.20
Asset Retirement Obligation, Liabilities Incurred	1.3	0.1
Asset Retirement Obligation, Liabilities Settled	-19.5	-13.8
Asset Retirement Obligation, Accretion Expense	25.1	23.8
Asset Retirement Obligation, Revision of Estimate	-5.5	2.6
Asset Retirement Obligation, Ending Balance	426.3	424.9
The Connecticut Light And Power Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	35	33.6
Asset Retirement Obligation, Liabilities Settled	-1.1	-0.7
Asset Retirement Obligation, Accretion Expense	1.9	2.2
Asset Retirement Obligation, Revision of Estimate	-0.5	-0.1
Asset Retirement Obligation, Ending Balance	35.3	35
NSTAR Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	32.8	31.4
Asset Retirement Obligation, Liabilities Settled	0	-0.1
Asset Retirement Obligation, Accretion Expense	1.5	1.5
Asset Retirement Obligation, Revision of Estimate		0
Asset Retirement Obligation, Ending Balance	34.3	32.8
Public Service Company Of New Hampshire [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	19.5	18.4
Asset Retirement Obligation, Liabilities Incurred		0
Asset Retirement Obligation, Accretion Expense	1.1	1.2
Asset Retirement Obligation, Revision of Estimate	0
Asset Retirement Obligation, Ending Balance	20.6	19.5
Western Massachusetts Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Beginning Balance	4.5	4.3
Asset Retirement Obligation, Liabilities Incurred	1.1
Asset Retirement Obligation, Accretion Expense	0.3	0.3
Asset Retirement Obligation, Revision of Estimate	0
Asset Retirement Obligation, Ending Balance	5.9	4.5
CYAPC and YAEC [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Ending Balance	$317.30	$318.80

SHORT_TERM_DEBT_Details

SHORT TERM DEBT (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Line of Credit Facility, Description	Credit Agreements and Commercial Paper Programs: NU parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas are parties to a five-year $1.45 billion revolving credit facility. The revolving credit facility is to be used primarily to backstop NU parent's $1.45 billion commercial paper program. The commercial paper program allows NU parent to issue commercial paper as a form of short-term debt. Effective July 23, 2014, NU parent, CL&P, PSNH, WMECO, NSTAR Gas and Yankee Gas extended the expiration date of their joint revolving credit facility for one additional year to September 6, 2019. CL&P has a borrowing sublimit of $600 million and PSNH and WMECO each have borrowing sublimits of $300 million. As of December 31, 2014 and 2013, NU had approximately $1.1 billion and $1.01 billion, respectively, in short-term borrowings outstanding under the NU parent commercial paper program, leaving $348.9 million and $435.5 million of available borrowing capacity as of December 31, 2014 and 2013, respectively. The weighted-average interest rate on these borrowings as of December 31, 2014 and 2013 was 0.43 percent and 0.24 percent, respectively, which is generally based on A2/P2 rated commercial paper. As of December 31, 2014, there were intercompany loans from NU of $133.4 million to CL&P, $90.5 million to PSNH and $21.4 million to WMECO. As of December 31, 2013, there were intercompany loans from NU of $287.3 million to CL&P and $86.5 million to PSNH. NSTAR Electric has a five-year $450 million revolving credit facility. This facility serves to backstop NSTAR Electric’s existing $450 million commercial paper program. Effective July 23, 2014, NSTAR Electric extended the expiration date of its revolving credit facility for one additional year to September 6, 2019. As of December 31, 2014 and 2013, NSTAR Electric had $302 million and $103.5 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $148 million and $346.5 million of available borrowing capacity as of December 31, 2014 and 2013, respectively. The weighted-average interest rate on these borrowings as of December 31, 2014 and 2013 was 0.27 percent and 0.13 percent, respectively, which is generally based on A2/P1 rated commercial paper.
Short Term Debt Regulatory Limits	The amount of short-term borrowings that may be incurred by CL&P, NSTAR Electric and WMECO is subject to periodic approval by the FERC. As a result of the NHPUC having jurisdiction over PSNH's short-term debt, PSNH is not currently required to obtain FERC approval for its short-term borrowings. On July 31, 2013, the FERC granted authorization to allow CL&P and WMECO to incur total short-term borrowings up to a maximum of $600 million and $300 million, respectively, effective January 1, 2014 through December 31, 2015. On June 11, 2014, the FERC granted authorization to allow NSTAR Electric to issue total short-term debt securities in an aggregate principal amount not to exceed $655 million outstanding at any one time, effective October 24, 2014 through October 23, 2016. CL&P's certificate of incorporation contains preferred stock provisions restricting the amount of unsecured debt that CL&P may incur, including limiting unsecured indebtedness with a maturity of less than 10 years to 10 percent of total capitalization. As of December 31, 2014, CL&P had $432.1 million of unsecured debt capacity available under this authorization.
Description of ST debt reclassified to LT	On January 15, 2015, NU parent issued $150 million of 1.60 percent Series G Senior Notes due to mature in 2018 and $300 million of 3.15 percent Series H Senior Notes, due to mature in 2025. The proceeds, net of issuance costs, were used to repay short-term borrowings outstanding under the NU commercial paper program. As the debt issuances refinanced short-term debt, the short-term debt was classified as Long-Term Debt as of December 31, 2014.	On January 2, 2014, Yankee Gas issued $100 million of Series L First Mortgage Bonds and $25 million of the proceeds was used to repay short-term borrowings outstanding under the NU commercial paper program. As the debt issuance refinanced short-term debt, these amounts were classified as Long-Term Debt on NU's balance sheet as of December 31, 2013.
Notes Payable	$956,825,000	$1,093,000,000
The Connecticut Light And Power Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	133,400,000	287,300,000
NSTAR Electric Company [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	148,000,000	346,500,000
CommercialPaper	302,000,000	103,500,000
WeightedAverageDiscountRatePercent	0.27%	0.13%
Notes Payable	302,000,000	103,500,000
Public Service Company Of New Hampshire [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	90,500,000	86,500,000
Western Massachusetts Electric Company [Member]
Line of Credit Facility [Line Items]
Notes Payable To Affiliated Companies	21,400,000	0
Nu Parent [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	348,900,000	435,500,000
CommercialPaper	1,100,000,000	1,010,000,000
WeightedAverageDiscountRatePercent	0.43%	0.24%
Notes Payable	$654,825,000	$1,014,500,000

LONGTERM_DEBT_Details

LONG-TERM DEBT (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	$245,583,000	$533,346,000
Long-Term Debt	8,606,017,000	7,776,833,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	216,700,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	200,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	745,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	810,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	800,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	4,956,600,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	7,728,300,000
Long-term Debt, Description	On September 15, 2014, CL&P repaid at maturity the $150 million of 4.80 percent 2004 Series A First Mortgage Bonds, using short-term borrowings. On April 24, 2014, CL&P issued $250 million of 4.30 percent 2014 Series A First Mortgage Bonds, due to mature in 2044. The proceeds, net of issuance costs, were used to repay short-term borrowings. On February 12, 2015, CL&P notified the trustee that it intends to purchase and cancel the bonds on April 1, 2015, after they have been tendered by the bondholders. On March 7, 2014, NSTAR Electric issued $300 million of 4.40 percent debentures, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $300 million of 4.875 percent debentures that matured on April 15, 2014. As of December 31, 2014 and 2013, the interest rate was 0.4721 percent and 0.478 percent, respectively. On July 15, 2014, PSNH repaid at maturity the $50 million of 5.25 percent Series L First Mortgage Bonds using short-term borrowings. On October 14, 2014, PSNH issued $75 million of first mortgage bonds at a yield of 3.144 percent due to mature in 2023. The first mortgage bonds are part of the same series of PSNH’s existing 3.50 percent Series S First Mortgage Bonds that were initially issued in November 2013. The proceeds, net of issuance costs, were used to repay short-term borrowings. On January 2, 2014, Yankee Gas issued $100 million of 4.82 percent Series L First Mortgage Bonds, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $75 million 4.80 percent Series G First Mortgage Bonds that matured on January 1, 2014 and to pay $25 million in short-term borrowings. As the debt issuance refinanced short-term debt, these amounts were classified as Long-Term Debt on NU's balance sheet as of December 31, 2013. On January 15, 2015, NU parent issued $150 million of 1.60 percent Series G Senior Notes due to mature in 2018 and $300 million of 3.15 percent Series H Notes, due to mature in 2025. The proceeds, net of issuance costs, were used to repay short-term borrowings outstanding under the NU commercial paper program. As the debt issuances refinanced short-term debt, the short-term debt was classified as Long-Term Debt as of December 31, 2014. Amount relates to the purchase price adjustment required to record the NSTAR long-term debt at fair value on the date of the merger. On July 15, 2014, PSNH repaid at maturity the $50 million of 5.25 percent Series L First Mortgage Bonds using short-term borrowings. On October 14, 2014, PSNH issued $75 million of first mortgage bonds at a yield of 3.144 percent due to mature in 2023. The first mortgage bonds are part of the same series of PSNH’s existing 3.50 percent Series S First Mortgage Bonds that were initially issued in November 2013. The proceeds, net of issuance costs, were used to repay short-term borrowings. The DOE is responsible for the selection and development of repositories for, and the disposal of, spent nuclear fuel and high-level radioactive waste. For nuclear fuel used to generate electricity prior to April 7, 1983 (Prior Period Spent Nuclear Fuel) for CL&P and WMECO, an accrual has been recorded for the full liability, and payment must be made by CL&P and WMECO to the DOE prior to the first delivery of spent fuel to the DOE. After the sale of Millstone, CL&P and WMECO remained responsible for their share of the disposal costs associated with the Prior Period Spent Nuclear Fuel. Until such payment to the DOE is made, the outstanding liability will continue to accrue interest at the 3-month Treasury bill yield rate. In addition, as a result of consolidating CYAPC, NU has consolidated $179.4 million in additional spent nuclear fuel obligations, including interest, as of both December 31, 2014 and 2013. The obligation due to the DOE for the disposal of CL&P's and WMECO's Prior Period Spent Nuclear Fuel and CYAPC's spent nuclear fuel obligation include accumulated interest costs of $350.4 million and $350.3 million ($178 million and $177.9 million for CL&P and $41.8 million and $41.7 million for WMECO) as of December 31, 2014 and 2013, respectively.
The Connecticut Light And Power Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	162,000,000	150,000,000
Long-Term Debt	2,679,951,000	2,591,208,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	162,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	0
Long-term Debt, Maturities, Repayments of Principal in Year Three	250,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	300,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	250,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,640,300,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	2,602,300,000
Long-term Debt, Description	On September 15, 2014, CL&P repaid at maturity the $150 million of 4.80 percent 2004 Series A First Mortgage Bonds, using short-term borrowings. On April 24, 2014, CL&P issued $250 million of 4.30 percent 2014 Series A First Mortgage Bonds, due to mature in 2044. The proceeds, net of issuance costs, were used to repay short-term borrowings. On February 12, 2015, CL&P notified the trustee that it intends to purchase and cancel the bonds on April 1, 2015, after they have been tendered by the bondholders. CL&P’s obligation to repay each series of PCRBs is secured by first mortgage bonds. Each such series of first mortgage bonds contains similar terms and provisions as the applicable series of PCRBs. If CL&P fails to meet its obligations under the first mortgage bonds, then the holder of the first mortgage bonds (the issuer of the PCRBs) would have rights under the first mortgage bonds. CL&P’s $62 million tax-exempt PCRBs, which are subject to mandatory tender for purchase on April 1, 2015 and carry a coupon rate of 1.55 percent during the current three-year fixed rate period, cannot be redeemed prior to its tender date. CL&P’s $120.5 million tax-exempt PCRBs will be subject to redemption at par on or after September 1, 2021. All other long-term debt securities are subject to make-whole provisions.
The Connecticut Light And Power Company [Member] | Series D 1994 Due 2024 7.875 % [Member]
Debt Instrument [Line Items]
Long-Term Debt	139,800,000	139,800,000
The Connecticut Light And Power Company [Member] | Series A 2004 Due 2014 4.800% [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	150,000,000
The Connecticut Light And Power Company [Member] | Series B 2004 Due 2034 5.750% [Member]
Debt Instrument [Line Items]
Long-Term Debt	130,000,000	130,000,000
The Connecticut Light And Power Company [Member] | Series A 2005 Due 2015 5.000% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
The Connecticut Light And Power Company [Member] | Series B 2005 Due 2035 5.625% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
The Connecticut Light And Power Company [Member] | Series A 2006 Due 2036 6.350% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000	250,000,000
The Connecticut Light And Power Company [Member] | Series A 2007 Due 2017 5.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000,000	150,000,000
The Connecticut Light And Power Company [Member] | Series B 2007 Due 2037 5.750% [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000,000	150,000,000
The Connecticut Light And Power Company [Member] | Series D 2007 Due 2037 6.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
The Connecticut Light And Power Company [Member] | Series A 2008 Due 2018 5.650% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000,000	300,000,000
The Connecticut Light And Power Company [Member] | Series A 2009 Due 2019 5.500% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000	250,000,000
The Connecticut Light And Power Company [Member] | One-Year Fixed Rate Tax Exempt Due 2031 [Member]
Debt Instrument [Line Items]
Long-Term Debt	62,000,000	62,000,000
The Connecticut Light And Power Company [Member] | Fees And Interest Due For Spent Nuclear Fuel Disposal Costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	244,500,000	244,400,000
The Connecticut Light And Power Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	2,419,800,000	2,319,800,000
The Connecticut Light And Power Company [Member] | Total Pollution Control Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	182,500,000	182,500,000
The Connecticut Light And Power Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	-4,800,000	-5,500,000
The Connecticut Light And Power Company [Member] | Serires C 2007 5.750% Due 2017 [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
The Connecticut Light And Power Company [Member] | Fixed Rate 4.375% due 2028 [Member]
Debt Instrument [Line Items]
Long-Term Debt	120,500,000	120,500,000
The Connecticut Light And Power Company [Member] | Series A Due 2023 2.5 percent [Member]
Debt Instrument [Line Items]
Long-Term Debt	400,000,000	400,000,000
The Connecticut Light And Power Company [Member] | Series A 2014 Due 2044 4.30% [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000
NSTAR Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	4,700,000	301,650,000
Long-Term Debt	1,792,712,000	1,499,417,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	4,700,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	200,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Three	400,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	0
Long-term Debt, Maturities, Repayments of Principal in Year Five	0
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,200,000,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	1,804,700,000
Long-term Debt, Description	On March 7, 2014, NSTAR Electric issued $300 million of 4.40 percent debentures, due to mature in 2044. The proceeds, net of issuance costs, were used to repay the $300 million of 4.875 percent debentures that matured on April 15, 2014. As of December 31, 2014 and 2013, the interest rate was 0.4721 percent and 0.478 percent, respectively.
NSTAR Electric Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	-7,300,000	-5,300,000
NSTAR Electric Company [Member] | Total Other [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,800,000,000	1,800,000,000
NSTAR Electric Company [Member] | Due 2014 4.875% [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	300,000,000
NSTAR Electric Company [Member] | Due 2022 2.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	400,000,000	400,000,000
NSTAR Electric Company [Member] | Due 2017 5.625% [Member]
Debt Instrument [Line Items]
Long-Term Debt	400,000,000	400,000,000
NSTAR Electric Company [Member] | Due 2036 5.75% [Member]
Debt Instrument [Line Items]
Long-Term Debt	200,000,000	200,000,000
NSTAR Electric Company [Member] | Due 2040 5.5% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000,000	300,000,000
NSTAR Electric Company [Member] | TaxExempt Sewage Facility Revenue Bonds Due 2015 [Member]
Debt Instrument [Line Items]
Long-Term Debt	4,700,000	6,400,000
NSTAR Electric Company [Member] | Variable Due 2016 [Member]
Debt Instrument [Line Items]
Long-Term Debt	200,000,000	200,000,000
NSTAR Electric Company [Member] | Series Due 2044 2.375% [Member]
Debt Instrument [Line Items]
Long-Term Debt	300,000,000
Public Service Company Of New Hampshire [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	0	50,000,000
Long-Term Debt	1,076,286,000	999,006,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	0
Long-term Debt, Maturities, Repayments of Principal in Year Two	0
Long-term Debt, Maturities, Repayments of Principal in Year Three	70,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Four	110,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	150,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	746,300,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	1,076,300,000
Long-term Debt, Description	On July 15, 2014, PSNH repaid at maturity the $50 million of 5.25 percent Series L First Mortgage Bonds using short-term borrowings. On October 14, 2014, PSNH issued $75 million of first mortgage bonds at a yield of 3.144 percent due to mature in 2023. The first mortgage bonds are part of the same series of PSNH’s existing 3.50 percent Series S First Mortgage Bonds that were initially issued in November 2013. The proceeds, net of issuance costs, were used to repay short-term borrowings. PSNH's obligation to repay the PCRBs is secured by first mortgage bonds and bond insurance. The first mortgage bonds contain similar terms and provisions as the PCRBs. If PSNH fails to meet its obligations under the first mortgage bonds, then the holder of the first mortgage bonds (the issuer of the PCRBs) would have rights under the first mortgage bonds. The PSNH Series A tax-exempt PCRBs are currently callable at 100 percent of par. The PCRBs bear interest at a rate that is periodically set pursuant to auctions. PSNH is not obligated to purchase these PCRBs, which mature in 2021, from the remarketing agent. As of December 31, 2014 and 2013, the interest rate was 0.175 percent and 0.088 percent, respectively.
Public Service Company Of New Hampshire [Member] | Series L 2004 Due 2014 5.25% [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	50,000,000
Public Service Company Of New Hampshire [Member] | Series M 2005 Due 2035 5.60% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Public Service Company Of New Hampshire [Member] | Series N 2007 Due 2017 6.15% [Member]
Debt Instrument [Line Items]
Long-Term Debt	70,000,000	70,000,000
Public Service Company Of New Hampshire [Member] | Series O 2008 Due 2018 6.00% [Member]
Debt Instrument [Line Items]
Long-Term Debt	110,000,000	110,000,000
Public Service Company Of New Hampshire [Member] | Series P 2009 Due 2019 4.50% [Member]
Debt Instrument [Line Items]
Long-Term Debt	150,000,000	150,000,000
Public Service Company Of New Hampshire [Member] | Adjustablerateseriesadue 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	89,300,000	89,300,000
Public Service Company Of New Hampshire [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	987,000,000	962,000,000
Public Service Company Of New Hampshire [Member] | Total Pollution Control Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	89,300,000	89,300,000
Public Service Company Of New Hampshire [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	-2,300,000
Public Service Company Of New Hampshire [Member] | Series Q 4.05% 2011 Due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	122,000,000	122,000,000
Public Service Company Of New Hampshire [Member] | Series R 2011 3.30% due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	160,000,000	160,000,000
Public Service Company Of New Hampshire [Member] | Series2013due2023member [Member]
Debt Instrument [Line Items]
Long-Term Debt	325,000,000	250,000,000
Western Massachusetts Electric Company [Member]
Debt Instrument [Line Items]
Long-Term Debt - Current Portion	50,000,000	0
Long-Term Debt	578,471,000	629,389,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	50,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Two	0
Long-term Debt, Maturities, Repayments of Principal in Year Three	0
Long-term Debt, Maturities, Repayments of Principal in Year Four	0
Long-term Debt, Maturities, Repayments of Principal in Year Five	0
Long-term Debt, Maturities, Repayments of Principal after Year Five	515,000,000
LongTermDebtMaturitiesRepaymentsOfPrincipalTotal	565,000,000
Western Massachusetts Electric Company [Member] | Senior Notes Series B 5.90% Due 2034 [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Western Massachusetts Electric Company [Member] | Senior Notes Series C 5.24% Due 2015 [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Western Massachusetts Electric Company [Member] | Senior Notes Series D 6.70% Due 2037 [Member]
Debt Instrument [Line Items]
Long-Term Debt	40,000,000	40,000,000
Western Massachusetts Electric Company [Member] | Senior Notes Series E 5.10% Due 2020 [Member]
Debt Instrument [Line Items]
Long-Term Debt	95,000,000	95,000,000
Western Massachusetts Electric Company [Member] | Fees And Interest Due For Spent Nuclear Fuel Disposal Costs [Member]
Debt Instrument [Line Items]
Long-Term Debt	57,400,000	57,300,000
Western Massachusetts Electric Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	6,100,000	7,100,000
Western Massachusetts Electric Company [Member] | Senior Notes Series F 3.50% due 2021 [Member]
Debt Instrument [Line Items]
Long-Term Debt	250,000,000	250,000,000
Western Massachusetts Electric Company [Member] | Total Pollution Control Notes and Other Notes [Member]
Debt Instrument [Line Items]
Long-Term Debt	565,000,000	565,000,000
Western Massachusetts Electric Company [Member] | Series g 2013 due 2023 member [Member]
Debt Instrument [Line Items]
Long-Term Debt	80,000,000	80,000,000
Yankee Gas Services Company [Member]
Debt Instrument [Line Items]
Long-Term Debt	370,600,000	345,700,000
Yankee Gas Services Company [Member] | Total First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Long-Term Debt	370,000,000	345,000,000
Yankee Gas Services Company [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	600,000	700,000
Yankee Gas Services Company [Member] | Series B due 2022 8.48% [Member]
Debt Instrument [Line Items]
Long-Term Debt	20,000,000	20,000,000
Yankee Gas Services Company [Member] | Series G due 2014 4.80% [Member]
Debt Instrument [Line Items]
Long-Term Debt	0	75,000,000
Yankee Gas Services Company [Member] | Series H due 2019 5.26% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Yankee Gas Services Company [Member] | Series I due 2035 5.35% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Yankee Gas Services Company [Member] | Series J due 2018 6.90% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000	100,000,000
Yankee Gas Services Company [Member] | Series K due 2020 4.87% [Member]
Debt Instrument [Line Items]
Long-Term Debt	50,000,000	50,000,000
Yankee Gas Services Company [Member] | Debentures 2014 Due 2044 4.40% [Member]
Debt Instrument [Line Items]
Long-Term Debt	100,000,000
NSTAR Gas Company [Member]
Debt Instrument [Line Items]
Long-Term Debt	210,000,000	210,000,000
NSTAR Gas Company [Member] | Series J Due 202 9.95% [Member]
Debt Instrument [Line Items]
Long-Term Debt	25,000,000	25,000,000
NSTAR Gas Company [Member] | Series K Due 2033 7.11% [Member]
Debt Instrument [Line Items]
Long-Term Debt	35,000,000	35,000,000
NSTAR Gas Company [Member] | Series M Due 2017 7.04% [Member]
Debt Instrument [Line Items]
Long-Term Debt	25,000,000	25,000,000
NSTAR Gas Company [Member] | Series N Due 2020 4.46% [Member]
Debt Instrument [Line Items]
Long-Term Debt	125,000,000	125,000,000
Subsidiaries [Member] | Unamortized Premiums and Discounts, Net [Member]
Debt Instrument [Line Items]
Long-Term Debt	-1,200,000	-1,300,000
Subsidiaries [Member] | Fair Value Adjustment [Member]
Debt Instrument [Line Items]
Long-Term Debt	202,300,000	230,700,000
Subsidiaries [Member] | Total Other [Member]
Debt Instrument [Line Items]
Long-Term Debt	1,725,700,000	1,304,400,000
Subsidiaries [Member] | Commerical Paper and Revolver Borrowings [Member]
Debt Instrument [Line Items]
Long-Term Debt	446,300,000	25,000,000
Subsidiaries [Member] | Debentures Due 2019 4.50% [Member]
Debt Instrument [Line Items]
Long-Term Debt	350,000,000	350,000,000
Subsidiaries [Member] | Spent Nuclear Fuel Obilgation CY [Member]
Debt Instrument [Line Items]
Long-Term Debt	179,400,000	179,400,000
Subsidiaries [Member] | FairValueAdjustmentMemberCurrentPortion [Member]
Debt Instrument [Line Items]
Long-Term Debt	28,900,000	31,700,000
Subsidiaries [Member] | Series F Due 2023 2.8% [Member]
Debt Instrument [Line Items]
Long-Term Debt	450,000,000	450,000,000
Subsidiaries [Member] | Series E Due 2018 1.45% [Member]
Debt Instrument [Line Items]
Long-Term Debt	$300,000,000	$300,000,000

PENSION_BENEFITS_AND_POSTRETIR1

PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	$1,638,558,000	$896,844,000
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year, Description	NU expects to make contributions of approximately $155 million in 2015, of which $5 million will be contributed by NSTAR Electric and $1 million will be contributed by PSNH. The remaining $149 million is expected to be contributed by other NU subsidiaries, primarily NUSCO. NU contributed $40 million to the PBOP Plans in 2014 and expects to make approximately $27 million in contributions in 2015. This amount will be funded into the 401(h) account and VEBAs up to the maximum tax-deductible level permitted
Description of allocation of assets to subsidiaries	Effective January 1, 2013, the NSTAR Pension Plan assets were transferred into the NUSCO Pension Plan master trust. The NUSCO Pension Plan is entitled to approximately 66 percent of each asset category in the master trust, the NSTAR Pension Plan is entitled to approximately 30 percent of each asset category in the master trust and the 401(h) plans are entitled to approximately four percent of each asset category in the master trust. For the years ended December 31, 2014 and 2013 the NUSCO Pension Plan was entitled to $2,803.6 million and $2,750.4 million respectively and the NSTAR Pension Plan was entitled to $1,322.9 million and $1,235.3 million, respectively. Also effective January 1, 2013, the NSTAR PBOP Plan assets were transferred into a master trust with the NUSCO PBOP Plan assets and assets were allocated to each plan. For the years ended December 31, 2014 and 2013, the NUSCO PBOP Plan was entitled to $399 million and $391 million, respectively, and the NSTAR PBOP Plan was entitled to $463.6 million and $435.5 million, respectively. CL&P, PSNH and WMECO are allocated a portion of the NUSCO Pension and PBOP Plan assets. NSTAR Electric is entitled to a portion of the NSTAR Pension and PBOP Plan assets.
Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	-4,676,500,000	-5,022,800,000
Defined Benefit Plan Service Cost	-79,900,000	-102,300,000	-84,300,000
Defined Benefit Plan Interest Cost	-225,700,000	-206,700,000	-198,300,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	-739,600,000	433,600,000
Defined Benefit Plan, Benefits Paid	230,300,000	216,600,000
Defined Benefit Plan, Payments SERP	5,200,000	5,100,000
Benefit Obligation Ending Balance	-5,486,200,000	-4,676,500,000	-5,022,800,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	79,900,000	102,300,000	84,300,000
Defined Benefit Plan Interest Cost	225,700,000	206,700,000	198,300,000
Defined Benefit Plan, Expected Return on Plan Assets	-310,800,000	-278,100,000	-220,900,000
Defined Benefit Plan, Amortization of Gains (Losses)	128,400,000	210,500,000	172,400,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	4,400,000	4,000,000	7,900,000
Defined Benefit Plan Net Periodic Benefit Cost	127,600,000	245,400,000	242,000,000
Amount Capitalized Defined Benefit Expense	35,200,000	73,200,000	70,600,000
us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlementsAndCurtailments			2,200,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	3,985,900,000	3,411,300,000
Defined Benefit Plan, Actual Return on Plan Assets	199,300,000	506,500,000
Defined Benefit Plan, Contributions by Employer	171,600,000	284,700,000
Defined Benefit Plan, Benefits Paid	-230,300,000	-216,600,000
Fair Value of Plan Assets Ending Balance	4,126,500,000	3,985,900,000	3,411,300,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-1,359,700,000	-690,600,000
Defined Benefit Plan, Accumulated Benefit Obligation	5,000,100,000	4,538,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	3,985,900,000	3,411,300,000
Fair Value of Plan Assets Ending Balance	4,126,500,000	3,985,900,000	3,411,300,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	55,900,000	-28,900,000
OCI Reclassification Adjustment for Net Actuarial Gain (Loss), before Tax	-5,600,000	-9,400,000
OCI Reclassification Adjustment for Amortization of Prior Service Cost, before tax	-200,000	-200,000
Gain Loss Arising During Year Regulatory	797,300,000	-635,200,000
Losses Reclassifed From RegAsset PreTax	-122,800,000	-201,200,000
AmortizationPriorServiceCostfromRegAsset	-4,200,000	-3,800,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Gains Losses included in AOCI, Pre Tax	93,500,000	43,200,000
Prior Service Cost balance in AOCI	800,000	1,000,000
RegulatoryAsset Gains Losses Before Tax	1,811,900,000	1,137,400,000
Regulatory Asset Prior Service Cost	13,200,000	17,400,000
Defined Benefit Amounts to be Reclassified from AOCI in next fiscal year
Future Amortization Of Gain Loss.	6,400,000
Future Amortization of Prior Service cost	200,000
Regulatory Asset Amortization Of Net Gains Losses	149,100,000
Regulatory Asset Defined Benefit Plan Amortization Of Net Prior ServiceCost	3,500,000
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	8.25%
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	244,500,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	253,600,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	268,900,000
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	273,400,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	285,400,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	1,591,100,000
Impact of Decrease in Discount Rate	In addition, the decreases in the discount rates resulted in an increase on NU's funded status liability of approximately $530 million. Partially offsetting these increases are the impact of other actuarial assumptions
Pension Plans Defined Benefit Member | MinimumMember
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.20%	5.03%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	3.50%	3.50%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.85%	4.13%	4.52%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	8.25%	8.25%	7.30%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	3.50%	3.50%	3.50%
Pension Plans Defined Benefit Member | MaximumMember
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.90%	5.03%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	3.50%	4.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate		4.24%	5.03%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase		4.00%	4.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets		0.00%	8.25%
Pension Plans Defined Benefit Member | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	443,700,000	434,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	443,700,000	434,800,000
Pension Plans Defined Benefit Member | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	1,748,400,000	1,865,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	1,748,400,000	1,865,400,000
Pension Plans Defined Benefit Member | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	1,850,700,000	1,560,000,000
Fair Value of Plan Assets Ending Balance	2,122,900,000	1,850,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,850,700,000	1,560,000,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	76,800,000	137,600,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	20,800,000	31,100,000
Defined Benefit Plan, Purchases, Sales, and Settlements	174,600,000	122,000,000
Fair Value of Plan Assets Ending Balance	2,122,900,000	1,850,700,000
Pension Plans Defined Benefit Member | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	386,700,000	396,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	386,700,000	396,700,000
Pension Plans Defined Benefit Member | Private Equity Funds [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	18,800,000	96,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	18,800,000	96,400,000
Pension Plans Defined Benefit Member | Private Equity Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	300,300,000	267,900,000
Fair Value of Plan Assets Ending Balance	367,900,000	300,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	300,300,000	267,900,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	14,000,000	15,400,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	13,900,000	13,700,000
Defined Benefit Plan, Purchases, Sales, and Settlements	39,700,000	3,300,000
Fair Value of Plan Assets Ending Balance	367,900,000	300,300,000
Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	1,293,600,000	1,206,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	1,293,600,000	1,206,200,000
Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	10,200,000	11,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	10,200,000	11,600,000
Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	561,400,000	605,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	561,400,000	605,100,000
Pension Plans Defined Benefit Member | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	589,500,000	315,100,000
Fair Value of Plan Assets Ending Balance	722,000,000	589,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	589,500,000	315,100,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	45,200,000	55,300,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	-6,200,000	-1,000,000
Defined Benefit Plan, Purchases, Sales, and Settlements	93,500,000	187,600,000
Fair Value of Plan Assets Ending Balance	722,000,000	589,500,000
TransferBetweenCategories		32,500,000
Pension Plans Defined Benefit Member | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	495,000,000	416,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	495,000,000	416,900,000
Pension Plans Defined Benefit Member | Hedge Funds [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	20,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	20,000,000
Pension Plans Defined Benefit Member | Hedge Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	416,900,000	418,900,000
Fair Value of Plan Assets Ending Balance	475,000,000	416,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	416,900,000	418,900,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	23,500,000	33,400,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	-15,200,000	0
Defined Benefit Plan, Purchases, Sales, and Settlements	49,800,000	-2,900,000
Fair Value of Plan Assets Ending Balance	475,000,000	416,900,000
TransferBetweenCategories		-32,500,000
Pension Plans Defined Benefit Member | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	397,800,000	376,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	397,800,000	376,700,000
Pension Plans Defined Benefit Member | Real Estate [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	0	0
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	0	0
Pension Plans Defined Benefit Member | Real Estate [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	132,000,000	88,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	132,000,000	88,200,000
Pension Plans Defined Benefit Member | Real Estate [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	288,500,000	235,400,000
Fair Value of Plan Assets Ending Balance	265,800,000	288,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	288,500,000	235,400,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	-3,600,000	12,900,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	28,300,000	6,200,000
Defined Benefit Plan, Purchases, Sales, and Settlements	-47,400,000	34,000,000
Fair Value of Plan Assets Ending Balance	265,800,000	288,500,000
Pension Plans Defined Benefit Member | Assetsrelatedto401h [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	188,500,000	165,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	188,500,000	165,000,000
Pension Plans Defined Benefit Member | Equity Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	1,741,900,000	1,754,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	1,741,900,000	1,754,400,000
Pension Plans Defined Benefit Member | Equity Member | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	414,700,000	326,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	414,700,000	326,800,000
Pension Plans Defined Benefit Member | Equity Member | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	1,035,000,000	1,172,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	1,035,000,000	1,172,100,000
Pension Plans Defined Benefit Member | Equity Member | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	255,500,000	322,700,000
Fair Value of Plan Assets Ending Balance	292,200,000	255,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	255,500,000	322,700,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	-2,300,000	20,600,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	0	12,200,000
Defined Benefit Plan, Purchases, Sales, and Settlements	39,000,000	-100,000,000
Fair Value of Plan Assets Ending Balance	292,200,000	255,500,000
PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	-1,038,000,000	-1,233,300,000
Defined Benefit Plan Service Cost	-12,500,000	-16,900,000	-15,700,000
Defined Benefit Plan Interest Cost	-49,500,000	-47,200,000	-49,000,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	-95,500,000	200,900,000
Defined Benefit Plan, Benefits Paid	47,600,000	58,500,000
Benefit Obligation Ending Balance	-1,147,900,000	-1,038,000,000	-1,233,300,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	12,500,000	16,900,000	15,700,000
Defined Benefit Plan Interest Cost	49,500,000	47,200,000	49,000,000
Defined Benefit Plan, Expected Return on Plan Assets	-63,300,000	-55,400,000	-39,200,000
Defined Benefit Plan, Amortization of Gains (Losses)	12,200,000	26,000,000	36,000,000
Defined Benefit Plan Amortization Of Transition Obligations Assets			12,200,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	-2,800,000	-2,100,000	-1,400,000
Defined Benefit Plan Net Periodic Benefit Cost	8,100,000	32,600,000	72,300,000
Amount Capitalized Defined Benefit Expense	1,400,000	8,800,000	26,600,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	826,500,000	709,100,000
Defined Benefit Plan, Actual Return on Plan Assets	43,700,000	118,300,000
Defined Benefit Plan, Contributions by Employer	40,000,000	57,600,000
Defined Benefit Plan, Benefits Paid	-47,600,000	-58,500,000
Fair Value of Plan Assets Ending Balance	862,600,000	826,500,000	709,100,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-285,300,000	-211,500,000
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Description of Direction and Pattern of Change for Assumed Health Care Cost Trend Rate	As of December 31, 2014 and 2013, the health care cost trend rate assumption used to determine the PBOP Plans’ funded status was 6.5 percent and 7 percent, respectively, subsequently decreasing to an ultimate rate of 4.5 percent in 2023.
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	111,200,000
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	-88,400,000
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components	5,300,000
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	-4,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	826,500,000	709,100,000
Fair Value of Plan Assets Ending Balance	862,600,000	826,500,000	709,100,000
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	400,000	-1,900,000
OCI Reclassification Adjustment for Net Actuarial Gain (Loss), before Tax	-600,000	-1,100,000
Gain Loss Arising During Year Regulatory	115,100,000	-262,000,000
Losses Reclassifed From RegAsset PreTax	-11,600,000	-24,900,000
AmortizationPriorServiceCostfromRegAsset	2,800,000	2,100,000
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Gains Losses included in AOCI, Pre Tax	6,000,000	6,200,000
RegulatoryAsset Gains Losses Before Tax	192,700,000	89,200,000
Regulatory Asset Prior Service Cost	-1,800,000	-4,600,000
Defined Benefit Amounts to be Reclassified from AOCI in next fiscal year
Future Amortization Of Gain Loss.	300,000
Regulatory Asset Amortization Of Net Gains Losses	6,900,000
Regulatory Asset Defined Benefit Plan Amortization Of Net Prior ServiceCost	-500,000
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	58,700,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	59,700,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	60,600,000
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	61,300,000
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	62,000,000
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	318,800,000
Impact of change in mortality assumption	During 2014, the Society of Actuaries released a series of updated mortality tables resulting from recent studies that measured mortality rates for various groups of individuals. The updated mortality tables released in 2014 increased life expectancy of plan participants by 3 to 5 years and have the effect of increasing the estimate of benefits to be provided to plan participants. The impact of this adoption on NU's funded status liability for the year ended December 31, 2014 was an increase of approximately $82 million.
Impact of Decrease in Discount Rate	In addition, the decreases in the discount rates resulted in an increase on NU's funded status liability of approximately $110 million. Partially offsetting these increases are the impact of other actuarial assumptions.
PBOP Plan | MinimumMember
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	4.22%	4.78%
Health Care Trend Assumption Used In Funded Status	6.50%	7.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.78%	4.04%	4.58%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	8.25%	7.30%
PBOP Plan | MaximumMember
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		5.10%
Health Care Trend Assumption Used In Funded Status		0.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	5.10%	4.35%	4.84%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	8.25%	8.25%	8.25%
PBOP Plan | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	120,200,000	121,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	120,200,000	121,400,000
PBOP Plan | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	302,200,000	310,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	302,200,000	310,700,000
PBOP Plan | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	229,400,000	146,000,000
Fair Value of Plan Assets Ending Balance	251,700,000	229,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	229,400,000	146,000,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	7,800,000	35,700,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	-2,100,000	100,000
Defined Benefit Plan, Purchases, Sales, and Settlements	16,600,000	47,600,000
Fair Value of Plan Assets Ending Balance	251,700,000	229,400,000
PBOP Plan | Private Equity Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	24,900,000	17,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	24,900,000	17,900,000
PBOP Plan | Private Equity Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	17,900,000	11,300,000
Fair Value of Plan Assets Ending Balance	24,900,000	17,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	17,900,000	11,300,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	1,300,000	1,500,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	100,000	200,000
Defined Benefit Plan, Purchases, Sales, and Settlements	5,600,000	4,900,000
Fair Value of Plan Assets Ending Balance	24,900,000	17,900,000
PBOP Plan | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	204,400,000	171,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	204,400,000	171,200,000
PBOP Plan | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	16,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	16,100,000
PBOP Plan | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	110,000,000	119,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	110,000,000	119,700,000
PBOP Plan | Fixed Income Debt Securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	51,500,000	32,100,000
Fair Value of Plan Assets Ending Balance	78,300,000	51,500,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	51,500,000	32,100,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	1,900,000	4,100,000
Defined Benefit Plan, Purchases, Sales, and Settlements	24,900,000	15,300,000
Fair Value of Plan Assets Ending Balance	78,300,000	51,500,000
PBOP Plan | Hedge Funds [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	58,400,000	57,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	58,400,000	57,000,000
PBOP Plan | Hedge Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	57,000,000	39,600,000
Fair Value of Plan Assets Ending Balance	58,400,000	57,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	57,000,000	39,600,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	1,400,000	5,400,000
Defined Benefit Plan, Purchases, Sales, and Settlements	0	12,000,000
Fair Value of Plan Assets Ending Balance	58,400,000	57,000,000
PBOP Plan | Real Estate [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	34,400,000	48,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	34,400,000	48,100,000
PBOP Plan | Real Estate [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	19,400,000	14,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	19,400,000	14,200,000
PBOP Plan | Real Estate [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	33,900,000	26,700,000
Fair Value of Plan Assets Ending Balance	15,000,000	33,900,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	33,900,000	26,700,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	-2,800,000	3,900,000
Defined Benefit Plan, Actual Return on Plan Assets Sold During Period	-2,200,000	-100,000
Defined Benefit Plan, Purchases, Sales, and Settlements	-13,900,000	3,400,000
Fair Value of Plan Assets Ending Balance	15,000,000	33,900,000
PBOP Plan | Cash And Cash Equivalents Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	0	11,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	0	11,100,000
PBOP Plan | Cash And Cash Equivalents Member | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	0	11,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	0	11,100,000
PBOP Plan | Assetsrelatedto401h [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	188,500,000	165,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	188,500,000	165,000,000
PBOP Plan | Equity Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	352,000,000	356,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	352,000,000	356,200,000
PBOP Plan | Equity Member | Fair Value Inputs Level 1 Member
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	104,100,000	110,300,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	104,100,000	110,300,000
PBOP Plan | Equity Member | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Ending Balance	172,800,000	176,800,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	172,800,000	176,800,000
PBOP Plan | Equity Member | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	69,100,000	36,300,000
Fair Value of Plan Assets Ending Balance	75,100,000	69,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	69,100,000	36,300,000
Defined Benefit Plan, Actual Return on Plan Assets Still Held	6,000,000	20,800,000
Defined Benefit Plan, Purchases, Sales, and Settlements	0	12,000,000
Fair Value of Plan Assets Ending Balance	75,100,000	69,100,000
NUSCOPensionandPBOP [Member] | United States Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	9.00%	9.00%
Target Allocation	24.00%	24.00%
NUSCOPensionandPBOP [Member] | International Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	9.00%	9.00%
Target Allocation	10.00%	10.00%
NUSCOPensionandPBOP [Member] | Emergining Markets Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	10.00%	10.00%
Target Allocation	6.00%	6.00%
NUSCOPensionandPBOP [Member] | Private Equity Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	13.00%	13.00%
Target Allocation	10.00%	10.00%
NUSCOPensionandPBOP [Member] | Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	5.00%	5.00%
Target Allocation	15.00%	15.00%
NUSCOPensionandPBOP [Member] | High Yield Fixed Income Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.50%	7.50%
Target Allocation	9.00%	9.00%
NUSCOPensionandPBOP [Member] | Emerging Markets Debt Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.50%	7.50%
Target Allocation	6.00%	6.00%
NUSCOPensionandPBOP [Member] | Hedge Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.00%	7.00%
Target Allocation	11.00%	11.00%
NUSCOPensionandPBOP [Member] | Real Estate [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Assumed Rate of Return on Defined Benefit Plan Assets	7.50%	7.50%
Target Allocation	9.00%	9.00%
The Connecticut Light And Power Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	273,854,000	95,895,000
The Connecticut Light And Power Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	-1,083,400,000	-1,178,000,000
ChangeInLiabilityDueToServiceCompanyMerger	26,400,000
Defined Benefit Plan Service Cost	-20,200,000	-24,900,000	-21,800,000
Defined Benefit Plan Interest Cost	-50,500,000	-48,300,000	-51,200,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	-161,000,000	110,700,000
Defined Benefit Plan, Benefits Paid	58,300,000	56,600,000
Defined Benefit Plan, Payments SERP	300,000	500,000
Benefit Obligation Ending Balance	-1,230,100,000	-1,083,400,000	-1,178,000,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	20,200,000	24,900,000	21,800,000
Defined Benefit Plan Interest Cost	50,500,000	48,300,000	51,200,000
Defined Benefit Plan, Expected Return on Plan Assets	-75,400,000	-73,800,000	-70,600,000
Defined Benefit Plan, Amortization of Gains (Losses)	33,700,000	55,900,000	49,600,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1,800,000	1,800,000	3,600,000
Defined Benefit Plan Net Periodic Benefit Cost	30,800,000	57,100,000	55,600,000
Related Intercompany Allocationsof Defined Benefit Expense	26,700,000	44,900,000	42,800,000
Amount Capitalized Defined Benefit Expense	17,600,000	28,000,000	26,800,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	1,016,300,000	937,600,000
Defined Benefit Plan, Actual Return on Plan Assets	49,200,000	135,300,000
Defined Benefit Plan, Contributions by Employer		0
Defined Benefit Plan, Benefits Paid	-58,300,000	-56,600,000
Fair Value of Plan Assets Ending Balance	980,800,000	1,016,300,000	937,600,000
ChangeInAssetsDueToServiceCompanyMerger	-26,400,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-249,300,000	-67,100,000
Defined Benefit Plan, Accumulated Benefit Obligation	1,101,400,000	1,058,000,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,016,300,000	937,600,000
Fair Value of Plan Assets Ending Balance	980,800,000	1,016,300,000	937,600,000
The Connecticut Light And Power Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	-180,400,000	-196,800,000
ChangeInLiabilityDueToServiceCompanyMerger	3,700,000
Defined Benefit Plan Service Cost	-2,200,000	-3,400,000	-3,000,000
Defined Benefit Plan Interest Cost	-8,100,000	-7,900,000	-9,200,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	3,500,000	13,300,000
Defined Benefit Plan, Benefits Paid	9,600,000	14,400,000
Benefit Obligation Ending Balance	-173,900,000	-180,400,000	-196,800,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	2,200,000	3,400,000	3,000,000
Defined Benefit Plan Interest Cost	8,100,000	7,900,000	9,200,000
Defined Benefit Plan, Expected Return on Plan Assets	-10,500,000	-10,100,000	-9,100,000
Defined Benefit Plan, Amortization of Gains (Losses)	4,200,000	7,400,000	7,500,000
Defined Benefit Plan Amortization Of Transition Obligations Assets			6,100,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0	0
Defined Benefit Plan Net Periodic Benefit Cost	4,000,000	8,600,000	16,700,000
Related Intercompany Allocationsof Defined Benefit Expense	3,800,000	7,100,000	7,900,000
Amount Capitalized Defined Benefit Expense	1,800,000	3,900,000	8,200,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	151,300,000	132,200,000
Defined Benefit Plan, Actual Return on Plan Assets	6,300,000	24,800,000
Defined Benefit Plan, Contributions by Employer	4,200,000	8,700,000
Defined Benefit Plan, Benefits Paid	-9,600,000	-14,400,000
Fair Value of Plan Assets Ending Balance	149,000,000	151,300,000	132,200,000
ChangeInAssetsDueToServiceCompanyMerger	-3,200,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-24,900,000	-29,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	151,300,000	132,200,000
Fair Value of Plan Assets Ending Balance	149,000,000	151,300,000	132,200,000
NSTAR Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	235,529,000	118,010,000
Defined Benefit Plan, Estimated Future Benefit Payments [Abstract]
Pension Contribution	120,306,000	82,000,000	25,000,000
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	-1,353,300,000	-1,430,000,000
ChangeInLiabilityDueToServiceCompanyMerger	479,900,000
Defined Benefit Plan Service Cost	-13,600,000	-33,100,000	-30,300,000
Defined Benefit Plan Interest Cost	-41,300,000	-58,000,000	-58,900,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	-107,000,000	96,600,000
Defined Benefit Plan, Benefits Paid	52,400,000	71,200,000
Defined Benefit Plan, Payments SERP	300,000
Benefit Obligation Ending Balance	-982,600,000	-1,353,300,000	-1,430,000,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	13,600,000	33,100,000	30,300,000
Defined Benefit Plan Interest Cost	41,300,000	58,000,000	58,900,000
Defined Benefit Plan, Expected Return on Plan Assets	-63,000,000	-84,400,000	-65,600,000
Defined Benefit Plan, Amortization of Gains (Losses)	23,500,000	58,100,000	63,100,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0	-300,000	-600,000
Defined Benefit Plan Net Periodic Benefit Cost	15,400,000	64,500,000	86,100,000
Related Intercompany Allocationsof Defined Benefit Expense	10,400,000	-8,400,000	-12,300,000
Amount Capitalized Defined Benefit Expense	7,900,000	28,900,000	30,700,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	1,235,300,000	1,069,100,000
Defined Benefit Plan, Actual Return on Plan Assets	36,500,000	155,400,000
Defined Benefit Plan, Contributions by Employer	101,000,000	82,000,000
Defined Benefit Plan, Benefits Paid	-52,400,000	-71,200,000
Fair Value of Plan Assets Ending Balance	879,000,000	1,235,300,000	1,069,100,000
ChangeInAssetsDueToServiceCompanyMerger	-441,400,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-103,600,000	-118,000,000
Defined Benefit Plan, Accumulated Benefit Obligation	910,400,000	1,280,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	1,235,300,000	1,069,100,000
Fair Value of Plan Assets Ending Balance	879,000,000	1,235,300,000	1,069,100,000
NSTAR Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
ChangeInLiabilityDueToServiceCompanyMerger	-395,500,000
Defined Benefit Plan Service Cost	-3,100,000
Defined Benefit Plan Interest Cost	-19,400,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	-68,600,000
Defined Benefit Plan, Benefits Paid	17,900,000
Benefit Obligation Ending Balance	-468,700,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,100,000
Defined Benefit Plan Interest Cost	19,400,000
Defined Benefit Plan, Expected Return on Plan Assets	-25,900,000
Defined Benefit Plan, Amortization of Gains (Losses)	-500,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	-1,900,000
Defined Benefit Plan Net Periodic Benefit Cost	-5,800,000	4,600,000	34,100,000
Related Intercompany Allocationsof Defined Benefit Expense	800,000
Amount Capitalized Defined Benefit Expense	-2,300,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Actual Return on Plan Assets	18,400,000
Defined Benefit Plan, Contributions by Employer	19,300,000
Defined Benefit Plan, Benefits Paid	-17,900,000
Fair Value of Plan Assets Ending Balance	336,500,000
ChangeInAssetsDueToServiceCompanyMerger	316,700,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-132,200,000
Level 3 Rollforwards
Fair Value of Plan Assets Ending Balance	336,500,000
NSTAR Electric Company [Member] | Supplemental Employee Retirement Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Net Periodic Benefit Cost		3,200,000	3,600,000
Public Service Company Of New Hampshire [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	93,243,000	15,272,000
Public Service Company Of New Hampshire [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	-529,000,000	-576,000,000
ChangeInLiabilityDueToServiceCompanyMerger	32,200,000
Defined Benefit Plan Service Cost	-9,700,000	-13,100,000	-11,800,000
Defined Benefit Plan Interest Cost	-23,800,000	-23,600,000	-24,400,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	-73,300,000	62,400,000
Defined Benefit Plan, Benefits Paid	22,800,000	21,100,000
Defined Benefit Plan, Payments SERP	100,000	200,000
Benefit Obligation Ending Balance	-580,700,000	-529,000,000	-576,000,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	9,700,000	13,100,000	11,800,000
Defined Benefit Plan Interest Cost	23,800,000	23,600,000	24,400,000
Defined Benefit Plan, Expected Return on Plan Assets	-38,100,000	-35,400,000	-28,200,000
Defined Benefit Plan, Amortization of Gains (Losses)	11,600,000	21,600,000	16,200,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	700,000	700,000	1,500,000
Defined Benefit Plan Net Periodic Benefit Cost	7,700,000	23,600,000	25,700,000
Related Intercompany Allocationsof Defined Benefit Expense	7,600,000	10,500,000	10,100,000
Amount Capitalized Defined Benefit Expense	3,000,000	7,300,000	7,900,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	528,600,000	386,600,000
Defined Benefit Plan, Actual Return on Plan Assets	24,800,000	54,800,000
Defined Benefit Plan, Contributions by Employer	0	108,300,000
Defined Benefit Plan, Benefits Paid	-22,800,000	-21,100,000
Fair Value of Plan Assets Ending Balance	498,400,000	528,600,000	386,600,000
ChangeInAssetsDueToServiceCompanyMerger	-32,200,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-82,300,000	-400,000
Defined Benefit Plan, Accumulated Benefit Obligation	524,500,000	520,100,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	528,600,000	386,600,000
Fair Value of Plan Assets Ending Balance	498,400,000	528,600,000	386,600,000
Public Service Company Of New Hampshire [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	-93,500,000	-100,200,000
ChangeInLiabilityDueToServiceCompanyMerger	4,300,000
Defined Benefit Plan Service Cost	-1,300,000	-2,300,000	-2,000,000
Defined Benefit Plan Interest Cost	-4,300,000	-4,000,000	-4,600,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	-1,100,000	7,200,000
Defined Benefit Plan, Benefits Paid	4,100,000	5,800,000
Benefit Obligation Ending Balance	-91,800,000	-93,500,000	-100,200,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	1,300,000	2,300,000	2,000,000
Defined Benefit Plan Interest Cost	4,300,000	4,000,000	4,600,000
Defined Benefit Plan, Expected Return on Plan Assets	-5,400,000	-5,200,000	-4,600,000
Defined Benefit Plan, Amortization of Gains (Losses)	2,200,000	3,600,000	3,600,000
Defined Benefit Plan Amortization Of Transition Obligations Assets			2,500,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0	0
Defined Benefit Plan Net Periodic Benefit Cost	2,400,000	4,700,000	8,100,000
Related Intercompany Allocationsof Defined Benefit Expense	1,000,000	1,600,000	2,000,000
Amount Capitalized Defined Benefit Expense	800,000	1,300,000	2,300,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	81,800,000	69,500,000
Defined Benefit Plan, Actual Return on Plan Assets	3,800,000	13,400,000
Defined Benefit Plan, Contributions by Employer	2,500,000	4,700,000
Defined Benefit Plan, Benefits Paid	-4,100,000	-5,800,000
Fair Value of Plan Assets Ending Balance	80,900,000	81,800,000	69,500,000
ChangeInAssetsDueToServiceCompanyMerger	-3,100,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-10,900,000	-11,700,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	81,800,000	69,500,000
Fair Value of Plan Assets Ending Balance	80,900,000	81,800,000	69,500,000
Western Massachusetts Electric Company [Member]
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Accrued Pension, PBOP and SERP	17,705,000	3,911,000
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	-223,900,000	-243,100,000
ChangeInLiabilityDueToServiceCompanyMerger	6,200,000
Defined Benefit Plan Service Cost	-3,500,000	-4,700,000	-4,100,000
Defined Benefit Plan Interest Cost	-10,300,000	-10,000,000	-10,500,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	-29,800,000	22,400,000
Defined Benefit Plan, Benefits Paid	11,900,000	11,500,000
Defined Benefit Plan, Payments SERP	0
Benefit Obligation Ending Balance	-249,400,000	-223,900,000	-243,100,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	3,500,000	4,700,000	4,100,000
Defined Benefit Plan Interest Cost	10,300,000	10,000,000	10,500,000
Defined Benefit Plan, Expected Return on Plan Assets	-17,900,000	-17,400,000	-16,400,000
Defined Benefit Plan, Amortization of Gains (Losses)	6,900,000	11,800,000	10,700,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit	400,000	400,000	800,000
Defined Benefit Plan Net Periodic Benefit Cost	3,200,000	9,500,000	9,700,000
Related Intercompany Allocationsof Defined Benefit Expense	5,100,000	8,000,000	8,100,000
Amount Capitalized Defined Benefit Expense	2,400,000	5,200,000	5,100,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	240,400,000	218,500,000
Defined Benefit Plan, Actual Return on Plan Assets	11,700,000	33,400,000
Defined Benefit Plan, Benefits Paid	-11,900,000	-11,500,000
Fair Value of Plan Assets Ending Balance	234,000,000	240,400,000	218,500,000
ChangeInAssetsDueToServiceCompanyMerger	-6,200,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-15,400,000	16,500,000
Defined Benefit Plan, Accumulated Benefit Obligation	226,400,000	220,600,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	240,400,000	218,500,000
Fair Value of Plan Assets Ending Balance	234,000,000	240,400,000	218,500,000
Western Massachusetts Electric Company [Member] | PBOP Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation Beginning Balance	-38,700,000	-42,500,000
ChangeInLiabilityDueToServiceCompanyMerger	1,000,000
Defined Benefit Plan Service Cost	-400,000	-700,000	-600,000
Defined Benefit Plan Interest Cost	-1,700,000	-1,700,000	-2,000,000
Defined Benefit Plan, Actuarial Net (Gains) Losses	1,300,000	3,300,000
Defined Benefit Plan, Benefits Paid	1,900,000	2,900,000
Benefit Obligation Ending Balance	-36,600,000	-38,700,000	-42,500,000
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Service Cost	400,000	700,000	600,000
Defined Benefit Plan Interest Cost	1,700,000	1,700,000	2,000,000
Defined Benefit Plan, Expected Return on Plan Assets	-2,300,000	-2,300,000	-2,100,000
Defined Benefit Plan, Amortization of Gains (Losses)	500,000	1,100,000	1,200,000
Defined Benefit Plan Amortization Of Transition Obligations Assets			1,300,000
Defined Benefit Plan Amortization Of Prior Service Cost Credit		0	0
Defined Benefit Plan Net Periodic Benefit Cost	300,000	1,200,000	3,000,000
Related Intercompany Allocationsof Defined Benefit Expense	700,000	1,300,000	1,500,000
Amount Capitalized Defined Benefit Expense	200,000	600,000	1,600,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets Beginning Balance	35,300,000	31,000,000
Defined Benefit Plan, Actual Return on Plan Assets	1,600,000	6,000,000
Defined Benefit Plan, Contributions by Employer	400,000	1,200,000
Defined Benefit Plan, Benefits Paid	-1,900,000	-2,900,000
Fair Value of Plan Assets Ending Balance	34,400,000	35,300,000	31,000,000
ChangeInAssetsDueToServiceCompanyMerger	-1,000,000
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Defined Benefit Plan, Funded Status of Plan	-2,200,000	-3,400,000
Level 3 Rollforwards
Fair Value of Plan Assets Beginning Balance	35,300,000	31,000,000
Fair Value of Plan Assets Ending Balance	$34,400,000	$35,300,000	$31,000,000

Defined_Contribution_Plans_Det

Defined Contribution Plans (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Contribution Plan, Cost Recognized	$29.70	$37	$25.70
The Connecticut Light And Power Company [Member]
Defined Contribution Plan, Cost Recognized	5	5.1	4.8
NSTAR Electric Company [Member]
Defined Contribution Plan, Cost Recognized	6.3	8.5	9
Western Massachusetts Electric Company [Member]
Defined Contribution Plan, Cost Recognized	1	1	0.9
Public Service Company Of New Hampshire [Member]
Defined Contribution Plan, Cost Recognized	$3.20	$3.30	$3.30

Employee_Stock_Ownership_Plan_

Employee Stock Ownership Plan (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Notes To Consolidated Financial Statements [Abstract]
Employee Stock Ownership Plan (ESOP), Compensation Expense	$22	$9.10	$8.90

ShareBased_Payments_Details

Share-Based Payments (Details) (USD $)	0 Months Ended	12 Months Ended
	Apr. 09, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		8,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant		3,112,020	3,440,590
AllocatedShareBasedCompensationExpense		$24,600,000	$27,000,000	$25,800,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		10,300,000	10,700,000	10,200,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized		15,700,000
Description Of Compensation Cost Not Yet Recognized Period For Recognition		This cost is expected to be recognized ratably over a weighted-average period of 1.65 years for NU, 1.68 years for CL&P, 1.69 years for NSTAR Electric, 1.71 years for PSNH and 1.68 years for WMECO.
Employee Service Share Based Compensation Awards Other Than Options Disclosures [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number		1,024,729	1,162,216
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value		$38.14	$36.58
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period		437,887
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value		$37.36
Share Based Compensation Arrangement By Share Based Payment Award Vested Outstanding Number		356,018
Share Based Compensation Arrangement By Share Based Payment Award Expected To Vest		973,493
Share-based Compensation Arrangement By Share-based Payment Award, Equity Instruments Other Than Options, NSTAR Deferred And Performance Awards Converted Upon Merger	421,775
Share-based Compensation Arrangement By Share-based Payment Award Equity Instruments Other Than Options NSTAR Deferred And Performance Awards Converted Upon Merger, Value	15.5
Share-based Compensation Arrangement By Share-based Payment Award, Equity Instruments Other Than Options, NSTAR RSU Retention Awards Converted To NU RSU's Upon Merger	195,619
Share-based Compensation Arrangement By Share-based Payment Award, Equity Instruments Other Than Options, NSTAR RSU Retention Awards Converted To NU RSU's Upon Merger, Value	7.2
Employee Service Share Based Compensation Stock Option Disclosures [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number		351,616	1,221,375	1,545,757	47,374
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		-869,759	-324,382	-1,166,511
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price		$26.69	$25.97	$24.92	$18.78
ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice		$25.68	$20.97	$22.53
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value		16,400,000	6,700,000	18,700,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value		9,400,000
Employee Service Share-based Compensation, Cash Received from Exercise of Stock Options		22,300,000
Employee Service Share-based Compensation, Tax Benefit Realized from Exercise of Stock Options		6,600,000
Share-based Compensation Arrangement By Share-based Payment Award, Options, Converted NSTAR Options Upon Merger	2,664,894
Share-based Compensation Arrangement By Share-based Payment Award, Options, Converted NSTAR Options Upon Merger, Weighted Average Exercise Price	$23.99
Share-based Compensation Arrangement By Share-based Payment Award, Options, Converted NSTAR Options Upon Merger, Total Value	30.5
Employee Service Share Based Compensation E S P P Disclosures [Abstract]
Stock Issued During Period, Shares, Employee Stock Purchase Plans		40,779	39,526
Employee Stock Purchase Plan Shares Remaining For Future Issuance		776,975	817,754
ESPP Discounted Purchase Price First Six Months		41.61	38.69
ESPP Discounted Purchase Price Second Six Months		41.71	42.19
Deferred Compensation Liability, Classified, Noncurrent [Abstract]
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent		57,500,000	51,300,000	54,600,000
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits		4,500,000	4,400,000	4,700,000
The Connecticut Light And Power Company [Member]
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]
AllocatedShareBasedCompensationExpense		8,100,000	6,800,000	4,800,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		3,400,000	2,700,000	1,900,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized		6,100,000
Deferred Compensation Liability, Classified, Noncurrent [Abstract]
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent		400,000	400,000	400,000
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits		2,100,000	2,500,000	2,600,000
NSTAR Electric Company [Member]
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]
AllocatedShareBasedCompensationExpense		7,400,000	7,500,000	7,400,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		3,100,000	3,000,000	2,900,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized		4,300,000
Deferred Compensation Liability, Classified, Noncurrent [Abstract]
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits		300,000
Public Service Company Of New Hampshire [Member]
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]
AllocatedShareBasedCompensationExpense		3,000,000	2,300,000	1,800,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		1,300,000	900,000	700,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized		2,000,000
Deferred Compensation Liability, Classified, Noncurrent [Abstract]
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent		2,600,000	2,300,000	2,500,000
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits		900,000	1,000,000	1,000,000
Western Massachusetts Electric Company [Member]
Employee Service Share-based Compensation, Aggregate Disclosures [Abstract]
AllocatedShareBasedCompensationExpense		1,300,000	1,300,000	1,000,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		500,000	500,000	400,000
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized		1,000,000
Deferred Compensation Liability, Classified, Noncurrent [Abstract]
Deferred Compensation Cash-based Arrangements, Liability, Classified, Noncurrent		200,000	100,000	200,000
Deferred Compensation Cash-based Arrangements, Expense Pertaining to Deferred Benefits		$400,000	$500,000	$500,000
Restricted Stock Units (RSUs) [Member]
Employee Service Share Based Compensation Awards Other Than Options Disclosures [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period		338,576	373,939	614,930
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period		-567,209	-891,129	-363,779
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period		-27,060	-29,689	-96,504
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Ending Balance		1,380,747	1,636,440	2,183,319	959,920
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value		$42.27	$39.56	$33.04
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Issued In Period Weighted Average Grant Date Fair Value		$33.48	$32.15	$29.05
ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue		$39.62	$33.75	$34.97
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Weighted Average Grant Date Fair Value Ending Balance		$35.67	$33.61	$31.99	$26.36
Share-based Compensation Arrangement By Share-based Payment Award Equity Instruments Other Than Options NSTAR Awards Converted To NU RSU's Upon Merger	617,394
Share-based Compensation Arrangement By Share-based Payment Award, Equity Instruments Other Than Options, NSTAR Awards Converted To NU RSU's Upon Merger, Weighted Average Grant Date Fair Value	$36.79
Employee Service Share Based Compensation Performance Share Disclosures [Abstract]
Share-based Compensation Arrangement By Share-based Payment Award, Performance Shares Converted To RSU's Upon Merger	451,358
Share-based Compensation Arrangement By Share-based Payment Award, Performance Shares Converted To RSU's Upon Merger, Weighted Average Grant-Date Fair Value	$34.32
Performance Shares [Member]
Employee Service Share Based Compensation Awards Other Than Options Disclosures [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period		193,396	191,961	225,935
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period		-2,009	-150,944	-106,773
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period		-6,171	-1,526
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Ending Balance		375,644	190,428	150,937	483,133
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value		$43.40	$40.96	$35.09
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Issued In Period Weighted Average Grant Date Fair Value		$41.46	$25.04	$24.52
ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue		$42.02	$40.93
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Weighted Average Grant Date Fair Value Ending Balance		$42.20	$40.96	$25.04	$29.18

INCOME_TAXES_Details

INCOME TAXES (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	$4,400,000	$8,800,000	($30,900,000)
Current State and Local Tax Expense (Benefit)	24,500,000	-9,400,000	17,600,000
Total Current Income Tax Expense (Benefit)	28,900,000	-600,000	-13,300,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	406,800,000	386,200,000	291,300,000
Deferred State and Local Income Tax Expense (Benefit)	36,500,000	45,400,000	800,000
Total Deferred Income Taxes	443,259,000	431,413,000	292,000,000
Investment Tax Credit	-3,900,000	-4,100,000	-3,900,000
Income Tax Expense	468,297,000	426,941,000	274,926,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	1,295,362,000	1,220,630,000	808,003,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	453,400,000	427,200,000	282,800,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	-5,600,000	-7,400,000	-10,800,000
Investment Tax Credit	-3,900,000	-4,100,000	-3,900,000
Income Tax Reconciliation, Tax Credits, Other	-3,500,000	-3,700,000	-3,800,000
Income Tax Reconciliation, State and Local Income Taxes	42,500,000	27,600,000	4,400,000
Income Tax Rate Reconciliation Deductions Employee Stock Ownership Plan Dividends	-8,000,000	-8,000,000	-6,400,000
Tax Asset Valuation Allowance / Reserve Adjustments	-2,900,000	-4,300,000	7,600,000
Income Tax Reconciliation, Other Adjustments	-3,700,000	-400,000	5,000,000
Income Tax Expense	468,297,000	426,941,000	274,926,000
Effective Income Tax Rate, Continuing Operations	36.20%	35.00%	34.00%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	632,200,000	435,200,000
Deferred Tax Assets, Derivative Instruments	199,600,000	272,900,000
Deferred Tax Assets, Regulatory Assets and Liabilities	366,700,000	272,700,000
Deferred Tax Assets, Allowance for Doubtful Accounts	60,500,000	65,000,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	10,000,000	16,200,000
Deferred Tax Assets, Operating Loss Carryforwards	59,100,000	158,000,000
Purchase Accounting Deferred Tax Asset	126,200,000	132,800,000
Deferred Tax Assets, Other	198,700,000	230,600,000
Deferred Tax Assets, Gross	1,653,000,000	1,583,400,000
Deferred Tax Assets, Valuation Allowance	5,100,000	24,300,000
Deferred Tax Assets, Net	1,647,900,000	1,559,100,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	4,215,900,000	3,806,500,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	109,600,000	95,100,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	1,277,900,000	1,146,700,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	240,200,000	248,200,000
Deferred Tax Liabilities Goodwill Regulatory Asset	203,200,000	211,500,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	32,600,000	30,100,000
Deferred Tax Liabilities, Other	196,300,000	157,100,000
Deferred Tax Liabilities, Total	6,275,700,000	5,695,200,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	38,200,000	83,100,000	73,500,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	9,300,000	8,200,000	10,300,000
Unrecognized Tax Benefits, Increases Resulting from Prior Period Tax Positions	300,000		100,000
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions		-1,100,000	-800,000
Unrecognized Tax Benefits, Decreases Resulting from Settlements with Taxing Authorities		-49,800,000
Unrecognized Tax Benefits, Reductions Resulting from Lapse of Applicable Statute of Limitations	-1,600,000	-2,200,000
Unrecognized Tax Benefits, Ending Balance	46,200,000	38,200,000	83,100,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense	400,000	-8,600,000	3,100,000
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	1,900,000	1,500,000
Income Tax Uncertainties [Abstract]
Income Tax Examination Description	Tax Positions: During 2014, NU did not resolve any of its uncertain tax positions.	During 2013, NU received a Final Determination from the Connecticut Department of Revenue Services (DRS) that concluded its audit of NU's Connecticut income tax returns for the years 2005 through 2008. The DRS Determination resulted in total NU and CL&P after-tax benefits of $13.6 million and $6.9 million, respectively, that included a reduction in NU and CL&P pre-tax interest expense of $8.7 million and $4 million, or $5.2 million and $2.4 million after-tax, respectively. Further, the income tax expense impact resulted in a tax benefit to NU and CL&P of $8.4 million and $4.5 million after-tax, respectively.
Tax Adjustments, Settlements - Total Impact		-13,600,000
Tax Adjustments, Settlements - Tax Expense Impact		-8,400,000
Tax Years [Abstract]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Other Information	NU estimates that during the next twelve months, differences of a non-timing nature could be resolved, resulting in a zero to $2 million decrease in unrecognized tax benefits by NU. These estimated changes are not expected to have a material impact on the earnings of NU. Other companies' impacts are not expected to be material.
Other Tax Disclosures [Abstract]
Other Information Pertaining To Income Taxes	2014 Federal Legislation: On December 19, 2014, the "Tax Increase Prevention Act of 2014" became law, which extended the accelerated deduction of depreciation to businesses through 2014. This extended stimulus provides NU with cash flow benefits of approximately $200 million (approximately $70 million at CL&P, $50 million at NSTAR Electric, $35 million at PSNH, and $15 million at WMECO) in 2015. 2013 Federal Legislation: On January 2, 2013, the "American Taxpayer Relief Act of 2012" became law, which extended the accelerated deduction of depreciation to businesses through 2013. This extended stimulus provided NU with cash flow benefits of approximately $300 million (approximately $95 million at CL&P, $85 million at NSTAR Electric, $35 million at PSNH, and $50 million at WMECO). On September 13, 2013, the Internal Revenue Service issued final Tangible Property regulations that are meant to simplify, clarify and make more administrable previously issued guidance. NU is in compliance with the new regulations, but continues to evaluate several new potential elections. 2013 Massachusetts: On July 24, 2013, Massachusetts enacted a law that changed the income tax rate applicable to utility companies effective January 1, 2014, from 6.5 percent to 8 percent. The tax law change required NU to remeasure its accumulated deferred income taxes and resulted in NU increasing its deferred tax liability with an offsetting regulatory asset of approximately $61 million at its utility companies ($46.3 million at NSTAR Electric and $9.8 million at WMECO).
Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	168,800,000	451,300,000
Tax Credit Carryforward, Amount	16,300,000	8,000,000
Charitable Contribution	19,400,000	33,700,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	40,600,000	12,100,000
Tax Credit Carryforward, Amount	99,700,000	104,700,000
Charitable Contribution	2,100,000	1,000,000
Tax Credit and Operating Loss Carryforward Valuation Allowance	4,400,000	23,700,000
The Connecticut Light And Power Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	-200,000	20,100,000	-47,800,000
Current State and Local Tax Expense (Benefit)	4,300,000	-6,700,000	3,100,000
Total Current Income Tax Expense (Benefit)	4,100,000	13,400,000	-44,700,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	138,000,000	114,900,000	141,500,000
Deferred State and Local Income Tax Expense (Benefit)	-7,100,000	15,100,000	-500,000
Total Deferred Income Taxes	130,949,000	130,038,000	140,993,000
Investment Tax Credit	-1,500,000	-1,700,000	-1,900,000
Income Tax Expense	133,451,000	141,663,000	94,437,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	421,205,000	421,075,000	304,162,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	147,400,000	147,400,000	106,500,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	-3,600,000	-7,000,000	-9,000,000
Investment Tax Credit	-1,500,000	-1,700,000	-1,900,000
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	4,400,000	5,000,000	100,000
Tax Asset Valuation Allowance / Reserve Adjustments	-6,300,000	400,000	1,600,000
Income Tax Reconciliation, Other Adjustments	-6,900,000	-2,400,000	-2,900,000
Income Tax Expense	133,451,000	141,663,000	94,437,000
Effective Income Tax Rate, Continuing Operations	31.70%	33.60%	31.00%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	129,000,000	56,000,000
Deferred Tax Assets, Derivative Instruments	193,000,000	272,400,000
Deferred Tax Assets, Regulatory Assets and Liabilities	73,900,000	61,500,000
Deferred Tax Assets, Allowance for Doubtful Accounts	32,300,000	31,200,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	3,100,000	4,700,000
Deferred Tax Assets, Operating Loss Carryforwards	0	51,000,000
Deferred Tax Assets, Other	53,800,000	75,300,000
Deferred Tax Assets, Gross	485,100,000	552,100,000
Deferred Tax Assets, Valuation Allowance	4,000,000	23,100,000
Deferred Tax Assets, Net	481,100,000	529,000,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,378,600,000	1,238,100,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	58,100,000	49,300,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	502,300,000	550,400,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	166,900,000	160,100,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	32,600,000	29,000,000
Deferred Tax Liabilities, Other	19,400,000	20,600,000
Deferred Tax Liabilities, Total	2,157,900,000	2,047,500,000
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Tax Credit Carryforward Description	In 2014, the Company recorded a reduction to its state credit carryforwards of $11 million (CL&P $10.1 million), net of tax, as a result of an update to reflect the amounts expired. Further, the Company decreased its valuation allowance reserve for state credits by $19.2 million at CL&P, net of tax, to reflect an update for expired state credits and latest estimate of usage.
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized Tax Benefits, Beginning Balance	11,400,000	49,000,000	46,500,000
Unrecognized Tax Benefits, Increases Resulting from Current Period Tax Positions	2,700,000	2,100,000	2,500,000
Unrecognized Tax Benefits, Increases Resulting from Prior Period Tax Positions	200,000		0
Unrecognized Tax Benefits, Decreases Resulting from Prior Period Tax Positions		-300,000	0
Unrecognized Tax Benefits, Decreases Resulting from Settlements with Taxing Authorities		-39,400,000
Unrecognized Tax Benefits, Ending Balance	14,300,000	11,400,000	49,000,000
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense and Accrued Interest [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Expense	0	-4,000,000	1,300,000
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	0	0
Income Tax Uncertainties [Abstract]
Income Tax Examination Description		During 2013, NU received a Final Determination from the Connecticut Department of Revenue Services (DRS) that concluded its audit of NU's Connecticut income tax returns for the years 2005 through 2008. The DRS Determination resulted in total NU and CL&P after-tax benefits of $13.6 million and $6.9 million, respectively, that included a reduction in NU and CL&P pre-tax interest expense of $8.7 million and $4 million, or $5.2 million and $2.4 million after-tax, respectively. Further, the income tax expense impact resulted in a tax benefit to NU and CL&P of $8.4 million and $4.5 million after-tax, respectively.
Tax Adjustments, Settlements - Total Impact		-6,900,000
Tax Adjustments, Settlements - Tax Expense Impact		-4,500,000
The Connecticut Light And Power Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	145,800,000
Tax Credit Carryforward, Amount	100,000	0
The Connecticut Light And Power Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	71,000,000	86,800,000
NSTAR Electric Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	75,000,000	95,800,000	93,500,000
Current State and Local Tax Expense (Benefit)	20,200,000	29,600,000	27,600,000
Total Current Income Tax Expense (Benefit)	95,200,000	125,400,000	121,100,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	88,000,000	49,800,000	11,400,000
Deferred State and Local Income Tax Expense (Benefit)	20,100,000	-1,000,000	-7,100,000
Total Deferred Income Taxes	108,133,000	48,808,000	4,264,000
Investment Tax Credit	-1,300,000	-1,300,000	-1,400,000
Income Tax Expense	201,981,000	172,866,000	123,966,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	505,069,000	441,412,000	314,208,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	176,800,000	154,500,000	110,000,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	-1,300,000	100,000	0
Investment Tax Credit	-1,300,000	-1,300,000	-1,400,000
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	26,200,000	18,600,000	13,400,000
Income Tax Reconciliation, Other Adjustments	1,600,000	1,000,000	2,000,000
Income Tax Expense	201,981,000	172,866,000	123,966,000
Effective Income Tax Rate, Continuing Operations	40.00%	39.20%	39.50%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	39,900,000	38,300,000
Deferred Tax Assets, Derivative Instruments	1,800,000	3,300,000
Deferred Tax Assets, Regulatory Assets and Liabilities	181,300,000	114,700,000
Deferred Tax Assets, Allowance for Doubtful Accounts	13,800,000	15,400,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	1,800,000	5,400,000
Deferred Tax Assets, Operating Loss Carryforwards	0	0
Deferred Tax Assets, Other	19,900,000	31,300,000
Deferred Tax Assets, Gross	258,500,000	208,400,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	1,296,900,000	1,179,400,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	25,000,000	25,300,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	276,000,000	276,200,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	35,500,000	36,000,000
Deferred Tax Liabilities Goodwill Regulatory Asset	174,400,000	181,600,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	500,000
Deferred Tax Liabilities, Other	33,500,000	26,400,000
Deferred Tax Liabilities, Total	1,841,300,000	1,725,400,000
NSTAR Electric Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	200,000	0
NSTAR Electric Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	0	0
Public Service Company Of New Hampshire [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	-22,600,000	-8,200,000	-900,000
Current State and Local Tax Expense (Benefit)	-100,000	3,600,000	3,400,000
Total Current Income Tax Expense (Benefit)	-22,700,000	-4,600,000	2,500,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	79,600,000	64,500,000	46,500,000
Deferred State and Local Income Tax Expense (Benefit)	15,200,000	11,200,000	12,000,000
Total Deferred Income Taxes	94,813,000	75,693,000	58,552,000
Investment Tax Credit	0	0	0
Income Tax Expense	72,135,000	71,101,000	60,993,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	186,079,000	182,498,000	157,875,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	65,100,000	63,900,000	55,300,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	300,000	600,000	-300,000
Investment Tax Credit	0	0	0
Income Tax Reconciliation, Tax Credits, Other	-3,500,000	-3,700,000	-3,800,000
Income Tax Reconciliation, State and Local Income Taxes	9,800,000	9,600,000	10,000,000
Income Tax Reconciliation, Other Adjustments	400,000	700,000	-200,000
Income Tax Expense	72,135,000	71,101,000	60,993,000
Effective Income Tax Rate, Continuing Operations	38.70%	39.00%	38.60%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	46,800,000	15,500,000
Deferred Tax Assets, Derivative Instruments	0	0
Deferred Tax Assets, Regulatory Assets and Liabilities	46,500,000	40,900,000
Deferred Tax Assets, Allowance for Doubtful Accounts	3,200,000	3,100,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	2,100,000	2,100,000
Deferred Tax Assets, Operating Loss Carryforwards	32,100,000	56,600,000
Deferred Tax Assets, Other	48,900,000	40,300,000
Deferred Tax Assets, Gross	179,600,000	158,500,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	596,600,000	526,600,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	7,400,000	7,100,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	147,600,000	109,300,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	15,900,000	16,300,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	0
Deferred Tax Liabilities, Other	35,600,000	28,000,000
Deferred Tax Liabilities, Total	803,100,000	687,300,000
Public Service Company Of New Hampshire [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	91,800,000	161,800,000
Tax Credit Carryforward, Amount	11,100,000	7,600,000
Public Service Company Of New Hampshire [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	0	0
Western Massachusetts Electric Company [Member]
Current Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	1,900,000	-53,400,000	-24,700,000
Current State and Local Tax Expense (Benefit)	1,800,000	4,200,000	3,400,000
Total Current Income Tax Expense (Benefit)	3,700,000	-49,200,000	-21,300,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	28,100,000	84,700,000	51,200,000
Deferred State and Local Income Tax Expense (Benefit)	6,000,000	2,300,000	2,700,000
Total Deferred Income Taxes	34,108,000	87,028,000	53,942,000
Investment Tax Credit	-500,000	-400,000	-500,000
Income Tax Expense	37,268,000	37,368,000	32,140,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	95,087,000	97,806,000	86,643,000
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	33,300,000	34,200,000	30,300,000
Tax Effect Of Differences [Abstract]
Income Tax Reconciliation, Nondeductible Expense, Depreciation	-200,000	0	200,000
Investment Tax Credit	-500,000	-400,000	-500,000
Income Tax Reconciliation, Tax Credits, Other	0	0	0
Income Tax Reconciliation, State and Local Income Taxes	5,000,000	4,200,000	4,000,000
Income Tax Reconciliation, Other Adjustments	-300,000	-600,000	-1,900,000
Income Tax Expense	37,268,000	37,368,000	32,140,000
Effective Income Tax Rate, Continuing Operations	39.20%	38.20%	37.10%
Deferred Tax Assets, Net [Abstract]
Deferred Tax Assets Employee Benefits	9,200,000	-1,800,000
Deferred Tax Assets, Derivative Instruments	0	-2,900,000
Deferred Tax Assets, Regulatory Assets and Liabilities	11,400,000	1,000,000
Deferred Tax Assets, Allowance for Doubtful Accounts	3,800,000	3,300,000
Deferred Tax Assets Tax Effect Tax Regulatory Liabilities	2,500,000	1,600,000
Deferred Tax Assets, Operating Loss Carryforwards	4,500,000	18,600,000
Deferred Tax Assets, Other	4,900,000	8,300,000
Deferred Tax Assets, Gross	36,300,000	28,100,000
Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	385,800,000	361,100,000
Deferred Tax Liabilities Resulting From Property Tax Accruals And Other	12,800,000	5,900,000
Deferred Tax Liabilities, Regulatory Amounts [Abstract]
Deferred Tax Liabilities Regulatory Assets Other Regulatory Deferrals	60,400,000	49,300,000
Deferred Tax Liabilities Regulatory Assets Tax Effect Tax Regulatory Assets	9,300,000	18,200,000
Deferred Tax Liabilities Regulatory Assets Derivative Assets	0	0
Deferred Tax Liabilities, Other	2,800,000	3,600,000
Deferred Tax Liabilities, Total	471,100,000	438,100,000
Western Massachusetts Electric Company [Member] | Federal [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	12,700,000	53,300,000
Tax Credit Carryforward, Amount	0	0
Western Massachusetts Electric Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards and Tax Credit Carryforwards [Abstract]
Operating Loss Carryforwards	0	0
Tax Credit Carryforward, Amount	$0	$0

Environmental_Matters_Details

Environmental Matters (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Site Contingency [Line Items]
Environmental Site Quantity	65	68
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	$35.40	$39.40
Accrual For Environmental Loss Contingencies Provision For New Losses	12.7	3.5
Accrual for Environmental Loss Contingencies, Payments	-4.8	-7.5
Ending Balance	43.3	35.4
Site Contingency [Abstract]
Information on range of possible loss and superfunds	CERCLA: Of the total environmental sites, 9 sites (1 for CL&P, 3 for NSTAR Electric and 3 for PSNH) are superfund sites under the federal Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA) and its amendments or state equivalents for which the Company has been notified that it is a potentially responsible party but for which the site assessment and remediation are not being managed by the Company.  As of December 31, 2014, a liability of $0.7 million accrued on these sites represents management's best estimate of its potential remediation costs with respect to these superfund sites. Included in the NU number of sites and reserve amounts above are former MGP sites that were operated several decades ago and manufactured gas from coal and other processes, which resulted in certain by-products remaining in the environment that may pose a potential risk to human health and the environment. The reserve balance related to these former MGP sites was $38.8 million and $31.4 million as of December 31, 2014 and 2013, respectively, and relates primarily to the natural gas business segment. The increase in the reserve balance for the MGP sites was due to the completion of the site assessment at three sites. The assessments provided new information related to the extent and nature of the contamination and the costs of required remediation. As of December 31, 2014, for 5 environmental sites (1 for CL&P, and 1 for WMECO) that are included in the Company's reserve for environmental costs, the information known and nature of the remediation options at those sites allow for the Company to estimate the range of losses for environmental costs. As of December 31, 2014, $17.7 million ($1 million for CL&P and $0.3 million for WMECO) had been accrued as a liability for these sites, which represent management's best estimates of the liabilities for environmental costs. These amounts are the best estimates with estimated ranges of additional losses from zero to $24 million. As of December 31, 2014, for 15 environmental sites (3 for CL&P, 3 for NSTAR Electric and 2 for PSNH) that are included in the Company’s reserve for environmental costs, management cannot reasonably estimate the exposure to loss in excess of the reserve, or range of loss, as these sites are under investigation and/or there is significant uncertainty as to what remedial actions, if any, the Company may be required to undertake. As of December 31, 2014, $13.4 million ($1.4 million for CL&P, $0.2 million for PSNH) had been accrued as a liability for these sites. As of December 31, 2014, for the remaining 45 environmental sites (12 for CL&P, 10 for NSTAR Electric, 11 for PSNH, and 3 for WMECO) that are included in the Company’s reserve for environmental costs, the $12.2 million accrual ($1.4 million for CL&P, $1.1 million for NSTAR Electric, $5 million for PSNH, and $0.2 million for WMECO) represents management’s best estimate of the liability and no additional loss is anticipated.
MGP Site accrual [Member]
Accrual for Environmental Loss Contingencies [Roll Forward]
Accrual For Environmental Loss Contingencies Component Amount	38.8	31.4
The Connecticut Light And Power Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	16	18
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	3.4	3.7
Accrual For Environmental Loss Contingencies Provision For New Losses	1	0.2
Accrual for Environmental Loss Contingencies, Payments	-0.6	-0.5
Ending Balance	3.8	3.4
NSTAR Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	13	12
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	1.2	1.7
Accrual For Environmental Loss Contingencies Provision For New Losses	0	0.2
Accrual for Environmental Loss Contingencies, Payments	-0.1	-0.7
Ending Balance	1.1	1.2
Public Service Company Of New Hampshire [Member]
Site Contingency [Line Items]
Environmental Site Quantity	13	15
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	5.4	4.9
Accrual For Environmental Loss Contingencies Provision For New Losses	0.1	1
Accrual for Environmental Loss Contingencies, Payments	-0.3	-0.5
Ending Balance	5.2	5.4
Western Massachusetts Electric Company [Member]
Site Contingency [Line Items]
Environmental Site Quantity	4	5
Accrual for Environmental Loss Contingencies [Roll Forward]
Beginning Balance	0.4	0.6
Accrual For Environmental Loss Contingencies Provision For New Losses	0.2	0
Accrual for Environmental Loss Contingencies, Payments	-0.1	-0.2
Ending Balance	$0.50	$0.40

LongTerm_Contractual_Arrangeme1

Long-Term Contractual Arrangements (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	$685.80
Unrecorded Unconditional Purchase Obligation, Due within Two Years	598.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	421.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	327.6
Unrecorded Unconditional Purchase Obligation, Due within Five Years	290
Unrecorded Unconditional Purchase Obligation, Due after Five Years	2,198.40
Unrecorded Unconditional Purchase Obligation, Total	4,521.60
Supply And Stranded Costs Obligations Incurred	99.2	141	216.8
Natural Gas Procurement Costs Incurred	482.5	349.8	243.1
Fuel Costs Incurred	120.5	112.6	105.2
Transmission Support Commitment Costs Incurred	25	24.9	24.8
Peaker CfD Expense Incurred	18.1	51.9	59.3
Renewable Energy Costs Incurred	114.4	91.3	48.7
Supply And Stranded Cost Obligations [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	196.6
Unrecorded Unconditional Purchase Obligation, Due within Two Years	169.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	101.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	65.3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	38.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	82.4
Unrecorded Unconditional Purchase Obligation, Total	653.5
Renewable Energy Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	204.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	240.5
Unrecorded Unconditional Purchase Obligation, Due within Three Years	239.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	204.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	202.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	1,994.60
Unrecorded Unconditional Purchase Obligation, Total	3,086.10
Peaker Contracts For Differences [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	26.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	25.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10.5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Total	62.2
Natural Gas Procurement Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	133.7
Unrecorded Unconditional Purchase Obligation, Due within Two Years	116.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	45.6
Unrecorded Unconditional Purchase Obligation, Due within Four Years	31.9
Unrecorded Unconditional Purchase Obligation, Due within Five Years	25.8
Unrecorded Unconditional Purchase Obligation, Due after Five Years	85.1
Unrecorded Unconditional Purchase Obligation, Total	438.4
Wood, Coal And Transporation Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	99
Unrecorded Unconditional Purchase Obligation, Due within Two Years	25.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	1.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	15
Unrecorded Unconditional Purchase Obligation, Total	151.1
Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	25.9
Recorded Unconditional Purchase Obligation Due in Second Year	21.6
Recorded Unconditional Purchase Obligation Due in Third Year	19
Recorded Unconditional Purchase Obligation Due in Fourth Year	21.2
Recorded Unconditional Purchase Obligation Due in Fifth Year	21.3
Recorded Unconditional Purchase Obligation Due after Fifth Year	21.3
Recorded Unconditional Purchase Obligation, Total	130.3
The Connecticut Light And Power Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	233.4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	241.8
Unrecorded Unconditional Purchase Obligation, Due within Three Years	169.1
Unrecorded Unconditional Purchase Obligation, Due within Four Years	123.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	106.4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	779.7
Unrecorded Unconditional Purchase Obligation, Total	1,653.80
Supply And Stranded Costs Obligations Incurred	63	77.6	158.2
Transmission Support Commitment Costs Incurred	9.9	9.8	9.6
Peaker CfD Expense Incurred	18.1	51.9	59.3
Renewable Energy Costs Incurred	0.7
The Connecticut Light And Power Company [Member] | Supply And Stranded Cost Obligations [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	134.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	136.8
Unrecorded Unconditional Purchase Obligation, Due within Three Years	79
Unrecorded Unconditional Purchase Obligation, Due within Four Years	42
Unrecorded Unconditional Purchase Obligation, Due within Five Years	25
Unrecorded Unconditional Purchase Obligation, Due after Five Years	43.6
Unrecorded Unconditional Purchase Obligation, Total	460.7
The Connecticut Light And Power Company [Member] | Renewable Energy Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	61.2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	70.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	71.3
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72.1
Unrecorded Unconditional Purchase Obligation, Due within Five Years	72.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	715.9
Unrecorded Unconditional Purchase Obligation, Total	1,062.90
The Connecticut Light And Power Company [Member] | Peaker Contracts For Differences [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	26.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	25.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10.5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation, Total	62.2
The Connecticut Light And Power Company [Member] | Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	10.2
Recorded Unconditional Purchase Obligation Due in Second Year	8.5
Recorded Unconditional Purchase Obligation Due in Third Year	7.5
Recorded Unconditional Purchase Obligation Due in Fourth Year	8.4
Recorded Unconditional Purchase Obligation Due in Fifth Year	8.4
Recorded Unconditional Purchase Obligation Due after Fifth Year	8.4
Recorded Unconditional Purchase Obligation, Total	51.4
The Connecticut Light And Power Company [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	1.4
Recorded Unconditional Purchase Obligation Due in Second Year	0.8
Recorded Unconditional Purchase Obligation Due in Third Year	0.8
Recorded Unconditional Purchase Obligation Due in Fourth Year	0.9
Recorded Unconditional Purchase Obligation Due in Fifth Year	0.9
Recorded Unconditional Purchase Obligation Due after Fifth Year	11.8
Recorded Unconditional Purchase Obligation, Total	16.6
NSTAR Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	128.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	121
Unrecorded Unconditional Purchase Obligation, Due within Three Years	107.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72
Unrecorded Unconditional Purchase Obligation, Due within Five Years	70.1
Unrecorded Unconditional Purchase Obligation, Due after Five Years	361.8
Unrecorded Unconditional Purchase Obligation, Total	861.1
Supply And Stranded Costs Obligations Incurred	7	32.4	36.3
Transmission Support Commitment Costs Incurred	7.7	7.7	7.6
Renewable Energy Costs Incurred	87.4	84.9	60.2
NSTAR Electric Company [Member] | Supply And Stranded Cost Obligations [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	34.3
Unrecorded Unconditional Purchase Obligation, Due within Two Years	14.1
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4.8
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5.5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5.5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	31.4
Unrecorded Unconditional Purchase Obligation, Total	95.6
NSTAR Electric Company [Member] | Renewable Energy Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	85.4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	99.9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	96.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	59.6
Unrecorded Unconditional Purchase Obligation, Due within Five Years	57.7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	319.8
Unrecorded Unconditional Purchase Obligation, Total	719.3
NSTAR Electric Company [Member] | Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	8.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	6.7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	5.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	6.6
Unrecorded Unconditional Purchase Obligation, Due within Five Years	6.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	6.6
Unrecorded Unconditional Purchase Obligation, Total	40.5
NSTAR Electric Company [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	0.5
Recorded Unconditional Purchase Obligation Due in Second Year	0.3
Recorded Unconditional Purchase Obligation Due in Third Year	0.3
Recorded Unconditional Purchase Obligation Due in Fourth Year	0.3
Recorded Unconditional Purchase Obligation Due in Fifth Year	0.3
Recorded Unconditional Purchase Obligation Due after Fifth Year	4
Recorded Unconditional Purchase Obligation, Total	5.7
Public Service Company Of New Hampshire [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	190.6
Unrecorded Unconditional Purchase Obligation, Due within Two Years	116.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	96.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	97.7
Unrecorded Unconditional Purchase Obligation, Due within Five Years	85
Unrecorded Unconditional Purchase Obligation, Due after Five Years	964
Unrecorded Unconditional Purchase Obligation, Total	1,550
Supply And Stranded Costs Obligations Incurred	26	29	30.5
Fuel Costs Incurred	120.5	112.6	105.2
Transmission Support Commitment Costs Incurred	5.3	5.3	5.2
Renewable Energy Costs Incurred	26.3	6.4	4.1
Public Service Company Of New Hampshire [Member] | Supply And Stranded Cost Obligations [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	28
Unrecorded Unconditional Purchase Obligation, Due within Two Years	18.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	18.1
Unrecorded Unconditional Purchase Obligation, Due within Four Years	17.8
Unrecorded Unconditional Purchase Obligation, Due within Five Years	7.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	7.4
Unrecorded Unconditional Purchase Obligation, Total	97.2
Public Service Company Of New Hampshire [Member] | Renewable Energy Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	57.7
Unrecorded Unconditional Purchase Obligation, Due within Two Years	67.9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	69
Unrecorded Unconditional Purchase Obligation, Due within Four Years	70.1
Unrecorded Unconditional Purchase Obligation, Due within Five Years	70.7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	932.4
Unrecorded Unconditional Purchase Obligation, Total	1,267.80
Public Service Company Of New Hampshire [Member] | Wood, Coal And Transporation Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	99
Unrecorded Unconditional Purchase Obligation, Due within Two Years	25.2
Unrecorded Unconditional Purchase Obligation, Due within Three Years	5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	1.9
Unrecorded Unconditional Purchase Obligation, Due after Five Years	15
Unrecorded Unconditional Purchase Obligation, Total	151.1
Public Service Company Of New Hampshire [Member] | Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	5.5
Recorded Unconditional Purchase Obligation Due in Second Year	4.6
Recorded Unconditional Purchase Obligation Due in Third Year	4
Recorded Unconditional Purchase Obligation Due in Fourth Year	4.5
Recorded Unconditional Purchase Obligation Due in Fifth Year	4.5
Recorded Unconditional Purchase Obligation Due after Fifth Year	4.5
Recorded Unconditional Purchase Obligation, Total	27.6
Public Service Company Of New Hampshire [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	0.4
Recorded Unconditional Purchase Obligation Due in Second Year	0.3
Recorded Unconditional Purchase Obligation Due in Third Year	0.3
Recorded Unconditional Purchase Obligation Due in Fourth Year	0.3
Recorded Unconditional Purchase Obligation Due in Fifth Year	0.3
Recorded Unconditional Purchase Obligation Due after Fifth Year	4.7
Recorded Unconditional Purchase Obligation, Total	6.3
Western Massachusetts Electric Company [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	2.4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	4.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4.2
Unrecorded Unconditional Purchase Obligation, Due within Four Years	4.3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	4.4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	31.3
Unrecorded Unconditional Purchase Obligation, Total	51
Supply And Stranded Costs Obligations Incurred	3.2	2	0.9
Transmission Support Commitment Costs Incurred	2.1	2.1	2
Western Massachusetts Electric Company [Member] | Renewable Energy Contracts [Member]
Long-term Purchase Commitment [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	2.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	2.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	2.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	2.4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	26.5
Unrecorded Unconditional Purchase Obligation, Total	36.1
Western Massachusetts Electric Company [Member] | Transmission Support Commitments [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	2.1
Recorded Unconditional Purchase Obligation Due in Second Year	1.8
Recorded Unconditional Purchase Obligation Due in Third Year	1.6
Recorded Unconditional Purchase Obligation Due in Fourth Year	1.7
Recorded Unconditional Purchase Obligation Due in Fifth Year	1.8
Recorded Unconditional Purchase Obligation Due after Fifth Year	1.8
Recorded Unconditional Purchase Obligation, Total	10.8
Western Massachusetts Electric Company [Member] | Yankee Companies Billings [Member]
Long-term Purchase Commitment [Line Items]
Recorded Unconditional Purchase Obligation Due within One Year	0.3
Recorded Unconditional Purchase Obligation Due in Second Year	0.2
Recorded Unconditional Purchase Obligation Due in Third Year	0.2
Recorded Unconditional Purchase Obligation Due in Fourth Year	0.2
Recorded Unconditional Purchase Obligation Due in Fifth Year	0.2
Recorded Unconditional Purchase Obligation Due after Fifth Year	3
Recorded Unconditional Purchase Obligation, Total	$4.10

Contractual_Obligations_with_Y

Contractual Obligations with Yankee Companies (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2013
Gain Contingency [Line Items]
Refund Obligation To Customers	As of December 31, 2014, CL&P's refund obligation to customers of $65.4 million was recorded as an offset to the deferred storm restoration costs regulatory asset, as directed by PURA. NSTAR Electric's, PSNH's and WMECO's refund obligation to customers of $29.1 million, $13.1 million and $18.1 million, respectively, was recorded as a regulatory liability in each company's respective regulatory tracker mechanisms. Refunds to customers for these DOE proceeds began in the third quarter of 2014. For further information, see Note 2, "Regulatory Accounting," to the financial statements.
MYAPC Phase One Damages Proceeds To Member Companies	On September 17, 2014, in accordance with the MYAPC refund plan, MYAPC returned a portion of the DOE Phase I Damages proceeds to the member companies, including CL&P, NSTAR Electric, PSNH, and WMECO, in the amount of $3.2 million, $1.1 million, $1.4 million and $0.8 million, respectively. These amounts reduced receivables at CL&P, NSTAR Electric, PSNH and WMECO.
CYAPC [Member] | DOE Phase I Damages [Member]
Gain Contingency [Line Items]
Awards Received from Litigation		$39.60
CYAPC [Member] | DOE Phase II Damages [Member]
Gain Contingency [Line Items]
Awards Received from Litigation	126.3
YAEC [Member] | DOE Phase I Damages [Member]
Gain Contingency [Line Items]
Awards Received from Litigation		38.3
YAEC [Member] | DOE Phase II Damages [Member]
Gain Contingency [Line Items]
Awards Received from Litigation	73.3
MYAPC [Member] | DOE Phase I Damages [Member]
Gain Contingency [Line Items]
Awards Received from Litigation		81.7
MYAPC [Member] | DOE Phase II Damages [Member]
Gain Contingency [Line Items]
Awards Received from Litigation	35.8
CYAPC and YAEC [Member] | DOE Phase I Damages [Member]
Gain Contingency [Line Items]
Increase In Marketable Securities due to CYAPC and YAEC			$77.90

Guarantees_and_Indemnification1

Guarantees and Indemnifications (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Various Subsidiary Surety Bonds And Performance Guarantees Expiring Between 2014 And 2016 [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	$60
Guarantee Of Rocky River Reality And Nusco Lease Payments For Real Estate And Vehicles Expiring 2019 Through 2024 [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	$14.40

Other_Contingencies_Details

Other Contingencies (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Loss Contingencies Line Items
Loss Contingency, Description	E.         FERC Base ROE Complaints E.         FERC Base ROE Complaints Beginning in 2011, several New England state attorneys general, state regulatory commissions, consumer advocates, consumer groups, municipal parties and other parties (the "Complainants") jointly filed three separate complaints at FERC. In the first complaint, filed in 2011, the Complainants alleged that the NETOs' base ROE of 11.14 percent that was utilized since 2006 was unjust and unreasonable, asserted that the rate was excessive due to changes in the capital markets, and sought an order to reduce it prospectively from the date of the final FERC order and for the 15-month period beginning October 1, 2011 to December 31, 2012 (the "first complaint refund period"). In the pursuant second and third complaints, filed in 2012 and 2014, respectively, the Complainants challenged the NETOs' base ROE and sought refunds for the 15-month periods beginning December 27, 2012 and July 31, 2014, respectively. In 2014, the FERC determined that the base ROE should be set at 10.57 percent for the first complaint refund period and that a utility’s total or maximum ROE should not exceed the top of the new zone of reasonableness (7.03 percent to 11.74 percent). The FERC ordered the NETOs to provide refunds to customers for the first complaint refund period and set the new base ROE of 10.57 percent prospectively from October 16, 2014. In late 2014, the NETOs made a compliance filing, and began refunding amounts from the first complaint period, inclusive of incentive ROE adders that exceeded the 11.74 percent as compared to the total company transmission ROE. Complainants have challenged the compliance filing. As a result of the actions taken by the FERC and other developments in this matter, NU recorded reserves in 2013 and 2014 to recognize the potential financial impacts of the first and second complaints. The Company is unable to determine any amount related to the third complaint. The following is a summary of the cumulative pre-tax reserves (excluding interest) established by the Company in 2013 and 2014: As of December 31, 2014, the cumulative reserves above do not reflect refunds totaling $4.8 million at NU, $2.7 million at CL&P, $1 million at NSTAR Electric, $0.5 million at PSNH and $0.6 million at WMECO for the first complaint refund period. The aggregate after-tax net charge to 2014 earnings resulting from the 2014 FERC orders totaled $22.4 million at NU, $12.4 million at CL&P, $4.9 million at NSTAR Electric, $1.7 million at PSNH and $3.4 million at WMECO. In 2013, the aggregate after-tax charge to earnings totaled $14.3 million at NU, $7.7 million at CL&P, $3.4 million at NSTAR Electric, $1.4 million at PSNH and $1.8 million at WMECO. Although management is uncertain on the final outcome on the second and third complaints regarding the base ROE and the incentive ROE adder, management believes the current reserves established are appropriate to reflect probable and reasonably estimable refunds. F.        2014 Comprehensive Settlement Agreement On December 31, 2014, NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General filed a comprehensive settlement agreement with the DPU. The comprehensive settlement agreement included resolution of the outstanding NSTAR Electric CPSL program filings for the periods 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the NSTAR Electric energy efficiency program filings regarding LBR for the periods 2008 through 2011. If approved by the DPU, NSTAR Electric and NSTAR Gas will be required to refund a total of $44.7 million to their respective customers, which was included in regulatory liabilities as of December 31, 2014. Upon the DPU's approval, NSTAR Electric will adjust its regulatory liabilities, which it expects will result in a benefit of $23 million in the first quarter of 2015. Management expects a response from the DPU in the first quarter of 2015. G.        Basic Service Bad Debt Adder In accordance with a generic 2005 DPU order, electric utilities in Massachusetts recover the energy-related portion of bad debt costs in their Basic Service rates. In 2007, NSTAR Electric filed its 2006 Basic Service reconciliation with the DPU proposing an adjustment related to the increase of its Basic Service bad debt charge-offs. The DPU issued an order approving the implementation of a revised Basic Service rate but instructed NSTAR Electric to reduce distribution rates by an amount equal to the increase in its Basic Service bad debt charge-offs. This adjustment to NSTAR Electric's distribution rates would eliminate the fully reconciling nature of the Basic Service bad debt adder. In 2010, NSTAR Electric filed an appeal of the DPU's order with the SJC. NSTAR Electric's position was that it had fully removed the collection of energy-related bad debt costs from its distribution rates effective January 1, 2006. Therefore, no further adjustment to distribution rates was warranted. In 2012, the SJC vacated the DPU order and remanded the matter to the DPU for further review. As of December 31, 2014, NSTAR Electric has a total deferred regulatory asset of approximately $33 million of costs associated with energy-related bad debt. On January 7, 2015, the DPU issued an order on remand stating that NSTAR Electric had, in fact, removed energy-related bad debt costs from distribution rates effective January 1, 2006. The DPU order approved NSTAR Electric's 2005 and 2006 reconciliation filings and ordered NSTAR Electric and the Massachusetts Attorney General to collaborate on the submission of a proposal for the reconciliation of energy-related bad debt costs for the open years of 2007 through 2014 by April 7, 2015. Management expects to present a proposal to the Attorney General in the first quarter of 2015 with a decision from the DPU later in 2015. H.        Litigation and Legal Proceedings NU, including CL&P, NSTAR Electric, PSNH and WMECO, are involved in legal, tax and regulatory proceedings regarding matters arising in the ordinary course of business, which involve management's assessment to determine the probability of whether a loss will occur and, if probable, its best estimate of probable loss. The Company records and discloses losses when these losses are probable and reasonably estimable, and discloses matters when losses are probable but not estimable or when losses are reasonably possible. Legal costs related to the defense of loss contingencies are expensed as incurred.
FERC ROE Reserve - First Complaint - Base ROE [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	$24.90	$23.70
Loss Contingency Loss in Period	1.2
CustomerRefundLiabilityCurrent	4.8
FERC ROE Reserve - First Complaint - Base ROE [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	13.3	12.8
Loss Contingency Loss in Period	0.5
CustomerRefundLiabilityCurrent	2.7
FERC ROE Reserve - First Complaint - Base ROE [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	6.1	5.7
Loss Contingency Loss in Period	0.4
CustomerRefundLiabilityCurrent	1
FERC ROE Reserve - First Complaint - Base ROE [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	2.4	2.3
Loss Contingency Loss in Period	0.1
CustomerRefundLiabilityCurrent	0.5
FERC ROE Reserve - First Complaint - Base ROE [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	3.1	2.9
Loss Contingency Loss in Period	0.2
CustomerRefundLiabilityCurrent	0.6
FERC ROE Reserve - Second Complaint - Base ROE [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	27.4	0
Loss Contingency Loss in Period	27.4
FERC ROE Reserve - Second Complaint - Base ROE [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	13.5	0
Loss Contingency Loss in Period	13.5
FERC ROE Reserve - Second Complaint - Base ROE [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	7.5	0
Loss Contingency Loss in Period	7.5
FERC ROE Reserve - Second Complaint - Base ROE [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	2.7	0
Loss Contingency Loss in Period	2.7
FERC ROE Reserve - Second Complaint - Base ROE [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	3.7	0
Loss Contingency Loss in Period	3.7
FERC ROE Reserve - Incentive - First and Second Complaint [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	8.4	0
Loss Contingency Loss in Period	8.4
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	6.7	0
Loss Contingency Loss in Period	6.7
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	0	0
Loss Contingency Loss in Period	0
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	0	0
Loss Contingency Loss in Period	0
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	1.7	0
Loss Contingency Loss in Period	1.7
FERC ROE Reserve - Cumulative [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	60.7	23.7
Loss Contingency Loss in Period	37
FERC ROE Reserve - Cumulative [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	33.5	12.8
Loss Contingency Loss in Period	20.7
FERC ROE Reserve - Cumulative [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	13.6	5.7
Loss Contingency Loss in Period	7.9
FERC ROE Reserve - Cumulative [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	5.1	2.3
Loss Contingency Loss in Period	2.8
FERC ROE Reserve - Cumulative [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	8.5	2.9
Loss Contingency Loss in Period	5.6
FERC ROE Reserve - Cumulative - After Tax Charge [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	22.4	14.3
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	12.4	7.7
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	4.9	3.4
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	1.7	1.4
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	$3.40	$1.80

LEASES_Details

LEASES (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Due, Current	$2.40
Capital Leases, Future Minimum Payments Due in Two Years	2.2
Capital Leases, Future Minimum Payments Due in Three Years	2.1
Capital Leases, Future Minimum Payments Due in Four Years	2.1
Capital Leases, Future Minimum Payments Due in Five Years	2
Capital Leases, Future Minimum Payments Due Thereafter	3.5
Capital Leases, Future Minimum Payments Due	14.3
Capital Leases, Future Minimum Payments, Interest Included in Payments	4.9
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments	9.4
Operating Leases, Rent Expense	16.3	14.8
Operating Leases, Future Minimum Payments Due, Current	20.1
Operating Leases, Future Minimum Payments, Due in Two Years	17.6
Operating Leases, Future Minimum Payments, Due in Three Years	14.6
Operating Leases, Future Minimum Payments, Due in Four Years	10.5
Operating Leases, Future Minimum Payments, Due in Five Years	8.6
Operating Leases, Future Minimum Payments, Due Thereafter	22.5
Operating Leases, Future Minimum Payments Due	93.9
The Connecticut Light And Power Company [Member]
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Due, Current	2
Capital Leases, Future Minimum Payments Due in Two Years	1.9
Capital Leases, Future Minimum Payments Due in Three Years	2
Capital Leases, Future Minimum Payments Due in Four Years	2
Capital Leases, Future Minimum Payments Due in Five Years	2
Capital Leases, Future Minimum Payments Due Thereafter	3.5
Capital Leases, Future Minimum Payments Due	13.4
Capital Leases, Future Minimum Payments, Interest Included in Payments	5
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments	8.4
Operating Leases, Rent Expense	8.1	8.2
Operating Leases, Future Minimum Payments Due, Current	4.3
Operating Leases, Future Minimum Payments, Due in Two Years	3.8
Operating Leases, Future Minimum Payments, Due in Three Years	2.6
Operating Leases, Future Minimum Payments, Due in Four Years	1.5
Operating Leases, Future Minimum Payments, Due in Five Years	1.1
Operating Leases, Future Minimum Payments, Due Thereafter	4
Operating Leases, Future Minimum Payments Due	17.3
NSTAR Electric Company [Member]
Operating Leased Assets [Line Items]
Operating Leases, Rent Expense	6.7	6.2
Operating Leases, Future Minimum Payments Due, Current	10
Operating Leases, Future Minimum Payments, Due in Two Years	8.8
Operating Leases, Future Minimum Payments, Due in Three Years	7.7
Operating Leases, Future Minimum Payments, Due in Four Years	5.8
Operating Leases, Future Minimum Payments, Due in Five Years	4.7
Operating Leases, Future Minimum Payments, Due Thereafter	10.4
Operating Leases, Future Minimum Payments Due	47.4
Public Service Company Of New Hampshire [Member]
Operating Leased Assets [Line Items]
Capital Leases, Future Minimum Payments Due, Current	0.4
Capital Leases, Future Minimum Payments Due in Two Years	0.3
Capital Leases, Future Minimum Payments Due in Three Years	0.1
Capital Leases, Future Minimum Payments Due in Four Years	0.1
Capital Leases, Future Minimum Payments Due in Five Years	0
Capital Leases, Future Minimum Payments Due Thereafter	0
Capital Leases, Future Minimum Payments Due	0.9
Capital Leases, Future Minimum Payments, Interest Included in Payments	0
Capital Leases, Future Minimum Payments, Present Value of Net Minimum Payments	0.9
Operating Leases, Rent Expense	1.7	2.5
Operating Leases, Future Minimum Payments Due, Current	1.1
Operating Leases, Future Minimum Payments, Due in Two Years	1
Operating Leases, Future Minimum Payments, Due in Three Years	0.8
Operating Leases, Future Minimum Payments, Due in Four Years	0.6
Operating Leases, Future Minimum Payments, Due in Five Years	0.5
Operating Leases, Future Minimum Payments, Due Thereafter	1.5
Operating Leases, Future Minimum Payments Due	5.5
Western Massachusetts Electric Company [Member]
Operating Leased Assets [Line Items]
Operating Leases, Rent Expense	2.9	3
Operating Leases, Future Minimum Payments Due, Current	1.2
Operating Leases, Future Minimum Payments, Due in Two Years	1
Operating Leases, Future Minimum Payments, Due in Three Years	0.8
Operating Leases, Future Minimum Payments, Due in Four Years	0.6
Operating Leases, Future Minimum Payments, Due in Five Years	0.6
Operating Leases, Future Minimum Payments, Due Thereafter	2.5
Operating Leases, Future Minimum Payments Due	$6.70

FAIR_VALUE_OF_FINANCIAL_INSTRU2

FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	$155,568,000	$155,568,000
Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	155,600,000	155,600,000
Other Long Term Debt	8,851,600,000	8,310,200,000
Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	153,600,000	152,700,000
Other Long Term Debt	9,451,200,000	8,443,100,000
The Connecticut Light And Power Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	116,200,000	116,200,000
The Connecticut Light And Power Company [Member] | Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	116,200,000	116,200,000
Other Long Term Debt	2,842,000,000	2,741,200,000
The Connecticut Light And Power Company [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	112,000,000	110,500,000
Other Long Term Debt	3,214,500,000	2,952,800,000
NSTAR Electric Company [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	43,000,000	43,000,000
NSTAR Electric Company [Member] | Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	43,000,000	43,000,000
Other Long Term Debt	1,797,400,000	1,801,100,000
NSTAR Electric Company [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	41,600,000	42,200,000
Other Long Term Debt	1,993,500,000	1,888,000,000
Public Service Company Of New Hampshire [Member] | Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other Long Term Debt	1,076,300,000	1,049,000,000
Public Service Company Of New Hampshire [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other Long Term Debt	1,137,900,000	1,073,900,000
Western Massachusetts Electric Company [Member] | Carrying Reported Amount Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other Long Term Debt	628,500,000	629,400,000
Western Massachusetts Electric Company [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other Long Term Debt	$689,400,000	$640,100,000

ACCUMULATED_OTHER_COMPREHENSIV2

ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				($46,031,000)				($72,900,000)	($46,031,000)	($72,900,000)
Other Comprehensive Income Before Reclassifications, Net of Tax									-33,900,000	18,500,000
Reclassification From OCI, Net Of Tax									5,900,000	8,400,000
Other Comprehensive Income (Loss), Net of Tax, Total									-27,978,000	26,823,000	-2,168,000
AOCI Ending Balance	-74,009,000				-46,031,000				-74,009,000	-46,031,000	-72,900,000
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									-362,106,000	-338,699,000	-329,945,000
Income Tax Expense									-468,297,000	-426,941,000	-274,926,000
Net Income Attributable to Controlling Interests	221,500,000	234,600,000	127,400,000	236,000,000	177,400,000	209,500,000	171,000,000	228,100,000	819,546,000	786,007,000	525,945,000
OCI Benefit Plans Before Reclassification Adjustments, Tax [Abstract]
OCI Benefit Plans Before Reclassification Adjustments TAX Effect									18,700,000	11,400,000	6,200,000
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									3,500,000
Pension SERP And PBOP Gain Loss To Be Reclassified Within Twelve Months									6,900,000
The Connecticut Light And Power Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				-1,387,000					-1,387,000
Other Comprehensive Income (Loss), Net of Tax, Total									456,000	413,000	451,000
AOCI Ending Balance	-931,000				-1,387,000				-931,000	-1,387,000
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									-147,421,000	-133,650,000	-133,127,000
Income Tax Expense									-133,451,000	-141,663,000	-94,437,000
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									700,000
Public Service Company Of New Hampshire [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				-8,550,000					-8,550,000
Other Comprehensive Income (Loss), Net of Tax, Total									1,181,000	1,105,000	1,177,000
AOCI Ending Balance	-7,369,000				-8,550,000				-7,369,000	-8,550,000
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									-45,349,000	-46,176,000	-50,228,000
Income Tax Expense									-72,135,000	-71,101,000	-60,993,000
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									2,000,000
Western Massachusetts Electric Company [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				-3,517,000					-3,517,000
Other Comprehensive Income (Loss), Net of Tax, Total									341,000	329,000	340,000
AOCI Ending Balance	-3,176,000				-3,517,000				-3,176,000	-3,517,000
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									-24,931,000	-24,851,000	-26,634,000
Income Tax Expense									-37,268,000	-37,368,000	-32,140,000
Future AOCI Reclassification Amounts [Abstract]
Cash Flow Hedge Loss To Be Reclassified Within Twelve Months									600,000
Qualified Cash Flow Hedging Instruments [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				-14,400,000				-16,400,000	-14,400,000	-16,400,000
Other Comprehensive Income Before Reclassifications, Net of Tax									0
Reclassification From OCI, Net Of Tax									2,000,000	2,000,000
Other Comprehensive Income (Loss), Net of Tax, Total									2,000,000	2,000,000
AOCI Ending Balance	-12,400,000				-14,400,000				-12,400,000	-14,400,000
Unrealized Gains Losses on Available for Sale Securities [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				400,000				1,300,000	400,000	1,300,000
Other Comprehensive Income Before Reclassifications, Net of Tax									300,000	-900,000
Other Comprehensive Income (Loss), Net of Tax, Total									300,000	-900,000
AOCI Ending Balance	700,000				400,000				700,000	400,000
Pension, SERP and PBOP Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss Net Of Tax RollForward [Abstract]
AOCI Beginning Balance				-32,000,000				-57,800,000	-32,000,000	-57,800,000
Other Comprehensive Income Before Reclassifications, Net of Tax									-34,200,000	19,400,000
Reclassification From OCI, Net Of Tax									3,900,000	6,400,000
Other Comprehensive Income (Loss), Net of Tax, Total									-30,300,000	25,800,000
AOCI Ending Balance	-62,300,000				-32,000,000				-62,300,000	-32,000,000
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Net Income Attributable to Controlling Interests									-5,900,000	-8,400,000	-7,800,000
Reclassification out of Accumulated Other Comprehensive Income [Member] | Qualified Cash Flow Hedging Instruments [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
Total Interest Expense									-3,400,000	-3,400,000	-3,300,000
Income Tax Expense									1,400,000	1,400,000	1,300,000
Net Income Attributable to Controlling Interests									-2,000,000	-2,000,000	-2,000,000
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension, SERP and PBOP Benefit Plans [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Abstract]
OCI Reclassification Adjustment for Net Actuarial Gain (Loss), before Tax									-6,200,000	-10,500,000	-8,900,000
OCI Reclassification for Transition Obligation											-200,000
OCI Reclassification Adjustment for Amortization of Prior Service Cost, before tax									-200,000	-200,000	-200,000
OCI Reclassification Adjustment for Pension, SERP and PBOP Plans, before Tax									-6,400,000	-10,700,000	-9,300,000
Income Tax Expense									2,500,000	4,300,000	3,500,000
Net Income Attributable to Controlling Interests									($3,900,000)	($6,400,000)	($5,800,000)

DIVIDEND_RESTRICTIONS_Details

DIVIDEND RESTRICTIONS (Details)	12 Months Ended
	Dec. 31, 2014
Line of Credit Facility, Dividend Restrictions	CL&P, NSTAR Electric, PSNH and WMECO are subject to Section 305 of the Federal Power Act that makes it unlawful for a public utility to make or pay a dividend from any funds "properly included in its capital account." Management believes that this Federal Power Act restriction, as applied to CL&P, NSTAR Electric, PSNH and WMECO, would not be construed or applied by the FERC to prohibit the payment of dividends for lawful and legitimate business purposes from retained earnings. In addition, certain state statutes may impose additional limitations on such companies and on Yankee Gas and NSTAR Gas. Such state law restrictions do not restrict payment of dividends from retained earnings or net income. Pursuant to the joint revolving credit agreement of NU, CL&P, PSNH, WMECO, Yankee Gas and NSTAR Gas, and the NSTAR Electric revolving credit agreement, each company is required to maintain consolidated total debt to total capitalization ratio of no greater than 65 percent at all times. As of December 31, 2014, all companies were in compliance with such covenant. The Retained Earnings balances subject to these restrictions were $2.4 billion for NU, $1.1 billion for CL&P, $1.5 billion for NSTAR Electric, $486.5 million for PSNH and $178.8 million for WMECO as of December 31, 2014. As of December 31, 2014, NU, CL&P, NSTAR Electric, PSNH, WMECO, Yankee Gas and NSTAR Gas were in compliance with all such provisions of the revolving credit agreements that may restrict the payment of dividends.
Public Service Company Of New Hampshire [Member]
Statutory Dividend Payment Restrictions Disclosure	PSNH is further required to reserve an additional amount under its FERC hydroelectric license conditions. As of December 31, 2014, $13 million of PSNH's Retained Earnings was subject to restriction under its FERC hydroelectric license conditions and PSNH was in compliance with this provision.

COMMON_SHARES_Details

COMMON SHARES (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Class Of Stock [Line Items]
Common Stock Par Or Stated Value Per Share	$5	$5
Common Stock Shares Authorized	380,000,000	380,000,000
Common Stock Shares Issued	333,359,172	333,113,492
Treasury Stock Shares	16,375,835	17,796,672
Common Stock, Shares, Outstanding	316,983,337	315,273,559
The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Common Stock Par Or Stated Value Per Share	$10	$10
Common Stock Shares Authorized	24,500,000	24,500,000
Common Stock Shares Issued	6,035,205	6,035,205
NSTAR Electric Company [Member]
Class Of Stock [Line Items]
Common Stock Par Or Stated Value Per Share	$1	$1
Common Stock Shares Authorized	100,000,000	100,000,000
Common Stock Shares Issued	100	100
Public Service Company Of New Hampshire [Member]
Class Of Stock [Line Items]
Common Stock Par Or Stated Value Per Share	$1	$1
Common Stock Shares Authorized	100,000,000	100,000,000
Common Stock Shares Issued	301	301
Western Massachusetts Electric Company [Member]
Class Of Stock [Line Items]
Common Stock Par Or Stated Value Per Share	$25	$25
Common Stock Shares Authorized	1,072,471	1,072,471
Common Stock Shares Issued	434,653	434,653

PREFFERED_STOCK_NOT_SUBJECT_TO1

PREFFERED STOCK NOT SUBJECT TO MANDATORY REDEMPTION (DETAILS) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Class Of Stock [Line Items]
Preferred Stock, Value, Outstanding	$155,600,000	$155,600,000
Cash Dividends on Preferred Stock	7,519,000	7,682,000	7,029,000
The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	2,324,000
Preferred Stock, Value, Outstanding	116,200,000	116,200,000
Cash Dividends on Preferred Stock	5,559,000	5,559,000	5,559,000
Preferred Stock Authorized Description	The CL&P and NSTAR Electric preferred stock is not subject to mandatory redemption and is presented as a noncontrolling interest of a subsidiary in NU’s financial statements. CL&P Preferred Stock: CL&P's charter authorizes it to issue up to 9 million shares of preferred stock ($50 par value per share).
NSTAR Electric Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	430,000
Preferred Stock, Value, Outstanding	43,000,000	43,000,000
Cash Dividends on Preferred Stock	1,961,000	2,123,000	1,960,000
Preferred Stock Authorized Description	NSTAR Electric is authorized to issue 2,890,000 shares ($100 par value per share).
Series 1947 $1.90 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	163,912
Preferred Stock, Redemption Price Per Share	$52.50
Preferred Stock, Value, Outstanding	8,200,000	8,200,000
Series 1947 $2.00 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	336,088
Preferred Stock, Redemption Price Per Share	$54
Preferred Stock, Value, Outstanding	16,800,000	16,800,000
Series 1949 $2.04 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	100,000
Preferred Stock, Redemption Price Per Share	$52
Preferred Stock, Value, Outstanding	5,000,000	5,000,000
Series 1949 $2.20 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	200,000
Preferred Stock, Redemption Price Per Share	$52.50
Preferred Stock, Value, Outstanding	10,000,000	10,000,000
Series 1949 3.90% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	160,000
Preferred Stock, Redemption Price Per Share	$50.50
Preferred Stock, Value, Outstanding	8,000,000	8,000,000
Series E 1954 $2.06 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	200,000
Preferred Stock, Redemption Price Per Share	$51
Preferred Stock, Value, Outstanding	10,000,000	10,000,000
Series F 1955 $2.09 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	100,000
Preferred Stock, Redemption Price Per Share	$51
Preferred Stock, Value, Outstanding	5,000,000	5,000,000
Series 1956 4.5% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	104,000
Preferred Stock, Redemption Price Per Share	$50.75
Preferred Stock, Value, Outstanding	5,200,000	5,200,000
Series 1958 4.96% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	100,000
Preferred Stock, Redemption Price Per Share	$50.50
Preferred Stock, Value, Outstanding	5,000,000	5,000,000
Series 1963 4.5% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	160,000
Preferred Stock, Redemption Price Per Share	$50.50
Preferred Stock, Value, Outstanding	8,000,000	8,000,000
Series 1967 5.28% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	200,000
Preferred Stock, Redemption Price Per Share	$51.43
Preferred Stock, Value, Outstanding	10,000,000	10,000,000
Series G of 1968 $3.24 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	300,000
Preferred Stock, Redemption Price Per Share	$51.84
Preferred Stock, Value, Outstanding	15,000,000	15,000,000
Series 1968 6.56% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	200,000
Preferred Stock, Redemption Price Per Share	$51.44
Preferred Stock, Value, Outstanding	10,000,000	10,000,000
Preferred Stock 4.25% [Member] | NSTAR Electric Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	180,000
Preferred Stock, Redemption Price Per Share	$103.63
Preferred Stock, Value, Outstanding	18,000,000	18,000,000
Preferred Stock 4.78% [Member] | NSTAR Electric Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	250,000
Preferred Stock, Redemption Price Per Share	$102.80
Preferred Stock, Value, Outstanding	$25,000,000	$25,000,000

COMMON_SHAREHOLDER_EQUIT_AND_N

COMMON SHAREHOLDER EQUIT AND NCI (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Shareholders' Equity, Beginning Balance				$9,611,528,000					$9,611,528,000
Net Income	223,600,000	236,500,000	129,200,000	237,800,000	179,200,000	211,400,000	173,100,000	230,000,000	827,065,000	793,689,000	533,077,000
Net Income Attributable to Noncontrolling Interests									7,519,000	7,682,000	7,132,000
Other Comprehensive Income (Loss), Net of Tax									-27,978,000	26,823,000	-2,168,000
Shareholders' Equity, Ending Balance	9,976,815,000				9,611,528,000				9,976,815,000	9,611,528,000
Common Shareholders Equity [Member]
Shareholders' Equity, Beginning Balance											4,012,700,000
Net Income									827,065,000	793,689,000	3,359,000
Dividends on Common Shares									-496,524,000	11,078,000	-375,527,000
Dividends on Preferred Stock									-7,519,000	-10,748,000	-7,029,000
Issuance of Common Shares									6,609,000	29,468,000	13,327,000
Other Transcations, Net									63,600,000	13,200,000	21,100,000
Net Income Attributable to Noncontrolling Interests											-103,000
Other Comprehensive Income (Loss), Net of Tax									-27,978,000		-2,168,000
Purchase Price of NSTAR											5,038,300,000
Other Impacts of Merger with NSTAR											3,400,000
Noncontrolling Interest Member
Shareholders' Equity, Beginning Balance								0		0	3,000,000
Contributions To Northern Pass Trans											300,000
Net Income Attributable to Noncontrolling Interests										0	100,000
Shareholders' Equity, Ending Balance					0					0	0
Other Impacts of Merger with NSTAR											-3,400,000
Total Equity [Member]
Shareholders' Equity, Beginning Balance				9,611,500,000				9,237,100,000	9,611,500,000	9,237,100,000	4,015,700,000
Net Income									827,100,000	793,700,000	533,100,000
Dividends on Common Shares									-496,500,000	-462,700,000	-375,500,000
Dividends on Preferred Stock									-7,500,000	-7,700,000	-7,000,000
Issuance of Common Shares									6,600,000	11,100,000	13,300,000
Contributions To Northern Pass Trans											300,000
Other Transcations, Net									63,600,000	13,200,000	21,100,000
Net Income Attributable to Noncontrolling Interests									0
Other Comprehensive Income (Loss), Net of Tax									-28,000,000	26,800,000	-2,200,000
Shareholders' Equity, Ending Balance	9,976,800,000				9,611,500,000				9,976,800,000	9,611,500,000	9,237,100,000
Purchase Price of NSTAR											5,038,300,000
Preferred Stock Member
Shareholders' Equity, Beginning Balance				155,600,000				155,600,000	155,600,000	155,600,000	116,200,000
Dividends on Preferred Stock									-7,500,000	-7,700,000	-7,000,000
Net Income Attributable to Noncontrolling Interests									7,500,000	7,700,000	7,000,000
Shareholders' Equity, Ending Balance	155,600,000				155,600,000				155,600,000	155,600,000	155,600,000
Other Impacts of Merger with NSTAR											$39,400,000

EARNINGS_PER_SHARE_Details

EARNINGS PER SHARE (Details) (USD $)	0 Months Ended	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Apr. 09, 2012	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Notes To Consolidated Financial Statements [Abstract]
Net Income Attributable to Controlling Interests		$221,500	$234,600	$127,400	$236,000	$177,400	$209,500	$171,000	$228,100	$819,546	$786,007	$525,945
Basic										316,136,748	315,311,387	277,209,819
Weighted Average Number Diluted Shares Outstanding Adjustment										1,280,666	899,773	783,812
Diluted										317,417,414	316,211,160	277,993,631
Earnings Per Share Basic And Diluted		$0.69	$0.74	$0.40	$0.75	$0.56	$0.66	$0.54	$0.72
Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount										3,643	1,575	4,266
Basic Earnings Per Common Share										$2.59	$2.49	$1.90
Diluted Earnings Per Common Share										$2.58	$2.49	$1.89
Common Stock Issuance Merger With Nstar [Abstract]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,049,000

SEGMENT_INFORMATION_Details

SEGMENT INFORMATION (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating Revenues	$1,881,200,000	$1,892,500,000	$1,677,600,000	$2,290,600,000	$1,777,700,000	$1,892,600,000	$1,635,900,000	$1,995,000,000	$7,741,856,000	$7,301,204,000	$6,273,787,000
Operating Income	430,200,000	440,900,000	294,000,000	467,700,000	360,600,000	399,300,000	350,600,000	418,900,000	1,632,849,000	1,529,435,000	1,118,206,000
Total Interest Expense									-362,106,000	-338,699,000	-329,945,000
Other Income/(Loss), Net									24,619,000	29,894,000	19,742,000
Income Tax Expense									-468,297,000	-426,941,000	-274,926,000
Net Income	223,600,000	236,500,000	129,200,000	237,800,000	179,200,000	211,400,000	173,100,000	230,000,000	827,065,000	793,689,000	533,077,000
Net Income Attributable to Noncontrolling Interests									-7,519,000	-7,682,000	-7,132,000
Net Income Attributable to Controlling Interests	221,500,000	234,600,000	127,400,000	236,000,000	177,400,000	209,500,000	171,000,000	228,100,000	819,546,000	786,007,000	525,945,000
Total Assets	29,777,975,000				27,795,537,000				29,777,975,000	27,795,537,000
Investments in Property, Plant and Equipment									1,603,744,000	1,456,787,000	1,472,272,000
Electric Distribution Member
Segment Reporting Information [Line Items]
Operating Revenues									5,663,400,000	5,362,300,000	4,716,500,000
Depreciation And Amortization									-384,600,000	-604,800,000	-530,300,000
Operating Expenses, Other									-4,366,200,000	-3,927,700,000	-3,585,400,000
Operating Income									912,600,000	829,800,000	600,800,000
Total Interest Expense									-191,600,000	-175,000,000	-165,600,000
Interest Income									5,100,000	4,100,000	2,800,000
Other Income/(Loss) Excluding Interest Income									10,700,000	12,900,000	8,900,000
Income Tax Expense									-269,700,000	-240,000,000	-150,200,000
Net Income									467,100,000	431,800,000	296,700,000
Net Income Attributable to Noncontrolling Interests									-4,700,000	-4,800,000	-4,400,000
Net Income Attributable to Controlling Interests									462,400,000	427,000,000	292,300,000
Total Assets	17,563,400,000				17,260,000,000				17,563,400,000	17,260,000,000
Investments in Property, Plant and Equipment									645,200,000	639,000,000	611,700,000
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Operating Revenues									1,007,300,000	855,800,000	572,900,000
Depreciation And Amortization									-68,100,000	-66,700,000	-49,100,000
Operating Expenses, Other									-786,700,000	-659,400,000	-445,200,000
Operating Income									152,500,000	129,700,000	78,600,000
Total Interest Expense									-34,000,000	-33,100,000	-31,300,000
Interest Income									0	0	0
Other Income/(Loss) Excluding Interest Income									200,000	800,000	400,000
Income Tax Expense									-46,400,000	-36,500,000	-16,900,000
Net Income									72,300,000	60,900,000	30,800,000
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									72,300,000	60,900,000	30,800,000
Total Assets	3,030,900,000				2,759,700,000				3,030,900,000	2,759,700,000
Investments in Property, Plant and Equipment									176,700,000	168,100,000	148,700,000
Transmission [Member]
Segment Reporting Information [Line Items]
Operating Revenues									1,018,200,000	978,700,000	861,500,000
Depreciation And Amortization									-150,500,000	-136,200,000	-109,200,000
Operating Expenses, Other									-302,100,000	-281,800,000	-251,600,000
Operating Income									565,600,000	560,700,000	500,700,000
Total Interest Expense									-104,100,000	-100,300,000	-96,700,000
Interest Income									900,000	700,000	400,000
Other Income/(Loss) Excluding Interest Income									10,300,000	10,900,000	7,300,000
Income Tax Expense									-174,500,000	-182,100,000	-159,200,000
Net Income									298,200,000	289,900,000	252,500,000
Net Income Attributable to Noncontrolling Interests									-2,800,000	-2,900,000	-2,800,000
Net Income Attributable to Controlling Interests									295,400,000	287,000,000	249,700,000
Total Assets	7,625,600,000				6,745,800,000				7,625,600,000	6,745,800,000
Investments in Property, Plant and Equipment									731,600,000	618,500,000	663,600,000
Other Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues									790,900,000	777,500,000	803,800,000
Depreciation And Amortization									-42,100,000	-62,200,000	-56,400,000
Operating Expenses, Other									-748,000,000	-715,000,000	-817,000,000
Operating Income									800,000	300,000	-69,600,000
Total Interest Expense									-36,600,000	-35,500,000	-43,600,000
Interest Income									3,600,000	5,400,000	7,100,000
Other Income/(Loss) Excluding Interest Income									916,000,000	858,900,000	795,000,000
Income Tax Expense									22,300,000	31,900,000	55,500,000
Net Income									906,100,000	861,000,000	744,400,000
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									906,100,000	861,000,000	744,400,000
Total Assets	12,682,500,000				11,842,400,000				12,682,500,000	11,842,400,000
Investments in Property, Plant and Equipment									50,200,000	31,200,000	48,300,000
Eliminations Segment [Member]
Segment Reporting Information [Line Items]
Operating Revenues									-737,900,000	-673,100,000	-680,900,000
Depreciation And Amortization									19,900,000	10,200,000	4,200,000
Operating Expenses, Other									719,300,000	671,800,000	684,400,000
Operating Income									1,300,000	8,900,000	7,700,000
Total Interest Expense									4,200,000	5,200,000	7,300,000
Interest Income									-3,600,000	-5,600,000	-7,100,000
Other Income/(Loss) Excluding Interest Income									-918,600,000	-858,200,000	-795,100,000
Income Tax Expense									0	-200,000	-4,100,000
Net Income									-916,700,000	-849,900,000	-791,300,000
Net Income Attributable to Noncontrolling Interests									0	0	0
Net Income Attributable to Controlling Interests									-916,700,000	-849,900,000	-791,300,000
Total Assets	-11,124,400,000				-10,812,400,000				-11,124,400,000	-10,812,400,000
Investments in Property, Plant and Equipment									0	0	0
Total [Member]
Segment Reporting Information [Line Items]
Operating Revenues									7,741,900,000	7,301,200,000	6,273,800,000
Depreciation And Amortization									-625,400,000	-859,700,000	-740,800,000
Operating Expenses, Other									-5,483,700,000	-4,912,100,000	-4,414,800,000
Operating Income									1,632,800,000	1,529,400,000	1,118,200,000
Total Interest Expense									-362,100,000	-338,700,000	-329,900,000
Interest Income									6,000,000	4,600,000	3,200,000
Other Income/(Loss) Excluding Interest Income									18,600,000	25,300,000	16,500,000
Income Tax Expense									-468,300,000	-426,900,000	-274,900,000
Net Income									827,000,000	793,700,000	533,100,000
Net Income Attributable to Noncontrolling Interests									-7,500,000	-7,700,000	-7,200,000
Net Income Attributable to Controlling Interests									819,500,000	786,000,000	525,900,000
Total Assets	29,778,000,000				27,795,500,000				29,778,000,000	27,795,500,000
Investments in Property, Plant and Equipment									$1,603,700,000	$1,456,800,000	$1,472,300,000

MERGER_OF_NU_AND_NSTAR_Details

MERGER OF NU AND NSTAR (Details) (USD $)	0 Months Ended	12 Months Ended
In Millions, except Share data, unless otherwise specified	Apr. 09, 2012	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013
Business Combination Consideration Transferred [Abstract]
Number of NSTAR Shares Outstanding	103,696,000
Exchange Ratio	1.312
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,049,000
Business Acquisition Share Price	$36.79
Business Acquisition Cost Of Acquired Entity Equity Interests Issued And Issuable	$5,005
Business Combination Consideration Transferred Other	33
Total Purchase Price, Business Combination Consideration Transferred	5,038
Business Acquisition, Number of Shares Awarded Vested Immediately Prior to Merger	109,000
Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Less Noncontrolling Interest [Abstract]
Goodwill Acquired During Period	3,232	3,200
Business Acquisition, Pro Forma Information [Abstract]
Business Acquisition, Pro Forma Revenue		7,004
Business Acquisition, Pro Forma Net Income		630
Business Acquisition, Pro Forma Earnings Per Share, Basic		$2
Business Acquisition, Pro Forma Earnings Per Share, Diluted		$1.99
Customer Rate Credits for Business Combination		46
Storm Cost Deferral Reduction		40
Establishment of Energy Efficiency Fund		15
Total Pre-Tax Settlement Agreement Impacts		101
Business Combination After Tax Transaction Costs		32
After Tax Settlement Costs Incurred		60
After-Tax Costs Transaction and Other Non Recurring Costs Excluded from Pro Forma Net Income		92
Business Combination, Description
Business Acquisition, Percentage of Voting Interests Acquired	100.00%
Common Stock, Shares, Outstanding			316,983,337	315,273,559
Business Acquisition, Description of Settlement Agreements		Regulatory Approvals: On February 15, 2012, NU and NSTAR reached comprehensive merger settlement agreements with the Massachusetts Attorney General and the DOER. The Attorney General settlement agreement covered a variety of rate-making and rate design issues, including a base distribution rate freeze through 2015 for NSTAR Electric, NSTAR Gas and WMECO and $15 million, $3 million and $3 million in the form of rate credits to their respective customers. On April 4, 2012, the DPU approved the settlement agreements and the merger of NU and NSTAR. On March 13, 2012, NU and NSTAR reached a comprehensive merger settlement agreement with both the Connecticut Attorney General and the Connecticut Office of Consumer Counsel. The settlement agreement covered a variety of matters, including a $25 million rate credit to CL&P customers, a CL&P base distribution rate freeze until December 1, 2014, and the establishment of a $15 million fund for energy efficiency and other initiatives to be disbursed at the direction of the DEEP. In the agreement, CL&P agreed to forego rate recovery of $40 million of the deferred storm restoration costs associated with restoration activities following Tropical Storm Irene and the October 2011 snowstorm. On April 2, 2012, the PURA approved the settlement agreement and the merger of NU and NSTAR.
The Connecticut Light And Power Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
Customer Rate Credits for Business Combination		25
Storm Cost Deferral Reduction		40
Total Pre-Tax Settlement Agreement Impacts		65
NSTAR Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
Customer Rate Credits for Business Combination		15
Total Pre-Tax Settlement Agreement Impacts		15
Western Massachusetts Electric Company [Member]
Business Acquisition, Pro Forma Information [Abstract]
Customer Rate Credits for Business Combination		3
Total Pre-Tax Settlement Agreement Impacts		$3

GOODWILL_Details

GOODWILL (Details) (USD $)	0 Months Ended	12 Months Ended
	Apr. 09, 2012	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2013
Goodwill [Line item]
Goodwill Acquired During Period	$3,232,000,000	$3,200,000,000
Goodwill			3,519,401,000	3,519,401,000
Yankee Gas Services Company [Member]
Goodwill [Line item]
Goodwill			300,000,000	300,000,000
Electric Distribution Member
Goodwill [Line item]
Goodwill			2,500,000,000	2,500,000,000
Transmission [Member]
Goodwill [Line item]
Goodwill			600,000,000	600,000,000
Natural Gas Distribution [Member]
Goodwill [Line item]
Goodwill			$400,000,000	$400,000,000

Recovered_Sheet2

QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Revenues	$1,881,200	$1,892,500	$1,677,600	$2,290,600	$1,777,700	$1,892,600	$1,635,900	$1,995,000	$7,741,856	$7,301,204	$6,273,787
Operating Income	430,200	440,900	294,000	467,700	360,600	399,300	350,600	418,900	1,632,849	1,529,435	1,118,206
Net Income	223,600	236,500	129,200	237,800	179,200	211,400	173,100	230,000	827,065	793,689	533,077
Net Income Attributable to Controlling Interests	221,500	234,600	127,400	236,000	177,400	209,500	171,000	228,100	819,546	786,007	525,945
Basic Earnings Per Common Share									$2.59	$2.49	$1.90
Diluted Earnings Per Common Share									$2.58	$2.49	$1.89
Earnings Per Share Basic And Diluted	$0.69	$0.74	$0.40	$0.75	$0.56	$0.66	$0.54	$0.72
The Connecticut Light And Power Company [Member]
Operating Revenues	675,100	695,600	587,300	734,600	600,500	648,400	569,300	624,100	2,692,582	2,442,341	2,407,449
Operating Income	159,000	146,200	92,100	158,000	119,200	133,900	136,800	149,700	555,250	539,576	426,989
Net Income	87,200	83,900	37,400	79,300	60,200	66,300	67,900	85,000	287,754	279,412	209,725
NSTAR Electric Company [Member]
Operating Revenues	581,100	727,900	561,500	666,200	576,900	753,900	570,400	592,300	2,536,677	2,493,479	2,300,997
Operating Income	132,000	206,600	121,500	118,400	109,200	192,000	112,500	94,500	578,456	508,156	381,416
Net Income	69,300	115,600	60,100	58,100	55,400	107,000	58,000	48,100	303,088	268,546	190,242
Public Service Company Of New Hampshire [Member]
Operating Revenues	224,400	223,700	211,600	299,800	226,900	218,600	216,100	273,800	959,500	935,402	988,013
Operating Income	60,000	56,400	49,000	64,000	56,200	56,600	54,300	58,100	229,383	225,219	205,095
Net Income	29,000	28,200	24,100	32,600	26,800	28,400	27,200	29,000	113,944	111,397	96,882
Western Massachusetts Electric Company [Member]
Operating Revenues	129,600	118,100	108,300	137,400	110,900	121,800	115,000	125,000	493,423	472,724	441,164
Operating Income	34,000	31,200	17,700	34,700	22,400	28,900	32,400	35,600	117,639	119,347	110,774
Net Income	$18,000	$14,700	$7,000	$18,100	$10,400	$15,000	$16,400	$18,600	$57,819	$60,438	$54,503

SCHEDULE_I_Balance_Sheets_Deta

SCHEDULE I - Balance Sheets (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Current Assets:
Cash	$38,703	$43,364	$45,748	$6,559
Receivables, Net	856,346	765,391
Taxes Receivable	337,307	16,629
Prepayments and Other Current Assets	226,194	197,659
Total Current Assets	2,692,465	2,087,049
Deferred Debits and Other Assets:
Other Long-Term Assets	349,957	365,692
Total Deferred Debits and Other Assets	8,438,469	8,132,302
Goodwill	3,519,401	3,519,401
Total Assets	29,777,975	27,795,537
Current Liabilities:
Notes Payable	956,825	1,093,000
Long-Term Debt - Current Portion	245,583	533,346
Accounts Payable - Current	868,231	742,251
Other Liabilities - Current	828,720	702,776
Total Current Liabilities	3,134,381	3,275,651
Deferred Credits and Other Liabilities:
Other Long-Term Liabilities	874,387	923,053
Total Deferred Credits and Other Liabilities	7,905,194	6,975,957
Capitalization:
Long-Term Debt	8,606,017	7,776,833
Common Shareholders' Equity:
Common Stock	1,666,796	1,665,351
Capital Surplus, Paid In	6,235,834	6,192,765
Retained Earnings	2,448,661	2,125,980
Accumulated Other Comprehensive Loss	-74,009	-46,031	-72,900
Treasury Stock	-300,467	-326,537
Common Shareholders' Equity	9,976,815	9,611,528
Total Capitalization	18,738,400	17,543,929
Total Liabilities and Capitalization	29,777,975	27,795,537
Nu Parent [Member]
Current Assets:
Cash	138	35	429	62
Notes Receivable from Affiliated Companies	741,150	871,050
Accounts Receivable from Affiliated Companies	6,725	95,513
Prepayments and Other Current Assets	41,366	59,905
Total Current Assets	789,379	1,026,503
Deferred Debits and Other Assets:
Equity Method Investments	8,387,976	7,733,051
Notes Receivable, Related Parties, Noncurrent	106,300	62,500
Deferred Tax Assets, Net, Noncurrent	177,908	205,779
Other Long-Term Assets	33,552	22,099
Total Deferred Debits and Other Assets	11,937,547	11,255,240
Goodwill	3,231,811	3,231,811
Total Assets	12,726,926	12,281,743
Current Liabilities:
Notes Payable	654,825	1,014,500
Long-Term Debt - Current Portion	28,883	31,696
Accounts Payable - Current	141	441
Accounts Payable to Affiliated Companies	150,268	144,026
Other Liabilities - Current	71,778	59,559
Total Current Liabilities	905,895	1,250,222
Deferred Credits and Other Liabilities:
Other Long-Term Liabilities	125,608	122,226
Total Deferred Credits and Other Liabilities	125,608	122,226
Capitalization:
Long-Term Debt	1,718,608	1,297,767
Common Shareholders' Equity:
Common Stock	1,666,796	1,665,351
Capital Surplus, Paid In	6,235,834	6,192,765
Retained Earnings	2,448,661	2,125,980
Accumulated Other Comprehensive Loss	-74,009	-46,031
Treasury Stock	-300,467	-326,537
Common Shareholders' Equity	9,976,815	9,611,528
Total Capitalization	11,695,423	10,909,295
Total Liabilities and Capitalization	$12,726,926	$12,281,743

SCHEDULE_I_Income_Statements_D

SCHEDULE I - Income Statements (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Operating Revenues	$1,881,200	$1,892,500	$1,677,600	$2,290,600	$1,777,700	$1,892,600	$1,635,900	$1,995,000	$7,741,856	$7,301,204	$6,273,787
Operating Expenses:
Operating Income	430,200	440,900	294,000	467,700	360,600	399,300	350,600	418,900	1,632,849	1,529,435	1,118,206
Total Interest Expense									362,106	338,699	329,945
Other Income [Abstract]
Income Before Income Tax Expense									1,295,362	1,220,630	808,003
Income Tax Expense									468,297	426,941	274,926
Net Income	223,600	236,500	129,200	237,800	179,200	211,400	173,100	230,000	827,065	793,689	533,077
Net Income Attributable to Noncontrolling Interests									7,519	7,682	7,132
Net Income Attributable to Controlling Interests	221,500	234,600	127,400	236,000	177,400	209,500	171,000	228,100	819,546	786,007	525,945
Basic Earnings Per Common Share									$2.59	$2.49	$1.90
Diluted Earnings Per Common Share									$2.58	$2.49	$1.89
Weighted Average Common Shares Outstanding:
Basic									316,136,748	315,311,387	277,209,819
Diluted									317,417,414	316,211,160	277,993,631
Nu Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Operating Revenues									0	8	2
Operating Expenses:
Other Operating Expenses									29,598	12,766	80,719
Operating Income									-29,598	-12,758	-80,717
Total Interest Expense									33,168	31,639	36,325
Other Income [Abstract]
Equity In Earnings Of Subsidiaries									848,435	785,650	579,221
Other Income/(Loss) Excluding Interest Income									1,830	5,062	6,080
Total Other Income									850,265	790,712	585,301
Income Before Income Tax Expense									787,499	746,315	468,259
Income Tax Expense									-32,047	-39,692	-57,789
Net Income									819,546	786,007	526,048
Net Income Attributable to Noncontrolling Interests									0	0	103
Net Income Attributable to Controlling Interests									$819,546	$786,007	$525,945
Basic Earnings Per Common Share									$2.59	$2.49	$1.90
Diluted Earnings Per Common Share									$2.58	$2.49	$1.89
Weighted Average Common Shares Outstanding:
Basic									316,136,748	315,311,387	277,209,819
Diluted									317,417,414	316,211,160	277,993,631

SCHEDULE_I_Cash_Flows_Details

SCHEDULE I - Cash Flows (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Activities:
Net Income	$223,600	$236,500	$129,200	$237,800	$179,200	$211,400	$173,100	$230,000	$827,065	$793,689	$533,077
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Deferred Income Tax Expense (Benefit)									443,259	431,413	292,000
Other Noncash Income (Expense)									39,523	56,071	42,852
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net									122,139	163,549	20,214
Accounts Payable									144,743	-54,619	-128,339
Taxes Receivable/(Accrued)									323,224	50,950	5,450
Other Current Assets and Liabilities									-15,797	22,623	-8,532
Net Cash Flows Provided by Operating Activities									1,635,473	1,663,539	1,161,229
Investing Activities:
Other Investing Activities									-14,380	-67,816	-35,683
Net Cash Flows Used in Investing Activities									-1,591,795	-1,441,223	-1,467,924
Financing Activities:
Cash Dividends on Common Stock									475,227	462,741	375,047
Increase/(Decrease) in Short-Term Debt									285,075	-397,000	825,000
Issuance of Long-Term Debt									725,000	1,680,000	850,000
Retirements of Long-Term Debt									576,551	929,885	839,136
Other Financing Activities									883	-25,253	6,529
Net Cash Flows Provided by/(Used in) Financing Activities									-48,339	-224,700	345,884
Supplemental Cash Flow Information [Abstract]
Interest Paid, Net of Amounts Capitalized									349,600	343,300	356,500
Income Taxes Paid									334,200	50,000	-12,800
Net Increase/(Decrease) in Cash									-4,661	-2,384	39,189
Cash - Beginning of Year				43,364				45,748	43,364	45,748	6,559
Cash - End of Year	38,703				43,364				38,703	43,364	45,748
Nu Parent [Member]
Operating Activities:
Net Income									819,546	786,007	526,048
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Equity In Earnings of Subsidiaries									-848,435	-785,650	-579,221
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries									609,800	407,837	374,584
Deferred Income Tax Expense (Benefit)									7,956	15,159	-15,350
Other Noncash Income (Expense)									9,409	29,169	-3,755
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net									88,800	14,704	-18,321
Accounts Payable									5,942	-7,058	48,332
Taxes Receivable/(Accrued)									23,178	13,295	-16,872
Other Current Assets and Liabilities									14,484	-1,411	60,182
Net Cash Flows Provided by Operating Activities									730,680	472,052	375,627
Investing Activities:
Capital Contributions To Subsidiaries									-437,553	-65,400	-81,431
Return Of Investment In Subsidiaries									0	0	8,207
(Increase) Decrease in Due from Related Parties									0	0	2,200
(Increase) Decrease In Notes Receivable From Affiliated Company									86,100	5,475	-704,475
Other Investing Activities									0	-1,862	-608
Net Cash Flows Used in Investing Activities									-351,453	-61,787	-776,107
Financing Activities:
Cash Dividends on Common Stock									-475,227	-462,741	-375,047
Increase/(Decrease) in Short-Term Debt									86,575	-135,500	733,500
Issuance of Long-Term Debt									0	750,000	300,000
Retirements of Long-Term Debt									0	-550,000	-263,000
Other Financing Activities									9,528	-12,418	5,394
Net Cash Flows Provided by/(Used in) Financing Activities									-379,124	-410,659	400,847
Supplemental Cash Flow Information [Abstract]
Interest Paid, Net of Amounts Capitalized									36,208	33,822	50,144
Income Taxes Paid									-86,804	-30,603	-27,126
Net Increase/(Decrease) in Cash									103	-394	367
Cash - Beginning of Year				35				429	35	429	62
Cash - End of Year	$138				$35				$138	$35	$429

SCHEDULE_I_Comprehensive_Incom

SCHEDULE I - Comprehensive Income (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Income	$223,600	$236,500	$129,200	$237,800	$179,200	$211,400	$173,100	$230,000	$827,065	$793,689	$533,077
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax									2,037	2,049	1,971
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax									315	-940	217
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax									-30,330	25,714	-4,356
Other Comprehensive Income (Loss), Net of Tax									-27,978	26,823	-2,168
Comprehensive Income Attributable to Noncontrolling Interests									-7,519	-7,682	-7,132
Comprehensive Income									791,568	812,830	523,777
NSTAR Electric Company [Member]
Net Income	69,300	115,600	60,100	58,100	55,400	107,000	58,000	48,100	303,088	268,546	190,242
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax									442
Other Comprehensive Income (Loss), Net of Tax									442
Comprehensive Income									303,530	268,546	190,242
Nu Parent [Member]
Net Income									819,546	786,007	526,048
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax									2,037	2,049	1,971
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax									315	-940	217
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax									-30,330	25,714	-4,356
Other Comprehensive Income (Loss), Net of Tax									-27,978	26,823	-2,168
Comprehensive Income Attributable to Noncontrolling Interests									0	0	-103
Comprehensive Income									$791,568	$812,830	$523,777

SCHEDULE_II_VALUATION_AND_QUAL

SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	$171,251	$165,549	$115,689
Valuation Allowances and Reserves, Charged to Cost and Expense	55,657	55,465	36,275
Valuation Allowances and Reserves, Charged to Other Accounts	51,227	37,744	34,761
Valuation Allowances and Reserves, Deductions	102,818	87,507	80,462
Valuation Allowances and Reserves, Ending Balance	175,317	171,251	165,549
Uncollectible Hardship Accounts Receivable Reserves
Impact Related To Nstar Merger	0	0	59,286
The Connecticut Light And Power Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	81,995	77,571	83,475
Valuation Allowances and Reserves, Charged to Cost and Expense	6,598	3,947	2,080
Valuation Allowances and Reserves, Charged to Other Accounts	39,706	27,258	27,084
Valuation Allowances and Reserves, Deductions	44,012	26,781	35,068
Valuation Allowances and Reserves, Ending Balance	84,287	81,995	77,571
NSTAR Electric Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	41,679	44,115	27,118
Valuation Allowances and Reserves, Charged to Cost and Expense	24,740	28,108	40,301
Valuation Allowances and Reserves, Charged to Other Accounts	627	0	0
Valuation Allowances and Reserves, Deductions	26,376	30,544	23,304
Valuation Allowances and Reserves, Ending Balance	40,670	41,679	44,115
Public Service Company Of New Hampshire [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	7,364	6,760	7,190
Valuation Allowances and Reserves, Charged to Cost and Expense	6,815	6,608	6,457
Valuation Allowances and Reserves, Charged to Other Accounts	797	779	2,481
Valuation Allowances and Reserves, Deductions	7,313	6,783	9,368
Valuation Allowances and Reserves, Ending Balance	7,663	7,364	6,760
Western Massachusetts Electric Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Beginning Balance	9,984	8,501	10,018
Valuation Allowances and Reserves, Charged to Cost and Expense	2,415	2,580	2,294
Valuation Allowances and Reserves, Charged to Other Accounts	3,608	4,299	2,428
Valuation Allowances and Reserves, Deductions	6,127	5,396	6,239
Valuation Allowances and Reserves, Ending Balance	9,880	9,984	8,501
Uncollectible Hardship Accounts Receivable Reserves
Yankee Gas Services Company [Member]
Valuation and Qualifying Accounts Disclosure [Line Items]
Uncollectible Hardship Accounts Receivable Reserves