NSTAR Electric (NSARO) 10-Q 4 Nov 13 2013 FY Quarterly report Financial data

DOCUMENT_AND_ENTITY_INFORMATIO

DOCUMENT AND ENTITY INFORMATION (USD $)	9 Months Ended
	Sep. 30, 2013	Oct. 31, 2013
Document And Entity [Line Items]	'	'
Entity Registrant Name	'NORTHEAST UTILITIES	'
Document Period End Date	30-Sep-13	'
Document Type	'10-Q	'
Entity Central Index Key	'0000072741	'
Current Fiscal Year End Date	'--12-31	'
Entity Common Stock, Shares Outstanding	'	315,094,075
Entity Public Float	$13,224,337,788	'
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	'2013	'
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	'0000023426	'
Current Fiscal Year End Date	'--12-31	'
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	'0000315256	'
Current Fiscal Year End Date	'--12-31	'
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	'0000106170	'
Current Fiscal Year End Date	'--12-31	'
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	'0000013372	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Non-accelerated Filer	'

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Current Assets:	'	'
Cash	$57,941	$45,748
Receivables, Net	784,498	792,822
Unbilled Revenues	174,097	216,040
Fuel, Materials and Supplies	304,698	267,713
Regulatory Assets Current	474,198	705,025
Prepayments and Other Current Assets	222,700	199,947
Total Current Assets	2,018,132	2,227,295
Property, Plant and Equipment, Net	17,187,896	16,605,010
Deferred Debits and Other Assets:	'	'
Regulatory Assets Long Term	4,882,381	5,132,411
Goodwill	3,519,401	3,519,401
Marketable Securities	468,094	400,329
Derivative Assets Noncurrent	88,887	90,612
Other Long-Term Assets	279,527	327,766
Total Deferred Debits and Other Assets	9,238,290	9,470,519
Total Assets	28,444,318	28,302,824
Current Liabilities:	'	'
Notes Payable to Banks	1,343,000	1,120,196
Long-Term Debt - Current Portion	608,346	763,338
Accounts Payable - Current	554,010	764,350
Regulatory Liability Current	224,416	134,115
Other Liabilities - Current	648,658	861,691
Total Current Liabilities	3,378,430	3,643,690
Rate Reduction Bonds	0	82,139
Deferred Credits and Other Liabilities:	'	'
Accumulated Deferred Income Taxes	3,954,246	3,463,347
Regulatory Liabilities Long-Term	520,732	540,162
Long-Term Derivative Liabilities	766,804	882,654
Accrued Pension, PBOP and SERP	1,808,896	2,130,497
Other Long-Term Liabilities	897,997	967,561
Total Deferred Credits and Other Liabilities	7,948,675	7,984,221
Capitalization:	'	'
Long-Term Debt	7,444,192	7,200,156
Noncontrolling Interest in Consolidated Subsidiary:	'	'
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	155,568	155,568
Common Shareholders' Equity:	'	'
Common Stock	1,665,098	1,662,547
Capital Surplus, Paid In	6,185,805	6,183,267
Retained Earnings	2,064,401	1,802,714
Accumulated Other Comprehensive Loss	-67,387	-72,854
Treasury Stock	-330,464	-338,624
Common Shareholders' Equity	9,517,453	9,237,050
Total Capitalization	17,117,213	16,592,774
Total Liabilities and Capitalization	28,444,318	28,302,824
The Connecticut Light And Power Company [Member]	'	'
Current Assets:	'	'
Cash	15,253	1
Receivables, Net	341,749	284,787
Accounts Receivable from Affiliated Companies	1,733	6,641
Unbilled Revenues	73,687	85,353
Regulatory Assets Current	147,076	185,858
Materials and Supplies	58,124	64,603
Prepayments and Other Current Assets	61,277	26,413
Total Current Assets	698,899	653,656
Property, Plant and Equipment, Net	6,326,225	6,152,959
Deferred Debits and Other Assets:	'	'
Regulatory Assets Long Term	2,021,974	2,158,363
Derivative Assets Noncurrent	88,018	90,612
Other Long-Term Assets	91,499	86,498
Total Deferred Debits and Other Assets	2,201,491	2,335,473
Total Assets	9,226,615	9,142,088
Current Liabilities:	'	'
Notes Payable To Affiliated Companies	342,900	99,296
Long-Term Debt - Current Portion	150,000	125,000
Accounts Payable - Current	170,683	262,857
Accounts Payable to Affiliated Companies	46,401	52,326
Obligations to Third Party Suppliers	65,580	67,344
Accrued Taxes	60,643	60,109
Regulatory Liability Current	81,988	32,119
Derivative Liabilities - Current	94,123	96,931
Other Liabilities - Current	78,520	125,662
Total Current Liabilities	1,090,838	921,644
Deferred Credits and Other Liabilities:	'	'
Accumulated Deferred Income Taxes	1,471,547	1,336,105
Regulatory Liabilities Long-Term	107,964	124,319
Long-Term Derivative Liabilities	756,437	865,571
Accrued Pension, PBOP and SERP	291,257	304,696
Other Long-Term Liabilities	160,368	197,434
Total Deferred Credits and Other Liabilities	2,787,573	2,828,125
Capitalization:	'	'
Long-Term Debt	2,591,012	2,737,790
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	1,641,487	1,640,149
Retained Earnings	940,647	839,628
Accumulated Other Comprehensive Loss	-1,494	-1,800
Common Shareholders' Equity	2,640,992	2,538,329
Total Capitalization	5,348,204	5,392,319
Total Liabilities and Capitalization	9,226,615	9,142,088
NSTAR Electric Company [Member]	'	'
Current Assets:	'	'
Cash	15,470	13,695
Receivables, Net	263,055	202,025
Accounts Receivable from Affiliated Companies	70,279	160,176
Unbilled Revenues	48,570	41,377
Regulatory Assets Current	189,754	347,081
Prepayments and Other Current Assets	54,105	28,086
Total Current Assets	641,233	792,440
Property, Plant and Equipment, Net	4,923,410	4,735,297
Deferred Debits and Other Assets:	'	'
Regulatory Assets Long Term	1,538,222	1,444,870
Other Long-Term Assets	59,267	87,382
Total Deferred Debits and Other Assets	1,597,489	1,532,252
Total Assets	7,162,132	7,059,989
Current Liabilities:	'	'
Notes Payable to Banks	156,000	276,000
Long-Term Debt - Current Portion	301,650	1,650
Accounts Payable - Current	157,375	168,611
Accounts Payable to Affiliated Companies	97,992	247,061
Deferred Tax Liabilities Current	32,049	104,668
Regulatory Liability Current	82,521	47,539
Other Liabilities - Current	128,846	144,433
Total Current Liabilities	956,433	989,962
Rate Reduction Bonds	0	43,493
Deferred Credits and Other Liabilities:	'	'
Accumulated Deferred Income Taxes	1,463,285	1,321,026
Regulatory Liabilities Long-Term	251,005	244,224
Accrued Pension, PBOP and SERP	380,688	360,932
Other Long-Term Liabilities	145,032	183,190
DueToRelatedPartiesNoncurrent	64,752	70,221
Total Deferred Credits and Other Liabilities	2,304,762	2,179,593
Capitalization:	'	'
Long-Term Debt	1,499,378	1,600,911
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	992,625	992,625
Retained Earnings	1,365,934	1,210,405
Common Shareholders' Equity	2,358,559	2,203,030
Total Capitalization	3,900,937	3,846,941
Total Liabilities and Capitalization	7,162,132	7,059,989
Public Service Company Of New Hampshire [Member]	'	'
Current Assets:	'	'
Cash	5,604	2,493
Receivables, Net	78,464	87,164
Accounts Receivable from Affiliated Companies	1,182	723
Unbilled Revenues	31,081	39,982
Taxes Receivable	12,074	17,177
Fuel, Materials and Supplies	125,801	95,345
Regulatory Assets Current	67,716	62,882
Prepayments and Other Current Assets	6,464	22,205
Total Current Assets	328,386	327,971
Property, Plant and Equipment, Net	2,409,039	2,352,515
Deferred Debits and Other Assets:	'	'
Regulatory Assets Long Term	301,368	351,059
Other Long-Term Assets	55,953	83,052
Total Deferred Debits and Other Assets	357,321	434,111
Total Assets	3,094,746	3,114,597
Current Liabilities:	'	'
Notes Payable To Affiliated Companies	228,500	63,300
Long-Term Debt - Current Portion	50,000	'
Accounts Payable - Current	60,814	62,864
Accounts Payable to Affiliated Companies	18,279	21,337
Regulatory Liability Current	23,394	23,002
Other Liabilities - Current	54,315	50,950
Renewable Portfolio Standards Compliance Obligation	6,701	17,383
Total Current Liabilities	442,003	238,836
Rate Reduction Bonds	0	29,294
Deferred Credits and Other Liabilities:	'	'
Accumulated Deferred Income Taxes	490,863	441,577
Regulatory Liabilities Long-Term	52,867	52,418
Accrued Pension, PBOP and SERP	104,557	220,129
Other Long-Term Liabilities	43,866	47,896
Total Deferred Credits and Other Liabilities	692,153	762,020
Capitalization:	'	'
Long-Term Debt	839,104	997,932
Common Shareholders' Equity:	'	'
Common Stock	0	0
Capital Surplus, Paid In	701,659	701,052
Retained Earnings	428,660	395,118
Accumulated Other Comprehensive Loss	-8,833	-9,655
Common Shareholders' Equity	1,121,486	1,086,515
Total Capitalization	1,960,590	2,084,447
Total Liabilities and Capitalization	3,094,746	3,114,597
Western Massachusetts Electric Company [Member]	'	'
Current Assets:	'	'
Cash	3,157	1
Receivables, Net	49,056	47,297
Accounts Receivable from Affiliated Companies	29,231	164
Unbilled Revenues	13,046	16,192
Taxes Receivable	2	15,513
Regulatory Assets Current	37,854	42,370
Marketable Securities - Current	24,570	27,352
Prepayments and Other Current Assets	10,195	7,963
Total Current Assets	167,111	156,852
Property, Plant and Equipment, Net	1,352,705	1,290,498
Deferred Debits and Other Assets:	'	'
Regulatory Assets Long Term	194,744	221,752
Marketable Securities	33,195	30,342
Other Long-Term Assets	20,246	23,625
Total Deferred Debits and Other Assets	248,185	275,719
Total Assets	1,768,001	1,723,069
Current Liabilities:	'	'
Notes Payable To Affiliated Companies	79,800	31,900
Long-Term Debt - Current Portion	0	55,000
Accounts Payable - Current	40,432	68,141
Accounts Payable to Affiliated Companies	7,521	7,103
Deferred Tax Liabilities Current	9,416	8,404
Regulatory Liability Current	22,400	21,037
Other Liabilities - Current	18,718	24,809
Total Current Liabilities	178,287	216,394
Rate Reduction Bonds	0	9,352
Deferred Credits and Other Liabilities:	'	'
Accumulated Deferred Income Taxes	392,360	303,111
Regulatory Liabilities Long-Term	11,914	9,686
Accrued Pension, PBOP and SERP	30,791	36,099
Other Long-Term Liabilities	26,503	40,148
Total Deferred Credits and Other Liabilities	461,568	389,044
Capitalization:	'	'
Long-Term Debt	549,617	550,270
Common Shareholders' Equity:	'	'
Common Stock	10,866	10,866
Capital Surplus, Paid In	390,645	390,412
Retained Earnings	180,618	160,577
Accumulated Other Comprehensive Loss	-3,600	-3,846
Common Shareholders' Equity	578,529	558,009
Total Capitalization	1,128,146	1,108,279
Total Liabilities and Capitalization	$1,768,001	$1,723,069

CONSOLIDATED_STATEMENTS_OF_INC

CONSOLIDATED STATEMENTS OF INCOME (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Operating Revenues	$1,892,590	$1,861,529	$5,523,475	$4,589,835
Operating Expenses:	'	'	'	'
Purchased Power and Transmission	645,881	602,751	1,881,992	1,540,110
Operations and Maintenance	386,700	395,531	1,089,960	1,187,471
Depreciation	149,105	144,475	463,635	369,798
Amortization of Regulatory Assets/(Liabilities), Net	70,046	43,835	178,668	74,851
Amortization of Rate Reduction Bonds	0	43,044	42,581	102,144
Energy Efficiency Programs	106,097	98,326	306,010	209,089
Taxes Other Than Income Taxes	135,499	120,662	391,846	319,559
Total Operating Expenses	1,493,328	1,448,624	4,354,692	3,803,022
Operating Income	399,262	412,905	1,168,783	786,813
Interest Expense:	'	'	'	'
Interest on Long-Term Debt	84,911	86,459	256,205	233,352
Interest on Rate Reduction Bonds	0	1,681	422	5,168
Other Interest	2,565	2,221	-6,044	7,336
Total Interest Expense	87,476	90,361	250,583	245,856
Other Income/(Loss), Net	8,945	4,324	21,655	14,904
Income Before Income Tax Expense	320,731	326,868	939,855	555,861
Income Tax Expense	109,351	117,360	325,442	199,379
Net Income	211,380	209,508	614,413	356,482
Net Income Attributable to Noncontrolling Interests	1,879	1,880	5,803	5,253
Net Income Attributable to Controlling Interests	209,501	207,628	608,610	351,229
Diluted Earnings Per Common Share	$0.66	$0.66	$1.93	$1.32
Basic Earnings Per Common Share	$0.66	$0.66	$1.93	$1.33
Common Stock Dividends Per Share Declared	$0.37	$0.34	$1.10	$0.97
Weighted Average Common Shares Outstanding:	'	'	'	'
Basic	315,291,346	314,806,441	315,191,752	264,636,636
Diluted	316,218,239	315,805,796	316,061,131	265,353,377
The Connecticut Light And Power Company [Member]	'	'	'	'
Operating Revenues	648,420	658,111	1,841,846	1,812,218
Operating Expenses:	'	'	'	'
Purchased Power and Transmission	253,152	241,046	667,266	658,743
Operations and Maintenance	127,104	141,913	359,759	480,286
Depreciation	44,786	41,863	132,356	124,451
Amortization of Regulatory Assets/(Liabilities), Net	-27	8,656	11,223	19,912
Energy Efficiency Programs	24,544	25,237	68,211	68,205
Taxes Other Than Income Taxes	64,979	59,687	182,676	168,667
Total Operating Expenses	514,538	518,402	1,421,491	1,520,264
Operating Income	133,882	139,709	420,355	291,954
Interest Expense:	'	'	'	'
Interest on Long-Term Debt	32,845	31,429	98,163	94,646
Other Interest	2,439	2,162	801	6,223
Total Interest Expense	35,284	33,591	98,964	100,869
Other Income/(Loss), Net	3,861	2,889	10,946	8,636
Income Before Income Tax Expense	102,459	109,007	332,337	199,721
Income Tax Expense	36,136	34,121	113,149	63,917
Net Income	66,323	74,886	219,188	135,804
NSTAR Electric Company [Member]	'	'	'	'
Operating Revenues	753,879	693,653	1,916,557	1,784,755
Operating Expenses:	'	'	'	'
Purchased Power and Transmission	255,244	222,753	659,140	622,265
Operations and Maintenance	97,069	83,329	277,261	340,547
Depreciation	45,441	42,494	136,323	127,692
Amortization of Regulatory Assets/(Liabilities), Net	72,740	41,888	173,289	87,912
Amortization of Rate Reduction Bonds	0	22,581	15,054	67,742
Energy Efficiency Programs	58,798	55,969	161,180	138,360
Taxes Other Than Income Taxes	32,610	30,520	95,275	89,689
Total Operating Expenses	561,902	499,534	1,517,522	1,474,207
Operating Income	191,977	194,119	399,035	310,548
Interest Expense:	'	'	'	'
Interest on Long-Term Debt	19,860	22,386	59,261	66,953
Interest on Rate Reduction Bonds	0	853	399	3,106
Other Interest	-1,324	-4,704	-8,011	-16,137
Total Interest Expense	18,536	18,535	51,649	53,922
Other Income/(Loss), Net	2,126	551	3,275	1,778
Income Before Income Tax Expense	175,567	176,135	350,661	258,404
Income Tax Expense	68,558	69,373	137,499	102,220
Net Income	107,009	106,762	213,162	156,184
Public Service Company Of New Hampshire [Member]	'	'	'	'
Operating Revenues	218,608	256,949	708,550	755,051
Operating Expenses:	'	'	'	'
Purchased Power and Transmission	46,668	76,008	197,765	239,173
Operations and Maintenance	69,477	67,547	191,606	200,960
Depreciation	22,919	22,264	68,433	65,282
Amortization of Regulatory Assets/(Liabilities), Net	225	-6,356	-1,745	-6,179
Amortization of Rate Reduction Bonds	0	16,112	19,748	43,855
Energy Efficiency Programs	3,990	4,030	11,036	10,824
Taxes Other Than Income Taxes	18,706	16,046	52,640	47,406
Total Operating Expenses	161,985	195,651	539,483	601,321
Operating Income	56,623	61,298	169,067	153,730
Interest Expense:	'	'	'	'
Interest on Long-Term Debt	10,345	11,434	32,951	34,537
Interest on Rate Reduction Bonds	0	564	-154	2,366
Other Interest	521	609	1,384	1,301
Total Interest Expense	10,866	12,607	34,181	38,204
Other Income/(Loss), Net	792	-353	2,454	2,237
Income Before Income Tax Expense	46,549	48,338	137,340	117,763
Income Tax Expense	18,196	21,106	52,797	48,037
Net Income	28,353	27,232	84,543	69,726
Western Massachusetts Electric Company [Member]	'	'	'	'
Operating Revenues	121,795	112,470	361,763	333,331
Operating Expenses:	'	'	'	'
Purchased Power and Transmission	38,797	32,028	111,095	105,297
Operations and Maintenance	26,148	24,765	70,213	75,214
Depreciation	9,426	7,464	27,707	22,154
Amortization of Regulatory Assets/(Liabilities), Net	-1,412	1,021	-598	634
Amortization of Rate Reduction Bonds	0	4,352	7,780	13,127
Energy Efficiency Programs	12,222	9,190	28,462	19,679
Taxes Other Than Income Taxes	7,696	5,505	20,188	15,365
Total Operating Expenses	92,877	84,325	264,847	251,470
Operating Income	28,918	28,145	96,916	81,861
Interest Expense:	'	'	'	'
Interest on Long-Term Debt	5,814	5,783	17,846	17,454
Interest on Rate Reduction Bonds	0	272	177	1,029
Other Interest	417	714	777	1,550
Total Interest Expense	6,231	6,769	18,800	20,033
Other Income/(Loss), Net	926	685	2,349	1,965
Income Before Income Tax Expense	23,613	22,061	80,465	63,793
Income Tax Expense	8,588	7,977	30,424	24,385
Net Income	$15,025	$14,084	$50,041	$39,408

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net Income	$211,380	$209,508	$614,413	$356,482
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract	'	'	'	'
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	509	516	1,539	1,455
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	-38	217	-810	411
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	1,611	1,445	4,738	4,611
Other Comprehensive Income (Loss), Net of Tax, Total	2,082	2,178	5,467	6,477
Comprehensive Income Attributable to Noncontrolling Interests	-1,879	-1,880	-5,803	-5,253
Comprehensive Income	211,583	209,806	614,077	357,706
The Connecticut Light And Power Company [Member]	'	'	'	'
Net Income	66,323	74,886	219,188	135,804
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract	'	'	'	'
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	111	111	333	333
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	-1	8	-27	14
Other Comprehensive Income (Loss), Net of Tax, Total	110	119	306	347
Comprehensive Income	66,433	75,005	219,494	136,151
NSTAR Electric Company [Member]	'	'	'	'
Net Income	107,009	106,762	213,162	156,184
Public Service Company Of New Hampshire [Member]	'	'	'	'
Net Income	28,353	27,232	84,543	69,726
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract	'	'	'	'
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	290	291	872	872
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	-2	13	-47	24
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	0	-2	-3	2
Other Comprehensive Income (Loss), Net of Tax, Total	288	302	822	898
Comprehensive Income	28,641	27,534	85,365	70,624
Western Massachusetts Electric Company [Member]	'	'	'	'
Net Income	15,025	14,084	50,041	39,408
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease Abstract	'	'	'	'
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	85	84	254	253
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	0	2	-8	4
Other Comprehensive Income (Loss), Net of Tax, Total	85	86	246	257
Comprehensive Income	$15,110	$14,170	$50,287	$39,665

CONSOLIDATED_STATEMENT_OF_CASH

CONSOLIDATED STATEMENT OF CASH FLOWS (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Operating Activities:	'	'
Net Income	$614,413	$356,482
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:	'	'
Depreciation	463,635	369,798
Deferred Income Tax Expense (Benefit)	334,225	186,181
Pension and PBOP Expense, Net of PBOP Contributions	146,803	160,209
Pension and Other Postretirement Benefit Contributions	-338,301	-237,123
Regulatory (Underrecoveries)/Overrecoveries, Net	66,239	-26,236
Amortization of Regulatory Assets/(Liabilities), Net	178,668	74,851
Amortization of Rate Reduction Bonds	42,581	102,144
Other Noncash Income (Expense)	3,158	6,640
Changes in Current Assets and Liabilities:	'	'
Receivables and Unbilled Revenues, Net	-98,432	-27,677
Inventory	-13,134	32,887
Taxes Receivable/(Accrued)	-28,609	26,302
Change in Accounts Payable	-112,512	-208,308
Other Current Assets and Liabilities	-81,766	-20,145
Net Cash Flows Provided by Operating Activities	1,176,968	796,005
Investing Activities:	'	'
Investments in Property, Plant and Equipment	-1,073,759	-1,081,750
Proceeds from Sales of Marketable Securities	487,729	232,911
Purchases of Marketable Securities	-541,070	-252,762
DecreaseInRestrictedCash	69,259	6,199
Other Investing Activities	-1,137	34,066
Net Cash Flows Used in Investing Activities	-1,058,978	-1,061,336
Financing Activities:	'	'
Cash Dividends on Common Stock	-341,720	-267,356
Cash Dividends on Preferred Stock	-5,802	-5,149
Increase/(Decrease) in Short-Term Debt	-172,000	654,250
Issuance of Long-Term Debt	1,350,000	300,000
Retirements of Long-Term Debt	-840,600	-267,561
Retirements of Rate Reduction Bonds	-82,139	-95,225
Other Financing Activities	-13,536	13,262
Net Cash Flows Provided by/(Used in) Financing Activities	-105,797	332,221
Net Increase/(Decrease) in Cash	12,193	66,890
Cash - Beginning of Year	45,748	6,559
Cash - End of Year	57,941	73,449
The Connecticut Light And Power Company [Member]	'	'
Operating Activities:	'	'
Net Income	219,188	135,804
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:	'	'
Depreciation	132,356	124,451
Deferred Income Tax Expense (Benefit)	89,084	97,224
Pension and PBOP Expense, Net of PBOP Contributions	16,182	18,394
Regulatory (Underrecoveries)/Overrecoveries, Net	24,061	-13,804
Amortization of Regulatory Assets/(Liabilities), Net	11,223	19,912
Other Noncash Income (Expense)	-8,759	-10,701
Changes in Current Assets and Liabilities:	'	'
Receivables and Unbilled Revenues, Net	-44,523	-21,632
Taxes Receivable/(Accrued)	841	21,410
Change in Accounts Payable	-101,949	-173,107
Other Current Assets and Liabilities	-29,106	-49,750
Net Cash Flows Provided by Operating Activities	308,598	148,201
Investing Activities:	'	'
Investments in Property, Plant and Equipment	-294,638	-332,323
Other Investing Activities	2,013	13,707
Net Cash Flows Used in Investing Activities	-292,625	-318,616
Financing Activities:	'	'
Cash Dividends on Common Stock	-114,000	-100,486
Cash Dividends on Preferred Stock	-4,169	-4,169
Increase/(Decrease) in Short-Term Debt	-89,000	-31,000
Issuance of Long-Term Debt	400,000	'
Increase/(Decrease) in Notes Payable to Affiliate	-62,200	314,275
Retirements of Long-Term Debt	-125,000	'
Other Financing Activities	-6,352	-1,636
Net Cash Flows Provided by/(Used in) Financing Activities	-721	176,984
Net Increase/(Decrease) in Cash	15,252	6,569
Cash - Beginning of Year	1	1
Cash - End of Year	15,253	6,570
NSTAR Electric Company [Member]	'	'
Operating Activities:	'	'
Net Income	213,162	156,184
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:	'	'
Bad Debt Expense	19,012	53,254
Depreciation	136,323	127,692
Deferred Income Tax Expense (Benefit)	26,358	-20,250
Pension and PBOP Expense, Net of PBOP Contributions	-55,195	1,394
Regulatory (Underrecoveries)/Overrecoveries, Net	-11,299	62,075
Amortization of Regulatory Assets/(Liabilities), Net	173,289	87,912
Amortization of Rate Reduction Bonds	15,054	67,742
Other Noncash Income (Expense)	-48,291	-29,154
Changes in Current Assets and Liabilities:	'	'
Receivables and Unbilled Revenues, Net	-80,575	-61,528
Inventory	7,961	7,264
Taxes Receivable/(Accrued)	-6,345	44,142
Change in Accounts Payable	6,856	-81,292
Other Current Assets and Liabilities	-19,547	58,890
IncreaseDecreaseInAccountsPayableRelatedParties	-59,173	-41,760
Net Cash Flows Provided by Operating Activities	317,590	432,565
Investing Activities:	'	'
Investments in Property, Plant and Equipment	-330,635	-298,424
DecreaseInRestrictedCash	37,899	25,234
Other Investing Activities	575	375
Net Cash Flows Used in Investing Activities	-292,161	-272,815
Financing Activities:	'	'
Cash Dividends on Common Stock	-56,000	-188,700
Cash Dividends on Preferred Stock	-1,633	-1,470
Increase/(Decrease) in Short-Term Debt	-120,000	104,500
Issuance of Long-Term Debt	200,000	0
Retirements of Long-Term Debt	-1,650	-688
Retirements of Rate Reduction Bonds	-43,493	-84,367
Other Financing Activities	-878	13,336
Net Cash Flows Provided by/(Used in) Financing Activities	-23,654	-157,389
Net Increase/(Decrease) in Cash	1,775	2,361
Cash - Beginning of Year	13,695	9,373
Cash - End of Year	15,470	11,734
Public Service Company Of New Hampshire [Member]	'	'
Operating Activities:	'	'
Net Income	84,543	69,726
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:	'	'
Depreciation	68,433	65,282
Deferred Income Tax Expense (Benefit)	57,066	39,108
Pension and PBOP Expense, Net of PBOP Contributions	20,427	19,508
Pension and Other Postretirement Benefit Contributions	-112,964	-94,169
Regulatory (Underrecoveries)/Overrecoveries, Net	-1,346	1,718
Amortization of Regulatory Assets/(Liabilities), Net	-1,745	-6,179
Amortization of Rate Reduction Bonds	19,748	43,855
Other Noncash Income (Expense)	7,165	18,699
Changes in Current Assets and Liabilities:	'	'
Receivables and Unbilled Revenues, Net	8,047	-4,274
Inventory	-30,456	20,178
Taxes Receivable/(Accrued)	5,103	4,506
Change in Accounts Payable	29,148	-18,567
Other Current Assets and Liabilities	7,220	18,358
Net Cash Flows Provided by Operating Activities	160,389	177,749
Investing Activities:	'	'
Investments in Property, Plant and Equipment	-155,676	-161,021
(Increase) Decrease In Notes Receivable From Affiliated Company	0	55,900
DecreaseInRestrictedCash	22,039	2,599
Other Investing Activities	-53	-99
Net Cash Flows Used in Investing Activities	-133,690	-102,621
Financing Activities:	'	'
Cash Dividends on Common Stock	-51,000	-74,675
Increase/(Decrease) in Short-Term Debt	165,200	44,200
Retirements of Long-Term Debt	-108,950	0
Retirements of Rate Reduction Bonds	-29,294	-41,265
Other Financing Activities	456	-349
Net Cash Flows Provided by/(Used in) Financing Activities	-23,588	-72,089
Net Increase/(Decrease) in Cash	3,111	3,039
Cash - Beginning of Year	2,493	56
Cash - End of Year	5,604	3,095
Western Massachusetts Electric Company [Member]	'	'
Operating Activities:	'	'
Net Income	50,041	39,408
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:	'	'
Depreciation	27,707	22,154
Deferred Income Tax Expense (Benefit)	79,401	30,565
Regulatory (Underrecoveries)/Overrecoveries, Net	11,685	-8,733
Amortization of Regulatory Assets/(Liabilities), Net	-598	634
Amortization of Rate Reduction Bonds	7,780	13,127
Other Noncash Income (Expense)	-544	1,755
Changes in Current Assets and Liabilities:	'	'
Receivables and Unbilled Revenues, Net	-32,231	-10,482
Taxes Receivable/(Accrued)	16,412	7,337
Change in Accounts Payable	20,260	-28,510
Other Current Assets and Liabilities	-9,857	-9,185
Net Cash Flows Provided by Operating Activities	170,056	58,070
Investing Activities:	'	'
Investments in Property, Plant and Equipment	-127,352	-218,184
Proceeds from Sales of Marketable Securities	53,552	65,131
Purchases of Marketable Securities	-54,042	-65,664
(Increase) Decrease In Notes Receivable From Affiliated Company	'	11,000
Other Investing Activities	7,401	308
Net Cash Flows Used in Investing Activities	-120,441	-207,409
Financing Activities:	'	'
Cash Dividends on Common Stock	-30,000	-9,431
Increase/(Decrease) in Notes Payable to Affiliate	47,900	172,500
Retirements of Long-Term Debt	-55,000	'
Retirements of Rate Reduction Bonds	-9,352	-13,141
Other Financing Activities	-7	-54
Net Cash Flows Provided by/(Used in) Financing Activities	-46,459	149,874
Net Increase/(Decrease) in Cash	3,156	535
Cash - Beginning of Year	1	1
Cash - End of Year	$3,157	$536

SUMMARY_OF_SIGNIFICANT_ACCOUNT

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	9 Months Ended
	Sep. 30, 2013
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 CONDENSED FINANCIAL STATEMENTS (Unaudited)
	Refer to the Glossary of Terms included in this combined Quarterly Report on Form 10-Q for abbreviations and acronyms used throughout the combined notes to the unaudited condensed financial statements.
	1.       SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
	A.       Basis of Presentation
	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 100 percent of the outstanding common shares of NSTAR and its subsidiaries. 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. NU's consolidated financial information does not include NSTAR and its subsidiaries' results of operations for the three months ended March 31, 2012. The information disclosed for NSTAR Electric represents its results of operations for the three and nine months ended September 30, 2013 and 2012, presented on a comparable basis.
	The unaudited condensed 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 unaudited condensed consolidated financial statements of NU, NSTAR Electric and PSNH and the unaudited condensed financial statements of CL&P and WMECO are herein collectively referred to as the "financial statements."
	The combined notes to the financial statements have been prepared pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures included in annual financial statements prepared in accordance with GAAP have been omitted pursuant to such rules and regulations. The accompanying financial statements should be read in conjunction with the entirety of this combined Quarterly Report on Form 10-Q, the first and second quarter 2013 combined Quarterly Reports on Form 10-Q and the 2012 combined Annual Report on Form 10-K of NU, CL&P, NSTAR Electric, PSNH and WMECO, which were filed with the SEC. 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.
	The financial statements contain, in the opinion of management, all adjustments (including normal, recurring adjustments) necessary to present fairly NU's, CL&P's, NSTAR Electric's, PSNH's and WMECO's financial position as of September 30, 2013 and December 31, 2012, the results of operations and comprehensive income for the three and nine months ended September 30, 2013 and 2012, and the cash flows for the nine months ended September 30, 2013 and 2012. The results of operations and comprehensive income for the three and nine months ended September 30, 2013 and 2012, and the cash flows for the nine months ended September 30, 2013 and 2012, are not necessarily indicative of the results expected for a full year. The demand for electricity and natural gas is affected by weather conditions, economic conditions, and consumer conservation (including company-sponsored energy efficiency programs). Electric energy sales and revenues are typically higher in the winter and summer months than in the spring and fall months. Natural gas sales and revenues are typically higher in the winter months than during other periods of the year.
	NU consolidates CYAPC and YAEC as 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. For CL&P, NSTAR Electric, PSNH and WMECO, the investment in CYAPC and YAEC continue to be accounted for under the equity method.
	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' energy delivery business is 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 period data were made in the accompanying balance sheets for NU, PSNH and WMECO, and the statements of cash flows for all companies presented. These reclassifications were made to conform to the current period's presentation.
	B.       Accounting Standards
	Recently Adopted Accounting Standards: In the first quarter of 2013, NU adopted the following Financial Accounting Standards Board's (FASB) final Accounting Standards Updates (ASU) relating to additional disclosure requirements:
	Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income: Requires entities to disclose additional information about items reclassified out of AOCI. The ASU does not change existing guidance on which items should be reclassified out of AOCI but requires disclosures about the components of AOCI and the amount of reclassification adjustments to be presented in one location. The ASU was effective beginning in the first quarter of 2013 and was applied prospectively. For further information, see Note 11, "Accumulated Other Comprehensive Income/(Loss)," to the financial statements. The ASU did not affect the calculation of net income, comprehensive income or EPS and did not have an impact on financial position, results of operations or cash flows.
	Clarifying the Scope of Disclosures about Offsetting Assets and Liabilities: Clarifies the scope of the offsetting disclosure requirements under GAAP. The disclosure requirements apply to derivative instruments, do not change existing guidance on which items should be offset in the balance sheets and require disclosures about the items that are offset. The ASU was effective beginning in the first quarter of 2013 with retrospective application. For further information, see Note 4, "Derivative Instruments," to the financial statements. The ASU did not have an impact on financial position, results of operations or cash flows.
	Accounting Standards Issued but not Yet Adopted: In July 2013, the FASB issued a final ASU that requires presentation of certain unrecognized tax benefits as reductions to deferred tax assets rather than as liabilities. Management is currently evaluating the balance sheet impact of implementing this standard. The ASU does not impact results of operations or cash flows.
	C.       Provision for Uncollectible Accounts
	NU, including CL&P, NSTAR Electric, PSNH and WMECO, presents its receivables at net realizable value by maintaining a provision for uncollectible amounts. This provision is determined based upon a variety of factors, including applying an estimated uncollectible account percentage to each receivable aging category, based upon historical collection and write-off experience and management's assessment of collectibility from individual customers. Management assesses the collectibility of receivables, and if circumstances change, collectibility estimates are adjusted accordingly. 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 provision for uncollectible accounts, which is included in Receivables, Net on the balance sheets, was as follows:
	(Millions of Dollars)			As of September 30, 2013			As of December 31, 2012
	NU			$	182.5		$	165.5
	CL&P				85.8			77.6
	NSTAR Electric				45.9			44.1
	PSNH				7.7			6.8
	WMECO				10.4			8.5
	D.       Fair Value Measurements
	Fair value measurement guidance is applied to derivative contracts recorded at fair value and to the marketable securities held in trusts. Fair value measurement guidance is also applied to investment valuations used to calculate the funded status of pension and PBOP plans and nonrecurring fair value measurements of nonfinancial assets such as goodwill and AROs.
	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," and Note 10, "Fair Value of Financial Instruments," to the financial statements.
	E.       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. For CL&P, NSTAR Electric, PSNH and WMECO, equity in earnings relate to investments in CYAPC, YAEC and MYAPC as well as NSTAR Electric's investment in two regional transmission companies, which are all accounted for on the equity method. On an NU consolidated basis, equity in earnings relate to the investment in MYAPC and NU's investment in two regional transmission companies.
	F.       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 Three Months Ended						For the Nine Months Ended
	(Millions of Dollars)	30-Sep-13			30-Sep-12			30-Sep-13		30-Sep-12
	NU	$	37.5		$	36.4		$	108.9	$	102
	CL&P		35.5			34.4			97.3		91.5
	Certain sales taxes are also 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.
	G.			Supplemental Cash Flow Information
	Non-cash investing activities include plant additions included in Accounts Payable as follows:
	(Millions of Dollars)				As of September 30, 2013			As of September 30, 2012
	NU				$	122.9		$	139.9
	CL&P					36.6			45.9
	NSTAR Electric					31.9			21.5
	PSNH					16.9			20.1
	WMECO					13.8			35.1
	H.       Severance Benefits
	In the third quarter of 2013, NU recorded severance benefit expenses of $9.2 million in connection with the partial outsourcing of information technology functions made as part of ongoing post-merger integration. As of September 30, 2013, the severance accrual totaled $14.2 million and was included in Other Current Liabilities on the accompanying balance sheet.

REGULATORY_ACCOUNTING

REGULATORY ACCOUNTING	9 Months Ended
	Sep. 30, 2013
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 reflect 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:
		As of September 30, 2013			As of December 31, 2012
	(Millions of Dollars)	NU			NU
	Benefit Costs	$	2,256.00		$	2,452.10
	Regulatory Assets Offsetting Derivative Liabilities		770.3			885.6
	Goodwill		531.1			537.6
	Storm Restoration Costs		621			547.7
	Income Taxes, Net		587.5			516.2
	Securitized Assets		37.4			232.6
	Contractual Obligations		170.9			217.6
	Buy Out Agreements for Power Contracts		76			92.9
	Regulatory Tracker Deferrals		163.3			190.1
	Asset Retirement Obligations		93			88.8
	Other Regulatory Assets		50.1			76.2
	Total Regulatory Assets		5,356.60			5,837.40
	Less: Current Portion		474.2			705
	Total Long-Term Regulatory Assets	$	4,882.40		$	5,132.40
			As of September 30, 2013									As of December 31, 2012
						NSTAR								NSTAR
	(Millions of Dollars)		CL&P			Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Benefit Costs		$	509.3		$	824.3	$	199.6	$	103.8	$	563.2	$	781.2	$	223.7	$	116
	Regulatory Assets Offsetting
		Derivative Liabilities		755.3			11.6		0.3		-		866.2		14.9		-		3
	Goodwill			-			455.9		-		-		-		461.5		-		-
	Storm Restoration Costs			439.4			114		27.9		39.7		413.9		55.8		34.5		43.5
	Income Taxes, Net			385.3			84.8		36.5		42.1		367.5		47.1		36.2		31
	Securitized Assets			-			37.4		-		-		-		205.1		19.7		7.8
	Contractual Obligations			20			6.4		-		4.6		64		22.8		-		14.9
	Buy Out Agreements for Power Contracts			-			70.1		5.9		-		-		85.9		7		-
	Regulatory Tracker Deferrals			-			83.6		52.5		21.5		12.2		71.4		49.3		31.9
	Asset Retirement Obligations			31.1			30.7		14.7		3.7		29.4		29.4		14.2		3.5
	Other Regulatory Assets			28.7			9.2		31.7		17.2		27.9		16.9		29.4		12.6
	Total Regulatory Assets			2,169.10			1,728.00		369.1		232.6		2,344.30		1,792.00		414		264.2
	Less: Current Portion			147.1			189.8		67.7		37.9		185.9		347.1		62.9		42.4
	Total Long-Term Regulatory Assets		$	2,022.00		$	1,538.20	$	301.4	$	194.7	$	2,158.40	$	1,444.90	$	351.1	$	221.8
	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 collect from customers. The storm restoration cost regulatory asset balance at CL&P, NSTAR Electric and WMECO primarily reflects costs incurred for Tropical Storm Irene, the October 2011 snowstorm, Storm Sandy and the February 2013 blizzard. For PSNH, costs incurred associated with these storms are recorded in Other Long-Term Assets. The storm restoration cost regulatory asset balance at PSNH primarily reflects costs incurred for storms in 2008 and 2010, which are currently being recovered in rates. Management believes the storm restoration costs meet the criteria for specific cost recovery in Connecticut, Massachusetts and New Hampshire and as a result, are probable of recovery. Each operating company is seeking recovery of these deferred storm restoration costs through its applicable regulatory recovery process.
	Regulatory Costs in Other Long-Term Assets: The Regulated companies had $95.1 million ($3.4 million for CL&P, $31.3 million for NSTAR Electric, $37.3 million for PSNH, and $7.9 million for WMECO) and $69.9 million ($3.9 million for CL&P, $25.4 million for NSTAR Electric, $35.7 million for PSNH, and $1.4 million for WMECO) of additional regulatory costs as of September 30, 2013 and December 31, 2012, respectively, which were included in Other Long-Term Assets on the balance sheets. These amounts represent incurred costs for which specific recovery has not yet been approved by the applicable regulatory agency. Management believes it is probable that these costs will ultimately be approved and recovered from customers.
	Regulatory Liabilities: The components of regulatory liabilities are as follows:
		As of September 30, 2013			As of December 31, 2012
	(Millions of Dollars)	NU			NU
	Cost of Removal	$	434.3		$	440.8
	Regulatory Tracker Deferrals		168.6			95.1
	AFUDC - Transmission		68.3			70
	Other Regulatory Liabilities		73.9			68.4
	Total Regulatory Liabilities		745.1			674.3
	Less: Current Portion		224.4			134.1
	Total Long-Term Regulatory Liabilities	$	520.7		$	540.2
			As of September 30, 2013									As of December 31, 2012
						NSTAR								NSTAR
	(Millions of Dollars)		CL&P			Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Cost of Removal		$	31.2		$	247.2	$	49.8	$	-	$	44.2	$	240.3	$	51.2	$	-
	Regulatory Tracker Deferrals			73.2			51		10.7		22.1		39.1		14.4		20.4		19
	AFUDC - Transmission			55			4		-		9.3		56.6		4.1		-		9.3
	Other Regulatory Liabilities			30.6			31.3		15.8		2.9		16.5		32.9		3.8		2.4
	Total Regulatory Liabilities			190			333.5		76.3		34.3		156.4		291.7		75.4		30.7
	Less: Current Portion			82			82.5		23.4		22.4		32.1		47.5		23		21
	Total Long-Term Regulatory Liabilities		$	108		$	251	$	52.9	$	11.9	$	124.3	$	244.2	$	52.4	$	9.7

PROPERTY_PLANT_AND_ACCUMULATED

PROPERTY, PLANT AND ACCUMULATED DEPRECIATION	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule of Public Utility Property, Plant, and Equipment [Text Block]	'
	3.       PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION
	The following tables summarize the NU, CL&P, NSTAR Electric, PSNH and WMECO investments in utility property, plant and equipment by asset category:
				As of September 30, 2013			As of December 31, 2012
	(Millions of Dollars)			NU			NU
	Distribution - Electric			$	11,735.40		$	11,438.20
	Distribution - Natural Gas				2,352.40			2,274.20
	Transmission				6,009.00			5,541.10
	Generation				1,142.10			1,146.60
	Electric and Natural Gas Utility				21,238.90			20,400.10
	Other (1)				505.2			429.3
	Property, Plant and Equipment, Gross				21,744.10			20,829.40
	Less: Accumulated Depreciation
		Electric and Natural Gas Utility			-5,331.00			-5,065.10
		Other			-192.9			-171.5
	Total Accumulated Depreciation				-5,523.90			-5,236.60
	Property, Plant and Equipment, Net				16,220.20			15,592.80
	Construction Work in Progress				967.7			1,012.20
	Total Property, Plant and Equipment, Net			$	17,187.90		$	16,605.00
	(1)       These assets represent unregulated property and are primarily comprised of building improvements at RRR, software, hardware and equipment at NUSCO and telecommunications assets at NSTAR Communications, Inc.
		As of September 30, 2013										As of December 31, 2012
					NSTAR									NSTAR
	(Millions of Dollars)	CL&P			Electric			PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Distribution	$	4,836.10		$	4,622.70		$	1,569.70	$	746.3	$	4,691.30	$	4,539.90	$	1,520.10	$	724.2
	Transmission		2,969.60			1,664.50			613.2		715.8		2,796.10		1,529.70		599.2		583.7
	Generation		-			-			1,121.00		21.1		-		-		1,125.50		21.1
	Property, Plant and
	Equipment, Gross		7,805.70			6,287.20			3,303.90		1,483.20		7,487.40		6,069.60		3,244.80		1,329.00
	Less: Accumulated Depreciation		-1,778.70			-1,634.50			-1,001.70		-265.7		-1,698.10		-1,540.10		-954		-252.1
	Property, Plant and Equipment, Net		6,027.00			4,652.70			2,302.20		1,217.50		5,789.30		4,529.50		2,290.80		1,076.90
	Construction Work in Progress		299.2			270.7			106.8		135.2		363.7		205.8		61.7		213.6
	Total Property, Plant and
	Equipment, Net	$	6,326.20		$	4,923.40		$	2,409.00	$	1,352.70	$	6,153.00	$	4,735.30	$	2,352.50	$	1,290.50

DERIVATIVE_INSTRUMENTS

DERIVATIVE INSTRUMENTS	9 Months Ended
	Sep. 30, 2013
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 for their customers, which are subject to price volatility. 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 purchases or normal sales" (normal) under the applicable accounting guidance.
	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 for the fair values of derivatives, as costs are recovered from, or refunded to, customers in their respective energy supply rates. For NU's remaining unregulated wholesale marketing contracts, changes in fair values of derivatives are included in Net Income. 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.
	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. Cash collateral posted or collected under master netting agreements is recorded as an offset to the derivative asset or liability. The following tables present the gross fair values of contracts categorized by risk type and the net amounts recorded as current or long-term derivative asset or liability:
			As of September 30, 2013
			Commodity Supply and						Net Amount Recorded as
	(Millions of Dollars)		Price Risk Management			Netting (1)			Derivative Asset/(Liability)
	Current Derivative Assets:
	Level 3:
		CL&P (1)	$	17.3		$	-10.1		$	7.2
		NSTAR Electric		0.4			-			0.4
		Other		1.3			-			1.3
	Total Current Derivative Assets		$	19		$	-10.1		$	8.9
	Long-Term Derivative Assets:
	Level 3:
		CL&P (1)	$	139.5		$	-51.5		$	88
		WMECO		0.9			-			0.9
	Total Long-Term Derivative Assets		$	140.4		$	-51.5		$	88.9
	Current Derivative Liabilities:
	Level 2:
		PSNH (1)	$	-0.5		$	0.2		$	-0.3
		Other (1) (2)		-7.4			-			-7.4
	Level 3:
		CL&P		-94.1			-			-94.1
		NSTAR Electric		-1.6			-			-1.6
		WMECO		-0.1			-			-0.1
	Total Current Derivative Liabilities		$	-103.7		$	0.2		$	-103.5
	Long-Term Derivative Liabilities:
	Level 3:
		CL&P	$	-756.4		$	-		$	-756.4
		NSTAR Electric		-10.4			-			-10.4
	Total Long-Term Derivative Liabilities		$	-766.8		$	-		$	-766.8
			As of December 31, 2012
			Commodity Supply and						Net Amount Recorded as
	(Millions of Dollars)		Price Risk Management			Netting (1)			Derivative Asset/(Liability)
	Current Derivative Assets:
	Level 2:
		Other (1)	$	0.2		$	-		$	0.2
	Level 3:
		CL&P (1)		17.7			-12			5.7
		Other		5.5			-			5.5
	Total Current Derivative Assets		$	23.4		$	-12		$	11.4
	Long-Term Derivative Assets:
	Level 3:
		CL&P (1)	$	159.7		$	-69.1		$	90.6
	Total Long-Term Derivative Assets		$	159.7		$	-69.1		$	90.6
	Current Derivative Liabilities:
	Level 2:
		Other (1) (2)	$	-19.9		$	0.6		$	-19.3
	Level 3:
		CL&P		-96.9			-			-96.9
		NSTAR Electric		-1			-			-1
	Total Current Derivative Liabilities		$	-117.8		$	0.6		$	-117.2
	Long-Term Derivative Liabilities:
	Level 2:
		Other (1)	$	-0.2		$	-		$	-0.2
	Level 3:
		CL&P		-865.6			-			-865.6
		NSTAR Electric		-13.9			-			-13.9
		WMECO		-3			-			-3
	Total Long-Term Derivative Liabilities		$	-882.7		$	-		$	-882.7
	Amounts represent derivative assets and liabilities which NU has 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.
	(2)       As of September 30, 2013 and December 31, 2012, NU had $1 million and $4.1 million, respectively, of cash posted related to these contracts, which was not offset against the derivative liability and is recorded as Prepayments and Other Current Assets on the balance sheets.
For further information on the fair value of derivative contracts, see Note 1D, "Summary of Significant Accounting Policies - Fair Value Measurements," to the financial statements.
Derivatives Not Designated as Hedges
Commodity Supply and Price Risk Management: As required by regulation, CL&P has capacity-related contracts with generation facilities. These contracts and similar UI contracts have an expected capacity of 787 MW. 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 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 forward 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. NSTAR Electric also has a capacity related contract for up to 35 MW per year that extends through 2019.
PSNH has electricity procurement contracts to purchase 0.2 million MWh of energy through November 2013.
WMECO has a renewable energy contract to purchase 0.1 million MWh of energy per year through 2029 with a facility that is expected to achieve commercial operation by June 2014.
As of September 30, 2013 and December 31, 2012, NU had NYMEX future contracts in order to reduce variability associated with the purchase price of approximately 10.2 million and 11.5 million MMBtu of natural gas, respectively.
As of September 30, 2013 and December 31, 2012, NU had approximately 5 thousand MWh and 24 thousand MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales are compared with supply contracts.
The following table presents the current change in fair value, primarily recovered through rates from customers, associated with NU's derivative contracts not designated as hedges:
Location of Amounts			Amounts Recognized on Derivatives
Recognized on Derivatives			For the Three Months Ended September 30,							For the Nine Months Ended September 30,
(Millions of Dollars)			2013			2012				2013			2012
NU
Balance Sheet:
	Regulatory Assets		$	0.3		$	11.7			$	48.8		$	-25
Statement of Income:
	Purchased Power, Fuel and Transmission			0.2			0.2				0.9			-0.8
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. The following summarizes the fair value of derivative contracts that were in a net liability position and subject to credit risk contingent features and the additional collateral that would be required to be posted by NU if the unsecured debt credit ratings of NU parent were downgraded to below investment grade:
	As of September 30, 2013									As of December 31, 2012
				Additional Collateral						Additional Collateral
	Fair Value Subject			Required if			Fair Value Subject			Required if
	to Credit Risk			Downgraded Below			to Credit Risk			Downgraded Below
(Millions of Dollars)	Contingent Features			Investment Grade			Contingent Features			Investment Grade
NU	$	-6.7		$	13.4		$	-15.3		$	17.4
Fair Value Measurements of Derivative Instruments
Valuation of Derivative Instruments: Derivative contracts classified as Level 2 in the fair value hierarchy relate to the financial contracts for natural gas futures, forward contracts to purchase energy at PSNH and the remaining unregulated wholesale marketing sourcing contracts. Prices are obtained from broker quotes and are based on actual market activity. The contracts are valued using the mid-point of the bid-ask spread. 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 approaches 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 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, NSTAR Electric's and WMECO'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 September 30, 2013				As of December 31, 2012
	Range		Period Covered		Range		Period Covered
Energy Prices:
NU	$45 - $93 per MWh		2018 - 2029		$43 - $90 per MWh		2018 - 2028
CL&P	$52 - $56 per MWh		2018 - 2020		$50 - $55 per MWh		2018 - 2020
WMECO	$45 - $93 per MWh		2018 - 2029		$43 - $90 per MWh		2018 - 2028
Capacity Prices:
NU	$1.40 - $10.53 per kW-Month		2017 - 2029		$1.40 - $10.53 per kW-Month		2016 - 2028
CL&P	$1.40 - $9.51 per kW-Month		2017 - 2026		$1.40 - $9.83 per kW-Month		2016 - 2026
NSTAR Electric	$1.40 - $3.39 per kW-Month		2017 - 2019		$1.40 - $3.39 per kW-Month		2016 - 2019
WMECO	$1.40 - $10.53 per kW-Month		2017 - 2029		$1.40 - $10.53 per kW-Month		2016 - 2028
Forward Reserve:
NU, CL&P	$3.00 per kW-Month		2013 - 2024		$0.35 - $0.90 per kW-Month		2013 - 2024
REC Prices:
NU	$25 - $87 per REC		2013 - 2029		$25 - $85 per REC		2013 - 2028
NSTAR Electric	$25 - $71 per REC		2013 - 2018		$25 - $71 per REC		2013 - 2018
WMECO	$25 - $87 per REC		2014 - 2029		$25 - $85 per REC		2013 - 2028
Exit price premiums of 10 percent through 32 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 power 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 for the three and nine months ended September 30, 2013 and 2012 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. The fair value as of January 1, 2012 reflects a reclassification of remaining unregulated wholesale marketing sourcing contracts that had previously been presented as a portfolio along with the unregulated wholesale marketing sales contract as Level 3 under the highest and best use valuation premise. These contracts are now classified within Level 2 of the fair value hierarchy.
		For the Three Months Ended September 30,						For the Nine Months Ended September 30,
		2013			2012			2013			2012
(Millions of Dollars)		NU			NU			NU			NU
Derivatives, Net:
Fair Value as of Beginning of Period		$	-788.1		$	-932.1		$	-878.6		$	-962.2
Liabilities Assumed due to Merger with NSTAR			-			-			-			-5.4
Transfer to Level 2			-			-			-			32.2
Net Realized/Unrealized Gains/(Losses) Included in:
	Net Income		1.2			-0.2			8.3			7.2
	Regulatory Assets		0.8			8.5			49.6			-30.1
Settlements			21.3			21.5			55.9			56
Fair Value as of End of Period		$	-764.8		$	-902.3		$	-764.8		$	-902.3
		For the Three Months Ended
		30-Sep-13									30-Sep-12
(Millions of Dollars)		CL&P			NSTAR Electric			WMECO			CL&P			NSTAR Electric(1)		WMECO
Derivatives, Net:
Fair Value as of Beginning of Period		$	-775.8		$	-13.1		$	-0.7		$	-910.7		$	-15.8	$	-13.5
Net Realized/Unrealized Gains/(Losses)
	Included in Regulatory Assets		-1.2			0.5			1.5			-2.8			1.4		9.8
Settlements			21.7			1			-			22.6			0.6		-
Fair Value as of End of Period		$	-755.3		$	-11.6		$	0.8		$	-890.9		$	-13.8	$	-3.7
		For the Nine Months Ended
		30-Sep-13									30-Sep-12
(Millions of Dollars)		CL&P			NSTAR Electric			WMECO			CL&P			NSTAR Electric(1)		WMECO
Derivatives, Net:
Fair Value as of Beginning of Period		$	-866.2		$	-14.9		$	-3		$	-931.6		$	-3.4	$	-7.3
Net Realized/Unrealized Gains/(Losses)
	Included in Regulatory Assets		45.1			0.6			3.8			-23.8			-13.2		3.6
Settlements			65.8			2.7			-			64.5			2.8		-
Fair Value as of End of Period		$	-755.3		$	-11.6		$	0.8		$	-890.9		$	-13.8	$	-3.7
NSTAR Electric amounts are included in NU consolidated from the date of the merger, April 10, 2012, through September 30, 2012.

MARKETABLE_SECURITIES

MARKETABLE SECURITIES	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Marketable Securities [Text Block]	'
	5.       MARKETABLE SECURITIES
	NU maintains a supplemental benefit trust to fund certain non-qualified executive retirement benefit obligations and WMECO maintains a spent nuclear fuel trust to fund WMECO's prior period spent nuclear fuel liability, each of which hold marketable securities. These trusts are not subject to regulatory oversight by state or federal agencies. NU's marketable securities also include legally restricted trusts for the decommissioning of nuclear power plants that are owned by CYAPC and YAEC.
	The Company elects to record mutual funds purchased by the NU supplemental benefit trust at fair value. As such, any change in fair value of these mutual funds is reflected in Net Income. These mutual funds, classified as Level 1 in the fair value hierarchy, totaled $54.3 million and $47 million as of September 30, 2013 and December 31, 2012, respectively, and are included in current Marketable Securities. Net gains on these securities of $3 million and $7.3 million for the three and nine months ended September 30, 2013, respectively, were recorded in Other Income, Net on the statements of income. These amounts were net gains of $1.9 million and $4.6 million for the three and nine months ended September 30, 2012, respectively. Dividend income is recorded when dividends are declared and is recorded in Other Income, Net on the statements of income. All other marketable securities are accounted for as available-for-sale.
	Available-for-Sale Securities: The following is a summary of NU's available-for-sale securities held in the NU supplemental benefit trust, WMECO's spent nuclear fuel trust and CYAPC's and YAEC's nuclear decommissioning trusts. These securities are recorded at fair value and included in current and long-term Marketable Securities on the balance sheets.
			As of September 30, 2013
						Pre-Tax			Pre-Tax
			Amortized			Unrealized			Unrealized
	(Millions of Dollars)		Cost			Gains(1)			Losses(1)			Fair Value
	NU
		Debt Securities (2)	$	306.1		$	1.5		$	-3.8		$	303.8
		Equity Securities (2)		164			40.9			-			204.9
	WMECO
		Debt Securities		57.8			-			-			57.8
			As of December 31, 2012
						Pre-Tax			Pre-Tax
			Amortized			Unrealized			Unrealized
	(Millions of Dollars)		Cost			Gains(1)			Losses(1)			Fair Value
	NU
		Debt Securities (2)	$	266.6		$	13.3		$	-0.1		$	279.8
		Equity Securities (2)		145.5			20			-			165.5
	WMECO
		Debt Securities		57.7			0.1			-0.1			57.7
	Unrealized gains and losses on debt securities for the NU supplemental benefit trust and WMECO spent nuclear fuel trust are recorded in AOCI and Other Long-Term Assets, respectively, on the balance sheets.
	(2)       NU's amounts include CYAPC's and YAEC's marketable securities held in nuclear decommissioning trusts of $403.1 million and $340.4 million as of September 30, 2013 and December 31, 2012, respectively, the majority of which are legally restricted and can only be used for the decommissioning of the nuclear power plants owned by these companies. In the first quarter of 2013, CYAPC and YAEC received cash from the DOE related to the litigation of storage costs for spent nuclear fuel, which was invested in the nuclear decommissioning trusts. Unrealized gains and losses for the nuclear decommissioning trusts are offset in Other Long-Term Liabilities on the balance sheets, with no impact on the statement of income. All of the equity securities accounted for as available-for-sale securities are held in these trusts.
	Unrealized Losses and Other-than-Temporary Impairment: There have been no significant unrealized losses, other-than-temporary impairments or credit losses for the NU supplemental benefit trust, the WMECO spent nuclear fuel trust, and the trusts held by CYAPC and YAEC. 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 the NU supplemental benefit trust, Other Long-Term Assets for the WMECO spent nuclear fuel trust, and offset in Other Long-Term Liabilities for CYAPC and YAEC. NU utilizes the specific identification basis method for the NU supplemental benefit trust securities and the average cost basis method for the WMECO spent nuclear fuel 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 September 30, 2013, 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)		$	67.1		$	65.3		$	24.4		$	24.6
	One to five years			76			76.6			26.4			26.3
	Six to ten years			58.4			57.3			2.5			2.5
	Greater than ten years			104.6			104.6			4.5			4.4
	Total Debt Securities		$	306.1		$	303.8		$	57.8		$	57.8
	(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						As of
	(Millions of Dollars)				30-Sep-13			31-Dec-12			30-Sep-13			31-Dec-12
	Level 1:
		Mutual Funds and Equities			$	259.2		$	212.5		$	-		$	-
		Money Market Funds				40			40.2			2.8			5.2
	Total Level 1				$	299.2		$	252.7		$	2.8		$	5.2
	Level 2:
		U.S. Government Issued Debt Securities
			(Agency and Treasury)		$	74.9		$	69.9		$	16.6		$	18.7
		Corporate Debt Securities				48.9			33			15.1			7
		Asset-Backed Debt Securities				30.4			28.5			9.6			10.9
		Municipal Bonds				93.7			93.8			8.8			11.6
		Other Fixed Income Securities				15.9			14.4			4.9			4.3
	Total Level 2				$	263.8		$	239.6		$	55		$	52.5
	Total Marketable Securities				$	563		$	492.3		$	57.8		$	57.7
	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.

SHORT_TERM_AND_LONG_TERM_DEBT

SHORT TERM AND LONG TERM DEBT	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Debt Disclosure	'
	6.       SHORT-TERM AND LONG-TERM DEBT
	Limits:  The amount of short-term borrowings that may be incurred by CL&P and WMECO is subject to periodic approval by the FERC. On July 31, 2013, the FERC approved the short-term debt application of CL&P and WMECO for issuances in the amounts of $600 million and $300 million, respectively, effective January 1, 2014 through December 31, 2015.
	Credit Agreements and Commercial Paper Programs: On September 6, 2013, NU parent, CL&P, NSTAR LLC, NSTAR Gas, PSNH, WMECO and Yankee Gas amended their joint five-year $1.15 billion revolving credit facility dated July 25, 2012, by increasing the aggregate principal amount available thereunder by $300 million to $1.45 billion, extending the expiration date from July 25, 2017 to September 6, 2018, and increasing CL&P's borrowing sublimit from $300 million to $600 million. At the same time, effective September 6, 2013, the CL&P $300 million revolving credit facility was terminated.
	On September 6, 2013, NSTAR Electric amended its five-year $450 million revolving credit facility dated July 25, 2012 by extending the expiration date from July 25, 2017 to September 6, 2018.
	On September 6, 2013, the NU parent $1.15 billion commercial paper program was increased by $300 million to $1.45 billion.
	As of September 30, 2013 and December 31, 2012, NU had approximately $1.2 billion and $1.15 billion, respectively, in short-term borrowings outstanding under the NU parent commercial paper program, which provides $263 million of available borrowing capacity as of September 30, 2013. The weighted-average interest rate on these borrowings as of September 30, 2013 and December 31, 2012 was 0.268 percent and 0.46 percent, respectively, which is generally based on money market rates. As of September 30, 2013, there were intercompany loans from NU of $342.9 million to CL&P, $228.5 million to PSNH and $79.8 million to WMECO. As of December 31, 2012, there were intercompany loans from NU of $405.1 million to CL&P, $63.3 million to PSNH, and $31.9 million to WMECO. As of September 30, 2013 and December 31, 2012, NSTAR Electric had $156 million and $276 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $294 million and $174 million, respectively, of available borrowing capacity. The weighted-average interest rate on these borrowings as of September 30, 2013 and December 31, 2012 was 0.134 percent and 0.31 percent, respectively, which is generally based on money market rates.
	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 Affiliated Companies and classified in current liabilities on the balance sheets.
	Long-Term Debt: On January 15, 2013, CL&P issued $400 million of Series A First and Refunding Mortgage Bonds with a coupon rate of 2.5 percent and a maturity date of January 15, 2023. The proceeds, net of issuance costs, were used to pay short-term borrowings outstanding under the CL&P credit agreement and the NU parent commercial paper program. Therefore, as of December 31, 2012, CL&P's credit agreement borrowings of $89 million and intercompany loans related to the commercial paper program of $305.8 million were classified as Long-Term Debt on the balance sheet.
	On May 1, 2013, PSNH redeemed at par approximately $109 million of the 2001 Series C PCRBs that were due to mature in 2021 using short-term debt.
	On May 13, 2013, NU parent issued $750 million of Senior Notes, consisting of $300 million of Series E Senior Notes at a coupon rate of 1.45 percent that will mature on May 1, 2018 and $450 million of Series F Senior Notes at a coupon rate of 2.80 percent that will mature on May 1, 2023. Part of the proceeds, net of issuance costs, was used to repay the NU parent $250 million Series C Senior Notes at a coupon rate of 5.65 percent that matured on June 1, 2013 and the NU parent $300 million floating rate Series D Senior Notes that matured on September 20, 2013. The remaining net proceeds were used to repay commercial paper borrowings and for other general corporate purposes.
	On May 17, 2013, NSTAR Electric issued $200 million of three-year floating rate debentures due to mature on May 17, 2016. The proceeds, net of issuance costs, were used to repay commercial paper borrowings and for general corporate purposes. The debentures have a coupon rate reset quarterly based on 3-month LIBOR plus a credit spread of 0.24 percent. The interest rate as of September 30, 2013 was 0.5032 percent.
	On September 1, 2013, WMECO repaid at maturity, $55 million of 5.00 percent Series A Senior Notes using short-term debt.
	On September 3, 2013, CL&P redeemed at par $125 million of 1.25 percent Series B 2011 PCRBs that were subject to mandatory tender for purchase using short-term debt.
	On September 20, 2013, NU parent repaid at maturity, $300 million of Floating Rate Series D Senior Notes with proceeds from NU parent's issuance on May 13, 2013 of $750 million of Series E and Series F Senior Notes.
	On August 29, 2013, NSTAR Electric filed an application with the DPU requesting authorization to issue up to $800 million in long-term debt for the two-year period ending December 31, 2015.
	On September 26, 2013, the NHPUC issued an order, effective October 8, 2013, approving PSNH's request to issue up to $315 million in long-term debt through December 31, 2014, and to refinance $89.3 million 2001 Series B PCRBs through its existing maturity of May 2021.
	Working Capital: NU, CL&P, NSTAR Electric, PSNH and WMECO use their available capital resources to fund their respective construction expenditures, meet debt requirements, pay operating costs, including storm-related costs, pay dividends and fund other corporate obligations, such as pension contributions. The current growth in NU's transmission construction expenditures utilizes a significant amount of cash for projects that have a long-term return on investment and recovery period. In addition, NU's Regulated companies operate in an environment where recovery of its electric and natural gas distribution construction expenditures takes place over an extended period of time. This impacts the timing of the revenue stream designed to fully recover the total investment plus a return on the equity portion of the cost and related financing costs. These factors have resulted in current liabilities exceeding current assets by approximately $1.4 billion, $392 million, $315 million, $114 million and $11 million at NU, CL&P, NSTAR Electric, PSNH and WMECO, respectively, as of September 30, 2013.
	As of September 30, 2013, approximately $577 million of NU's current liabilities related to long-term debt that will be paid in the next 12 months, primarily consisting of $150 million for CL&P, $302 million for NSTAR Electric and $50 million for PSNH. NU, with its strong credit ratings, has several options available in the financial markets to repay or refinance these maturities with the issuance of new long-term debt. NU, CL&P, NSTAR Electric, PSNH and WMECO will reduce their short-term borrowings with cash received from operating cash flows and/or with the issuance of new long-term debt, as deemed appropriate given capital requirements and maintenance of NU's credit rating and profile. Management expects the future operating cash flows of NU, CL&P, NSTAR Electric, PSNH and WMECO along with the access to financial markets, will be sufficient to meet any future operating requirements and forecasted capital investment opportunities.

PENSION_BENEFITS_AND_POSTRETIR

PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Pension and Other Postretirement Benefits Disclosure [Text Block]	'
	7.       Pension Benefits and Postretirement Benefits Other than Pensions
	The components of net periodic benefit expense for the Pension Plans (including the SERP Plans) and PBOP Plans, the portion of pension and PBOP amounts capitalized related to employees working on capital projects, and intercompany allocations not included in the net periodic benefit expense are as follows:
		Pension and SERP				Pension and SERP
		For the Three Months Ended				For the Nine Months Ended
		30-Sep-13		30-Sep-12		30-Sep-13		30-Sep-12
	(Millions of Dollars)	NU		NU		NU		NU
	Service Cost	$	25.6	$	23	$	76.7	$	61.1
	Interest Cost		51.7		53.3		155		144.7
	Expected Return on Plan Assets		-69.5		-59.5		-208.5		-161.3
	Actuarial Loss		52.4		47.4		158.1		125
	Prior Service Cost		1.1		2		3		6.1
	Total Net Periodic Benefit Expense	$	61.3	$	66.2	$	184.3	$	175.6
	Capitalized Pension Expense	$	18.3	$	19.2	$	54.9	$	49.5
		PBOP				PBOP
		For the Three Months Ended				For the Nine Months Ended
		30-Sep-13		30-Sep-12		30-Sep-13		30-Sep-12
	(Millions of Dollars)	NU		NU		NU		NU
	Service Cost	$	4.2	$	4.4	$	12.6	$	11.3
	Interest Cost		11.8		14.3		35.4		34.4
	Expected Return on Plan Assets		-13.9		-11.1		-41.6		-28.1
	Actuarial Loss		6.5		10.3		19.5		25.5
	Prior Service Credit		-0.5		-0.5		-1.5		-0.9
	Net Transition Obligation Cost		-		3.1		-		9
	Total Net Periodic Benefit Expense	$	8.1	$	20.5	$	24.4	$	51.2
	Capitalized PBOP Expense	$	2.6	$	5.1	$	7.6	$	14.9
		Pension and SERP
		For the Three Months Ended September 30, 2013								For the Three Months Ended September 30, 2012
				NSTAR								NSTAR
	(Millions of Dollars)	CL&P		Electric(1)		PSNH		WMECO		CL&P		Electric(1)		PSNH		WMECO
	Service Cost	$	6.3	$	8.3	$	3.3	$	1.2	$	5.4	$	7.6	$	2.9	$	1
	Interest Cost		12.1		14.5		5.8		2.5		12.9		14.7		6.1		2.6
	Expected Return on Plan Assets		-18.4		-21.1		-9.2		-4.3		-17.7		-16.4		-7.2		-4.1
	Actuarial Loss		13.9		14.4		5.4		2.9		12.6		15.7		4.2		2.7
	Prior Service Cost/(Credit)		0.4		-		0.1		0.1		0.9		-0.1		0.4		0.2
	Total Net Periodic Benefit Expense	$	14.3	$	16.1	$	5.4	$	2.4	$	14.1	$	21.5	$	6.4	$	2.4
	Intercompany Allocations	$	11.4	$	-2.1	$	2.6	$	2	$	10.7	$	-3	$	2.4	$	2.1
	Capitalized Pension Expense	$	7	$	9.8	$	1.7	$	1.3	$	6.8	$	8.4	$	1.9	$	1.3
		Pension and SERP
		For the Nine Months Ended September 30, 2013								For the Nine Months Ended September 30, 2012
				NSTAR								NSTAR
	(Millions of Dollars)	CL&P		Electric(1)		PSNH		WMECO		CL&P		Electric(1)		PSNH		WMECO
	Service Cost	$	18.7	$	24.8	$	9.8	$	3.5	$	16.3	$	22.7	$	8.8	$	3.1
	Interest Cost		36.3		43.5		17.8		7.5		38.5		44.2		18.3		7.9
	Expected Return on Plan Assets		-55.4		-63.3		-26.2		-13		-52.8		-49.2		-21.1		-12.3
	Actuarial Loss		42		43.6		16.2		8.9		37		47.3		12.1		8
	Prior Service Cost/(Credit)		1.4		-0.2		0.4		0.3		2.7		-0.4		1.1		0.6
	Total Net Periodic Benefit Expense	$	43	$	48.4	$	18	$	7.2	$	41.7	$	64.6	$	19.2	$	7.3
	Intercompany Allocations	$	33.6	$	-6.2	$	7.8	$	6	$	32	$	-9.2	$	7.5	$	6
	Capitalized Pension Expense	$	21	$	21.6	$	5.6	$	3.9	$	20.2	$	23.6	$	5.8	$	3.7
		PBOP
		For the Three Months Ended September 30, 2013						For the Three Months Ended September 30, 2012
	(Millions of Dollars)	CL&P			PSNH		WMECO	CL&P		PSNH		WMECO
	Service Cost	$	0.9	$	0.6	$	0.2	$	0.8	$	0.5	$	0.1
	Interest Cost		2		1		0.4		2.3		1.1		0.5
	Expected Return on Plan Assets		-2.5		-1.3		-0.6		-2.3		-1.1		-0.5
	Actuarial Loss		1.8		0.9		0.3		1.9		0.9		0.3
	Net Transition Obligation Cost		-		-		-		1.5		0.6		0.3
	Total Net Periodic Benefit Expense	$	2.2	$	1.2	$	0.3	$	4.2	$	2	$	0.7
	Intercompany Allocations	$	1.7	$	0.4	$	0.3	$	2	$	0.5	$	0.4
	Capitalized PBOP Expense	$	1.3	$	0.4	$	0.3	$	2.1	$	0.6	$	0.4
		PBOP
		For the Nine Months Ended September 30, 2013						For the Nine Months Ended September 30, 2012
	(Millions of Dollars)	CL&P			PSNH		WMECO	CL&P		PSNH		WMECO
Service Cost	$	2.6	$	1.7	$	0.5	$	2.2	$	1.5	$	0.4
Interest Cost		5.9		3.1		1.3		6.9		3.4		1.5
Expected Return on Plan Assets		-7.6		-3.9		-1.7		-6.8		-3.4		-1.6
Actuarial Loss		5.6		2.7		0.8		5.7		2.7		0.9
Net Transition Obligation Cost		-		-		-		4.6		1.9		1.1
Total Net Periodic Benefit Expense	$	6.5	$	3.6	$	0.9	$	12.6	$	6.1	$	2.3
Intercompany Allocations	$	5.3	$	1.2	$	1	$	5.9	$	1.5	$	1.1
Capitalized PBOP Expense	$	3.7	$	1.1	$	0.7	$	6.2	$	1.7	$	1.1
NSTAR Electric's pension amounts do not include SERP expense. NSTAR Electric pension amounts are included in NU consolidated from the date of the merger, April 10, 2012, through September 30, 2012.
The net periodic postretirement expense allocated to NSTAR Electric was $1.2 million and $8.5 million for the three months ended September 30, 2013 and 2012, respectively, and $3.5 million and $25.6 million for the nine months ended September 30, 2013 and 2012, respectively.
Contributions: For the nine months ended September 30, 2013, NU contributed $202.7 million to the NUSCO Pension Plan, $108.3 million of which was contributed by PSNH, and NSTAR Electric contributed $82 million to the NSTAR Pension Plan. NU contributed $53.6 million to the PBOP Plans for the nine months ended September 30, 2013.

INCOME_TAXES

INCOME TAXES	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Income Tax Disclosure [Text Block]	'
	8.       INCOME TAXES
	2013 Massachusetts: On July 24, 2013, Massachusetts enacted a law that changes 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 deferred taxes and resulted in NU increasing its deferred tax liability with an offsetting regulatory asset of approximately $61 million at its utility companies ($46.4 million at NSTAR Electric and $9.8 million at WMECO).
	2013 Federal: On September 13, 2013, the Internal Revenue Service issued final Tangible Property regulations. The final regulations are meant to simplify, clarify and make more administrable the previously issued temporary and proposed regulations. In the third quarter of 2013, CL&P recorded an after-tax valuation allowance of $10.5 million against its deferred tax assets as a result of these regulations. NU continues to evaluate the implications of these new regulations, including several new elections. Therefore, a change to the valuation allowance at CL&P could result once NU completes the review of the impact of the final regulations.

COMMITMENTS_AND_CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Commitments and Contingencies Disclosure [Text Block]	'
	9.       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.
	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 September 30, 2013							As of December 31, 2012
						Reserve							Reserve
			Number of Sites			(in millions)				Number of Sites			(in millions)
	NU			68		$	36.5				77		$	39.4
	CL&P			18			3.5				19			3.7
	NSTAR Electric			12			1.2				16			1.7
	PSNH			15			5.6				16			4.9
	WMECO			5			0.4				6			0.6
	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 $32.4 million and $34.5 million as of September 30, 2013 and December 31, 2012, respectively, and relates primarily to the natural gas business segment.
	B.       Long-Term Contractual Arrangements
	Yankee Billings: As a result of the change in forecasted life of spent nuclear fuel decommissioning obligations, as well as proceeds received from the DOE in January 2013 arising from the spent nuclear fuel litigation, estimated future annual costs of Yankee Billings as of September 30, 2013 are reflected in the table below.
	Renewable Energy: Renewable energy contracts include non-cancelable commitments under contracts of CL&P for the purchase of energy and capacity from renewable energy facilities.
		October - December
	(Millions of Dollars)	2013			2014			2015			2016			2017		Thereafter		Total
	Yankee Billings
	CL&P	$	0.4		$	1.5		$	1.3		$	0.8		$	0.8	$	13.1	$	17.9
	NSTAR Electric		0.2			0.7			0.5			0.2			0.3		4.5		6.4
	PSNH		0.1			0.3			0.4			0.3			0.3		5.2		6.6
	WMECO		0.1			0.4			0.4			0.2			0.2		3.3		4.6
	Renewable Energy
	CL&P		1.2			49.9			50.9			51.4			52		626		831.4
	Other Long-Term Renewable Energy Contracts: On September 20, 2013, NSTAR Electric and WMECO, along with two other Massachusetts utilities, signed a long-term commitment, as required by state regulation, to purchase wind power from six wind farms in Maine and New Hampshire for a combined estimated generating capacity of approximately 550 MW. Over the life of the 15- to 20-year contracts, the utilities will pay an average price of less than $0.08 per kWh. On September 19, 2013, CL&P, along with another Connecticut utility, signed long-term commitments, as required by state regulation, to purchase approximately 250 MW of wind power from a Maine wind farm and 20 MW of solar power from sites in Connecticut, at a combined average price of less than $0.08 per kWh. The table above does not include these commitments for the purchase of renewable energy, as such commitments are contingent on the future construction of the respective energy facilities.
	C.       Deferred Contractual Obligations
	Spent Nuclear Fuel Litigation - DOE Phase I Damages - On May 1, 2013, CYAPC, YAEC and MYAPC filed applications with the FERC to reduce rates in their wholesale power contracts through the application of the DOE proceeds for the benefit of customers. In its June 27, 2013 order, FERC granted the proposed rate reductions, and changes to the terms of the wholesale power contracts to become 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 or will be passed on to customers.
	D.       Guarantees and Indemnifications
	NU parent, or NSTAR LLC, as applicable, 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, 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 or cash flows from operations 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 September 30, 2013:
					Maximum Exposure
	Subsidiary		Description		(in millions)				Expiration Dates
	Various		Surety Bonds		$	33			2013 - 2015 (1)
	Various		New England Hydro Companies' Long-Term Debt		$	4			Unspecified
	NUSCO and RRR		Lease Payments for Vehicles and Real Estate		$	18.8			2019 and 2024
	NU Enterprises		Surety Bonds, Performance Guarantees and Insurance Bond		$	62.3	-2		-2
	(1)       Surety bond expiration dates reflect termination dates, the majority of which will be renewed or extended.
	(2)       The maximum exposure includes $3.8 million related to performance guarantees on wholesale purchase contracts, which expire December 31, 2013. Also included in the maximum exposure is $57.5 million relating to surety bonds covering ongoing projects, which expire upon project completion. The remaining $1 million is related to an insurance bond with no expiration date that is billed annually.
	Many of the underlying contracts that NU parent guarantees, as well as 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, or NSTAR LLC, as applicable, are downgraded.
	E. FERC Base ROE Complaint
	On September 30, 2011, several New England state attorneys general, state regulatory commissions, consumer advocates and other parties filed a joint complaint with the FERC under Sections 206 and 306 of the Federal Power Act alleging that the base ROE used in calculating formula rates for transmission service under the ISO-NE Open Access Transmission Tariff by NETOs, including CL&P, NSTAR Electric, PSNH and WMECO, is unjust and unreasonable. The complainants asserted that the current 11.14 percent rate, which became effective in 2006, is excessive due to changes in the capital markets and are seeking an order to reduce the rate, which would be effective October 1, 2011. In response, the NETOs filed testimony and analysis based on standard FERC methodology and precedent, demonstrating that the base ROE of 11.14 percent remained just and reasonable. The FERC set the case for trial before a FERC ALJ after settlement negotiations were unsuccessful in August 2012.
	Hearings before the FERC ALJ were held in May 2013, followed by the filing of briefs by the complainants, the Massachusetts municipal electric utilities (late interveners to the case), the FERC trial staff and the NETOs. The NETOs recommended that the current base ROE of 11.14 percent should remain in effect for the refund period (October 1, 2011 through December 31, 2012) and the prospective period (beginning when FERC issues its final decision). The complainants, the Massachusetts municipal electric utilities, and the FERC trial staff each recommended a base ROE of 9 percent or below.
	On August 6, 2013, the FERC ALJ issued an initial decision, finding that the current base ROE is not reasonable under the standard application of FERC methodology, but leaving policy considerations and additional adjustments to the FERC. Using the established FERC methodology, the FERC ALJ determined that a separate base ROE should be set for the refund period and the prospective period. The FERC ALJ found those base ROEs to be 10.6 percent and 9.7 percent, respectively. The FERC may adjust the prospective period base ROE in its final decision to reflect movement in 10-year Treasury bond rates from when the case was filed (April 2013) to the date of the final decision. The parties filed briefs on this decision to the FERC, and a decision from the FERC is expected in 2014. Though NU cannot predict the ultimate outcome of this proceeding, during the third quarter of 2013, the Company recorded a series of reserves at its electric subsidiaries to recognize the potential financial impact from the FERC ALJ's initial decision for the refund period. As a result, the aggregate after-tax charge to earnings totaled $14.3 million at NU. This represents reserves of $7.7 million at CL&P, $3.4 million at NSTAR Electric, $1.4 million at PSNH and $1.8 million at WMECO.
	On December 27, 2012, several additional parties filed a separate complaint concerning the NETOs' base ROE with the FERC. This complaint seeks to reduce the NETOs' base ROE effective January 1, 2013, effectively extending the refund period for an additional 15 months, and to consolidate this complaint with the joint complaint filed on September 30, 2011. The NETOs have asked the FERC to reject this complaint. The FERC has not yet acted on, and management is unable to predict the ultimate outcome or the estimated impacts on financial position, results of operations or cash flows, of this complaint.
	Management expects the CL&P, NSTAR Electric, PSNH, and WMECO aggregate shareholder equity invested in their transmission facilities to be approximately $2.4 billion at the end of 2013. As a result, each 10 basis point change in the prospective period authorized base ROE would change annual consolidated earnings by an approximate $2.4 million.
	F. DPU Safety and Reliability Programs - CPSL
	Since 2006, NSTAR Electric has been recovering incremental costs related to the DPU-approved Safety and Reliability Programs. From 2006 through 2011, cumulative costs associated with the CPSL program resulted in an incremental revenue requirement to customers of approximately $83 million. These amounts included incremental operations and maintenance costs and the related revenue requirement for specific capital investments relative to the CPSL programs.
	On May 28, 2010, the DPU issued an order on NSTAR Electric's 2006 CPSL cost recovery filing (the May 2010 Order). In October 2010, NSTAR Electric filed a reconciliation of the cumulative CPSL program activity for the periods 2006 through 2009 with the DPU in order to determine a proposed rate adjustment. The DPU allowed the proposed rates to go into effect January 1, 2011, subject to final reconciliation of CPSL program costs through a future DPU proceeding. In February 2013, NSTAR Electric updated the October 2010 filing with final activity through 2011. NSTAR Electric recorded its 2006 through 2011 revenues under the CPSL programs based on the May 2010 Order.
	NSTAR Electric cannot predict the timing of a final DPU order related to its CPSL filings for the period 2006 through 2011. While management does not believe that any subsequent DPU order would result in revenues that are materially different than the amounts already recognized, it is reasonably possible that an order could have a material impact on NSTAR Electric's results of operations, financial position and cash flows.
	G. Basic Service Bad Debt Adder
	In accordance with a generic 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. In 2012, the SJC vacated the DPU order and remanded the matter to the DPU for further review.
	NSTAR Electric deferred approximately $34 million of costs associated with energy-related bad debt as a regulatory asset through 2011 as NSTAR Electric had concluded that it was probable that these costs would ultimately be recovered from customers. Due to the delays and duration of the proceedings, NSTAR Electric concluded that while an ultimate outcome on the matter in its favor remained "more likely than not," it could no longer be deemed "probable." As a result, NSTAR Electric recognized a reserve related to the regulatory asset in the first quarter of 2012. NSTAR Electric will continue to maintain the reserve until the ultimate outcome of the proceeding has been concluded with the DPU.

FAIR_VALUE_OF_FINANCIAL_INSTRU

FAIR VALUE OF FINANCIAL INSTRUMENTS	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Fair Value Disclosures [Text Block]	'
	10.       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, Long-Term Debt and Rate Reduction Bonds: 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 fixed-rate long-term debt securities and RRBs 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. Adjustable rate long-term debt securities are assumed to have a fair value equal to their carrying value. 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 September 30, 2013				As of December 31, 2012
			NU				NU
			Carrying		Fair		Carrying		Fair
	(Millions of Dollars)		Amount		Value		Amount		Value
	Preferred Stock Not
		Subject to Mandatory Redemption	$	155.6	$	152.2	$	155.6	$	152.2
	Long-Term Debt			8,052.50		8,267.20		7,963.50		8,640.70
	Rate Reduction Bonds			-		-		82.1		83
			As of September 30, 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	$	110.3	$	43	$	41.9	$	-	$	-	$	-	$	-
	Long-Term Debt			2,741.00		2,992.00		1,801.00		1,881.90		889.1		934.7		549.6		562.1
			As of December 31, 2012
			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	$	43	$	42.2	$	-	$	-	$	-	$	-
	Long-Term Debt			2,862.80		3,295.40		1,602.60		1,818.80		997.9		1,088.00		605.3		660.4
	Rate Reduction Bonds			-		-		43.5		43.9		29.3		29.6		9.4		9.5
	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 1D, "Summary of Significant Accounting Policies - Fair Value Measurements," and 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.

ACCUMULATED_OTHER_COMPREHENSIV

ACCUMULATED OTHER COMPREHENSIVE INCOME	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Comprehensive Income Note [Text Block]	'
	11.       ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS)
	The changes in accumulated other comprehensive income/(loss) by component, net of tax, is as follows:
			For the Nine Months Ended September 30, 2013
	(Millions of Dollars)		Qualified Cash Flow Hedging Instruments			Unrealized Gains/(Losses) on Available-for-Sale Securities			Pension, SERP and PBOP Benefit Plans		Total
	AOCI as of January 1, 2013		$	-16.4		$	1.3		$	-57.8	$	-72.9
	Other Comprehensive Income Before Reclassifications			-			-0.8			-		-0.8
	Amounts Reclassified from AOCI			1.5			-			4.8		6.3
	Net Other Comprehensive Income			1.5			-0.8			4.8		5.5
	AOCI as of September 30, 2013		$	-14.9		$	0.5		$	-53	$	-67.4
	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 following table sets forth the amounts reclassified from AOCI by component and the affected line item on the statements of income:
		For the Three Months Ended			For the Nine Months Ended
		30-Sep-13			30-Sep-13
		Amount Reclassified			Amount Reclassified			Statements of Income
	(Millions of Dollars)	from AOCI			from AOCI			Line Item Impacted
	Qualified Cash Flow Hedging Instruments	$	-0.8		$	-2.5		Interest Expense
	Tax Benefit		0.3			1		Income Tax Expense
	Qualified Cash Flow Hedging Instruments, Net of Tax	$	-0.5		$	-1.5
	Pension, SERP and PBOP Benefit Plan Costs:
	Amortization of Actuarial Losses	$	-2.5		$	-7.3		-1
	Amortization of Prior Service Cost		0			-0.1		-1
	Total Pension, SERP and PBOP Benefit Plan Costs		-2.5			-7.4		-1
	Tax Benefit		0.9			2.6		Income Tax Expense
	Pension, SERP and PBOP Benefit Plan Costs, Net of Tax	$	-1.6		$	-4.8
	Total Amount Reclassified from AOCI, Net of Tax	$	-2.1		$	-6.3
	These AOCI amounts are included in the computation of net periodic Pension, SERP and PBOP costs. See Note 7, "Pension Benefits and Postretirement Benefits Other Than Pensions," for further information.

COMMON_SHARES

COMMON SHARES	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule of Stock by Class [Text Block]	'
	12.       COMMON SHARES
	The following table sets forth the NU common shares and the shares of CL&P, NSTAR Electric, PSNH and WMECO common stock authorized and issued and the respective par values:
		Shares
		Authorized				Issued
		Per Share		As of		As of
		Par Value		30-Sep-13	31-Dec-12	30-Sep-13	31-Dec-12
	NU	$	5	380,000,000	380,000,000	333,019,517		332,509,383
	CL&P	$	10	24,500,000	24,500,000	6,035,205		6,035,205
	NSTAR Electric	$	1	100,000,000	100,000,000	100		100
	PSNH	$	1	100,000,000	100,000,000	301		301
	WMECO	$	25	1,072,471	1,072,471	434,653		434,653
	As of September 30, 2013 and December 31, 2012, 18,137,017 and 18,455,749 NU common shares were held as treasury shares, respectively.

COMMON_SHARESHOLDERS_EQUITY_AN

COMMON SHARESHOLDERS' EQUITY AND NONCONTROLLING INTERESTS (NU)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Stockholders' Equity Note Disclosure [Text Block]	'
	13		COMMON SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS
	A summary of the changes in Common Shareholders' Equity and Noncontrolling Interests of NU is as follows:
				For the Three Months Ended
				30-Sep-13				30-Sep-12
						Noncontrolling								Noncontrolling
						Interest -								Interest -
				Common		Preferred		Common		Non-				Preferred
				Shareholders'		Stock of		Shareholders'		Controlling		Total		Stock of
	(Millions of Dollars)			Equity		Subsidiaries		Equity		Interest		Equity		Subsidiaries
	Balance - Beginning of Period			$	9,406.60	$	155.6	$	9,067.60	$	-	$	9,067.60	$	155.6
	Net Income				211.4		-		209.5		-		209.5		-
	Dividends on Common Shares				-114.9		-		-107.6		-		-107.6		-
	Dividends on Preferred Stock				-1.9		-1.9		-1.9		-		-1.9		-1.9
	Issuance of Common Shares				1.4		-		0.8		-		0.8		-
	Other Transactions, Net				12.8		-		6.3		-		6.3		-
	Net Income Attributable to
		Noncontrolling Interests			-		1.9		-		-		-		1.9
	Other Comprehensive Income				2.1		-		2.2		-		2.2		-
	Balance - End of Period			$	9,517.50	$	155.6	$	9,176.90	$	-	$	9,176.90	$	155.6
				For the Nine Months Ended
				30-Sep-13				30-Sep-12
						Noncontrolling								Noncontrolling
						Interest -								Interest -
				Common		Preferred		Common		Non-				Preferred
				Shareholders'		Stock of		Shareholders'		Controlling		Total		Stock of
	(Millions of Dollars)			Equity		Subsidiaries		Equity		Interest		Equity		Subsidiaries
	Balance - Beginning of Period			$	9,237.10	$	155.6	$	4,012.70	$	3	$	4,015.70	$	116.2
	Net Income				614.4		-		356.5		-		356.5		-
	Purchase Price of NSTAR				-		-		5,038.30		-		5,038.30		-
	Other Equity Impacts of
		Merger with NSTAR			-		-		3.4		-3.4		-		39.4
	Dividends on Common Shares				-346.9		-		-267.8		-		-267.8		-
	Dividends on Preferred Stock				-5.8		-5.8		-5.1		-		-5.1		-5.1
	Issuance of Common Shares				10.2		-		12.2		-		12.2		-
	Contributions to NPT				-		-		-		0.3		0.3		-
	Other Transactions, Net				3		-		20.3		-		20.3		-
	Net Income Attributable to
		Noncontrolling Interests			-		5.8		-0.1		0.1		-		5.1
	Other Comprehensive Income				5.5		-		6.5		-		6.5		-
	Balance - End of Period			$	9,517.50	$	155.6	$	9,176.90	$	0	$	9,176.90	$	155.6

EARNINGS_PER_SHARE

EARNINGS PER SHARE	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Earnings Per Share [Text Block]	'
	14.       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 if certain share-based compensation awards are converted into common shares. There were no antidilutive share awards outstanding for the three months ended September 30, 2013 and 2012. For the nine months ended September 30, 2013 and 2012, there were 2,100 and 5,688, respectively, antidilutive share awards excluded from the computation.
	The following table sets forth the components of basic and diluted EPS:
			For the Three Months Ended				For the Nine Months Ended
	(Millions of Dollars, except share information)		30-Sep-13		30-Sep-12		30-Sep-13		30-Sep-12
	Net Income Attributable to Controlling Interest		$	209.5	$	207.6	$	608.6	$	351.2
	Weighted Average Common Shares Outstanding:
		Basic		315,291,346		314,806,441		315,191,752		264,636,636
		Dilutive Effect		926,893		999,355		869,379		716,741
		Diluted		316,218,239		315,805,796		316,061,131		265,353,377
	Basic EPS		$	0.66	$	0.66	$	1.93	$	1.33
	Diluted EPS		$	0.66	$	0.66	$	1.93	$	1.32
	On April 10, 2012, NU issued approximately 136 million common shares as a result of the merger with NSTAR, which are reflected in weighted average common shares outstanding for all periods presented.
	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	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Segment Reporting Disclosure [Text Block]	'
	15.       SEGMENT INFORMATION
	Presentation: NU is organized between the Electric Distribution, Electric Transmission and Natural Gas Distribution 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 segments represented substantially all of NU's total consolidated revenues for the three and nine months ended September 30, 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 segment includes the generation activities of PSNH and WMECO.
	Other operations in the tables below 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 income and expense related to the cash and debt of NU parent and NSTAR LLC, respectively, 2) the revenues and expenses of NU's service companies, most of which are eliminated in consolidation, 3) the operations of CYAPC and YAEC, and 4) the results of other subsidiaries, which are comprised of NU Enterprises, NSTAR Communications, Inc., RRR (a real estate subsidiary), the non-energy-related subsidiaries of Yankee and the remaining operations of HWP.
	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 the combined Electric Distribution, Electric Transmission and Natural Gas Distribution segments, 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. Therefore, separate Transmission and Distribution information is not disclosed for CL&P, NSTAR Electric, PSNH or WMECO. NU's operating segments and reporting units are consistent with its reportable business segments.
	NSTAR amounts are included in NU consolidated as of April 10, 2012.
	NU's segment information for the three and nine months ended September 30, 2013 and 2012 is as follows:
			For the Three Months Ended September 30, 2013
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues		$	1,508.60	$	97.1	$	234.1	$	212.5	$	-159.7	$	1,892.60
	Depreciation and Amortization			-159.6		-16.4		-34.5		-11.2		2.6		-219.1
	Other Operating Expenses			-1,064.10		-89.4		-73.4		-206.8		159.5		-1,274.20
	Operating Income/(Loss)			284.9		-8.7		126.2		-5.5		2.4		399.3
	Net Income/(Loss) Attributable
		to Controlling Interest		156.9		-10.4		58.6		313.1		-308.7		209.5
			For the Nine Months Ended September 30, 2013
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues		$	4,104.40	$	613	$	721.5	$	650.4	$	-565.8	$	5,523.50
	Depreciation and Amortization			-488.7		-50.5		-100.9		-52		7.2		-684.9
	Other Operating Expenses			-2,952.40		-483.6		-199.1		-599		564.3		-3,669.80
	Operating Income/(Loss)			663.3		78.9		421.5		-0.6		5.7		1,168.80
	Net Income Attributable
		to Controlling Interest		347.5		34.1		215.4		868.7		-857.1		608.6
	Cash Flows Used for
		Investments in Plant		501.9		91.2		458.2		22.5		-		1,073.80
			For the Three Months Ended September 30, 2012
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues		$	1,483.70	$	91.3	$	235.6	$	219.5	$	-168.6	$	1,861.50
	Depreciation and Amortization			-172.6		-12.6		-29.7		-17.5		1.1		-231.3
	Other Operating Expenses			-1,027.40		-77.2		-66.3		-216.8		170.4		-1,217.30
	Operating Income/(Loss)			283.7		1.5		139.6		-14.8		2.9		412.9
	Net Income/(Loss) Attributable
		to Controlling Interest		150.5		-4.4		71.1		313.9		-323.5		207.6
			For the Nine Months Ended September 30, 2012
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues		$	3,499.70	$	361.5	$	627.2	$	582.9	$	-481.5	$	4,589.80
	Depreciation and Amortization			-398.1		-32.7		-79.5		-39.1		2.6		-546.8
	Other Operating Expenses			-2,654.40		-292.9		-179.5		-614.5		485.1		-3,256.20
	Operating Income/(Loss)			447.2		35.9		368.2		-70.7		6.2		786.8
	Net Income Attributable
		to Controlling Interest		212.1		8.3		181.1		511.6		-561.9		351.2
	Cash Flows Used for
		Investments in Plant		461.3		105.9		476		38.6		-		1,081.80
	The following table summarizes NU's segmented total assets:
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	As of September 30, 2013			17,912.90		2,656.80		6,566.10		19,446.90		-18,138.40		28,444.30
	As of December 31, 2012			18,047.30		2,717.40		6,187.70		18,832.60		-17,482.20		28,302.80

SIGNIFCANT_ACCOUNTING_POLICIES

SIGNIFCANT ACCOUNTING POLICIES (Policies)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Organization Consolidation And Presentation Of Financial Statements Disclosure Text Block	'
	A.       Basis of Presentation
	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 100 percent of the outstanding common shares of NSTAR and its subsidiaries. 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. NU's consolidated financial information does not include NSTAR and its subsidiaries' results of operations for the three months ended March 31, 2012. The information disclosed for NSTAR Electric represents its results of operations for the three and nine months ended September 30, 2013 and 2012, presented on a comparable basis.
	The unaudited condensed 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 unaudited condensed consolidated financial statements of NU, NSTAR Electric and PSNH and the unaudited condensed financial statements of CL&P and WMECO are herein collectively referred to as the "financial statements."
	The combined notes to the financial statements have been prepared pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures included in annual financial statements prepared in accordance with GAAP have been omitted pursuant to such rules and regulations. The accompanying financial statements should be read in conjunction with the entirety of this combined Quarterly Report on Form 10-Q, the first and second quarter 2013 combined Quarterly Reports on Form 10-Q and the 2012 combined Annual Report on Form 10-K of NU, CL&P, NSTAR Electric, PSNH and WMECO, which were filed with the SEC. 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.
	The financial statements contain, in the opinion of management, all adjustments (including normal, recurring adjustments) necessary to present fairly NU's, CL&P's, NSTAR Electric's, PSNH's and WMECO's financial position as of September 30, 2013 and December 31, 2012, the results of operations and comprehensive income for the three and nine months ended September 30, 2013 and 2012, and the cash flows for the nine months ended September 30, 2013 and 2012. The results of operations and comprehensive income for the three and nine months ended September 30, 2013 and 2012, and the cash flows for the nine months ended September 30, 2013 and 2012, are not necessarily indicative of the results expected for a full year. The demand for electricity and natural gas is affected by weather conditions, economic conditions, and consumer conservation (including company-sponsored energy efficiency programs). Electric energy sales and revenues are typically higher in the winter and summer months than in the spring and fall months. Natural gas sales and revenues are typically higher in the winter months than during other periods of the year.
	NU consolidates CYAPC and YAEC as 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. For CL&P, NSTAR Electric, PSNH and WMECO, the investment in CYAPC and YAEC continue to be accounted for under the equity method.
	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' energy delivery business is 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 period data were made in the accompanying balance sheets for NU, PSNH and WMECO, and the statements of cash flows for all companies presented. These reclassifications were made to conform to the current period's presentation.
Receivables Policy Text Block	'
	NU, including CL&P, NSTAR Electric, PSNH and WMECO, presents its receivables at net realizable value by maintaining a provision for uncollectible amounts. This provision is determined based upon a variety of factors, including applying an estimated uncollectible account percentage to each receivable aging category, based upon historical collection and write-off experience and management's assessment of collectibility from individual customers. Management assesses the collectibility of receivables, and if circumstances change, collectibility estimates are adjusted accordingly. Receivable balances are written off against the provision for uncollectible accounts when the accounts are terminated and these balances are deemed to be uncollectible.
Fair Value Of Financial Instruments Policy	'
	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.
	The following methods and assumptions were used to estimate the fair value of each of the following financial instruments:
	Preferred Stock, Long-Term Debt and Rate Reduction Bonds: 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 fixed-rate long-term debt securities and RRBs 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. Adjustable rate long-term debt securities are assumed to have a fair value equal to their carrying value. The fair values provided in the tables below are classified as Level 2 within the fair value hierarchy.
Other Taxes Policy	'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 Certain sales taxes are also 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 reflect 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	'
	4.       DERIVATIVE INSTRUMENTS
	The Regulated companies purchase and procure energy and energy-related products for their customers, which are subject to price volatility. 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 purchases or normal sales" (normal) under the applicable accounting guidance.
	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 for the fair values of derivatives, as costs are recovered from, or refunded to, customers in their respective energy supply rates. For NU's remaining unregulated wholesale marketing contracts, changes in fair values of derivatives are included in Net Income. 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.
	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. Cash collateral posted or collected under master netting agreements is recorded as an offset to the derivative asset or liability.
	Fair Value Measurements of Derivative Instruments
	Valuation of Derivative Instruments: Derivative contracts classified as Level 2 in the fair value hierarchy relate to the financial contracts for natural gas futures, forward contracts to purchase energy at PSNH and the remaining unregulated wholesale marketing sourcing contracts. Prices are obtained from broker quotes and are based on actual market activity. The contracts are valued using the mid-point of the bid-ask spread. 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 approaches 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 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.
Marketable Securities Policy	'
	5.       MARKETABLE SECURITIES
	NU maintains a supplemental benefit trust to fund certain non-qualified executive retirement benefit obligations and WMECO maintains a spent nuclear fuel trust to fund WMECO's prior period spent nuclear fuel liability, each of which hold marketable securities. These trusts are not subject to regulatory oversight by state or federal agencies. NU's marketable securities also include legally restricted trusts for the decommissioning of nuclear power plants that are owned by CYAPC and YAEC.
	The Company elects to record mutual funds purchased by the NU supplemental benefit trust at fair value. As such, any change in fair value of these mutual funds is reflected in Net Income. These mutual funds, classified as Level 1 in the fair value hierarchy, totaled $54.3 million and $47 million as of September 30, 2013 and December 31, 2012, respectively, and are included in current Marketable Securities. Net gains on these securities of $3 million and $7.3 million for the three and nine months ended September 30, 2013, respectively, were recorded in Other Income, Net on the statements of income. These amounts were net gains of $1.9 million and $4.6 million for the three and nine months ended September 30, 2012, respectively. Dividend income is recorded when dividends are declared and is recorded in Other Income, Net on the statements of income. 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.
Earnings Per Share Policy Text Block	'
	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 if certain share-based compensation awards are converted into common shares.
	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 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 segments represented substantially all of NU's total consolidated revenues for the three and nine months ended September 30, 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 segment includes the generation activities of PSNH and WMECO.
	Other operations in the tables below 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 income and expense related to the cash and debt of NU parent and NSTAR LLC, respectively, 2) the revenues and expenses of NU's service companies, most of which are eliminated in consolidation, 3) the operations of CYAPC and YAEC, and 4) the results of other subsidiaries, which are comprised of NU Enterprises, NSTAR Communications, Inc., RRR (a real estate subsidiary), the non-energy-related subsidiaries of Yankee and the remaining operations of HWP.
	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.

Provision_for_Uncollectible_Ac

Provision for Uncollectible Accounts (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule Of Accounts Notes Loans And Financing Receivable Text Block	'
	(Millions of Dollars)	As of September 30, 2013		As of December 31, 2012
	NU	$	182.5	$	165.5
	CL&P		85.8		77.6
	NSTAR Electric		45.9		44.1
	PSNH		7.7		6.8
	WMECO		10.4		8.5

Other_Taxes_Tables

Other Taxes (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
State Of Connecticut Gross Earnings Taxes [Table Text Block]	'
		For the Three Months Ended				For the Nine Months Ended
	(Millions of Dollars)	30-Sep-13		30-Sep-12		30-Sep-13		30-Sep-12
	NU	$	37.5	$	36.4	$	108.9	$	102
	CL&P		35.5		34.4		97.3		91.5

Supplemental_Cash_Flow_Informa

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]	'
	G.	Supplemental Cash Flow Information
	Non-cash investing activities include plant additions included in Accounts Payable as follows:
	(Millions of Dollars)		As of September 30, 2013		As of September 30, 2012
	NU		$	122.9	$	139.9
	CL&P			36.6		45.9
	NSTAR Electric			31.9		21.5
	PSNH			16.9		20.1
	WMECO			13.8		35.1

REGULATORY_ACCOUNTING_Tables

REGULATORY ACCOUNTING (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule Of Regulatory Assets Text Block	'
		As of September 30, 2013			As of December 31, 2012
	(Millions of Dollars)	NU			NU
	Benefit Costs	$	2,256.00		$	2,452.10
	Regulatory Assets Offsetting Derivative Liabilities		770.3			885.6
	Goodwill		531.1			537.6
	Storm Restoration Costs		621			547.7
	Income Taxes, Net		587.5			516.2
	Securitized Assets		37.4			232.6
	Contractual Obligations		170.9			217.6
	Buy Out Agreements for Power Contracts		76			92.9
	Regulatory Tracker Deferrals		163.3			190.1
	Asset Retirement Obligations		93			88.8
	Other Regulatory Assets		50.1			76.2
	Total Regulatory Assets		5,356.60			5,837.40
	Less: Current Portion		474.2			705
	Total Long-Term Regulatory Assets	$	4,882.40		$	5,132.40
			As of September 30, 2013									As of December 31, 2012
						NSTAR								NSTAR
	(Millions of Dollars)		CL&P			Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Benefit Costs		$	509.3		$	824.3	$	199.6	$	103.8	$	563.2	$	781.2	$	223.7	$	116
	Regulatory Assets Offsetting
		Derivative Liabilities		755.3			11.6		0.3		-		866.2		14.9		-		3
	Goodwill			-			455.9		-		-		-		461.5		-		-
	Storm Restoration Costs			439.4			114		27.9		39.7		413.9		55.8		34.5		43.5
	Income Taxes, Net			385.3			84.8		36.5		42.1		367.5		47.1		36.2		31
	Securitized Assets			-			37.4		-		-		-		205.1		19.7		7.8
	Contractual Obligations			20			6.4		-		4.6		64		22.8		-		14.9
	Buy Out Agreements for Power Contracts			-			70.1		5.9		-		-		85.9		7		-
	Regulatory Tracker Deferrals			-			83.6		52.5		21.5		12.2		71.4		49.3		31.9
	Asset Retirement Obligations			31.1			30.7		14.7		3.7		29.4		29.4		14.2		3.5
	Other Regulatory Assets			28.7			9.2		31.7		17.2		27.9		16.9		29.4		12.6
	Total Regulatory Assets			2,169.10			1,728.00		369.1		232.6		2,344.30		1,792.00		414		264.2
	Less: Current Portion			147.1			189.8		67.7		37.9		185.9		347.1		62.9		42.4
	Total Long-Term Regulatory Assets		$	2,022.00		$	1,538.20	$	301.4	$	194.7	$	2,158.40	$	1,444.90	$	351.1	$	221.8
Schedule Of Regulatory Liabilities Text Block	'
		As of September 30, 2013			As of December 31, 2012
	(Millions of Dollars)	NU			NU
	Cost of Removal	$	434.3		$	440.8
	Regulatory Tracker Deferrals		168.6			95.1
	AFUDC - Transmission		68.3			70
	Other Regulatory Liabilities		73.9			68.4
	Total Regulatory Liabilities		745.1			674.3
	Less: Current Portion		224.4			134.1
	Total Long-Term Regulatory Liabilities	$	520.7		$	540.2
			As of September 30, 2013									As of December 31, 2012
						NSTAR								NSTAR
	(Millions of Dollars)		CL&P			Electric		PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Cost of Removal		$	31.2		$	247.2	$	49.8	$	-	$	44.2	$	240.3	$	51.2	$	-
	Regulatory Tracker Deferrals			73.2			51		10.7		22.1		39.1		14.4		20.4		19
	AFUDC - Transmission			55			4		-		9.3		56.6		4.1		-		9.3
	Other Regulatory Liabilities			30.6			31.3		15.8		2.9		16.5		32.9		3.8		2.4
	Total Regulatory Liabilities			190			333.5		76.3		34.3		156.4		291.7		75.4		30.7
	Less: Current Portion			82			82.5		23.4		22.4		32.1		47.5		23		21
	Total Long-Term Regulatory Liabilities		$	108		$	251	$	52.9	$	11.9	$	124.3	$	244.2	$	52.4	$	9.7

PROPERTY_PLANT_AND_ACCUMULATED1

PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule of Public Utility Property, Plant, and Equipment [Text Block]	'
	3.       PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION
	The following tables summarize the NU, CL&P, NSTAR Electric, PSNH and WMECO investments in utility property, plant and equipment by asset category:
				As of September 30, 2013			As of December 31, 2012
	(Millions of Dollars)			NU			NU
	Distribution - Electric			$	11,735.40		$	11,438.20
	Distribution - Natural Gas				2,352.40			2,274.20
	Transmission				6,009.00			5,541.10
	Generation				1,142.10			1,146.60
	Electric and Natural Gas Utility				21,238.90			20,400.10
	Other (1)				505.2			429.3
	Property, Plant and Equipment, Gross				21,744.10			20,829.40
	Less: Accumulated Depreciation
		Electric and Natural Gas Utility			-5,331.00			-5,065.10
		Other			-192.9			-171.5
	Total Accumulated Depreciation				-5,523.90			-5,236.60
	Property, Plant and Equipment, Net				16,220.20			15,592.80
	Construction Work in Progress				967.7			1,012.20
	Total Property, Plant and Equipment, Net			$	17,187.90		$	16,605.00
	(1)       These assets represent unregulated property and are primarily comprised of building improvements at RRR, software, hardware and equipment at NUSCO and telecommunications assets at NSTAR Communications, Inc.
		As of September 30, 2013										As of December 31, 2012
					NSTAR									NSTAR
	(Millions of Dollars)	CL&P			Electric			PSNH		WMECO		CL&P		Electric		PSNH		WMECO
	Distribution	$	4,836.10		$	4,622.70		$	1,569.70	$	746.3	$	4,691.30	$	4,539.90	$	1,520.10	$	724.2
	Transmission		2,969.60			1,664.50			613.2		715.8		2,796.10		1,529.70		599.2		583.7
	Generation		-			-			1,121.00		21.1		-		-		1,125.50		21.1
	Property, Plant and
	Equipment, Gross		7,805.70			6,287.20			3,303.90		1,483.20		7,487.40		6,069.60		3,244.80		1,329.00
	Less: Accumulated Depreciation		-1,778.70			-1,634.50			-1,001.70		-265.7		-1,698.10		-1,540.10		-954		-252.1
	Property, Plant and Equipment, Net		6,027.00			4,652.70			2,302.20		1,217.50		5,789.30		4,529.50		2,290.80		1,076.90
	Construction Work in Progress		299.2			270.7			106.8		135.2		363.7		205.8		61.7		213.6
	Total Property, Plant and
	Equipment, Net	$	6,326.20		$	4,923.40		$	2,409.00	$	1,352.70	$	6,153.00	$	4,735.30	$	2,352.50	$	1,290.50

DERIVATIVE_INSTRUMENTS_Tables

DERIVATIVE INSTRUMENTS (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule Of Derivative Instruments In Statement Of Financial Position Fair Value Text Block	'
			As of September 30, 2013
			Commodity Supply and						Net Amount Recorded as
	(Millions of Dollars)		Price Risk Management			Netting (1)			Derivative Asset/(Liability)
	Current Derivative Assets:
	Level 3:
		CL&P (1)	$	17.3		$	-10.1		$	7.2
		NSTAR Electric		0.4			-			0.4
		Other		1.3			-			1.3
	Total Current Derivative Assets		$	19		$	-10.1		$	8.9
	Long-Term Derivative Assets:
	Level 3:
		CL&P (1)	$	139.5		$	-51.5		$	88
		WMECO		0.9			-			0.9
	Total Long-Term Derivative Assets		$	140.4		$	-51.5		$	88.9
	Current Derivative Liabilities:
	Level 2:
		PSNH (1)	$	-0.5		$	0.2		$	-0.3
		Other (1) (2)		-7.4			-			-7.4
	Level 3:
		CL&P		-94.1			-			-94.1
		NSTAR Electric		-1.6			-			-1.6
		WMECO		-0.1			-			-0.1
	Total Current Derivative Liabilities		$	-103.7		$	0.2		$	-103.5
	Long-Term Derivative Liabilities:
	Level 3:
		CL&P	$	-756.4		$	-		$	-756.4
		NSTAR Electric		-10.4			-			-10.4
	Total Long-Term Derivative Liabilities		$	-766.8		$	-		$	-766.8
			As of December 31, 2012
			Commodity Supply and						Net Amount Recorded as
	(Millions of Dollars)		Price Risk Management			Netting (1)			Derivative Asset/(Liability)
	Current Derivative Assets:
	Level 2:
		Other (1)	$	0.2		$	-		$	0.2
	Level 3:
		CL&P (1)		17.7			-12			5.7
		Other		5.5			-			5.5
	Total Current Derivative Assets		$	23.4		$	-12		$	11.4
	Long-Term Derivative Assets:
	Level 3:
		CL&P (1)	$	159.7		$	-69.1		$	90.6
	Total Long-Term Derivative Assets		$	159.7		$	-69.1		$	90.6
	Current Derivative Liabilities:
	Level 2:
		Other (1) (2)	$	-19.9		$	0.6		$	-19.3
	Level 3:
		CL&P		-96.9			-			-96.9
		NSTAR Electric		-1			-			-1
	Total Current Derivative Liabilities		$	-117.8		$	0.6		$	-117.2
	Long-Term Derivative Liabilities:
	Level 2:
		Other (1)	$	-0.2		$	-		$	-0.2
	Level 3:
		CL&P		-865.6			-			-865.6
		NSTAR Electric		-13.9			-			-13.9
		WMECO		-3			-			-3
	Total Long-Term Derivative Liabilities		$	-882.7		$	-		$	-882.7
Schedule Of Derivative Instruments Gain Loss In Statement Of Financial Performance Text Block	'
	Location of Amounts			Amounts Recognized on Derivatives
	Recognized on Derivatives			For the Three Months Ended September 30,							For the Nine Months Ended September 30,
	(Millions of Dollars)			2013			2012				2013			2012
	NU
	Balance Sheet:
		Regulatory Assets		$	0.3		$	11.7			$	48.8		$	-25
	Statement of Income:
		Purchased Power, Fuel and Transmission			0.2			0.2				0.9			-0.8
Schedule Of Credit Risk Contingent Features [Table Text Block]	'
		As of September 30, 2013									As of December 31, 2012
					Additional Collateral						Additional Collateral
		Fair Value Subject			Required if			Fair Value Subject			Required if
		to Credit Risk			Downgraded Below			to Credit Risk			Downgraded Below
	(Millions of Dollars)	Contingent Features			Investment Grade			Contingent Features			Investment Grade
	NU	$	-6.7		$	13.4		$	-15.3		$	17.4
Rollforward Of Net Derivative Asset Liabilities Valued Using Unobservable Inputs [Table Text Block]	'
			For the Three Months Ended September 30,						For the Nine Months Ended September 30,
			2013			2012			2013			2012
	(Millions of Dollars)		NU			NU			NU			NU
	Derivatives, Net:
	Fair Value as of Beginning of Period		$	-788.1		$	-932.1		$	-878.6		$	-962.2
	Liabilities Assumed due to Merger with NSTAR			-			-			-			-5.4
	Transfer to Level 2			-			-			-			32.2
	Net Realized/Unrealized Gains/(Losses) Included in:
		Net Income		1.2			-0.2			8.3			7.2
		Regulatory Assets		0.8			8.5			49.6			-30.1
	Settlements			21.3			21.5			55.9			56
	Fair Value as of End of Period		$	-764.8		$	-902.3		$	-764.8		$	-902.3
			For the Three Months Ended
			30-Sep-13									30-Sep-12
	(Millions of Dollars)		CL&P			NSTAR Electric			WMECO			CL&P			NSTAR Electric(1)		WMECO
	Derivatives, Net:
	Fair Value as of Beginning of Period		$	-775.8		$	-13.1		$	-0.7		$	-910.7		$	-15.8	$	-13.5
	Net Realized/Unrealized Gains/(Losses)
		Included in Regulatory Assets		-1.2			0.5			1.5			-2.8			1.4		9.8
	Settlements			21.7			1			-			22.6			0.6		-
	Fair Value as of End of Period		$	-755.3		$	-11.6		$	0.8		$	-890.9		$	-13.8	$	-3.7
			For the Nine Months Ended
			30-Sep-13									30-Sep-12
	(Millions of Dollars)		CL&P			NSTAR Electric			WMECO			CL&P			NSTAR Electric(1)		WMECO
	Derivatives, Net:
	Fair Value as of Beginning of Period		$	-866.2		$	-14.9		$	-3		$	-931.6		$	-3.4	$	-7.3
	Net Realized/Unrealized Gains/(Losses)
		Included in Regulatory Assets		45.1			0.6			3.8			-23.8			-13.2		3.6
	Settlements			65.8			2.7			-			64.5			2.8		-
	Fair Value as of End of Period		$	-755.3		$	-11.6		$	0.8		$	-890.9		$	-13.8	$	-3.7
Fairvalueinputsliabilitiesquantitativeinformationtabletextblock	'
		As of September 30, 2013				As of December 31, 2012
		Range		Period Covered		Range		Period Covered
	Energy Prices:
	NU	$45 - $93 per MWh		2018 - 2029		$43 - $90 per MWh		2018 - 2028
	CL&P	$52 - $56 per MWh		2018 - 2020		$50 - $55 per MWh		2018 - 2020
	WMECO	$45 - $93 per MWh		2018 - 2029		$43 - $90 per MWh		2018 - 2028
	Capacity Prices:
	NU	$1.40 - $10.53 per kW-Month		2017 - 2029		$1.40 - $10.53 per kW-Month		2016 - 2028
	CL&P	$1.40 - $9.51 per kW-Month		2017 - 2026		$1.40 - $9.83 per kW-Month		2016 - 2026
	NSTAR Electric	$1.40 - $3.39 per kW-Month		2017 - 2019		$1.40 - $3.39 per kW-Month		2016 - 2019
	WMECO	$1.40 - $10.53 per kW-Month		2017 - 2029		$1.40 - $10.53 per kW-Month		2016 - 2028
	Forward Reserve:
	NU, CL&P	$3.00 per kW-Month		2013 - 2024		$0.35 - $0.90 per kW-Month		2013 - 2024
	REC Prices:
	NU	$25 - $87 per REC		2013 - 2029		$25 - $85 per REC		2013 - 2028
	NSTAR Electric	$25 - $71 per REC		2013 - 2018		$25 - $71 per REC		2013 - 2018
	WMECO	$25 - $87 per REC		2014 - 2029		$25 - $85 per REC		2013 - 2028

MARKETABLE_SECURITIES_Tables

MARKETABLE SECURITIES (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule Of Available For Sale Securities Reconciliation [Table TextBlock]	'
			As of September 30, 2013
						Pre-Tax			Pre-Tax
			Amortized			Unrealized			Unrealized
	(Millions of Dollars)		Cost			Gains(1)			Losses(1)			Fair Value
	NU
		Debt Securities (2)	$	306.1		$	1.5		$	-3.8		$	303.8
		Equity Securities (2)		164			40.9			-			204.9
	WMECO
		Debt Securities		57.8			-			-			57.8
			As of December 31, 2012
						Pre-Tax			Pre-Tax
			Amortized			Unrealized			Unrealized
	(Millions of Dollars)		Cost			Gains(1)			Losses(1)			Fair Value
	NU
		Debt Securities (2)	$	266.6		$	13.3		$	-0.1		$	279.8
		Equity Securities (2)		145.5			20			-			165.5
	WMECO
		Debt Securities		57.7			0.1			-0.1			57.7
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)		$	67.1		$	65.3		$	24.4		$	24.6
	One to five years			76			76.6			26.4			26.3
	Six to ten years			58.4			57.3			2.5			2.5
	Greater than ten years			104.6			104.6			4.5			4.4
	Total Debt Securities		$	306.1		$	303.8		$	57.8		$	57.8
Fair Value Heirarchy [Table Text Block]	'
					NU						WMECO
					As of						As of
	(Millions of Dollars)				30-Sep-13			31-Dec-12			30-Sep-13			31-Dec-12
	Level 1:
		Mutual Funds and Equities			$	259.2		$	212.5		$	-		$	-
		Money Market Funds				40			40.2			2.8			5.2
	Total Level 1				$	299.2		$	252.7		$	2.8		$	5.2
	Level 2:
		U.S. Government Issued Debt Securities
			(Agency and Treasury)		$	74.9		$	69.9		$	16.6		$	18.7
		Corporate Debt Securities				48.9			33			15.1			7
		Asset-Backed Debt Securities				30.4			28.5			9.6			10.9
		Municipal Bonds				93.7			93.8			8.8			11.6
		Other Fixed Income Securities				15.9			14.4			4.9			4.3
	Total Level 2				$	263.8		$	239.6		$	55		$	52.5
	Total Marketable Securities				$	563		$	492.3		$	57.8		$	57.7

Recovered_Sheet1

Pension Benefits and Postretirement Benefits Other Than Pensions (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule Of Net Benefit Costs [Table Text Block]	'
		Pension and SERP				Pension and SERP
		For the Three Months Ended				For the Nine Months Ended
		30-Sep-13		30-Sep-12		30-Sep-13		30-Sep-12
	(Millions of Dollars)	NU		NU		NU		NU
	Service Cost	$	25.6	$	23	$	76.7	$	61.1
	Interest Cost		51.7		53.3		155		144.7
	Expected Return on Plan Assets		-69.5		-59.5		-208.5		-161.3
	Actuarial Loss		52.4		47.4		158.1		125
	Prior Service Cost		1.1		2		3		6.1
	Total Net Periodic Benefit Expense	$	61.3	$	66.2	$	184.3	$	175.6
	Capitalized Pension Expense	$	18.3	$	19.2	$	54.9	$	49.5
		PBOP				PBOP
		For the Three Months Ended				For the Nine Months Ended
		30-Sep-13		30-Sep-12		30-Sep-13		30-Sep-12
	(Millions of Dollars)	NU		NU		NU		NU
	Service Cost	$	4.2	$	4.4	$	12.6	$	11.3
	Interest Cost		11.8		14.3		35.4		34.4
	Expected Return on Plan Assets		-13.9		-11.1		-41.6		-28.1
	Actuarial Loss		6.5		10.3		19.5		25.5
	Prior Service Credit		-0.5		-0.5		-1.5		-0.9
	Net Transition Obligation Cost		-		3.1		-		9
	Total Net Periodic Benefit Expense	$	8.1	$	20.5	$	24.4	$	51.2
	Capitalized PBOP Expense	$	2.6	$	5.1	$	7.6	$	14.9
		Pension and SERP
		For the Three Months Ended September 30, 2013								For the Three Months Ended September 30, 2012
				NSTAR								NSTAR
	(Millions of Dollars)	CL&P		Electric(1)		PSNH		WMECO		CL&P		Electric(1)		PSNH		WMECO
	Service Cost	$	6.3	$	8.3	$	3.3	$	1.2	$	5.4	$	7.6	$	2.9	$	1
	Interest Cost		12.1		14.5		5.8		2.5		12.9		14.7		6.1		2.6
	Expected Return on Plan Assets		-18.4		-21.1		-9.2		-4.3		-17.7		-16.4		-7.2		-4.1
	Actuarial Loss		13.9		14.4		5.4		2.9		12.6		15.7		4.2		2.7
	Prior Service Cost/(Credit)		0.4		-		0.1		0.1		0.9		-0.1		0.4		0.2
	Total Net Periodic Benefit Expense	$	14.3	$	16.1	$	5.4	$	2.4	$	14.1	$	21.5	$	6.4	$	2.4
	Intercompany Allocations	$	11.4	$	-2.1	$	2.6	$	2	$	10.7	$	-3	$	2.4	$	2.1
	Capitalized Pension Expense	$	7	$	9.8	$	1.7	$	1.3	$	6.8	$	8.4	$	1.9	$	1.3
		Pension and SERP
		For the Nine Months Ended September 30, 2013								For the Nine Months Ended September 30, 2012
				NSTAR								NSTAR
	(Millions of Dollars)	CL&P		Electric(1)		PSNH		WMECO		CL&P		Electric(1)		PSNH		WMECO
	Service Cost	$	18.7	$	24.8	$	9.8	$	3.5	$	16.3	$	22.7	$	8.8	$	3.1
	Interest Cost		36.3		43.5		17.8		7.5		38.5		44.2		18.3		7.9
	Expected Return on Plan Assets		-55.4		-63.3		-26.2		-13		-52.8		-49.2		-21.1		-12.3
	Actuarial Loss		42		43.6		16.2		8.9		37		47.3		12.1		8
	Prior Service Cost/(Credit)		1.4		-0.2		0.4		0.3		2.7		-0.4		1.1		0.6
	Total Net Periodic Benefit Expense	$	43	$	48.4	$	18	$	7.2	$	41.7	$	64.6	$	19.2	$	7.3
	Intercompany Allocations	$	33.6	$	-6.2	$	7.8	$	6	$	32	$	-9.2	$	7.5	$	6
	Capitalized Pension Expense	$	21	$	21.6	$	5.6	$	3.9	$	20.2	$	23.6	$	5.8	$	3.7
		PBOP
		For the Three Months Ended September 30, 2013						For the Three Months Ended September 30, 2012
	(Millions of Dollars)	CL&P			PSNH		WMECO	CL&P		PSNH		WMECO
	Service Cost	$	0.9	$	0.6	$	0.2	$	0.8	$	0.5	$	0.1
	Interest Cost		2		1		0.4		2.3		1.1		0.5
	Expected Return on Plan Assets		-2.5		-1.3		-0.6		-2.3		-1.1		-0.5
	Actuarial Loss		1.8		0.9		0.3		1.9		0.9		0.3
	Net Transition Obligation Cost		-		-		-		1.5		0.6		0.3
	Total Net Periodic Benefit Expense	$	2.2	$	1.2	$	0.3	$	4.2	$	2	$	0.7
	Intercompany Allocations	$	1.7	$	0.4	$	0.3	$	2	$	0.5	$	0.4
	Capitalized PBOP Expense	$	1.3	$	0.4	$	0.3	$	2.1	$	0.6	$	0.4
		PBOP
		For the Nine Months Ended September 30, 2013						For the Nine Months Ended September 30, 2012
	(Millions of Dollars)	CL&P			PSNH		WMECO	CL&P		PSNH		WMECO
	Service Cost	$	2.6	$	1.7	$	0.5	$	2.2	$	1.5	$	0.4
	Interest Cost		5.9		3.1		1.3		6.9		3.4		1.5
Expected Return on Plan Assets		-7.6		-3.9		-1.7		-6.8		-3.4		-1.6
Actuarial Loss		5.6		2.7		0.8		5.7		2.7		0.9
Net Transition Obligation Cost		-		-		-		4.6		1.9		1.1
Total Net Periodic Benefit Expense	$	6.5	$	3.6	$	0.9	$	12.6	$	6.1	$	2.3
Intercompany Allocations	$	5.3	$	1.2	$	1	$	5.9	$	1.5	$	1.1
Capitalized PBOP Expense	$	3.7	$	1.1	$	0.7	$	6.2	$	1.7	$	1.1

Environmental_Matters_Tablels

Environmental Matters (Tablels)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule Of Environmental Loss Contingencies By Site Text Block	'
		As of September 30, 2013				As of December 31, 2012
				Reserve				Reserve
		Number of Sites		(in millions)		Number of Sites		(in millions)
	NU		68	$	36.5		77	$	39.4
	CL&P		18		3.5		19		3.7
	NSTAR Electric		12		1.2		16		1.7
	PSNH		15		5.6		16		4.9
	WMECO		5		0.4		6		0.6

LongTerm_Contractual_Arrangeme

Long-Term Contractual Arrangements (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Long-term Purchase Commitment [Table Text Block]	'
		October - December
	(Millions of Dollars)	2013		2014		2015		2016		2017		Thereafter		Total
	Yankee Billings
	CL&P	$	0.4	$	1.5	$	1.3	$	0.8	$	0.8	$	13.1	$	17.9
	NSTAR Electric		0.2		0.7		0.5		0.2		0.3		4.5		6.4
	PSNH		0.1		0.3		0.4		0.3		0.3		5.2		6.6
	WMECO		0.1		0.4		0.4		0.2		0.2		3.3		4.6
	Renewable Energy
	CL&P		1.2		49.9		50.9		51.4		52		626		831.4

Guarantees_and_Indemnification

Guarantees and Indemnifications (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule Of Guarantee Obligations Text Block	'
			Maximum Exposure
	Subsidiary	Description	(in millions)			Expiration Dates
	Various	Surety Bonds	$	33		2013 - 2015 (1)
	Various	New England Hydro Companies' Long-Term Debt	$	4		Unspecified
	NUSCO and RRR	Lease Payments for Vehicles and Real Estate	$	18.8		2019 and 2024
	NU Enterprises	Surety Bonds, Performance Guarantees and Insurance Bond	$	62.3	-2	-2

FAIR_VALUE_OF_FINANCIAL_INSTRU1

FAIR VALUE OF FINANCIAL INSTRUMENTS (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Fair Value By Balance Sheet Grouping Text Block	'
			As of September 30, 2013				As of December 31, 2012
			NU				NU
			Carrying		Fair		Carrying		Fair
	(Millions of Dollars)		Amount		Value		Amount		Value
	Preferred Stock Not
		Subject to Mandatory Redemption	$	155.6	$	152.2	$	155.6	$	152.2
	Long-Term Debt			8,052.50		8,267.20		7,963.50		8,640.70
	Rate Reduction Bonds			-		-		82.1		83
			As of September 30, 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	$	110.3	$	43	$	41.9	$	-	$	-	$	-	$	-
	Long-Term Debt			2,741.00		2,992.00		1,801.00		1,881.90		889.1		934.7		549.6		562.1
			As of December 31, 2012
			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	$	43	$	42.2	$	-	$	-	$	-	$	-
	Long-Term Debt			2,862.80		3,295.40		1,602.60		1,818.80		997.9		1,088.00		605.3		660.4
	Rate Reduction Bonds			-		-		43.5		43.9		29.3		29.6		9.4		9.5

ACCUMULATED_OTHER_COMPREHENSIV1

ACCUMULATED OTHER COMPREHENSIVE INCOME (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule Of Accumulated Other Comprehensive Income Loss Table Text Block	'
			For the Nine Months Ended September 30, 2013
	(Millions of Dollars)		Qualified Cash Flow Hedging Instruments			Unrealized Gains/(Losses) on Available-for-Sale Securities			Pension, SERP and PBOP Benefit Plans		Total
	AOCI as of January 1, 2013		$	-16.4		$	1.3		$	-57.8	$	-72.9
	Other Comprehensive Income Before Reclassifications			-			-0.8			-		-0.8
	Amounts Reclassified from AOCI			1.5			-			4.8		6.3
	Net Other Comprehensive Income			1.5			-0.8			4.8		5.5
	AOCI as of September 30, 2013		$	-14.9		$	0.5		$	-53	$	-67.4
		For the Three Months Ended			For the Nine Months Ended
		30-Sep-13			30-Sep-13
		Amount Reclassified			Amount Reclassified			Statements of Income
	(Millions of Dollars)	from AOCI			from AOCI			Line Item Impacted
	Qualified Cash Flow Hedging Instruments	$	-0.8		$	-2.5		Interest Expense
	Tax Benefit		0.3			1		Income Tax Expense
	Qualified Cash Flow Hedging Instruments, Net of Tax	$	-0.5		$	-1.5
	Pension, SERP and PBOP Benefit Plan Costs:
	Amortization of Actuarial Losses	$	-2.5		$	-7.3		-1
	Amortization of Prior Service Cost		0			-0.1		-1
	Total Pension, SERP and PBOP Benefit Plan Costs		-2.5			-7.4		-1
	Tax Benefit		0.9			2.6		Income Tax Expense
	Pension, SERP and PBOP Benefit Plan Costs, Net of Tax	$	-1.6		$	-4.8
	Total Amount Reclassified from AOCI, Net of Tax	$	-2.1		$	-6.3

COMMON_SHARES_Tables

COMMON SHARES (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule of Stock by Class [Text Block]	'
	12.       COMMON SHARES
	The following table sets forth the NU common shares and the shares of CL&P, NSTAR Electric, PSNH and WMECO common stock authorized and issued and the respective par values:
		Shares
		Authorized				Issued
		Per Share		As of		As of
		Par Value		30-Sep-13	31-Dec-12	30-Sep-13	31-Dec-12
	NU	$	5	380,000,000	380,000,000	333,019,517		332,509,383
	CL&P	$	10	24,500,000	24,500,000	6,035,205		6,035,205
	NSTAR Electric	$	1	100,000,000	100,000,000	100		100
	PSNH	$	1	100,000,000	100,000,000	301		301
	WMECO	$	25	1,072,471	1,072,471	434,653		434,653
	As of September 30, 2013 and December 31, 2012, 18,137,017 and 18,455,749 NU common shares were held as treasury shares, respectively.

COMMON_SHARESHOLDERS_EQUITY_AN1

COMMON SHARESHOLDERS' EQUITY AND NONCONTROLLING INTERESTS (NU) (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Minority Interest Disclosure Text Block	'
	13		COMMON SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS
	A summary of the changes in Common Shareholders' Equity and Noncontrolling Interests of NU is as follows:
				For the Three Months Ended
				30-Sep-13				30-Sep-12
						Noncontrolling								Noncontrolling
						Interest -								Interest -
				Common		Preferred		Common		Non-				Preferred
				Shareholders'		Stock of		Shareholders'		Controlling		Total		Stock of
	(Millions of Dollars)			Equity		Subsidiaries		Equity		Interest		Equity		Subsidiaries
	Balance - Beginning of Period			$	9,406.60	$	155.6	$	9,067.60	$	-	$	9,067.60	$	155.6
	Net Income				211.4		-		209.5		-		209.5		-
	Dividends on Common Shares				-114.9		-		-107.6		-		-107.6		-
	Dividends on Preferred Stock				-1.9		-1.9		-1.9		-		-1.9		-1.9
	Issuance of Common Shares				1.4		-		0.8		-		0.8		-
	Other Transactions, Net				12.8		-		6.3		-		6.3		-
	Net Income Attributable to
		Noncontrolling Interests			-		1.9		-		-		-		1.9
	Other Comprehensive Income				2.1		-		2.2		-		2.2		-
	Balance - End of Period			$	9,517.50	$	155.6	$	9,176.90	$	-	$	9,176.90	$	155.6
				For the Nine Months Ended
				30-Sep-13				30-Sep-12
						Noncontrolling								Noncontrolling
						Interest -								Interest -
				Common		Preferred		Common		Non-				Preferred
				Shareholders'		Stock of		Shareholders'		Controlling		Total		Stock of
	(Millions of Dollars)			Equity		Subsidiaries		Equity		Interest		Equity		Subsidiaries
	Balance - Beginning of Period			$	9,237.10	$	155.6	$	4,012.70	$	3	$	4,015.70	$	116.2
	Net Income				614.4		-		356.5		-		356.5		-
	Purchase Price of NSTAR				-		-		5,038.30		-		5,038.30		-
	Other Equity Impacts of
		Merger with NSTAR			-		-		3.4		-3.4		-		39.4
	Dividends on Common Shares				-346.9		-		-267.8		-		-267.8		-
	Dividends on Preferred Stock				-5.8		-5.8		-5.1		-		-5.1		-5.1
	Issuance of Common Shares				10.2		-		12.2		-		12.2		-
	Contributions to NPT				-		-		-		0.3		0.3		-
	Other Transactions, Net				3		-		20.3		-		20.3		-
	Net Income Attributable to
		Noncontrolling Interests			-		5.8		-0.1		0.1		-		5.1
	Other Comprehensive Income				5.5		-		6.5		-		6.5		-
	Balance - End of Period			$	9,517.50	$	155.6	$	9,176.90	$	0	$	9,176.90	$	155.6

EARNINGS_PER_SHARE_Tables

EARNINGS PER SHARE (Tables)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
Schedule Of Earnings Per Share Basic And Diluted [Table Text Block]	'
			For the Three Months Ended				For the Nine Months Ended
	(Millions of Dollars, except share information)		30-Sep-13		30-Sep-12		30-Sep-13		30-Sep-12
	Net Income Attributable to Controlling Interest		$	209.5	$	207.6	$	608.6	$	351.2
	Weighted Average Common Shares Outstanding:
		Basic		315,291,346		314,806,441		315,191,752		264,636,636
		Dilutive Effect		926,893		999,355		869,379		716,741
		Diluted		316,218,239		315,805,796		316,061,131		265,353,377
	Basic EPS		$	0.66	$	0.66	$	1.93	$	1.33
	Diluted EPS		$	0.66	$	0.66	$	1.93	$	1.32

SEGMENT_INFORMATION_Tables

SEGMENT INFORMATION (Tables)	9 Months Ended
	Sep. 30, 2013
Segment Reporting Information [Line Items]	'
Schedule Of Segment Reporting Information By Segment Text Block	'
			For the Three Months Ended September 30, 2013
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues		$	1,508.60	$	97.1	$	234.1	$	212.5	$	-159.7	$	1,892.60
	Depreciation and Amortization			-159.6		-16.4		-34.5		-11.2		2.6		-219.1
	Other Operating Expenses			-1,064.10		-89.4		-73.4		-206.8		159.5		-1,274.20
	Operating Income/(Loss)			284.9		-8.7		126.2		-5.5		2.4		399.3
	Net Income/(Loss) Attributable
		to Controlling Interest		156.9		-10.4		58.6		313.1		-308.7		209.5
			For the Nine Months Ended September 30, 2013
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues		$	4,104.40	$	613	$	721.5	$	650.4	$	-565.8	$	5,523.50
	Depreciation and Amortization			-488.7		-50.5		-100.9		-52		7.2		-684.9
	Other Operating Expenses			-2,952.40		-483.6		-199.1		-599		564.3		-3,669.80
	Operating Income/(Loss)			663.3		78.9		421.5		-0.6		5.7		1,168.80
	Net Income Attributable
		to Controlling Interest		347.5		34.1		215.4		868.7		-857.1		608.6
	Cash Flows Used for
		Investments in Plant		501.9		91.2		458.2		22.5		-		1,073.80
			For the Three Months Ended September 30, 2012
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues		$	1,483.70	$	91.3	$	235.6	$	219.5	$	-168.6	$	1,861.50
	Depreciation and Amortization			-172.6		-12.6		-29.7		-17.5		1.1		-231.3
	Other Operating Expenses			-1,027.40		-77.2		-66.3		-216.8		170.4		-1,217.30
	Operating Income/(Loss)			283.7		1.5		139.6		-14.8		2.9		412.9
	Net Income/(Loss) Attributable
		to Controlling Interest		150.5		-4.4		71.1		313.9		-323.5		207.6
			For the Nine Months Ended September 30, 2012
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	Operating Revenues		$	3,499.70	$	361.5	$	627.2	$	582.9	$	-481.5	$	4,589.80
	Depreciation and Amortization			-398.1		-32.7		-79.5		-39.1		2.6		-546.8
	Other Operating Expenses			-2,654.40		-292.9		-179.5		-614.5		485.1		-3,256.20
	Operating Income/(Loss)			447.2		35.9		368.2		-70.7		6.2		786.8
	Net Income Attributable
		to Controlling Interest		212.1		8.3		181.1		511.6		-561.9		351.2
	Cash Flows Used for
		Investments in Plant		461.3		105.9		476		38.6		-		1,081.80
	The following table summarizes NU's segmented total assets:
			Electric		Natural Gas
	(Millions of Dollars)		Distribution		Distribution		Transmission		Other		Eliminations		Total
	As of September 30, 2013			17,912.90		2,656.80		6,566.10		19,446.90		-18,138.40		28,444.30
	As of December 31, 2012			18,047.30		2,717.40		6,187.70		18,832.60		-17,482.20		28,302.80

Basis_of_Presentation_Details

Basis of Presentation (Details)	9 Months Ended
	Sep. 30, 2013
Description of Consolidation of Yankee Companies	'NU consolidates CYAPC and YAEC as 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. For CL&P, NSTAR Electric, PSNH and WMECO, the investment in CYAPC and YAEC continue to be accounted for under the equity method.

Provision_for_Uncollectible_Ac1

Provision for Uncollectible Accounts (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Accounts Notes And Loans Receivable [Line Items]	'	'
Allowance For Doubtful Accounts Receivable Current	$182.50	$165.50
The Connecticut Light And Power Company [Member]	'	'
Accounts Notes And Loans Receivable [Line Items]	'	'
Allowance For Doubtful Accounts Receivable Current	85.8	77.6
NSTAR Electric Company [Member]	'	'
Accounts Notes And Loans Receivable [Line Items]	'	'
Allowance For Doubtful Accounts Receivable Current	45.9	44.1
Public Service Company Of New Hampshire [Member]	'	'
Accounts Notes And Loans Receivable [Line Items]	'	'
Allowance For Doubtful Accounts Receivable Current	7.7	6.8
Western Massachusetts Electric Company [Member]	'	'
Accounts Notes And Loans Receivable [Line Items]	'	'
Allowance For Doubtful Accounts Receivable Current	$10.40	$8.50

Other_Taxes_Details

Other Taxes (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Excise And Sales Taxes	$37.50	$36.40	$108.90	$102
The Connecticut Light And Power Company [Member]	'	'	'	'
Excise And Sales Taxes	$35.50	$34.40	$97.30	$91.50

Supplemental_Cash_Flow_Informa1

Supplemental Cash Flow Information (Details) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Capital Expenditures Incurred But Not Yet Paid	$122.90	$139.90
The Connecticut Light And Power Company [Member]	'	'
Capital Expenditures Incurred But Not Yet Paid	36.6	45.9
NSTAR Electric Company [Member]	'	'
Capital Expenditures Incurred But Not Yet Paid	31.9	21.5
Public Service Company Of New Hampshire [Member]	'	'
Capital Expenditures Incurred But Not Yet Paid	16.9	20.1
Western Massachusetts Electric Company [Member]	'	'
Capital Expenditures Incurred But Not Yet Paid	$13.80	$35.10

Severance_Benefits_Details

Severance Benefits (Details)	9 Months Ended
	Sep. 30, 2013
Notes To Consolidated Financial Statements [Abstract]	'
DescriptionOfPostemploymentBenefits	'Severance BenefitsIn the third quarter of 2013, NU recorded severance benefit expenses of $9.2 million in connection with the partial outsourcing of information technology functions made as part of ongoing post-merger integration. As of September 30, 2013, the severance accrual totaled $14.2 million and was included in Other Current Liabilities on the accompanying balance sheet.

MERGER_OF_NU_AND_NSTAR_Details

MERGER OF NU AND NSTAR (Details) (USD $)	0 Months Ended
In Thousands, except Share data, unless otherwise specified	Apr. 09, 2012	Sep. 30, 2013	Dec. 31, 2012
Business Combination Consideration Transferred [Abstract]	'	'	'
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,000,000	'	'
Business Combination Recognized Identifiable Assets Acquired Goodwill And Liabilities Assumed Less Noncontrolling Interest [Abstract]	'	'	'
Goodwill	'	$3,519,401	$3,519,401
Goodwill Roll Forward [Abstract]	'	'	'
Goodwill, Beginning Balance	'	3,519,401	3,519,401
Goodwill, Ending Balance	'	$3,519,401	$3,519,401

REGULATORY_ACCOUNTING_Details

REGULATORY ACCOUNTING (Details) (USD $)	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Regulatory Asset [Line Items]	'	'
Regulatory Assets Current	$474,198,000	$705,025,000
Regulatory Assets Long Term	4,882,381,000	5,132,411,000
Regulatory Assets, Total	5,356,600,000	5,837,400,000
Regulatory Assets And Liabilities Other Disclosures Abstract	'	'
Description of Regulatory Costs Not yet Approved	'Regulatory Costs in Other Long-Term Assets: The Regulated companies had $95.1 million ($3.4 million for CL&P, $31.3 million for NSTAR Electric, $37.3 million for PSNH, and $7.9 million for WMECO) and $69.9 million ($3.9 million for CL&P, $25.4 million for NSTAR Electric, $35.7 million for PSNH, and $1.4 million for WMECO) of additional regulatory costs as of September 30, 2013 and December 31, 2012, respectively, which were included in Other Long-Term Assets on the balance sheets. These amounts represent incurred costs for which specific recovery has not yet been approved by the applicable regulatory agency. Management believes it is probable that these costs will ultimately be approved and recovered from customers.	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liability Current	224,416,000	134,115,000
Regulatory Liabilities Long-Term	520,732,000	540,162,000
Regulatory Liabilities, Total	745,100,000	674,300,000
Cost Of Removal [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	434,300,000	440,800,000
Regulatory Tracker Deferrals Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	168,600,000	95,100,000
AFUDC Transmission Incentive [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	68,300,000	70,000,000
Other Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	73,900,000	68,400,000
The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets Current	147,076,000	185,858,000
Regulatory Assets Long Term	2,021,974,000	2,158,363,000
Regulatory Assets, Total	2,169,100,000	2,344,300,000
Regulatory Liabilities [Line Items]	'	'
Regulatory Liability Current	81,988,000	32,119,000
Regulatory Liabilities Long-Term	107,964,000	124,319,000
Regulatory Liabilities, Total	190,000,000	156,400,000
The Connecticut Light And Power Company [Member] | Cost Of Removal [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	31,200,000	44,200,000
The Connecticut Light And Power Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	73,200,000	39,100,000
The Connecticut Light And Power Company [Member] | AFUDC Transmission Incentive [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	55,000,000	56,600,000
The Connecticut Light And Power Company [Member] | Other Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	30,600,000	16,500,000
NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets Current	189,754,000	347,081,000
Regulatory Assets Long Term	1,538,222,000	1,444,870,000
Regulatory Assets, Total	1,728,000,000	1,792,000,000
Regulatory Liabilities [Line Items]	'	'
Regulatory Liability Current	82,521,000	47,539,000
Regulatory Liabilities Long-Term	251,005,000	244,224,000
Regulatory Liabilities, Total	333,500,000	291,700,000
NSTAR Electric Company [Member] | Cost Of Removal [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	247,200,000	240,300,000
NSTAR Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	51,000,000	14,400,000
NSTAR Electric Company [Member] | AFUDC Transmission Incentive [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	4,000,000	4,100,000
NSTAR Electric Company [Member] | Other Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	31,300,000	32,900,000
Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets Current	67,716,000	62,882,000
Regulatory Assets Long Term	301,368,000	351,059,000
Regulatory Assets, Total	369,100,000	414,000,000
Regulatory Liabilities [Line Items]	'	'
Regulatory Liability Current	23,394,000	23,002,000
Regulatory Liabilities Long-Term	52,867,000	52,418,000
Regulatory Liabilities, Total	76,300,000	75,400,000
Public Service Company Of New Hampshire [Member] | Cost Of Removal [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	49,800,000	51,200,000
Public Service Company Of New Hampshire [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	10,700,000	20,400,000
Public Service Company Of New Hampshire [Member] | Other Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	15,800,000	3,800,000
Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets Current	37,854,000	42,370,000
Regulatory Assets Long Term	194,744,000	221,752,000
Regulatory Assets, Total	232,600,000	264,200,000
Regulatory Liabilities [Line Items]	'	'
Regulatory Liability Current	22,400,000	21,037,000
Regulatory Liabilities Long-Term	11,914,000	9,686,000
Regulatory Liabilities, Total	34,300,000	30,700,000
Western Massachusetts Electric Company [Member] | Cost Of Removal [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	0	0
Western Massachusetts Electric Company [Member] | Regulatory Tracker Deferrals Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	22,100,000	19,000,000
Western Massachusetts Electric Company [Member] | AFUDC Transmission Incentive [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	9,300,000	9,300,000
Western Massachusetts Electric Company [Member] | Other Regulatory Liabilities [Member]	'	'
Regulatory Liabilities [Line Items]	'	'
Regulatory Liabilities, Total	2,900,000	2,400,000
Benefit Costs [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	2,256,000,000	2,452,100,000
Benefit Costs [Member] | The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	509,300,000	563,200,000
Benefit Costs [Member] | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	824,300,000	781,200,000
Benefit Costs [Member] | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	199,600,000	223,700,000
Benefit Costs [Member] | Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	103,800,000	116,000,000
Regulatory Assets Offsetting Derivative Liabilities [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	770,300,000	885,600,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	755,300,000	866,200,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	11,600,000	14,900,000
Regulatory Assets Offsetting Derivative Liabilities [Member] | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	300,000	0
Regulatory Assets Offsetting Derivative Liabilities [Member] | Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	0	3,000,000
Goodwill Regulatory Asset [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	531,100,000	537,600,000
Goodwill Regulatory Asset [Member] | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	455,900,000	461,500,000
Storm Restoration Costs [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	621,000,000	547,700,000
Storm Restoration Costs [Member] | The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	439,400,000	413,900,000
Storm Restoration Costs [Member] | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	114,000,000	55,800,000
Storm Restoration Costs [Member] | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	27,900,000	34,500,000
Storm Restoration Costs [Member] | Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	39,700,000	43,500,000
Deferred Income Tax Charges Member	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	587,500,000	516,200,000
Deferred Income Tax Charges Member | The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	385,300,000	367,500,000
Deferred Income Tax Charges Member | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	84,800,000	47,100,000
Deferred Income Tax Charges Member | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	36,500,000	36,200,000
Deferred Income Tax Charges Member | Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	42,100,000	31,000,000
Securitized Assets [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	37,400,000	232,600,000
Securitized Assets [Member] | The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	0	0
Securitized Assets [Member] | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	37,400,000	205,100,000
Securitized Assets [Member] | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	0	19,700,000
Securitized Assets [Member] | Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	0	7,800,000
Unrecovered Contractual Obligations [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	170,900,000	217,600,000
Unrecovered Contractual Obligations [Member] | The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	20,000,000	64,000,000
Unrecovered Contractual Obligations [Member] | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	6,400,000	22,800,000
Unrecovered Contractual Obligations [Member] | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	0	0
Unrecovered Contractual Obligations [Member] | Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	4,600,000	14,900,000
Power Contracts Buy Out Agreements [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	76,000,000	92,900,000
Power Contracts Buy Out Agreements [Member] | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	70,100,000	85,900,000
Power Contracts Buy Out Agreements [Member] | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	5,900,000	7,000,000
Regulatory Tracker Deferrals Regulatory Assets [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	163,300,000	190,100,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	0	12,200,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	83,600,000	71,400,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	52,500,000	49,300,000
Regulatory Tracker Deferrals Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	21,500,000	31,900,000
Asset Retirement Obligation Costs Member	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	93,000,000	88,800,000
Asset Retirement Obligation Costs Member | The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	31,100,000	29,400,000
Asset Retirement Obligation Costs Member | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	30,700,000	29,400,000
Asset Retirement Obligation Costs Member | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	14,700,000	14,200,000
Asset Retirement Obligation Costs Member | Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	3,700,000	3,500,000
Other Regulatory Assets [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	50,100,000	76,200,000
Other Regulatory Assets [Member] | The Connecticut Light And Power Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	28,700,000	27,900,000
Other Regulatory Assets [Member] | NSTAR Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	9,200,000	16,900,000
Other Regulatory Assets [Member] | Public Service Company Of New Hampshire [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	31,700,000	29,400,000
Other Regulatory Assets [Member] | Western Massachusetts Electric Company [Member]	'	'
Regulatory Asset [Line Items]	'	'
Regulatory Assets, Total	$17,200,000	$12,600,000

PROPERTY_PLANT_AND_ACCUMULATED2

PROPERTY, PLANT AND ACCUMULATED DEPRECIATION (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Public Utility Property Plant And Equipment [Line Items]	'	'
Public Utilities Property Plant And Equipment Distribution	$11,735,400,000	$11,438,200,000
Public Utilities Property Plant And Equipment Distribution Natural Gas	2,352,400,000	2,274,200,000
Public Utilities Property Plant And Equipment Transmission	6,009,000,000	5,541,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,142,100,000	1,146,600,000
Public Utilities Property Plant And Equipment Electric And Natural Gas Utility Total	21,238,900,000	20,400,100,000
Public Utilities Property Plant And Equipment Other Property Plant And Equipment	505,200,000	429,300,000
Public Utilities Property Plant And Equipment Plant In Service	21,744,100,000	20,829,400,000
ElectricAndNaturalGasUtilityAccumulatedDepreciation	-5,331,000,000	-5,065,100,000
OtherAccumulatedDepreciation	-192,900,000	-171,500,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-5,523,900,000	-5,236,600,000
Property Plant And Equipment Net Excluding Construction Work In Progress	16,220,200,000	15,592,800,000
Public Utilities Property Plant And Equipment Construction Work In Progress	967,700,000	1,012,200,000
Property, Plant and Equipment, Net	17,187,896,000	16,605,010,000
The Connecticut Light And Power Company [Member]	'	'
Public Utility Property Plant And Equipment [Line Items]	'	'
Public Utilities Property Plant And Equipment Distribution	4,836,100,000	4,691,300,000
Public Utilities Property Plant And Equipment Transmission	2,969,600,000	2,796,100,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	7,805,700,000	7,487,400,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-1,778,700,000	-1,698,100,000
Property Plant And Equipment Net Excluding Construction Work In Progress	6,027,000,000	5,789,300,000
Public Utilities Property Plant And Equipment Construction Work In Progress	299,200,000	363,700,000
Property, Plant and Equipment, Net	6,326,225,000	6,152,959,000
NSTAR Electric Company [Member]	'	'
Public Utility Property Plant And Equipment [Line Items]	'	'
Public Utilities Property Plant And Equipment Distribution	4,622,700,000	4,539,900,000
Public Utilities Property Plant And Equipment Transmission	1,664,500,000	1,529,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	0	0
Public Utilities Property Plant And Equipment Plant In Service	6,287,200,000	6,069,600,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-1,634,500,000	-1,540,100,000
Property Plant And Equipment Net Excluding Construction Work In Progress	4,652,700,000	4,529,500,000
Public Utilities Property Plant And Equipment Construction Work In Progress	270,700,000	205,800,000
Property, Plant and Equipment, Net	4,923,410,000	4,735,297,000
Public Service Company Of New Hampshire [Member]	'	'
Public Utility Property Plant And Equipment [Line Items]	'	'
Public Utilities Property Plant And Equipment Distribution	1,569,700,000	1,520,100,000
Public Utilities Property Plant And Equipment Transmission	613,200,000	599,200,000
Public Utilities Property Plant And Equipment Generation Or Processing	1,121,000,000	1,125,500,000
Public Utilities Property Plant And Equipment Plant In Service	3,303,900,000	3,244,800,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-1,001,700,000	-954,000,000
Property Plant And Equipment Net Excluding Construction Work In Progress	2,302,200,000	2,290,800,000
Public Utilities Property Plant And Equipment Construction Work In Progress	106,800,000	61,700,000
Property, Plant and Equipment, Net	2,409,039,000	2,352,515,000
Western Massachusetts Electric Company [Member]	'	'
Public Utility Property Plant And Equipment [Line Items]	'	'
Public Utilities Property Plant And Equipment Distribution	746,300,000	724,200,000
Public Utilities Property Plant And Equipment Transmission	715,800,000	583,700,000
Public Utilities Property Plant And Equipment Generation Or Processing	21,100,000	21,100,000
Public Utilities Property Plant And Equipment Plant In Service	1,483,200,000	1,329,000,000
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	-265,700,000	-252,100,000
Property Plant And Equipment Net Excluding Construction Work In Progress	1,217,500,000	1,076,900,000
Public Utilities Property Plant And Equipment Construction Work In Progress	135,200,000	213,600,000
Property, Plant and Equipment, Net	$1,352,705,000	$1,290,498,000

DERIVATIVE_INSTURMENTS_Details

DERIVATIVE INSTURMENTS (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Liabilities, Noncurrent	($766,804,000)	'	($766,804,000)	'	($882,654,000)
Derivative Assets Noncurrent	88,887,000	'	88,887,000	'	90,612,000
Current Derivative Assets Consolidated	8,900,000	'	8,900,000	'	11,400,000
DerivativeCollateralRightToReclaimCash	1,000,000	'	1,000,000	'	4,100,000
Consolidated Current Derivative Liabilities	103,500,000	'	103,500,000	'	117,200,000
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract	'	'	'	'	'
Description of Derivative Activity Volume	'	'	'As of September 30, 2013 and December 31, 2012, NU had approximately 5 thousand MWh and 24 thousand MWh, respectively, of supply volumes remaining in its unregulated wholesale portfolio when expected sales are compared with supply contracts.	'	'
Derivative Instruments Not Designated As Hedging Instruments Gain Loss Net Abstract	'	'	'	'	'
Derivative Gain Loss On Derivative Net	200,000	200,000	900,000	-800,000	'
Derivative Instruments Gain Loss Recognized In Regulatory Asset Liability	300,000	11,700,000	48,800,000	-25,000,000	'
Derivative Credit Risk Related Contingent Features Abstract	'	'	'	'	'
Derivative Net Liability Position Aggregate Fair Value	-6,700,000	'	-6,700,000	'	-15,300,000
Additional Collateral Aggregate Fair Value	13,400,000	'	13,400,000	'	17,400,000
MinimumMember	'	'	'	'	'
FairValueInputsQuantitativeInformation[Abstract]	'	'	'	'	'
FairValueInputsCapacityPrices	1.4	'	1.4	'	1.4
FairValueInputsEnergyPrices	45	'	45	'	43
FairValueInputsRenewableEnergyCreditPrices	25	'	25	'	25
FairValueInputsForwardReserve	3	'	3	'	0.35
FairValueInputsExitPricePremium	'	'	'0.1	'	'
MaximumMember	'	'	'	'	'
FairValueInputsQuantitativeInformation[Abstract]	'	'	'	'	'
FairValueInputsCapacityPrices	10.53	'	10.53	'	10.53
FairValueInputsEnergyPrices	93	'	93	'	90
FairValueInputsRenewableEnergyCreditPrices	87	'	87	'	85
FairValueInputsForwardReserve	3	'	3	'	0.9
FairValueInputsExitPricePremium	'	'	'0.32	'	'
The Connecticut Light And Power Company [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Liabilities - Current	-94,123,000	'	-94,123,000	'	-96,931,000
Derivative Liabilities, Noncurrent	-756,437,000	'	-756,437,000	'	-865,571,000
Derivative Assets Noncurrent	88,018,000	'	88,018,000	'	90,612,000
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 has capacity-related contracts with generation facilities. These contracts and similar UI contracts have an expected capacity of 787 MW. 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 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 forward 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.	'	'
The Connecticut Light And Power Company [Member] | MinimumMember	'	'	'	'	'
FairValueInputsQuantitativeInformation[Abstract]	'	'	'	'	'
FairValueInputsCapacityPrices	1.4	'	1.4	'	1.4
FairValueInputsEnergyPrices	52	'	52	'	50
FairValueInputsForwardReserve	3	'	3	'	0.35
The Connecticut Light And Power Company [Member] | MaximumMember	'	'	'	'	'
FairValueInputsQuantitativeInformation[Abstract]	'	'	'	'	'
FairValueInputsCapacityPrices	9.51	'	9.51	'	9.83
FairValueInputsEnergyPrices	56	'	56	'	55
FairValueInputsForwardReserve	3	'	3	'	0.9
NSTAR Electric Company [Member]	'	'	'	'	'
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract	'	'	'	'	'
Description of Derivative Activity Volume	'	'	'NSTAR Electric has a renewable energy contract to purchase 0.1 million MWh of energy per year through 2018. NSTAR Electric also has a capacity related contract for up to 35 MW per year that extends through 2019. PSNH has electricity procurement contracts to purchase 0.2 million MWh of energy through November 2013.	'	'
NSTAR Electric Company [Member] | MinimumMember	'	'	'	'	'
FairValueInputsQuantitativeInformation[Abstract]	'	'	'	'	'
FairValueInputsCapacityPrices	1.4	'	1.4	'	1.4
FairValueInputsRenewableEnergyCreditPrices	25	'	25	'	25
NSTAR Electric Company [Member] | MaximumMember	'	'	'	'	'
FairValueInputsQuantitativeInformation[Abstract]	'	'	'	'	'
FairValueInputsCapacityPrices	3.39	'	3.39	'	3.39
FairValueInputsRenewableEnergyCreditPrices	71	'	71	'	71
Public Service Company Of New Hampshire [Member]	'	'	'	'	'
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract	'	'	'	'	'
Description of Derivative Activity Volume	'	'	'PSNH has electricity procurement contracts to purchase 0.2 million MWh of energy through November 2013.	'	'
Western Massachusetts Electric Company [Member]	'	'	'	'	'
Other Derivatives Not Designated As Hedging Instruments At Fair Value Net Total Abstract	'	'	'	'	'
Description of Derivative Activity Volume	'	'	'WMECO has a renewable energy contract to purchase 0.1 million MWh of energy per year through 2029 with a facility that is expected to achieve commercial operation by June 2014.	'	'
Western Massachusetts Electric Company [Member] | MinimumMember	'	'	'	'	'
FairValueInputsQuantitativeInformation[Abstract]	'	'	'	'	'
FairValueInputsCapacityPrices	1.4	'	1.4	'	1.4
FairValueInputsEnergyPrices	45	'	45	'	43
FairValueInputsRenewableEnergyCreditPrices	25	'	25	'	25
Western Massachusetts Electric Company [Member] | MaximumMember	'	'	'	'	'
FairValueInputsQuantitativeInformation[Abstract]	'	'	'	'	'
FairValueInputsCapacityPrices	10.53	'	10.53	'	10.53
FairValueInputsEnergyPrices	93	'	93	'	90
FairValueInputsRenewableEnergyCreditPrices	87	'	87	'	85
Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Liabilities - Current	-300,000	'	-300,000	'	'
Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	'	'	'	'	200,000
Derivative Liabilities - Current	-7,400,000	'	-7,400,000	'	-19,300,000
Derivative Liabilities, Noncurrent	'	'	'	'	-200,000
Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	7,200,000	'	7,200,000	'	5,700,000
Derivative Liabilities - Current	-94,100,000	'	-94,100,000	'	-96,900,000
Derivative Liabilities, Noncurrent	-756,400,000	'	-756,400,000	'	-865,600,000
Derivative Assets Noncurrent	88,000,000	'	88,000,000	'	90,600,000
Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	400,000	'	400,000	'	'
Derivative Liabilities - Current	-1,600,000	'	-1,600,000	'	-1,000,000
Derivative Liabilities, Noncurrent	-10,400,000	'	-10,400,000	'	-13,900,000
Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Liabilities - Current	-100,000	'	-100,000	'	'
Derivative Liabilities, Noncurrent	'	'	'	'	-3,000,000
Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	1,300,000	'	1,300,000	'	5,500,000
Netting And Collateral [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	-10,100,000	'	-10,100,000	'	-12,000,000
Derivative Liabilities - Current	200,000	'	200,000	'	600,000
Derivative Assets Noncurrent	-51,500,000	'	-51,500,000	'	-69,100,000
Netting And Collateral [Member] | Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Liabilities - Current	200,000	'	200,000	'	'
Netting And Collateral [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Liabilities - Current	0	'	0	'	600,000
Netting And Collateral [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	-10,100,000	'	-10,100,000	'	-12,000,000
Derivative Assets Noncurrent	-51,500,000	'	-51,500,000	'	-69,100,000
Commodity [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	19,000,000	'	19,000,000	'	23,400,000
Derivative Liabilities - Current	-103,700,000	'	-103,700,000	'	-117,800,000
Derivative Liabilities, Noncurrent	-766,800,000	'	-766,800,000	'	-882,700,000
Derivative Assets Noncurrent	140,400,000	'	140,400,000	'	159,700,000
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Public Service Company Of New Hampshire [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Liabilities - Current	-500,000	'	-500,000	'	'
Commodity [Member] | Fair Value Inputs Level 2 [Member] | Subsidiaries [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	'	'	'	'	200,000
Derivative Liabilities - Current	-7,400,000	'	-7,400,000	'	-19,900,000
Derivative Liabilities, Noncurrent	'	'	'	'	-200,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | The Connecticut Light And Power Company [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	17,300,000	'	17,300,000	'	17,700,000
Derivative Liabilities - Current	-94,100,000	'	-94,100,000	'	-96,900,000
Derivative Liabilities, Noncurrent	-756,400,000	'	-756,400,000	'	-865,600,000
Derivative Assets Noncurrent	139,500,000	'	139,500,000	'	159,700,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | NSTAR Electric Company [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	400,000	'	400,000	'	'
Derivative Liabilities - Current	-1,600,000	'	-1,600,000	'	-1,000,000
Derivative Liabilities, Noncurrent	-10,400,000	'	-10,400,000	'	-13,900,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Western Massachusetts Electric Company [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Liabilities - Current	-100,000	'	-100,000	'	'
Derivative Liabilities, Noncurrent	'	'	'	'	-3,000,000
Commodity [Member] | Fair Value Inputs Level 3 [Member] | Subsidiaries [Member]	'	'	'	'	'
Derivative Instrument Detail Abstract	'	'	'	'	'
Derivative Assets - Current	$1,300,000	'	$1,300,000	'	$5,500,000

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]	'	'	'	'
Beginning Balance	($788.10)	($932.10)	($878.60)	($962.20)
DerivativeLiabilityRecordedAtMergerDateLevel3	'	0	'	-5.4
us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3	'	0	'	32.2
Net Realized/Unrealized Gains Losses included in Net Income	1.2	-0.2	8.3	7.2
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	0.8	8.5	49.6	-30.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	21.3	21.5	55.9	56
Ending Balance	-764.8	-902.3	-764.8	-902.3
The Connecticut Light And Power Company [Member]	'	'	'	'
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]	'	'	'	'
Beginning Balance	-775.8	-910.7	-866.2	-931.6
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	-1.2	-2.8	45.1	-23.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	21.7	22.6	65.8	64.5
Ending Balance	-755.3	-890.9	-755.3	-890.9
NSTAR Electric Company [Member]	'	'	'	'
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]	'	'	'	'
Beginning Balance	-13.1	-15.8	-14.9	-3.4
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	0.5	1.4	0.6	-13.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	1	0.6	2.7	2.8
Ending Balance	-11.6	-13.8	-11.6	-13.8
Western Massachusetts Electric Company [Member]	'	'	'	'
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]	'	'	'	'
Beginning Balance	-0.7	-13.5	-3	-7.3
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Gain Loss Included In Reg Asset Liability	1.5	9.8	3.8	3.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	'	'	0	0
Ending Balance	$0.80	($3.70)	$0.80	($3.70)

MARKETABLE_SECURITIES_Details

MARKETABLE SECURITIES (Details) (USD $)

3 Months Ended

9 Months Ended

9 Months Ended

12 Months Ended

9 Months Ended

12 Months Ended

9 Months Ended

12 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Fair Value Inputs Level 1 Member

Fair Value Inputs Level 1 Member

Fair Value Inputs Level 1 Member

Fair Value Inputs Level 1 Member

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

US Government Debt Securities Member

US Government Debt Securities Member

US Government Debt Securities Member

US Government Debt Securities Member

Corporate Bond Securities Member

Corporate Bond Securities Member

Corporate Bond Securities Member

Corporate Bond Securities Member

Asset Backed Securities Member

Asset Backed Securities Member

Asset Backed Securities Member

Asset Backed Securities Member

Municipal Bonds Member

Municipal Bonds Member

Municipal Bonds Member

Municipal Bonds Member

Other Debt Securities Member

Other Debt Securities Member

Other Debt Securities Member

Other Debt Securities Member

Cash And Cash Equivalents Member

Cash And Cash Equivalents Member

Cash And Cash Equivalents Member

Cash And Cash Equivalents Member

Mutual Funds [Member]

Mutual Funds [Member]

Debt Securities

Debt Securities

Debt Securities

Debt Securities

Equity Securities

Equity Securities

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 2 [Member]

Fair Value Inputs Level 1 Member

Fair Value Inputs Level 1 Member

Fair Value Inputs Level 1 Member

Fair Value Inputs Level 1 Member

Fair Value Inputs Level 1 Member

Fair Value Inputs Level 1 Member

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Western Massachusetts Electric Company [Member]

Schedule Of Available For Sale Securities [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fair Value Of Securities Under Fair Value Option

$54.30

'

$54.30

'

$47

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fair Value, Option, Changes in Fair Value, Gain (Loss)

3

1.9

7.3

4.6

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Investments, Debt and Equity Securities [Abstract]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Available For Sale Securities Fair Value Disclosure

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

303.8

279.8

57.8

57.7

204.9

165.5

Available-for-sale Securities, Gross Unrealized Gains

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1.5

13.3

0

0.1

40.9

20

AvailableForSaleSecuritiesGrossUnrealizedLoss

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-3.8

-0.1

0

-0.1

'

'

Available For Sale Securities Amortized Cost

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

306.1

266.6

57.8

57.7

164

145.5

Marketable Securities, Restricted

403.1

'

403.1

'

340.4

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Available For Sale Securities Debt Maturities Fair Value Abstract

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Available For Sale Securities Debt Maturities After Five Through Ten Years Fair Value

57.3

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57.3

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2.5

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Available For Sale Securities Debt Maturities After Ten Years Fair Value

104.6

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104.6

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4.4

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Available For Sale Securities Debt Maturities After One Year Through Five Years Fair Value

76.6

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76.6

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26.3

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Available For Sale Securities Debt Maturities Within One Year Fair Value

65.3

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65.3

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Available For Sale Securities Debt Maturities Fair Value

303.8

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57.8

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Available For Sale Securities Debt Maturities Amortized Cost Abstract

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Available For Sale Securities Debt Maturities After One Through Five Years Amortized Cost

76

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76

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26.4

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Available For Sale Securities Debt Maturities Within One Year Amortized Cost

67.1

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67.1

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24.4

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Available For Sale Securities Debt Maturities After Ten Years Amortized Cost

104.6

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104.6

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4.5

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Available For Sale Securities Debt Maturities After Five Through Ten Years Amortized Cost

58.4

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58.4

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2.5

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Available For Sale Securities Debt Maturities Amortized Cost

306.1

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306.1

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57.8

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Fair Value Assets And Liabilities Measured On Recurring Basis Abstract

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Investments Fair Value Disclosure

$563

'

$563

'

$492.30

$57.80

$57.70

$299.20

$252.70

$2.80

$5.20

$263.80

$239.60

$55

$52.50

$74.90

$69.90

$16.60

$18.70

$48.90

$33

$15.10

$7

$30.40

$28.50

$9.60

$10.90

$93.70

$93.80

$8.80

$11.60

$15.90

$14.40

$4.90

$4.30

$40

$40.20

$2.80

$5.20

$259.20

$212.50

'

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'

SHORT_TERM_AND_LONG_TERM_DEBT_

SHORT TERM AND LONG TERM DEBT (Details)	9 Months Ended
	Sep. 30, 2013
Line of Credit Facility [Line Items]	'
Description of commercial paper program	'On September 6, 2013, NU parent, CL&P, NSTAR LLC, NSTAR Gas, PSNH, WMECO and Yankee Gas amended their joint five-year $1.15 billion revolving credit facility dated July 25, 2012, by increasing the aggregate principal amount available thereunder by $300 million to $1.45 billion, extending the expiration date from July 25, 2017 to September 6, 2018, and increasing CL&P's borrowing sublimit from $300 million to $600 million. At the same time, effective September 6, 2013, the CL&P $300 million revolving credit facility was terminated. On September 6, 2013, NSTAR Electric amended its five-year $450 million revolving credit facility dated July 25, 2012 by extending the expiration date from July 25, 2017 to September 6, 2018. On September 6, 2013, the NU parent $1.15 billion commercial paper program was increased by $300 million to $1.45 billion. As of September 30, 2013 and December 31, 2012, NU had approximately $1.2 billion and $1.15 billion, respectively, in short-term borrowings outstanding under the NU parent commercial paper program, which provides $263 million of available borrowing capacity as of September 30, 2013. The weighted-average interest rate on these borrowings as of September 30, 2013 and December 31, 2012 was 0.268 percent and 0.46 percent, respectively, which is generally based on money market rates. As of September 30, 2013, there were intercompany loans from NU of $342.9 million to CL&P, $228.5 million to PSNH and $79.8 million to WMECO. As of December 31, 2012, there were intercompany loans from NU of $405.1 million to CL&P, $63.3 million to PSNH, and $31.9 million to WMECO. As of September 30, 2013 and December 31, 2012, NSTAR Electric had $156 million and $276 million, respectively, in short-term borrowings outstanding under its commercial paper program, leaving $294 million and $174 million, respectively, of available borrowing capacity. The weighted-average interest rate on these borrowings as of September 30, 2013 and December 31, 2012 was 0.134 percent and 0.31 percent, respectively, which is generally based on money market rates. 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 Affiliated Companies and classified in current liabilities on the balance sheets.
Description of Working Capital	'Working Capital: NU, CL&P, NSTAR Electric, PSNH and WMECO use their available capital resources to fund their respective construction expenditures, meet debt requirements, pay operating costs, including storm-related costs, pay dividends and fund other corporate obligations, such as pension contributions. The current growth in NU’s transmission construction expenditures utilizes a significant amount of cash for projects that have a long-term return on investment and recovery period. In addition, NU’s Regulated companies operate in an environment where recovery of its electric and natural gas distribution construction expenditures takes place over an extended period of time. This impacts the timing of the revenue stream designed to fully recover the total investment plus a return on the equity portion of the cost and related financing costs. These factors have resulted in current liabilities exceeding current assets by approximately $1.4 billion, $392 million, $315 million, $114 million and $11 million at NU, CL&P, NSTAR Electric, PSNH and WMECO, respectively, as of September 30, 2013. As of September 30, 2013, approximately $577 million of NU's current liabilities related to long-term debt that will be paid in the next 12 months, primarily consisting of $150 million for CL&P, $302 million for NSTAR Electric and $50 million for PSNH. NU, with its strong credit ratings, has several options available in the financial markets to repay or refinance these maturities with the issuance of new long-term debt. NU, CL&P, NSTAR Electric, PSNH and WMECO will reduce their short-term borrowings with cash received from operating cash flows and/or with the issuance of new long-term debt, as deemed appropriate given capital requirements and maintenance of NU's credit rating and profile. Management expects the future operating cash flows of NU, CL&P, NSTAR Electric, PSNH and WMECO along with the access to financial markets, will be sufficient to meet any future operating requirements and forecasted capital investment opportunities.
Long-term Debt, Description	'Long-Term Debt: On January 15, 2013, CL&P issued $400 million of Series A First and Refunding Mortgage Bonds with a coupon rate of 2.5 percent and a maturity date of January 15, 2023. The proceeds, net of issuance costs, were used to pay short-term borrowings outstanding under the CL&P credit agreement and the NU parent commercial paper program. Therefore, as of December 31, 2012, CL&P's credit agreement borrowings of $89 million and intercompany loans related to the commercial paper program of $305.8 million were classified as Long-Term Debt on the balance sheet. On May 1, 2013, PSNH redeemed at par approximately $109 million of the 2001 Series C PCRBs that were due to mature in 2021 using short-term debt. On May 13, 2013, NU parent issued $750 million of Senior Notes, consisting of $300 million of Series E Senior Notes at a coupon rate of 1.45 percent that will mature on May 1, 2018 and $450 million of Series F Senior Notes at a coupon rate of 2.80 percent that will mature on May 1, 2023. Part of the proceeds, net of issuance costs, was used to repay the NU parent $250 million Series C Senior Notes at a coupon rate of 5.65 percent that matured on June 1, 2013 and the NU parent $300 million floating rate Series D Senior Notes that matured on September 20, 2013. The remaining net proceeds were used to repay commercial paper borrowings and for other general corporate purposes. On May 17, 2013, NSTAR Electric issued $200 million of three-year floating rate debentures due to mature on May 17, 2016. The proceeds, net of issuance costs, were used to repay commercial paper borrowings and for general corporate purposes. The debentures have a coupon rate reset quarterly based on 3-month LIBOR plus a credit spread of 0.24 percent. The interest rate as of September 30, 2013 was 0.5032 percent. On September 1, 2013, WMECO repaid at maturity, $55 million of 5.00 percent Series A Senior Notes using short-term debt. On September 3, 2013, CL&P redeemed at par $125 million of 1.25 percent Series B 2011 PCRBs that were subject to mandatory tender for purchase using short-term debt. On September 20, 2013, NU parent repaid at maturity, $300 million of Floating Rate Series D Senior Notes with proceeds from NU parent’s issuance on May 13, 2013 of $750 million of Series E and Series F Senior Notes. On August 29, 2013, NSTAR Electric filed an application with the DPU requesting authorization to issue up to $800 million in long-term debt for the two-year period ending December 31, 2015. On September 26, 2013, the NHPUC issued an order, effective October 8, 2013, approving PSNH's request to issue up to $315 million in long-term debt through December 31, 2014, and to refinance $89.3 million 2001 Series B PCRBs through its existing maturity of May 2021.
The Connecticut Light And Power Company [Member]	'
Line of Credit Facility [Line Items]	'
Long-term Debt, Description	'On January 15, 2013, CL&P issued $400 million of Series A First and Refunding Mortgage Bonds with a coupon rate of 2.5 percent and a maturity date of January 15, 2023. The proceeds, net of issuance costs, were used to pay short-term borrowings outstanding under the CL&P credit agreement and the NU parent commercial paper program. Therefore, as of December 31, 2012, CL&P's credit agreement borrowings of $89 million and intercompany loans related to the commercial paper program of $305.8 million were classified as Long-Term Debt on the balance sheet. On September 3, 2013, CL&P redeemed at par $125 million of 1.25 percent Series B 2011 PCRBs that were subject to mandatory tender for purchase using short-term debt.
NSTAR Electric Company [Member]	'
Line of Credit Facility [Line Items]	'
Long-term Debt, Description	'On May 17, 2013, NSTAR Electric issued $200 million of three-year floating rate debentures due to mature on May 17, 2016. The proceeds, net of issuance costs, were used to repay commercial paper borrowings and for general corporate purposes. The debentures have a coupon rate reset quarterly based on 3-month LIBOR plus a credit spread of 0.24 percent. The interest rate as of September 30, 2013 was 0.5032 percent.
Public Service Company Of New Hampshire [Member]	'
Line of Credit Facility [Line Items]	'
Long-term Debt, Description	'On May 1, 2013, PSNH redeemed at par approximately $109 million of the 2001 Series C PCRBs that were due to mature in 2021 using short-term debt.
Western Massachusetts Electric Company [Member]	'
Line of Credit Facility [Line Items]	'
Long-term Debt, Description	'On September 1, 2013, WMECO repaid at maturity, $55 million of 5.00 percent Series A Senior Notes using short-term debt.

PENSION_BENEFITS_AND_POSTRETIR1

PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Defined Benefit Plan Service Cost	$25.60	$23	$76.70	$61.10
Defined Benefit Plan Interest Cost	51.7	53.3	155	144.7
Defined Benefit Plan, Expected Return on Plan Assets	-69.5	-59.5	-208.5	-161.3
Defined Benefit Plan, Amortization of Gains (Losses)	52.4	47.4	158.1	125
Defined Benefit Plan Amortization Of Prior Service Cost Credit	1.1	2	3	6.1
Defined Benefit Plan Net Periodic Benefit Cost	61.3	66.2	184.3	175.6
Amount Capitalized Defined Benefit Expense	18.3	19.2	54.9	49.5
DescriptionOfDefinedContributionPensionAndOtherPostretirementPlans	'	'	': For the nine months ended September 30, 2013, NU contributed $202.7 million to the NUSCO Pension Plan, $108.3 million of which was contributed by PSNH, and NSTAR Electric contributed $82 million to the NSTAR Pension Plan.	'
Other Postretirement Benefit Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Defined Benefit Plan Service Cost	4.2	4.4	12.6	11.3
Defined Benefit Plan Interest Cost	11.8	14.3	35.4	34.4
Defined Benefit Plan, Expected Return on Plan Assets	-13.9	-11.1	-41.6	-28.1
Defined Benefit Plan, Amortization of Gains (Losses)	6.5	10.3	19.5	25.5
Defined Benefit Plan Amortization Of Transition Obligations Assets	'	3.1	'	9
Defined Benefit Plan Amortization Of Prior Service Cost Credit	-0.5	-0.5	-1.5	-0.9
Defined Benefit Plan Net Periodic Benefit Cost	8.1	20.5	24.4	51.2
Amount Capitalized Defined Benefit Expense	2.6	5.1	7.6	14.9
DescriptionOfDefinedContributionPensionAndOtherPostretirementPlans	'	'	' NU contributed $53.6 million to the PBOP Plans for the nine months ended September 30, 2013	'
The Connecticut Light And Power Company [Member] | Pension Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Defined Benefit Plan Service Cost	6.3	5.4	18.7	16.3
Defined Benefit Plan Interest Cost	12.1	12.9	36.3	38.5
Defined Benefit Plan, Expected Return on Plan Assets	-18.4	-17.7	-55.4	-52.8
Defined Benefit Plan, Amortization of Gains (Losses)	13.9	12.6	42	37
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.4	0.9	1.4	2.7
Defined Benefit Plan Net Periodic Benefit Cost	14.3	14.1	43	41.7
Related Intercompany Allocationsof Defined Benefit Expense	11.4	10.7	33.6	32
Amount Capitalized Defined Benefit Expense	7	6.8	21	20.2
The Connecticut Light And Power Company [Member] | Other Postretirement Benefit Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Defined Benefit Plan Service Cost	0.9	0.8	2.6	2.2
Defined Benefit Plan Interest Cost	2	2.3	5.9	6.9
Defined Benefit Plan, Expected Return on Plan Assets	-2.5	-2.3	-7.6	-6.8
Defined Benefit Plan, Amortization of Gains (Losses)	1.8	1.9	5.6	5.7
Defined Benefit Plan Amortization Of Transition Obligations Assets	'	1.5	0	4.6
Defined Benefit Plan Net Periodic Benefit Cost	2.2	4.2	6.5	12.6
Related Intercompany Allocationsof Defined Benefit Expense	1.7	2	5.3	5.9
Amount Capitalized Defined Benefit Expense	1.3	2.1	3.7	6.2
NSTAR Electric Company [Member] | Pension Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Defined Benefit Plan Service Cost	8.3	7.6	24.8	22.7
Defined Benefit Plan Interest Cost	14.5	14.7	43.5	44.2
Defined Benefit Plan, Expected Return on Plan Assets	-21.1	-16.4	-63.3	-49.2
Defined Benefit Plan, Amortization of Gains (Losses)	14.4	15.7	43.6	47.3
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0	-0.1	-0.2	-0.4
Defined Benefit Plan Net Periodic Benefit Cost	16.1	21.5	48.4	64.6
Related Intercompany Allocationsof Defined Benefit Expense	-2.1	-3	-6.2	-9.2
Amount Capitalized Defined Benefit Expense	9.8	8.4	21.6	23.6
NSTAR Electric Company [Member] | Other Postretirement Benefit Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Related Intercompany Allocationsof Defined Benefit Expense	1.2	8.5	3.5	25.6
Public Service Company Of New Hampshire [Member] | Pension Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Defined Benefit Plan Service Cost	3.3	2.9	9.8	8.8
Defined Benefit Plan Interest Cost	5.8	6.1	17.8	18.3
Defined Benefit Plan, Expected Return on Plan Assets	-9.2	-7.2	-26.2	-21.1
Defined Benefit Plan, Amortization of Gains (Losses)	5.4	4.2	16.2	12.1
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.1	0.4	0.4	1.1
Defined Benefit Plan Net Periodic Benefit Cost	5.4	6.4	18	19.2
Related Intercompany Allocationsof Defined Benefit Expense	2.6	2.4	7.8	7.5
Amount Capitalized Defined Benefit Expense	1.7	1.9	5.6	5.8
Public Service Company Of New Hampshire [Member] | Other Postretirement Benefit Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Defined Benefit Plan Service Cost	0.6	0.5	1.7	1.5
Defined Benefit Plan Interest Cost	1	1.1	3.1	3.4
Defined Benefit Plan, Expected Return on Plan Assets	-1.3	-1.1	-3.9	-3.4
Defined Benefit Plan, Amortization of Gains (Losses)	0.9	0.9	2.7	2.7
Defined Benefit Plan Amortization Of Transition Obligations Assets	'	0.6	0	1.9
Defined Benefit Plan Net Periodic Benefit Cost	1.2	2	3.6	6.1
Related Intercompany Allocationsof Defined Benefit Expense	0.4	0.5	1.2	1.5
Amount Capitalized Defined Benefit Expense	0.4	0.6	1.1	1.7
Western Massachusetts Electric Company [Member] | Pension Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Defined Benefit Plan Service Cost	1.2	1	3.5	3.1
Defined Benefit Plan Interest Cost	2.5	2.6	7.5	7.9
Defined Benefit Plan, Expected Return on Plan Assets	-4.3	-4.1	-13	-12.3
Defined Benefit Plan, Amortization of Gains (Losses)	2.9	2.7	8.9	8
Defined Benefit Plan Amortization Of Prior Service Cost Credit	0.1	0.2	0.3	0.6
Defined Benefit Plan Net Periodic Benefit Cost	2.4	2.4	7.2	7.3
Related Intercompany Allocationsof Defined Benefit Expense	2	2.1	6	6
Amount Capitalized Defined Benefit Expense	1.3	1.3	3.9	3.7
Western Massachusetts Electric Company [Member] | Other Postretirement Benefit Plans Defined Benefit Member	'	'	'	'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'	'	'	'
Defined Benefit Plan Service Cost	0.2	0.1	0.5	0.4
Defined Benefit Plan Interest Cost	0.4	0.5	1.3	1.5
Defined Benefit Plan, Expected Return on Plan Assets	-0.6	-0.5	-1.7	-1.6
Defined Benefit Plan, Amortization of Gains (Losses)	0.3	0.3	0.8	0.9
Defined Benefit Plan Amortization Of Transition Obligations Assets	'	0.3	0	1.1
Defined Benefit Plan Net Periodic Benefit Cost	0.3	0.7	0.9	2.3
Related Intercompany Allocationsof Defined Benefit Expense	0.3	0.4	1	1.1
Amount Capitalized Defined Benefit Expense	$0.30	$0.40	$0.70	$1.10

INCOME_TAXES_Details

INCOME TAXES (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]	'	'	'	'
Total Deferred Income Taxes	'	'	$334,225	$186,181
Income Tax Expense	109,351	117,360	325,442	199,379
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]	'	'	'	'
Income Before Income Tax Expense	320,731	326,868	939,855	555,861
Income Tax Expense	109,351	117,360	325,442	199,379
Other Tax Disclosures [Abstract]	'	'	'	'
Other Information Pertaining To Income Taxes	'	'	'2013 Massachusetts: On July 24, 2013, Massachusetts enacted a law that changes 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 deferred taxes and resulted in NU increasing its deferred tax liability with an offsetting regulatory asset of approximately $61 million at its utility companies ($46.4 million at NSTAR Electric and $9.8 million at WMECO). 2013 Federal: On September 13, 2013, the Internal Revenue Service issued final Tangible Property regulations. The final regulations are meant to simplify, clarify and make more administrable the previously issued temporary and proposed regulations. In the third quarter of 2013, CL&P recorded an after-tax valuation allowance of $10.5 million against its deferred tax assets as a result of these regulations. NU continues to evaluate the implications of these new regulations, including several new elections. Therefore, a change to the valuation allowance at CL&P could result once NU completes the review of the impact of the final regulations.	'
The Connecticut Light And Power Company [Member]	'	'	'	'
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]	'	'	'	'
Total Deferred Income Taxes	'	'	89,084	97,224
Income Tax Expense	36,136	34,121	113,149	63,917
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]	'	'	'	'
Income Before Income Tax Expense	102,459	109,007	332,337	199,721
Income Tax Expense	36,136	34,121	113,149	63,917
NSTAR Electric Company [Member]	'	'	'	'
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]	'	'	'	'
Total Deferred Income Taxes	'	'	26,358	-20,250
Income Tax Expense	68,558	69,373	137,499	102,220
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]	'	'	'	'
Income Before Income Tax Expense	175,567	176,135	350,661	258,404
Income Tax Expense	68,558	69,373	137,499	102,220
Public Service Company Of New Hampshire [Member]	'	'	'	'
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]	'	'	'	'
Total Deferred Income Taxes	'	'	57,066	39,108
Income Tax Expense	18,196	21,106	52,797	48,037
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]	'	'	'	'
Income Before Income Tax Expense	46,549	48,338	137,340	117,763
Income Tax Expense	18,196	21,106	52,797	48,037
Western Massachusetts Electric Company [Member]	'	'	'	'
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]	'	'	'	'
Total Deferred Income Taxes	'	'	79,401	30,565
Income Tax Expense	8,588	7,977	30,424	24,385
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]	'	'	'	'
Income Before Income Tax Expense	23,613	22,061	80,465	63,793
Income Tax Expense	$8,588	$7,977	$30,424	$24,385

Environmental_Matters_Details

Environmental Matters (Details) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Site Contingency [Line Items]	'	'
Environmental Site Quantity	68	77
Accrual for Environmental Loss Contingencies [Roll Forward]	'	'
Beginning Balance	$39.40	'
Ending Balance	36.5	39.4
MGP Site accrual [Member]	'	'
Accrual for Environmental Loss Contingencies [Roll Forward]	'	'
Ending Balance	32.4	34.5
The Connecticut Light And Power Company [Member]	'	'
Site Contingency [Line Items]	'	'
Environmental Site Quantity	18	19
Accrual for Environmental Loss Contingencies [Roll Forward]	'	'
Beginning Balance	3.7	'
Ending Balance	3.5	3.7
NSTAR Electric Company [Member]	'	'
Site Contingency [Line Items]	'	'
Environmental Site Quantity	12	16
Accrual for Environmental Loss Contingencies [Roll Forward]	'	'
Beginning Balance	1.7	'
Ending Balance	1.2	1.7
Public Service Company Of New Hampshire [Member]	'	'
Site Contingency [Line Items]	'	'
Environmental Site Quantity	15	16
Accrual for Environmental Loss Contingencies [Roll Forward]	'	'
Beginning Balance	4.9	'
Ending Balance	5.6	4.9
Western Massachusetts Electric Company [Member]	'	'
Site Contingency [Line Items]	'	'
Environmental Site Quantity	5	6
Accrual for Environmental Loss Contingencies [Roll Forward]	'	'
Beginning Balance	0.6	'
Ending Balance	$0.40	$0.60

LongTerm_Contractual_Arrangeme1

Long-Term Contractual Arrangements (Details) (USD $)	Sep. 30, 2013
In Millions, unless otherwise specified
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	$1.20
Unrecorded Unconditional Purchase Obligation, Due within Two Years	49.9
Unrecorded Unconditional Purchase Obligation, Due within Three Years	50.9
Unrecorded Unconditional Purchase Obligation, Due within Four Years	51.4
Unrecorded Unconditional Purchase Obligation, Due within Five Years	52
Unrecorded Unconditional Purchase Obligation, Due after Five Years	626
Unrecorded Unconditional Purchase Obligation, Total	831.4
The Connecticut Light And Power Company [Member] | Yankee Companies Billings [Member]	'
Long-term Purchase Commitment [Line Items]	'
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	1.5
Unrecorded Unconditional Purchase Obligation, Due within Three Years	1.3
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0.8
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0.8
Unrecorded Unconditional Purchase Obligation, Due after Five Years	13.1
Unrecorded Unconditional Purchase Obligation, Total	17.9
NSTAR Electric Company [Member] | Yankee Companies Billings [Member]	'
Long-term Purchase Commitment [Line Items]	'
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0.7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0.5
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	4.5
Unrecorded Unconditional Purchase Obligation, Total	6.4
Public Service Company Of New Hampshire [Member] | Yankee Companies Billings [Member]	'
Long-term Purchase Commitment [Line Items]	'
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0.3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	5.2
Unrecorded Unconditional Purchase Obligation, Total	6.6
Western Massachusetts Electric Company [Member] | Yankee Companies Billings [Member]	'
Long-term Purchase Commitment [Line Items]	'
Unrecorded Unconditional Purchase Obligation, Due within One Year	0.1
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0.4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0.4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0.2
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0.2
Unrecorded Unconditional Purchase Obligation, Due after Five Years	3.3
Unrecorded Unconditional Purchase Obligation, Total	$4.60

Guarantees_and_Indemnification1

Guarantees and Indemnifications (Details) (USD $)	Sep. 30, 2013
In Millions, unless otherwise specified
Guarantee Of Financial Obligations Of Npt [Member]	'
Guarantee Obligations [Line Items]	'
Guarantee Obligations Maximum Exposure	$25
Various Subsidiary Surety Bonds And Performance Guarantees Expiring Between 2013 And 2015 [Member]	'
Guarantee Obligations [Line Items]	'
Guarantee Obligations Maximum Exposure	33
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	18.8
Guarantee Of Nuei Surety Bonds Insurance Bonds And Performance Guarantees [Member]	'
Guarantee Obligations [Line Items]	'
Guarantee Obligations Maximum Exposure	62.3
Guarantee Of Ngs Insurance Bond [Member]	'
Guarantee Obligations [Line Items]	'
Guarantee Obligations Maximum Exposure	1
Guarantee Of Es Boulos Surety Bonds Expiring Upon Project Completion [Member]	'
Guarantee Obligations [Line Items]	'
Guarantee Obligations Maximum Exposure	57.5
Guarantee Of Select Energy Wholesale Purchase Contracts [Member]	'
Guarantee Obligations [Line Items]	'
Guarantee Obligations Maximum Exposure	3.8
Guarantee Of Debt Of Ne Hydros [Member]	'
Guarantee Obligations [Line Items]	'
Guarantee Obligations Maximum Exposure	$4

Other_Contingencies_Details

Other Contingencies (Details) (Unfavorable Regulatory Action [Member])	9 Months Ended
	Sep. 30, 2013
Unfavorable Regulatory Action [Member]	'
Loss Contingencies Line Items	'
Loss Contingency, Description	'     FERC Base ROE Complaint On September 30, 2011, several New England state attorneys general, state regulatory commissions, consumer advocates and other parties filed a joint complaint with the FERC under Sections 206 and 306 of the Federal Power Act alleging that the base ROE used in calculating formula rates for transmission service under the ISO-NE Open Access Transmission Tariff by NETOs, including CL&P, NSTAR Electric, PSNH and WMECO, is unjust and unreasonable. The complainants asserted that the current 11.14 percent rate, which became effective in 2006, is excessive due to changes in the capital markets and are seeking an order to reduce the rate, which would be effective October 1, 2011. In response, the NETOs filed testimony and analysis based on standard FERC methodology and precedent, demonstrating that the base ROE of 11.14 percent remained just and reasonable. The FERC set the case for trial before a FERC ALJ after settlement negotiations were unsuccessful in August 2012. Hearings before the FERC ALJ were held in May 2013, followed by the filing of briefs by the complainants, the Massachusetts municipal electric utilities (late interveners to the case), the FERC trial staff and the NETOs. The NETOs recommended that the current base ROE of 11.14 percent should remain in effect for the refund period (October 1, 2011 through December 31, 2012) and the prospective period (beginning when FERC issues its final decision). The complainants, the Massachusetts municipal electric utilities, and the FERC trial staff each recommended a base ROE of 9 percent or below. On August 6, 2013, the FERC ALJ issued an initial decision, finding that the current base ROE is not reasonable under the standard application of FERC methodology, but leaving policy considerations and additional adjustments to the FERC. Using the established FERC methodology, the FERC ALJ determined that a separate base ROE should be set for the refund period and the prospective period. The FERC ALJ found those base ROEs to be 10.6 percent and 9.7 percent, respectively. The FERC may adjust the prospective period base ROE in its final decision to reflect movement in 10-year Treasury bond rates from when the case was filed (April 2013) to the date of the final decision. The parties filed briefs on this decision to the FERC, and a decision from the FERC is expected in 2014. Though NU cannot predict the ultimate outcome of this proceeding, during the third quarter of 2013, the Company recorded a series of reserves at its electric subsidiaries to recognize the potential financial impact from the FERC ALJ's initial decision for the refund period. As a result, the aggregate after-tax charge to earnings totaled $14.3 million at NU. This represents reserves of $7.7 million at CL&P, $3.4 million at NSTAR Electric, $1.4 million at PSNH and $1.8 million at WMECO. On December 27, 2012, several additional parties filed a separate complaint concerning the NETOs' base ROE with the FERC. This complaint seeks to reduce the NETOs’ base ROE effective January 1, 2013, effectively extending the refund period for an additional 15 months, and to consolidate this complaint with the joint complaint filed on September 30, 2011. The NETOs have asked the FERC to reject this complaint. The FERC has not yet acted on, and management is unable to predict the ultimate outcome or the estimated impacts on financial position, results of operations or cash flows, of this complaint. Management expects the CL&P, NSTAR Electric, PSNH, and WMECO aggregate shareholder equity invested in their transmission facilities to be approximately $2.4 billion at the end of 2013. As a result, each 10 basis point change in the prospective period authorized base ROE would change annual consolidated earnings by an approximate $2.4 million. F. DPU Safety and Reliability Programs - CPSL Since 2006, NSTAR Electric has been recovering incremental costs related to the DPU-approved Safety and Reliability Programs. From 2006 through 2011, cumulative costs associated with the CPSL program resulted in an incremental revenue requirement to customers of approximately $83 million. These amounts included incremental operations and maintenance costs and the related revenue requirement for specific capital investments relative to the CPSL programs. On May 28, 2010, the DPU issued an order on NSTAR Electric’s 2006 CPSL cost recovery filing (the May 2010 Order). In October 2010, NSTAR Electric filed a reconciliation of the cumulative CPSL program activity for the periods 2006 through 2009 with the DPU in order to determine a proposed rate adjustment. The DPU allowed the proposed rates to go into effect January 1, 2011, subject to final reconciliation of CPSL program costs through a future DPU proceeding. In February 2013, NSTAR Electric updated the October 2010 filing with final activity through 2011. NSTAR Electric recorded its 2006 through 2011 revenues under the CPSL programs based on the May 2010 Order. NSTAR Electric cannot predict the timing of a final DPU order related to its CPSL filings for the period 2006 through 2011. While management does not believe that any subsequent DPU order would result in revenues that are materially different than the amounts already recognized, it is reasonably possible that an order could have a material impact on NSTAR Electric’s results of operations, financial position and cash flows. G. Basic Service Bad Debt Adder In accordance with a generic 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. In 2012, the SJC vacated the DPU order and remanded the matter to the DPU for further review. NSTAR Electric deferred approximately $34 million of costs associated with energy-related bad debt as a regulatory asset through 2011 as NSTAR Electric had concluded that it was probable that these costs would ultimately be recovered from customers. Due to the delays and duration of the proceedings, NSTAR Electric concluded that while an ultimate outcome on the matter in its favor remained "more likely than not," it could no longer be deemed "probable." As a result, NSTAR Electric recognized a reserve related to the regulatory asset in the first quarter of 2012. NSTAR Electric will continue to maintain the reserve until the ultimate outcome of the proceeding has been concluded with the DPU.

FAIR_VALUE_OF_FINANCIAL_INSTRU2

FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]	'	'
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	$155,568,000	$155,568,000
Rate Reduction Bonds	0	82,139,000
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
Rate Reduction Bonds	0	82,100,000
Other Long Term Debt	8,052,500,000	7,963,500,000
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	152,200,000	152,200,000
Rate Reduction Bonds	0	83,000,000
Other Long Term Debt	8,267,200,000	8,640,700,000
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	2,741,000,000	2,862,800,000
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	110,300,000	110,000,000
Other Long Term Debt	2,992,000,000	3,295,400,000
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
Rate Reduction Bonds	0	43,493,000
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
Rate Reduction Bonds	'	43,500,000
Other Long Term Debt	1,801,000,000	1,602,600,000
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	41,900,000	42,200,000
Rate Reduction Bonds	'	43,900,000
Other Long Term Debt	1,881,900,000	1,818,800,000
Public Service Company Of New Hampshire [Member]	'	'
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]	'	'
Rate Reduction Bonds	0	29,294,000
Public Service Company Of New Hampshire [Member] | Carrying Reported Amount Fair Value Disclosure [Member]	'	'
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]	'	'
Rate Reduction Bonds	'	29,300,000
Other Long Term Debt	889,100,000	997,900,000
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]	'	'
Rate Reduction Bonds	'	29,600,000
Other Long Term Debt	934,700,000	1,088,000,000
Western Massachusetts Electric Company [Member]	'	'
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]	'	'
Rate Reduction Bonds	0	9,352,000
Western Massachusetts Electric Company [Member] | Carrying Reported Amount Fair Value Disclosure [Member]	'	'
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]	'	'
Rate Reduction Bonds	'	9,400,000
Other Long Term Debt	549,600,000	605,300,000
Western Massachusetts Electric Company [Member] | Estimate Of Fair Value Fair Value Disclosure [Member]	'	'
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]	'	'
Rate Reduction Bonds	'	9,500,000
Other Long Term Debt	$562,100,000	$660,400,000

ACCUMULATED_OTHER_COMPREHENSIV2

ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
AOCI Cash Flow Hedging Component [Abstract]	'	'	'	'
Beginning Balance -AOCI From CF Hedges Effect Net Of Tax	'	'	($16,400,000)	'
OCI Amortization of Cash Flow Hedge, Net of Tax	500,000	'	1,500,000	'
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	-509,000	-516,000	-1,539,000	-1,455,000
Ending Balance - AOCI From CFHedges Effect Net Of Tax	-14,900,000	'	-14,900,000	'
AOCI Unrealized GainLoss component [Abstract]	'	'	'	'
AOCI Unrealized Gains/Losses on Other Securities	'	'	1,300,000	'
OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentBeforeReclassificationAdjustmentsNetOfTax	'	'	-800,000	'
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	-38,000	217,000	-810,000	411,000
AOCI Unrealized Gains/Losses on Other Securities-Ending Balance	500,000	'	500,000	'
AOCI Defined Benefit Component [Abstract]	'	'	'	'
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	'	'	-57,800,000	'
OCI Reclassification Of Defined Benefit Plans Net Gain Loss Net of Tax	1,600,000	'	4,800,000	'
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	1,611,000	1,445,000	4,738,000	4,611,000
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	-53,000,000	'	-53,000,000	'
AOCI Rollforward [Abstract]	'	'	'	'
AOCI Beginning Balance	'	'	-72,854,000	'
AmountReclassifiedFromAOCI	2,100,000	'	6,300,000	'
OtherComprehensiveIncomeBeforeReclassifications	'	'	-800,000	'
Other Comprehensive Income (Loss), Net of Tax, Total	2,082,000	2,178,000	5,467,000	6,477,000
AOCI ending Balance	-67,387,000	'	-67,387,000	'
Reclassication of Items out of AOCI [Abstract]	'	'	'	'
OCI Amortization of Cash Flow Hedges, Before Tax	-800,000	'	-2,500,000	'
OCI Amortization of Cash Flow Hedge, Net of Tax	-500,000	'	-1,500,000	'
OCI Amortization of Prior Service Cost	'	'	-100,000	'
OCI Reclass, Defined Benefit, Net GainLoss before Tax	-2,500,000	'	-7,300,000	'
OCI Defined Benefit Plan Tax	900,000	'	2,600,000	'
OCI Reclassification Of Defined Benefit Plans Net Gain Loss Net of Tax	-1,600,000	'	-4,800,000	'
AmountReclassifiedFromAOCI	-2,100,000	'	-6,300,000	'
OCI Reclass CF Hedging Tax	300,000	'	1,000,000	'
The Connecticut Light And Power Company [Member]	'	'	'	'
AOCI Cash Flow Hedging Component [Abstract]	'	'	'	'
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	-111,000	-111,000	-333,000	-333,000
AOCI Unrealized GainLoss component [Abstract]	'	'	'	'
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	-1,000	8,000	-27,000	14,000
AOCI Rollforward [Abstract]	'	'	'	'
AOCI Beginning Balance	'	'	-1,800,000	'
Other Comprehensive Income (Loss), Net of Tax, Total	110,000	119,000	306,000	347,000
AOCI ending Balance	-1,494,000	'	-1,494,000	'
Public Service Company Of New Hampshire [Member]	'	'	'	'
AOCI Cash Flow Hedging Component [Abstract]	'	'	'	'
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	-290,000	-291,000	-872,000	-872,000
AOCI Unrealized GainLoss component [Abstract]	'	'	'	'
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	-2,000	13,000	-47,000	24,000
AOCI Defined Benefit Component [Abstract]	'	'	'	'
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	0	-2,000	-3,000	2,000
AOCI Rollforward [Abstract]	'	'	'	'
AOCI Beginning Balance	'	'	-9,655,000	'
Other Comprehensive Income (Loss), Net of Tax, Total	288,000	302,000	822,000	898,000
AOCI ending Balance	-8,833,000	'	-8,833,000	'
Western Massachusetts Electric Company [Member]	'	'	'	'
AOCI Cash Flow Hedging Component [Abstract]	'	'	'	'
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	-85,000	-84,000	-254,000	-253,000
AOCI Unrealized GainLoss component [Abstract]	'	'	'	'
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	0	2,000	-8,000	4,000
AOCI Rollforward [Abstract]	'	'	'	'
AOCI Beginning Balance	'	'	-3,846,000	'
Other Comprehensive Income (Loss), Net of Tax, Total	85,000	86,000	246,000	257,000
AOCI ending Balance	($3,600,000)	'	($3,600,000)	'

COMMON_SHARES_Details

COMMON SHARES (Details) (USD $)	0 Months Ended
	Apr. 09, 2012	Sep. 30, 2013	Dec. 31, 2012
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	'	333,019,517	332,509,383
Treasury Stock Shares	'	18,137,017	18,455,749
Common Stock Issuance Merger With Nstar [Abstract]	'	'	'
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,000,000	'	'
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

COMMON_SHAREHOLDER_EQUIT_AND_N

COMMON SHAREHOLDER EQUIT AND NCI (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Shareholders' Equity, Beginning Balance	'	'	$9,237,050,000	'
Net Income	211,380,000	209,508,000	614,413,000	356,482,000
Net Income Attributable to Noncontrolling Interests	1,879,000	1,880,000	5,803,000	5,253,000
Other Comprehensive Income (Loss), Net of Tax	2,082,000	2,178,000	5,467,000	6,477,000
Shareholders' Equity, Ending Balance	9,517,453,000	'	9,517,453,000	'
Common Shareholders Equity [Member]	'	'	'	'
Shareholders' Equity, Beginning Balance	9,406,600,000	9,067,600,000	9,237,100,000	4,012,700,000
Net Income	211,400,000	209,500,000	614,400,000	356,500,000
Dividends on Common Shares	-114,900,000	-107,600,000	-346,900,000	-267,800,000
Dividends, Preferred Stock	-1,900,000	-1,900,000	-5,800,000	-5,100,000
Issuance of Common Shares	1,400,000	800,000	10,200,000	12,200,000
Other Transcations, Net	12,800,000	6,300,000	3,000,000	20,300,000
Net Income Attributable to Noncontrolling Interests	'	'	'	-100,000
Other Comprehensive Income (Loss), Net of Tax	2,100,000	2,200,000	5,500,000	6,500,000
Shareholders' Equity, Ending Balance	9,517,500,000	9,176,900,000	9,517,500,000	9,176,900,000
Purchase Price of NSTAR	'	'	'	5,038,300,000
Other Impacts of Merger with NSTAR	'	'	'	3,400,000
Noncontrolling Interest Member	'	'	'	'
Shareholders' Equity, Beginning Balance	'	'	'	3,000,000
Contributions To Northern Pass Trans	'	'	'	300,000
Net Income Attributable to Noncontrolling Interests	'	'	'	100,000
Other Impacts of Merger with NSTAR	'	'	'	-3,400,000
Total Equity [Member]	'	'	'	'
Shareholders' Equity, Beginning Balance	'	9,067,600,000	'	4,015,700,000
Net Income	'	209,500,000	'	356,500,000
Dividends on Common Shares	'	-107,600,000	'	-267,800,000
Dividends, Preferred Stock	'	-1,900,000	'	-5,100,000
Issuance of Common Shares	'	800,000	'	12,200,000
Contributions To Northern Pass Trans	'	'	'	300,000
Other Transcations, Net	'	6,300,000	'	20,300,000
Other Comprehensive Income (Loss), Net of Tax	'	2,200,000	'	6,500,000
Shareholders' Equity, Ending Balance	'	9,176,900,000	'	9,176,900,000
Purchase Price of NSTAR	'	'	'	5,038,300,000
Preferred Stock Member	'	'	'	'
Shareholders' Equity, Beginning Balance	155,600,000	155,600,000	155,600,000	116,200,000
Dividends, Preferred Stock	-1,900,000	-1,900,000	-5,800,000	-5,100,000
Net Income Attributable to Noncontrolling Interests	1,900,000	1,900,000	5,800,000	5,100,000
Shareholders' Equity, Ending Balance	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		9 Months Ended
In Thousands, except Share data, unless otherwise specified	Apr. 09, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Notes To Consolidated Financial Statements [Abstract]	'	'	'	'	'
Net Income Attributable to Controlling Interests	'	$209,501	$207,628	$608,610	$351,229
Basic	'	315,291,346	314,806,441	315,191,752	264,636,636
Weighted Average Number Diluted Shares Outstanding Adjustment	'	926,893	999,355	869,379	716,741
Diluted	'	316,218,239	315,805,796	316,061,131	265,353,377
Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount	'	0	0	2,100	5,688
Basic Earnings Per Common Share	'	$0.66	$0.66	$1.93	$1.33
Diluted Earnings Per Common Share	'	$0.66	$0.66	$1.93	$1.32
Common Stock Issuance Merger With Nstar [Abstract]	'	'	'	'	'
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,000,000	'	'	'	'

SEGMENT_INFORMATION_Details

SEGMENT INFORMATION (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]	'	'	'	'	'
Operating Revenues	$1,892,590,000	$1,861,529,000	$5,523,475,000	$4,589,835,000	'
Operating Income	399,262,000	412,905,000	1,168,783,000	786,813,000	'
Total Interest Expense	-87,476,000	-90,361,000	-250,583,000	-245,856,000	'
Income Tax Expense	-109,351,000	-117,360,000	-325,442,000	-199,379,000	'
Net Income	211,380,000	209,508,000	614,413,000	356,482,000	'
Net Income Attributable to Noncontrolling Interests	-1,879,000	-1,880,000	-5,803,000	-5,253,000	'
Net Income Attributable to Controlling Interests	209,501,000	207,628,000	608,610,000	351,229,000	'
Total Assets	28,444,318,000	'	28,444,318,000	'	28,302,824,000
Investments in Property, Plant and Equipment	'	'	1,073,759,000	1,081,750,000	'
Electric Distribution Member	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'
Operating Revenues	1,508,600,000	1,483,700,000	4,104,400,000	3,499,700,000	'
Depreciation And Amortization	-159,600,000	-172,600,000	-488,700,000	-398,100,000	'
Operating Expenses, Other	-1,064,100,000	-1,027,400,000	-2,952,400,000	-2,654,400,000	'
Operating Income	284,900,000	283,700,000	663,300,000	447,200,000	'
Net Income Attributable to Controlling Interests	156,900,000	150,500,000	347,500,000	212,100,000	'
Total Assets	17,912,900,000	'	17,912,900,000	'	18,047,300,000
Investments in Property, Plant and Equipment	'	'	501,900,000	461,300,000	'
Natural Gas Distribution [Member]	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'
Operating Revenues	97,100,000	91,300,000	613,000,000	361,500,000	'
Depreciation And Amortization	-16,400,000	-12,600,000	-50,500,000	-32,700,000	'
Operating Expenses, Other	-89,400,000	-77,200,000	-483,600,000	-292,900,000	'
Operating Income	-8,700,000	1,500,000	78,900,000	35,900,000	'
Net Income Attributable to Controlling Interests	-10,400,000	-4,400,000	34,100,000	8,300,000	'
Total Assets	2,656,800,000	'	2,656,800,000	'	2,717,400,000
Investments in Property, Plant and Equipment	'	'	91,200,000	105,900,000	'
Transmission [Member]	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'
Operating Revenues	234,100,000	235,600,000	721,500,000	627,200,000	'
Depreciation And Amortization	-34,500,000	-29,700,000	-100,900,000	-79,500,000	'
Operating Expenses, Other	-73,400,000	-66,300,000	-199,100,000	-179,500,000	'
Operating Income	126,200,000	139,600,000	421,500,000	368,200,000	'
Net Income Attributable to Controlling Interests	58,600,000	71,100,000	215,400,000	181,100,000	'
Total Assets	6,566,100,000	'	6,566,100,000	'	6,187,700,000
Investments in Property, Plant and Equipment	'	'	458,200,000	476,000,000	'
Other Segment [Member]	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'
Operating Revenues	212,500,000	219,500,000	650,400,000	582,900,000	'
Depreciation And Amortization	-11,200,000	-17,500,000	-52,000,000	-39,100,000	'
Operating Expenses, Other	-206,800,000	-216,800,000	-599,000,000	-614,500,000	'
Operating Income	-5,500,000	-14,800,000	-600,000	-70,700,000	'
Net Income Attributable to Controlling Interests	313,100,000	313,900,000	868,700,000	511,600,000	'
Total Assets	19,446,900,000	'	19,446,900,000	'	18,832,600,000
Investments in Property, Plant and Equipment	'	'	22,500,000	38,600,000	'
Eliminations Segment [Member]	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'
Operating Revenues	-159,700,000	-168,600,000	-565,800,000	-481,500,000	'
Depreciation And Amortization	2,600,000	1,100,000	7,200,000	2,600,000	'
Operating Expenses, Other	159,500,000	170,400,000	564,300,000	485,100,000	'
Operating Income	2,400,000	2,900,000	5,700,000	6,200,000	'
Net Income Attributable to Controlling Interests	-308,700,000	-323,500,000	-857,100,000	-561,900,000	'
Total Assets	-18,138,400,000	'	-18,138,400,000	'	-17,482,200,000
Investments in Property, Plant and Equipment	'	'	0	0	'
Total [Member]	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'
Operating Revenues	1,892,600,000	1,861,500,000	5,523,500,000	4,589,800,000	'
Depreciation And Amortization	-219,100,000	-231,300,000	-684,900,000	-546,800,000	'
Operating Expenses, Other	-1,274,200,000	-1,217,300,000	-3,669,800,000	-3,256,200,000	'
Operating Income	399,300,000	412,900,000	1,168,800,000	786,800,000	'
Net Income Attributable to Controlling Interests	209,500,000	207,600,000	608,600,000	351,200,000	'
Total Assets	28,444,300,000	'	28,444,300,000	'	28,302,800,000
Investments in Property, Plant and Equipment	'	'	$1,073,800,000	$1,081,800,000	'