WGL (WGL) 10-Q 7 May 15 2015 Q2 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	6 Months Ended
	Mar. 31, 2015	Apr. 30, 2015
Document Information [Line Items]
Entity Registrant Name	WGL Holdings Inc.
Entity Central Index Key	1103601
Document Type	10-Q
Document Period End Date	31-Mar-15
Amendment Flag	FALSE
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	-21
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		49,728,662
Washington Gas Light Company
Document Information [Line Items]
Entity Registrant Name	Washington Gas Light Company
Entity Central Index Key	104819
Document Type	10-Q
Document Period End Date	31-Mar-15
Amendment Flag	FALSE
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	-21
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		0

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (Unaudited) (USD $)	Mar. 31, 2015	Sep. 30, 2014
In Thousands, unless otherwise specified
WGL Holdings, Inc.
Property, Plant and Equipment
At original cost	$4,727,512	$4,582,764
Accumulated depreciation and amortization	-1,275,721	-1,268,319
Net property, plant and equipment	3,451,791	3,314,445
Current Assets
Cash and cash equivalents	9,287	8,811
Receivables
Accounts receivable	491,339	222,253
Gas costs and other regulatory assets	5,972	3,752
Unbilled revenues	200,747	96,314
Allowance for doubtful accounts	-23,088	-23,341
Net receivables	674,970	298,978
Materials and supplies—principally at average cost	20,822	23,647
Storage gas	96,123	333,602
Deferred income taxes	20,999	26,664
Prepaid taxes	10,631	66,578
Other prepayments	45,313	34,269
Derivatives	23,481	18,331
Other	22,704	24,635
Total current assets	924,330	835,515
Regulatory assets
Gas costs	228,958	191,346
Pension and other post-retirement benefits	181,200	192,981
Other	72,487	71,638
Prepaid post-retirement benefits	104,140	96,385
Derivatives	35,063	18,739
Investments in direct financing leases, capital leases	20,227	18,159
Investments in unconsolidated affiliates	114,682	100,528
Other	17,485	16,763
Total deferred charges and other assets	774,242	706,539
Total Assets	5,150,363	4,856,499
Capitalization
Common shareholders’ equity	1,303,684	1,246,576
Washington Gas Light Company preferred stock	28,173	28,173
Long-term debt	950,469	679,228
Total capitalization	2,282,326	1,953,977
Current Liabilities
Current maturities of long-term debt	45,000	20,000
Notes payable	175,000	453,500
Accounts payable and other accrued liabilities	324,046	313,221
Wages payable	21,566	19,995
Accrued interest	7,990	3,488
Dividends declared	23,330	22,449
Customer deposits and advance payments	82,620	68,318
Gas costs and other regulatory liabilities	82,107	22,563
Accrued taxes	43,918	14,133
Derivatives	47,695	48,555
Other	47,800	34,063
Total current liabilities	901,072	1,020,285
Deferred Credits
Unamortized investment tax credits	115,106	99,351
Deferred income taxes	687,442	660,908
Accrued pensions and benefits	123,914	120,446
Asset retirement obligations	178,670	175,203
Regulatory liabilities
Accrued asset removal costs	336,774	327,388
Other post-retirement benefits	82,264	86,428
Other	20,262	17,588
Derivatives	337,837	294,745
Other	84,696	100,180
Total deferred credits	1,966,965	1,882,237
Commitments and Contingencies (Note 13)
Total Capitalization and Liabilities	5,150,363	4,856,499
Washington Gas Light Company
Property, Plant and Equipment
At original cost	4,336,433	4,250,194
Accumulated depreciation and amortization	-1,229,492	-1,228,130
Net property, plant and equipment	3,106,941	3,022,064
Current Assets
Cash and cash equivalents	4,909	1,060
Receivables
Accounts receivable	295,220	121,419
Gas costs and other regulatory assets	5,972	3,752
Unbilled revenues	109,165	20,881
Allowance for doubtful accounts	-18,898	-19,209
Net receivables	391,459	126,843
Materials and supplies—principally at average cost	20,777	23,600
Storage gas	41,281	156,083
Deferred income taxes	15,969	22,916
Prepaid taxes	7,090	16,137
Other prepayments	12,281	14,272
Receivables from associated companies	3,387	4,821
Derivatives	13,877	3,884
Other	142	0
Total current assets	511,172	369,616
Regulatory assets
Gas costs	228,958	191,346
Pension and other post-retirement benefits	180,179	191,896
Other	72,424	71,584
Prepaid post-retirement benefits	103,361	95,660
Derivatives	14,944	9,455
Other	12,063	13,457
Total deferred charges and other assets	611,929	573,398
Total Assets	4,230,042	3,965,078
Capitalization
Common shareholders’ equity	1,151,088	1,050,166
Washington Gas Light Company preferred stock	28,173	28,173
Long-term debt	702,199	679,228
Total capitalization	1,881,460	1,757,567
Current Liabilities
Current maturities of long-term debt	45,000	20,000
Notes payable	0	89,000
Accounts payable and other accrued liabilities	166,859	176,467
Wages payable	19,778	18,290
Accrued interest	4,025	3,488
Dividends declared	20,222	19,722
Customer deposits and advance payments	82,475	68,318
Gas costs and other regulatory liabilities	82,107	22,563
Accrued taxes	62,095	24,610
Payables to associated companies	68,942	54,685
Derivatives	31,465	33,858
Other	6,257	7,199
Total current liabilities	589,225	538,200
Deferred Credits
Unamortized investment tax credits	6,056	6,479
Deferred income taxes	692,523	619,946
Accrued pensions and benefits	122,399	118,954
Asset retirement obligations	177,200	173,775
Regulatory liabilities
Accrued asset removal costs	336,774	327,388
Other post-retirement benefits	81,689	85,814
Other	20,262	17,588
Derivatives	288,587	260,789
Other	33,867	58,578
Total deferred credits	1,759,357	1,669,311
Commitments and Contingencies (Note 13)
Total Capitalization and Liabilities	$4,230,042	$3,965,078

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Income (Unaudited) (USD $)	3 Months Ended		6 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
WGL Holdings, Inc.
OPERATING REVENUES
Utility	$606,505	$702,255	$988,217	$1,088,796
Non-utility	395,228	471,995	762,753	765,751
Total Operating Revenues	1,001,733	1,174,250	1,750,970	1,854,547
OPERATING EXPENSES
Utility cost of gas	310,138	459,107	439,842	645,988
Non-utility cost of energy-related sales	356,535	426,286	693,103	731,637
Operation and maintenance	104,287	99,699	196,667	187,841
Depreciation and Amortization	30,103	27,304	59,463	53,894
General taxes and other assessments	57,784	57,121	97,167	97,742
Total Operating Expenses	858,847	1,069,517	1,486,242	1,717,102
OPERATING INCOME	142,886	104,733	264,728	137,445
Equity in Earnings of Unconsolidated Affiliates	1,832	543	2,976	1,033
Other income (expenses)—net	338	342	-4,017	561
Interest expense	13,254	9,525	25,564	18,517
INCOME BEFORE INCOME TAXES	131,802	96,093	238,123	120,522
INCOME TAX EXPENSE	50,017	34,550	92,120	40,020
NET INCOME	81,785	61,543	146,003	80,502
Dividends on preferred stock	330	330	660	660
NET INCOME APPLICABLE TO COMMON STOCK	81,455	61,213	145,343	79,842
WEIGHTED AVERAGE COMMON SHARES OUTSTANDING
Basic (in shares)	49,720	51,875	49,851	51,846
Diluted (in shares)	49,983	51,899	50,055	51,864
EARNINGS PER AVERAGE COMMON SHARE
Basic (in dollars per share)	$1.64	$1.18	$2.92	$1.54
Diluted (in dollars per share)	$1.63	$1.18	$2.90	$1.54
DIVIDENDS DECLARED PER COMMON SHARE (in dollars per share)	$0.46	$0.44	$0.90	$0.86
Washington Gas Light Company
OPERATING REVENUES
Utility	615,694	716,808	1,002,887	1,107,223
OPERATING EXPENSES
Utility cost of gas	319,328	473,508	454,493	664,203
Operation and maintenance	85,680	76,440	160,637	148,512
Depreciation and Amortization	26,880	25,401	53,484	50,442
General taxes and other assessments	53,862	53,421	89,706	90,799
Total Operating Expenses	485,750	628,770	758,320	953,956
OPERATING INCOME	129,944	88,038	244,567	153,267
Other income (expenses)—net	-169	90	-619	-90
Interest expense	10,564	9,393	20,828	18,272
INCOME BEFORE INCOME TAXES	119,211	78,735	223,120	134,905
INCOME TAX EXPENSE	44,517	29,229	83,475	46,592
NET INCOME	74,694	49,506	139,645	88,313
Dividends on preferred stock	330	330	660	660
NET INCOME APPLICABLE TO COMMON STOCK	$74,364	$49,176	$138,985	$87,653

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $)	3 Months Ended				6 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014		Mar. 31, 2015		Mar. 31, 2014
WGL Holdings, Inc.
Net Income (Loss)	$81,785		$61,543		$146,003		$80,502
OTHER COMPREHENSIVE INCOME (LOSS), BEFORE INCOME TAXES:
Qualified cash flow hedging instruments	222	[1]	0	[1]	-8,042	[1]	0	[1]
Pension and other post-retirement benefit plans
Change in prior service credit	-171	[2]	-35	[2]	-342	[2]	-69	[2]
Change in actuarial net loss (gain)	491	[2]	-164	[2]	974	[2]	200	[2]
Total pension and other post-retirement benefit plans	320		-199		632		131
Current-period other comprehensive income (loss)	542		-199		-7,410		131
INCOME TAX EXPENSE (BENEFIT) RELATED TO OTHER COMPREHENSIVE INCOME	219		-79		-3,728		52
OTHER COMPREHENSIVE INCOME (LOSS)	323		-120		-3,682		79
COMPREHENSIVE INCOME	82,108		61,423		142,321		80,581
Washington Gas Light Company
Net Income (Loss)	74,694		49,506		139,645		88,313
Pension and other post-retirement benefit plans
Change in prior service credit	-171	[3]	-35	[3]	-342	[3]	-69	[3]
Change in actuarial net loss (gain)	491	[3]	-164	[3]	974	[3]	200	[3]
Current-period other comprehensive income (loss)	320		-199		632		131
INCOME TAX EXPENSE (BENEFIT) RELATED TO OTHER COMPREHENSIVE INCOME	127		-79		250		52
OTHER COMPREHENSIVE INCOME (LOSS)	193		-120		382		79
COMPREHENSIVE INCOME	$74,887		$49,386		$140,027		$88,392
[1]	Cash flow hedging instruments represent interest rate swaps agreements on debt issuances. Refer to Note 8- Derivative and weather-related instruments for further discussion of the interest rate swap agreements.
[2]	These accumulated other comprehensive income components are included in the computation of net periodic benefit cost. Refer to Note 14- Pension and other post-retirement benefit plans for additional details.
[3]	These accumulated other comprehensive income components are included in the computation of net periodic benefit cost. Refer to Note 14-Pension and other post-retirement benefit plans for additional details.

Condensed_Consolidated_Stateme2

Condensed Consolidated Statement of Cash Flows (Unaudited) (USD $)	6 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
WGL Holdings, Inc.
OPERATING ACTIVITIES
Net income	$146,003	$80,502
ADJUSTMENTS TO RECONCILE NET INCOME TO NET CASH PROVIDED BY OPERATING ACTIVITIES
Depreciation and Amortization	59,463	53,894
Amortization of:
Other regulatory assets and liabilities—net	684	-5,896
Debt related costs	638	288
Deferred income taxes—net	62,916	-16,248
Accrued/deferred pension and other post-retirement benefit cost	13,092	18,380
Compensation expense related to stock-based awards	4,127	3,923
Provision for doubtful accounts	9,888	8,850
Impairment loss	5,625	770
Other non-cash charges—net	1,160	822
CHANGES IN ASSETS AND LIABILITIES
Accounts receivable and unbilled revenues—net	-381,157	-471,422
Gas costs and other regulatory assets/liabilities—net	57,324	78,252
Storage gas	237,479	273,138
Prepaid taxes	55,947	6,645
Other prepayments	-11,044	-33,529
Accounts payable and other accrued liabilities	47,054	108,629
Wages payable	1,571	991
Customer deposits and advance payments	14,302	-14,713
Unamortized investment tax credits	10,683	37,364
Accrued taxes	29,785	15,659
Accrued interest	4,502	-24
Other current assets	4,756	6,476
Other current liabilities	13,737	-9,774
Deferred gas costs—net	-37,612	-198,829
Deferred assets—other	-8,399	-597
Deferred liabilities—other	-56,851	22,703
Derivatives	20,758	281,905
Other—net	-629	-6,018
Net Cash Provided by Operating Activities	305,802	242,141
FINANCING ACTIVITIES
Common stock issued	0	-732
Long-term debt issued	296,481	75,253
Long-term debt retired	0	-37,000
Debt issuance costs	-2,744	0
Notes payable issued (retired) —net	-278,500	-58,600
Dividends on common stock and preferred stock	-44,656	-44,199
Repurchase of common stock	-41,485	0
Other financing activities—net	0	-515
Net Cash Used in Financing Activities	-70,904	-65,793
INVESTING ACTIVITIES
Capital expenditures (excluding AFUDC)	-214,306	-147,472
Investments in non-utility interests	-22,810	-21,194
Distributions from non-utility interests	2,694	817
Net Cash Used in Investing Activities	-234,422	-167,849
INCREASE IN CASH AND CASH EQUIVALENTS	476	8,499
Cash and Cash Equivalents at Beginning of Year	8,811	3,478
Cash and Cash Equivalents at End of Period	9,287	11,977
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Income taxes paid—net	2,011	10,020
Interest paid	20,533	18,240
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES
Project debt financing activities—net	-2,032	253
Capital expenditure accruals included in accounts payable and other accrued liabilities	17,221	14,483
Dividends paid in common stock	2,634	2,617
Washington Gas Light Company
OPERATING ACTIVITIES
Net income	139,645	88,313
ADJUSTMENTS TO RECONCILE NET INCOME TO NET CASH PROVIDED BY OPERATING ACTIVITIES
Depreciation and Amortization	53,484	50,442
Amortization of:
Other regulatory assets and liabilities—net	684	-6,141
Debt related costs	644	565
Deferred income taxes—net	49,602	-17,185
Accrued/deferred pension and other post-retirement benefit cost	12,786	18,208
Compensation expense related to stock-based awards	8,654	1,812
Provision for doubtful accounts	1,330	7,895
Impairment loss	0	770
Other non-cash charges—net	3,036	1,283
CHANGES IN ASSETS AND LIABILITIES
Accounts receivable and unbilled revenues—net	-262,292	-378,643
Gas costs and other regulatory assets/liabilities—net	57,324	78,252
Storage gas	114,802	86,341
Other prepayments	11,039	-5,647
Accounts payable and other accrued liabilities	35,205	106,067
Wages payable	1,488	1,224
Customer deposits and advance payments	14,157	-14,713
Accrued taxes	37,485	76,173
Accrued interest	537	-24
Other current assets	2,680	4,988
Other current liabilities	-942	13,711
Deferred gas costs—net	-37,612	-198,829
Deferred assets—other	-10,247	-8,729
Deferred liabilities—other	-2,976	13,527
Derivatives	9,923	257,151
Other—net	5	-305
Net Cash Provided by Operating Activities	240,441	176,506
FINANCING ACTIVITIES
Long-term debt issued	50,000	75,253
Long-term debt retired	0	-37,000
Debt issuance costs	-722	0
Notes payable issued (retired) —net	-89,000	-63,500
Dividends on common stock and preferred stock	-39,320	-38,764
Net Cash Used in Financing Activities	-79,042	-64,011
INVESTING ACTIVITIES
Capital expenditures (excluding AFUDC)	-157,550	-104,576
Net Cash Used in Investing Activities	-157,550	-104,576
INCREASE IN CASH AND CASH EQUIVALENTS	3,849	7,919
Cash and Cash Equivalents at Beginning of Year	1,060	0
Cash and Cash Equivalents at End of Period	4,909	7,919
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Income taxes paid—net	1,850	1,240
Interest paid	15,797	17,995
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES
Project debt financing activities—net	-2,032	253
Capital expenditure accruals included in accounts payable and other accrued liabilities	$12,388	$8,474

Accounting_Policies

Accounting Policies	6 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Accounting Policies
	NOTE 1. ACCOUNTING POLICIES
	Basis of Presentation
	WGL Holdings, Inc. (WGL) is a holding company that owns all of the shares of common stock of Washington Gas Light Company (Washington Gas), a regulated natural gas utility, all of the shares of common stock of Washington Gas Resources Corporation (Washington Gas Resources) and Hampshire Gas Company (Hampshire). Washington Gas Resources owns all of the shares of common stock of four non-utility subsidiaries that include WGL Energy Services, Inc. (WGL Energy Services), WGL Energy Systems, Inc. (WGL Energy Systems), WGL Midstream, Inc. (WGL Midstream) and WGSW, Inc. (WGSW). Except where the content clearly indicates otherwise, “WGL,” “we,” “us” or “our” refers to the holding company or the consolidated entity of WGL Holdings, Inc. and all of its subsidiaries. Unless otherwise noted, these notes apply equally to WGL and Washington Gas.
	The condensed consolidated financial statements have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (SEC). Therefore, certain financial information and note disclosures accompanying annual financial statements prepared in accordance with generally accepted accounting principles in the United States of America (GAAP) are omitted in this interim report. The interim consolidated financial statements and accompanying notes should be read in conjunction with the combined Annual Report on Form 10-K for WGL and Washington Gas for the fiscal year ended September 30, 2014. Due to the seasonal nature of our businesses, the results of operations for the periods presented in this report are not necessarily indicative of actual results for the full fiscal years ending September 30, 2015 and 2014 of either WGL or Washington Gas.
	The accompanying unaudited financial statements for WGL and Washington Gas reflect all normal recurring adjustments that are necessary, in our opinion, to present fairly the results of operations in accordance with GAAP.
	For a complete description of our accounting policies, refer to Note 1 of the Notes to Consolidated Financial Statements of the combined Annual Report on Form 10-K for WGL and Washington Gas for the fiscal year ended September 30, 2014.
	Storage Gas Valuations
	For Washington Gas and WGL Energy Services, storage gas inventory is stated at the lower-of-cost or market as determined using the first-in, first-out method. For WGL Midstream, storage gas inventory is stated at the lower-of-cost or market using the weighted average cost method. Interim period inventory losses attributable to lower-of-cost or market adjustments may be reversed if the market value of the inventory is recovered by the end of the same fiscal year.
	For the three months ended March 31, 2015, Washington Gas did not record any lower-of-cost or market adjustments. For the six months ended March 31, 2015, Washington Gas recorded a decrease to net income due to a lower-of-cost or market adjustment of $0.7 million. For the three and six months ended March 31, 2014, Washington Gas recorded a decrease to net income due to a lower-of-cost or market adjustment of $0.4 million. For the three and six months ended March 31, 2015 and 2014, WGL Energy Services did not record any lower-of-cost or market adjustments. For the three and six months ended March 31, 2015, WGL Midstream recorded a decrease to net income due to a lower-of-cost or market adjustment of $3.0 million and $20.5 million, respectively. For the three and six months ended March 31, 2014, WGL Midstream did not record any lower-of-cost or market adjustments.
	ACCOUNTING STANDARDS ADOPTED IN FISCAL YEAR 2015
	Standard	Description	Date of adoption	Effect on the financial statements or other significant matters
	ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (Topic 740)	The standard requires an unrecognized tax benefit, or a portion of an unrecognized tax benefit to be presented in the financial statements as a reduction to a deferred tax asset for net operating loss carryforwards, a similar loss, or a tax credit carryforward.	1-Oct-14	As a result of the implementation of this standard, we reduced our deferred tax assets by a portion of our unrecognized tax benefits. The adoption of this standard did not have a material effect on our financial statements.
	OTHER NEWLY ISSUED ACCOUNTING STANDARDS
	Standard	Description	Date of adoption	Effect on the financial statements or other significant matters
	ASU 2015-03, Simplifying the Presentation of Debt Issuance Cost (Topic 835)	The standard requires an entity to present debt issuance costs in the balance sheet as a direct deduction of the debt liability in a manner consistent with its accounting treatment of debt discounts.	1-Oct-16	We are in the process of evaluating the impact the adoption of this standard will have on our financial statements.
	ASU 2014-09, Revenue from Contracts with Customers (Topic 606)	The standard establishes a comprehensive revenue recognition model clarifying the method used to determine the timing and requirements for revenue recognition from contracts with customers. The disclosure requirements under the new standard will enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers.	1-Oct-17	We are in the process of evaluating the impact the adoption of this standard will have on our financial statements.

Accounts_Payable_and_Other_Acc

Accounts Payable and Other Accrued Liabilities	6 Months Ended
	Mar. 31, 2015
Accounts Payable and Accrued Liabilities [Abstract]
Accounts Payable and Other Accrued Liabilities
	NOTE 2. ACCOUNTS PAYABLE AND OTHER ACCRUED LIABILITIES
	The tables below provide details for the amounts included in “Accounts payable and other accrued liabilities” on the balance sheets for both WGL and Washington Gas.
	WGL Holdings, Inc.
	(In millions)	31-Mar-15			30-Sep-14
	Accounts payable—trade	$	274.4		$	278.8
	Employee benefits and payroll accruals	21			19.8
	Other accrued liabilities	28.6			14.6
	Total	$	324		$	313.2
	Washington Gas Light Company
	(In millions)	March 31, 2015		September 30, 2014
	Accounts payable—trade	126.4		146.4
	Employee benefits and payroll accruals	19.7		18.2
	Other accrued liabilities	20.8		11.9
	Total	166.9		176.5

ShortTerm_Debt

Short-Term Debt	6 Months Ended
	Mar. 31, 2015
Short-term Debt [Abstract]
Short-Term Debt
	NOTE 3. SHORT-TERM DEBT
	WGL and Washington Gas satisfy their short-term financing requirements through the sale of commercial paper or through bank borrowings. Due to the seasonal nature of the regulated utility and retail energy-marketing segments, short-term financing requirements can vary significantly during the year. Revolving credit agreements are maintained to support outstanding commercial paper and to permit short-term borrowing flexibility. The policy of each WGL and Washington Gas, is to maintain bank credit facilities in amounts equal to or greater than the expected maximum commercial paper position. The following is a summary of committed credit available at March 31, 2015 and September 30, 2014.
	Committed Credit Available (In millions)
	March 31, 2015	WGL(b)			Washington Gas			Total Consolidated
	Committed credit agreements
	Unsecured revolving credit facility, expires December 19, 2019(a)	$	450		$	350		$	800
	Less: Commercial Paper	(175.0		)	—			(175.0		)
	Net committed credit available	$	275		$	350		$	625
	Weighted average interest rate	0.26		%	—		%	0.26		%
	September 30, 2014
	Committed credit agreements
	Unsecured revolving credit facility, expires April 3, 2017(a)	$	450		$	350		$	800
	Less: Commercial Paper	(364.5		)	(89.0		)	(453.5		)
	Net committed credit available	$	85.5		$	261		$	346.5
	Weighted average interest rate	0.2		%	0.13		%	0.19		%
	(a)	Both WGL and Washington Gas have the right to request extensions with the banks’ approval. WGL’s revolving credit facility permits it to borrow an additional $100 million, with the banks’ approval, for a total of $550 million. Washington Gas’ revolving credit facility permits it to borrow an additional $100 million, with the banks’ approval, for a total of $450 million.
	(b)	WGL includes WGL Holdings and all subsidiaries other than Washington Gas.
	In December 2014, both WGL and Washington Gas entered into a first amendment to their respective credit agreements, each dated April 3, 2012. The amendments extend the maturity date of the credit facilities from April 3, 2017 to December 19, 2019, subject to maintaining active regulatory financing approvals. The credit agreements, as amended, each have the right to request two one-year-extensions, with the banks' approval. At March 31, 2015 and September 30, 2014, there were no outstanding bank loans from WGL’s or Washington Gas’ revolving credit facilities.

LongTerm_Debt

Long-Term Debt	6 Months Ended
	Mar. 31, 2015
Long-term Debt, by Current and Noncurrent [Abstract]
Long-Term Debt
	NOTE 4. LONG-TERM DEBT
	UNSECURED NOTES
	WGL and Washington Gas issue long-term notes with individual terms regarding interest rates, maturities and call or put options. These notes can have maturity dates of one or more years from the date of issuance.
	At March 31, 2015 and September 30, 2014, WGL had the capacity under a shelf registration to issue an unspecified amount of long-term senior notes debt securities and Washington Gas had the capacity to issue up to $275.0 million of additional long-term Medium-Term Notes (MTNs).
	The following tables show the outstanding notes as of March 31, 2015 and September 30, 2014.
	Senior Notes, MTNs and Private Placement Notes Outstanding
	($ In millions)	WGL(a)			Washington Gas				Total Consolidated
	March 31, 2015
	Long-term notes(b)	$	250		$	741			$	991
	Weighted average interest rate	3.66		%	5.56		%		5.06		%
	September 30, 2014
	Long-term notes(b)	$	—		$	691			$	691
	Weighted average interest rate	n/a			5.65		%		5.65		%
	(a)	WGL includes WGL Holdings and all subsidiaries other than Washington Gas.
	(b)	Includes Senior Notes for WGL and both MTNs and private placement notes for Washington Gas. Represents face value including current maturities.
	The following tables show senior notes, MTN and private placement issuances and retirements for the six months ended March 31, 2015 and 2014.
	Senior Notes, MTNs and Private Placement Issuances and Retirements
	($ In millions)	Principal(b)			Interest			Effective			Nominal
					Rate			Cost(c)			Maturity Date
	Six Months Ended March 31, 2015
	WGL(a)
	Issuances:
	10/24/14	$	100		2.25	%		2.42	%		11/1/19
	10/24/14	125			4.6	%		5.11	%		11/1/44
	12/16/14	25			4.6	%		5.53	%		11/1/44
	Total	$	250
	Washington Gas
	Issuances:
	12/15/14	$	50		4.24	%		4.41	%		12/15/44
	Total	$	50
	Total consolidated issuances	$	300
	Six Months Ended March 31, 2014
	Washington Gas
	Issuances:
	12/5/13	$	75		5	%		4.95	%		12/15/43
	Total	$	75
	Retirements:
	11/7/13	$	37		4.88	%		n/a			11/7/13
	Total	$	37
	(a)	WGL includes WGL Holdings and all subsidiaries other than Washington Gas.
	(b)	Represents face amount.
	(c)	The estimated effective cost of the issued notes, including consideration of issuance fees and hedge costs.

Common_Shareholders_Equity

Common Shareholders' Equity	6 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Common Shareholders' Equity
	NOTE 5. COMMON SHAREHOLDERS’ EQUITY
	The tables below reflect the components of “Common shareholders’ equity” for WGL and “Common shareholder’s equity” for Washington Gas for the six months ended March 31, 2015.
	WGL Holdings, Inc.
	Components of Common Shareholders’ Equity
	(In thousands)	Common Stock			Paid-In		Retained			Accumulated Other Comprehensive Loss, Net of Taxes			Total
		Amount			Capital		Earnings
	Balance at September 30, 2014	$	525,932		$	11,847	$	716,758		$	(7,961	)	$	1,246,576
	Net income	—			—		146,003			—			146,003
	Other comprehensive loss	—			—		—			(3,682		)	(3,682		)
	Repurchase of common stock	(41,485		)	—		—			—			(41,485		)
	Stock-based compensation	1,009			800		—			—			1,809
	Dividends declared:
	Common stock	—			—		(44,877		)	—			(44,877		)
	Preferred stock	—			—		(660		)	—			(660		)
	Balance at March 31, 2015	$	485,456		$	12,647	$	817,224		$	(11,643	)	$	1,303,684
	Washington Gas Light Company
	Components of Common Shareholder’s Equity
	(In thousands)	Common Stock			Paid-In		Retained			Accumulated Other			Total
		Amount			Capital		Earnings			Comprehensive
										Loss, Net of Taxes
	Balance at September 30, 2014	$	46,479		$	480,620	$	529,480		$	(6,413	)	$	1,050,166
	Net income	—			—		139,645			—			139,645
	Other comprehensive income	—			—					382			382
	Stock-based compensation	—			715		—			—			715
	Dividends declared:
	Common stock	—			—		(39,160		)	—			(39,160		)
	Preferred stock	—			—		(660		)	—			(660		)
	Balance at March 31, 2015	$	46,479		$	481,335	$	629,305		$	(6,031	)	$	1,151,088
	WGL had 49,728,662 and 50,656,553 shares issued of common stock at March 31, 2015 and September 30, 2014, respectively. Washington Gas had 46,479,536 shares issued of common stock at both March 31, 2015 and September 30, 2014.

Earnings_Per_Share

Earnings Per Share	6 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Earnings Per Share
	NOTE 6. EARNINGS PER SHARE
	Basic EPS of WGL is computed by dividing net income by the weighted average number of common shares outstanding during the reported period. Diluted EPS assumes the issuance of common shares pursuant to stock-based compensation plans at the beginning of the applicable period unless the effect of such issuance would be anti-dilutive. The following table reflects the computation of our basic and diluted EPS for the three and six months ended March 31, 2015 and 2014.
	Basic and Diluted EPS
	(in thousands, except per share data)	Net Income		Shares	Per Share
		Applicable to			Amount
		Common Stock
	Three Months Ended March 31, 2015
	Basic EPS	$	81,455	49,720	$	1.64
	Stock-based compensation plans	—		263
	Diluted EPS	$	81,455	49,983	$	1.63
	Three Months Ended March 31, 2014
	Basic EPS	$	61,213	51,875	$	1.18
	Stock-based compensation plans	—		24
	Diluted EPS	$	61,213	51,899	$	1.18
	Six Months Ended March 31, 2015
	Basic EPS	$	145,343	49,851	$	2.92
	Stock-based compensation plans	—		204
	Diluted EPS	$	145,343	50,055	$	2.9
	Six Months Ended March 31, 2014
	Basic EPS	$	79,842	51,846	$	1.54
	Stock-based compensation plans	—		18
	Diluted EPS	$	79,842	51,864	$	1.54
	There were no anti-dilutive shares for the three or six months ended March 31, 2015 and 2014.

Income_Taxes

Income Taxes	6 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes
	NOTE 7. INCOME TAXES
	As of March 31, 2015 and September 30, 2014, our uncertain tax positions were approximately $30.2 million and $32.6 million, respectively, primarily due to the change in tax accounting for repairs. If the amounts of unrecognized tax benefits are eventually realized, it would not materially impact the effective tax rate. It is reasonably possible that the amount of the unrecognized tax benefit with respect to some of WGL’s and Washington Gas’ uncertain tax positions will significantly increase or decrease in the next 12 months, however at this time, an estimate of the range of reasonably possible outcomes cannot be determined.
	Under ASC Topic 740, Income Taxes, Washington Gas recognizes any accrued interest associated with uncertain tax positions in interest expense and recognizes any accrued penalties associated with uncertain tax positions in other expenses in the statements of income. At March 31, 2015, we did not have an accrual of interest expense related to uncertain tax positions. At September 30, 2014, we had a total accrual of $0.1 million of interest expense related to uncertain tax positions included in other deferred credits in the accompanying balance sheets.
	WGL files a consolidated federal tax return and various other state returns. We are no longer subject to income tax examinations by the Internal Revenue Service for years ended prior to September 30, 2011. Substantially all state income tax years in major jurisdictions are closed for years ended prior to September 30, 2010.
	Washington Gas charged the Maryland portion of the Medicare Part D (Med D) regulatory asset to tax expense during the fiscal year ended September 30, 2012 based on positions taken by the Maryland Public Service Commission (PSC of MD) in Washington Gas’ rate case during that fiscal year that did not permit recovery. Washington Gas received an order in the first quarter of fiscal year ended September 30, 2014 from the PSC of MD that did allow recovery of the Med D regulatory asset over a five year amortization period beginning December 2013. Therefore, the reinstatement of the regulatory asset is reflected in the effective tax rate for the six months ended March 31, 2014.

Derivative_and_Weather_Related

Derivative and Weather Related Instruments	6 Months Ended
	Mar. 31, 2015
Derivative Instrument Detail [Abstract]
Derivative and Weather-Related Instruments
	NOTE 8. DERIVATIVE AND WEATHER-RELATED INSTRUMENTS
	DERIVATIVE INSTRUMENTS
	Regulated Utility Operations
	Washington Gas enters into contracts that qualify as derivative instruments and are accounted for under ASC Topic 815. These derivative instruments are recorded at fair value on our balance sheet and Washington Gas does not designate any derivatives as hedges under ASC Topic 815. Washington Gas’ derivative instruments relate to: (i) Washington Gas’ asset optimization program; (ii) managing price risk associated with the purchase of gas to serve utility customers and (iii) managing interest rate risk.
	Asset Optimization. Washington Gas optimizes the value of its long-term natural gas transportation and storage capacity resources during periods when these resources are not being used to physically serve utility customers. Specifically, Washington Gas utilizes its transportation capacity assets to benefit from favorable natural gas prices between different geographic locations and utilizes its storage capacity assets to benefit from favorable natural gas prices between different time periods. As part of this asset optimization program, Washington Gas enters into physical and financial derivative transactions in the form of forward, futures and option contracts with the primary objective of locking in operating margins that Washington Gas will ultimately realize. The derivatives used under this program are subject to mark-to-market accounting treatment while the capacity and transportation resources are not.
	Regulatory sharing mechanisms allow the profit from these transactions to be shared between Washington Gas’ shareholders and customers; therefore, any changes in fair value are recorded through earnings, or as regulatory assets or liabilities to the extent that gains and losses associated with these derivative instruments will be included in the rates charged to customers when they are realized. Valuation changes for the portion of net profits to be retained for shareholders may cause significant period-to-period volatility in earnings from unrealized gains and losses. This volatility does not change the locked-in operating margins that Washington Gas expects to ultimately realize from these transactions through the use of its storage and transportation capacity resources.
	All physically and financially settled contracts under our asset optimization program are reported on a net basis in the statements of income in “Utility cost of gas.” Total net margins recorded to “Utility cost of gas” after sharing and management fees associated with all asset optimization transactions for the three months ended March 31, 2015 was a loss of $14.0 million including an unrealized loss of $28.0 million. During the three months ended March 31, 2014 we recorded a loss of $60.4 million including an unrealized loss of $77.9 million. Total net margins recorded for the six months ended March 31, 2015 was a gain of $17.1 million including an unrealized loss of $2.9 million. During the six months ended March 31, 2014, we recorded a loss of $80.2 million including an unrealized loss of $104.1 million.
	Managing Price Risk. To manage price risk associated with acquiring natural gas supply for utility customers, Washington Gas enters into forward contracts, option contracts, financial contracts and other contracts, as authorized by its regulators. These instruments are accounted for as derivative instruments. Any gains and losses associated with these derivatives are recorded as regulatory liabilities or assets, respectively, to reflect the rate treatment for these economic hedging activities.
	Managing Interest-Rate Risk. Washington Gas utilizes derivative instruments that are designed to minimize the risk of interest-rate volatility associated with planned issuances of debt securities. Any gains and losses associated with these types of derivatives are recorded as regulatory liabilities or assets, respectively, and amortized in accordance with regulatory requirements, typically over the life of the newly issued debt.
	Non-Utility Operations
	Managing Price Risk. WGL Energy Services enters into certain derivative contracts as part of managing the price risk associated with the sale and purchase of natural gas and electricity. WGL Midstream enters into derivative contracts for the purpose of optimizing its storage and transportation capacity as well as managing the transportation and storage assets on behalf of third parties. Derivative instruments are recorded at fair value on our consolidated balance sheets. WGL Energy Services and WGL Midstream do not designate these derivatives as hedges under ASC Topic 815; therefore, changes in the fair value of these derivative instruments are reflected in the earnings of our non-utility operations and may cause significant period-to-period volatility in earnings. The storage and transportation capacities utilized by WGL Midstream are not considered to be derivative instruments, and thus they are not recorded at fair value on our consolidated balance sheets.
	Managing Interest-Rate Risk. WGL utilizes derivative instruments that are designed to minimize the risk of interest-rate volatility associated with future debt issuances. WGL elected cash flow hedge accounting for its interest rate derivative instruments, which settled with the issuance of the related debt issuance in the first quarter of 2015. The effective portion of the gains and losses on the hedge were recorded within other comprehensive income and are being amortized over the life of the debt (through 2044). The amortization will be minimal for fiscal 2015.
	Consolidated Operations
	Reflected in the tables below is information for WGL as well as Washington Gas. The information for WGL includes derivative instruments for both utility and non-utility operations.
	At March 31, 2015 and September 30, 2014, respectively, the absolute notional amounts of our derivatives were as follows:
	Absolute Notional Amounts
	of Open Positions on Derivative Instruments
	March 31, 2015	Notional Amounts
	Derivative transactions	WGL Holdings			Washington Gas
	Natural Gas (In millions of therms)
	Asset Optimization	21,936.90			14,409.40
	Retail sales	9.6			—
	Other risk-management activities	1,745.70			1,389.10
	Electricity (In millions of kWhs)
	Retail sales	4,126.90			—
	Other risk-management activities	15,842.90			—
	30-Sep-14
	Derivative transactions
	Natural Gas (In millions of therms)
	Asset Optimization	20,593.30			13,740.90
	Retail sales	44.7			—
	Other risk-management activities	1,641.30			1,398.20
	Electricity (In millions of kWhs)
	Retail sales	3,831.40			—
	Other risk-management activities	16,734.10			—
	Warrants (In millions of shares)	4.6			—
	Interest Rate Swaps (In millions of dollars)	150			—
	The following tables present the balance sheet classification for all derivative instruments as of March 31, 2015 and September 30, 2014.
	WGL Holdings, Inc.
	Balance Sheet Classification of Derivative Instruments
	(In millions)	Derivative Instruments Not Designated as Hedging Instruments							Derivative Instruments Designated as Hedging Instruments
	As of March 31, 2015	Gross				Gross			Gross			Gross			Netting of			Total(a)
		Derivative				Derivative			Derivative			Derivative			Collateral
		Assets				Liabilities			Assets			Liabilities
	Current Assets—Derivatives	$	42.1			$	(16.5	)	$	—		$	—		$	(2.1	)	$	23.5
	Deferred Charges and Other Assets—Derivatives	35.1				—			—			—			—			35.1
	Current Liabilities—Derivatives	4.3				(58.0		)	—			—			6			(47.7		)
	Deferred Credits—Derivatives	12				(352.7		)	—			—			2.9			(337.8		)
	Total	$	93.5			$	(427.2	)	$	—		$	—		$	6.8		$	(326.9	)
	As of September 30, 2014
	Current Assets—Derivatives	$	20.8			$	(2.5	)	$	—		$	—		$	—		$	18.3
	Deferred Charges and Other Assets—Derivatives	18.7				—			—			—			—			18.7
	Current Liabilities—Derivatives	15.4				(70.3		)	—			(1.7		)	8			(48.6		)
	Deferred Credits—Derivatives	17.7				(316.4		)	—			—			4			(294.7		)
	Total	$	72.6			$	(389.2	)	$	—		$	(1.7	)	$	12		$	(306.3	)
	Washington Gas Light Company
	Balance Sheet Classification of Derivative Instruments(b)
	(In millions)
	As of March 31, 2015	Gross				Gross			Netting of			Total(a)
		Derivative				Derivative			Collateral
		Assets				Liabilities
	Current Assets—Derivatives	$	21.9			$	(5.9	)	$	(2.1	)	$	13.9
	Deferred Charges and Other Assets—Derivatives	14.9				—			—			14.9
Current Liabilities—Derivatives	—				(31.5		)	—			(31.5		)
Deferred Credits—Derivatives	0.9				(289.5		)	—			(288.6		)
Total	$	37.7			$	(326.9	)	$	(2.1	)	$	(291.3	)
As of September 30, 2014
Current Assets—Derivatives	$	3.9			$	—		$	—		$	3.9
Deferred Charges and Other Assets—Derivatives	9.5				—			—			9.5
Current Liabilities—Derivatives	8.6				(43.2		)	0.7			(33.9		)
Deferred Credits—Derivatives	4.2				(265.2		)	0.2			(260.8		)
Total	$	26.2			$	(308.4	)	$	0.9		$	(281.3	)
(a) WGL has elected to offset the fair value of recognized derivative instruments against the right to reclaim or the obligation to return collateral for derivative instruments executed under the same master netting arrangement in accordance with ASC 815. All recognized derivative contracts and associated financial collateral subject to a master netting arrangement or similar that is eligible for offset under ASC 815 have been presented net in the balance sheet.
(b) Washington Gas did not have any derivative instruments outstanding that were designated as hedging instruments at March 31, 2015 or September 30, 2014.
The following table presents all gains and losses associated with derivative instruments for the three and six months ended March 31, 2015 and 2014.
Gains and Losses on Derivative Instruments
(In millions)	WGL Holdings, Inc.							Washington Gas
								Light Company
Three Months Ended March 31,	2015				2014			2015			2014
Recorded to income
Operating revenues—non-utility	$	(1.1	)		$	(35.8	)	$	—		$	—
Utility cost of gas	(35.8		)		(96.4		)	(35.8		)	(96.4		)
Non-utility cost of energy-related sales	10.5				27.6			—			—
Other income-net	—				—			—			—
Interest expense(a)	(0.1		)		—			—			—
Recorded to regulatory assets
Gas costs	(52.1		)		(133.7		)	(52.1		)	(133.7		)
Other	—				1.2			—			1.2
Recorded to other comprehensive income	0.2				—			—			—
Total	$	(78.4	)		$	(237.1	)	$	(87.9	)	$	(228.9	)
Six Months Ended March 31,	2015				2014			2015			2014
Recorded to income
Operating revenues—non-utility	$	74.3			$	(84.3	)	$	—		$	—
Utility cost of gas	(10.0		)		(123.7		)	(10.0		)	(123.7		)
Non-utility cost of energy-related sales	(39.1		)		43.9			—			—
Other income-net	—				0.1			—			—
Interest expense(b)	(0.5		)		—			—			—
Recorded to regulatory assets
Gas costs	(23.9		)		(212.0		)	(23.9		)	(212.0		)
Other	—				1.2			—			1.2
Recorded to other comprehensive income(c)	(8.0		)		—			—			—
Total	$	(7.2	)		$	(374.8	)	$	(33.9	)	$	(334.5	)
(a) Represents $(0.1) million of amortization of amounts previously recorded to Accumulated Other Comprehensive Income.
(b) Represents $(0.4) million of ineffectiveness for our cash flow hedge and $(0.1) million of amortization of amounts previously recorded to Accumulated Other Comprehensive Income.
(c) Represents the effective portion of our cash flow hedge of $(8.1) million less $(0.1) million of amortization that was reclassified to interest expense.
Collateral
WGL utilizes standardized master netting agreements, which facilitate the netting of cash flows into a single net exposure for a given counterparty. As part of these master netting agreements, cash, letters of credit and parental guarantees may be required to be posted or obtained from counterparties in order to mitigate credit risk related to both derivatives and non-derivative positions. Under WGL’s offsetting policy, collateral balances are offset against the related counterparties’ derivative positions to the extent the application would not result in the over-collateralization of those derivative positions on the balance sheet. At March 31, 2015, Washington Gas, WGL Energy Services and WGL Midstream posted $3.0 million, $16.7 million and $8.6 million, respectively, of collateral deposits with counterparties that were not offset against open and settled derivative contracts. At September 30, 2014, Washington Gas, WGL Energy Services and WGL Midstream posted $8.2 million, $5.7 million and $11.4 million, respectively, of collateral deposits with counterparties that were not offset against open and settled derivative contracts. In addition, at March 31, 2015 and September 30, 2014, Washington Gas held $0.4 million and $2.5 million, respectively of cash collateral representing an obligation to counterparties that was not offset against open and settled derivative contracts. Any collateral posted that is not offset against open and settled derivative contracts is included in “Other prepayments” in the accompanying balance sheet. Collateral received and not offset against open and settled derivative contracts is included in “Customer deposits and advance payments” in the accompanying balance sheet.
Certain derivative instruments of Washington Gas, WGL Energy Services and WGL Midstream contain contract provisions that require collateral to be posted if the credit rating of Washington Gas or WGL falls below certain levels or if counterparty exposure to WGL Energy Services or WGL Midstream exceeds a certain level. Due to counterparty exposure levels, at March 31, 2015, WGL Energy Services posted $9.0 million of collateral related to its derivative liabilities that contained credit-related contingent features. At September 30, 2014, WGL Energy Services posted $5.3 million of collateral related to these aforementioned derivative liabilities. Washington Gas and WGL Midstream were not required to post any collateral related to its derivative liabilities that contained credit-related contingent features at March 31, 2015 or September 30, 2014. The following table shows the aggregate fair value of all derivative instruments with credit-related contingent features that are in a liability position, as well as the maximum amount of collateral that would be required if the most intrusive credit-risk-related contingent features underlying these agreements were triggered on March 31, 2015 and September 30, 2014, respectively.
Potential Collateral Requirements for Derivative Liabilities
with Credit-Risk-Contingent Features
(In millions)	WGL Holdings, Inc.				Washington Gas
March 31, 2015
Derivative liabilities with credit-risk-contingent features	$	40.8			$	7.3
Maximum potential collateral requirements	32.3				7.3
September 30, 2014
Derivative liabilities with credit-risk-contingent features	$	28.8			$	20.6
Maximum potential collateral requirements	16.5				16.1
Washington Gas, WGL Energy Services and WGL Midstream do not enter into derivative contracts for speculative purposes.
Concentration of Credit Risk
We are exposed to credit risk from derivative instruments with wholesale counterparties, which is represented by the fair value of these instruments at the reporting date. We actively monitor and work to minimize counterparty concentration risk through various practices. At March 31, 2015, three counterparties represented over 10% of Washington Gas’ credit exposure to wholesale derivative counterparties for a total credit risk of $22.5 million; two counterparties represented over 10% of WGL Energy Services’ credit exposure to wholesale counterparties for a total credit risk of $1.2 million; and two counterparties represented over 10% of WGL Midstream’s credit exposure to wholesale counterparties for a total credit risk of $15.1 million.
WEATHER-RELATED INSTRUMENTS
Washington Gas did not use any weather-related instruments during the three and six months ended March 31, 2015 and 2014.
WGL Energy Services utilizes weather-related instruments for managing the financial effects of weather risks. These instruments cover a portion of WGL Energy Services’ estimated revenue or energy-related cost exposure to variations in heating or cooling degree days. These contracts provide for payment to WGL Energy Services of a fixed-dollar amount for every degree day over or under specific levels during the calculation period depending upon the type of contract executed. For the three months ended March 31, 2015 and 2014, WGL Energy Services recorded pre-tax gains of $4.9 million and $6.4 million, respectively, related to these contracts. For the six months ended March 31, 2015 and 2014, WGL Energy Services recorded pre-tax gains of $3.2 million and $5.4 million, respectively, related to these contracts.

Fair_Value_Measurements

Fair Value Measurements	6 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements
	NOTE 9. FAIR VALUE MEASUREMENTS
	We measure the fair value of our financial assets and liabilities using a combination of the income and market approach in accordance with ASC Topic 820. These financial assets and liabilities primarily consist of derivatives recorded on our balance sheet under ASC Topic 815 and short-term investments, commercial paper and long-term debt outstanding required to be disclosed at fair value. Under ASC Topic 820, fair value is defined as the exit price, representing the amount that would be received in the sale of an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. To value our financial instruments, we use market data or assumptions that market participants would use, including assumptions about credit risk (both our own credit risk and the counterparty’s credit risk) and the risks inherent in the inputs to valuation.
	We enter into derivative contracts in the futures and over-the-counter (OTC) wholesale and retail markets. These markets are the principal markets for the respective wholesale and retail contracts. Our relevant market participants are our existing counterparties and others who have participated in energy transactions at our delivery points. These participants have access to the same market data as WGL. We value our derivative contracts based on an “in-exchange” premise, and valuations are generally based on pricing service data or indicative broker quotes depending on the market location. We measure the net credit exposure at the counterparty level where the right to set-off exists. The net exposure is determined using the mark-to-market exposure adjusted for collateral, letters of credit and parent guarantees. We use published default rates from Standard & Poor’s Ratings Services and Moody’s Investors Service as inputs for determining credit adjustments.
	ASC Topic 820 establishes a fair value hierarchy that prioritizes the inputs used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities and the lowest priority to unobservable inputs. The three levels of the fair value hierarchy under ASC Topic 820 are described below:
	Level 1.Level 1 of the fair value hierarchy consists of assets or liabilities that are valued using observable inputs based upon unadjusted quoted prices in active markets for identical assets or liabilities at the reporting date. Level 1 assets and liabilities primarily include exchange traded derivatives and securities. WGL did not have any Level 1 derivatives at March 31, 2015 or September 30, 2014.
	Level 2.Level 2 of the fair value hierarchy consists of assets or liabilities that are valued using directly or indirectly observable inputs either corroborated with market data or based on exchange traded market data. Level 2 includes fair values based on industry-standard valuation techniques that consider various assumptions: (i) quoted forward prices, including the use of mid-market pricing within a bid/ask spread; (ii) discount rates; (iii) implied volatility and (iv) other economic factors. Substantially all of these assumptions are observable throughout the full term of the instrument, can be derived from observable data or are supported by observable levels at which transactions are executed in the relevant market. At March 31, 2015 and September 30, 2014, Level 2 financial assets and liabilities included energy-related derivatives such as financial contracts, options and physical forward contracts for deliveries at active market locations.
	Level 3.Level 3 of the fair value hierarchy consists of assets or liabilities that are valued using significant unobservable inputs at the reporting date. These unobservable assumptions reflect our assumptions about estimates that market participants would use in pricing the asset or liability, including natural gas basis prices, annualized volatilities of natural gas prices, and electricity congestion prices. A significant change to any one of these inputs in isolation could result in a significant upward or downward fluctuation in the fair value measurement. These inputs may be used with industry standard valuation methodologies that result in our best estimate of fair value for the assets or liabilities at the reporting date.
	Our Risk Analysis and Mitigation (RA&M) Group determines the valuation policies and procedures. The RA&M Group reports to WGL’s Chief Financial Officer. In accordance with WGL’s valuation policy, we may utilize a variety of valuation methodologies to fair value Level 3 derivative contracts including internally developed valuation inputs and pricing models. The prices used in our valuations are corroborated using multiple pricing sources, and we periodically conduct assessments to determine whether each valuation model is appropriate for its intended purpose. The RA&M Group also evaluates changes in fair value measurements on a daily basis.
	At March 31, 2015 and September 30, 2014, Level 3 derivative assets and liabilities included: (i) physical contracts valued at illiquid market locations with no observable market data; (ii) long-dated positions where observable pricing is not available over the life of the contract; (iii) contracts valued using historical spot price volatility assumptions; (iv) valuations using indicative broker quotes for inactive market locations and (v) non-publicly traded stock warrants.
	The following tables set forth financial instruments recorded at fair value as of March 31, 2015 and September 30, 2014, respectively. A financial instrument’s classification within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Our assessment of the significance of a particular input to the fair value measurement requires judgment, and may affect the valuation of fair value assets and liabilities and their placement within the fair value hierarchy.
	WGL Holdings, Inc.
	Fair Value Measurements Under the Fair Value Hierarchy
	(In millions)	Level 1				Level 2				Level 3			Total
	At March 31, 2015
	Assets
	Natural gas related derivatives	$	—			$	32			$	39.9		$	71.9
	Electricity related derivatives	—				2.9				18.7			21.6
	Total Assets	$	—			$	34.9			$	58.6		$	93.5
	Liabilities
	Natural gas related derivatives	$	—			$	(35.2	)		$	(359.5	)	$	(394.7	)
	Electricity related derivatives	—				(3.3		)		(29.2		)	(32.5		)
	Total Liabilities	$	—			$	(38.5	)		$	(388.7	)	$	(427.2	)
	At September 30, 2014
	Assets
	Natural gas related derivatives	$	—			$	22.7			$	33.7		$	56.4
	Electricity related derivatives	—				0.3				15.9			16.2
	Total Assets	$	—			$	23			$	49.6		$	72.6
	Liabilities
	Natural gas related derivatives	$	—			$	(39.8	)		$	(328.4	)	$	(368.2	)
	Electricity related derivatives	—				(0.1		)		(20.9		)	(21.0		)
	Interest rate derivatives	—				(1.7		)		—			(1.7		)
	Total Liabilities	$	—			$	(41.6	)		$	(349.3	)	$	(390.9	)
	Washington Gas Light Company
	Fair Value Measurements Under the Fair Value Hierarchy
	(In millions)	Level 1				Level 2				Level 3			Total
	At March 31, 2015
	Assets
	Natural gas related derivatives	$	—			$	22.6			$	15.1		$	37.7
	Total Assets	$	—			$	22.6			$	15.1		$	37.7
	Liabilities
	Natural gas related derivatives	$	—			$	(12.0	)		$	(314.9	)	$	(326.9	)
	Total Liabilities	$	—			$	(12.0	)		$	(314.9	)	$	(326.9	)
	At September 30, 2014
	Assets
	Natural gas related derivatives	$	—			$	13.5			$	12.7		$	26.2
	Total Assets	$	—			$	13.5			$	12.7		$	26.2
	Liabilities
	Natural gas related derivatives	$	—			$	(25.1	)		$	(283.3	)	$	(308.4	)
	Total Liabilities	$	—			$	(25.1	)		$	(283.3	)	$	(308.4	)
	The following table includes quantitative information about the significant unobservable inputs used in the fair value measurement of our Level 3 financial instruments and the respective fair values of the net derivative asset and liability positions, by contract type, as of March 31, 2015 and September 30, 2014.
	Quantitative Information about Level 3 Fair Value Measurements
			Net Fair Value		Valuation Techniques		Unobservable Inputs		Range
			March 31, 2015
	WGL Holdings, Inc.		(In millions)
	Natural gas related derivatives		($319.60)		Discounted Cash Flow		Natural Gas Basis Price		($1.630) - $4.760
							(per dekatherm)
					Option Model		Natural Gas Basis Price		($0.215) - $0.039
							(per dekatherm)
							Annualized Volatility of Spot Market Natural Gas		30.5% - 51.1%
	Electricity related derivatives		($10.50)		Discounted Cash Flow		Electricity Congestion Price		($3.103) - $82.600
							(per megawatt hour)
	Washington Gas Light Company
	Natural gas related derivatives		($299.80)		Discounted Cash Flow		Natural Gas Basis Price		($1.630) - $4.268
(per dekatherm)
		Net Fair Value
		September 30, 2014
WGL Holdings, Inc.
Natural gas related derivatives		($294.70)		Discounted Cash Flow		Natural Gas Basis Price		($2.101) - $6.154
						(per dekatherm)
				Option Model		Natural Gas Basis Price		($1.675) - $6.154
						(per dekatherm)
						Annualized Volatility of Spot Market Natural Gas		30.9% - 589.6%
Electricity related derivatives		($5.00)		Discounted Cash Flow		Electricity Congestion Price		($2.85) - $90.95
						(per megawatt hour)
Washington Gas Light Company
Natural gas related derivatives		($270.60)		Discounted Cash Flow		Natural Gas Basis Price		($2.101) - $6.154
						(per dekatherm)
The following tables are a summary of the changes in the fair value of our derivative instruments that are measured at net fair value on a recurring basis in accordance with ASC Topic 820 using significant Level 3 inputs during the three and six months ended March 31, 2015 and 2014, respectively.
WGL Holdings, Inc.
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
(In millions)	Natural Gas				Electricity				Warrants			Total
	Related				Related
	Derivatives				Derivatives
Three Months Ended March 31, 2015
Balance at January 1, 2015	$	(252.6	)		$	(11.5	)		$	—		$	(264.1	)
Realized and unrealized gains (losses)
Recorded to income	(37.7		)		0.4				—			(37.3		)
Recorded to regulatory assets—gas costs	(53.9		)		—				—			(53.9		)
Transfers into Level 3	5.4				—				—			5.4
Transfers out of Level 3	1.4				—				—			1.4
Purchases	—				0.2				—			0.2
Settlements	17.8				0.4				—			18.2
Balance at March 31, 2015	$	(319.6	)		$	(10.5	)		$	—		$	(330.1	)
Three Months Ended March 31, 2014
Balance at January 1, 2014	$	(264.5	)		$	(1.7	)		$	1.2		$	(265.0	)
Realized and unrealized gains (losses)
Recorded to income	(85.9		)		11.2				—			(74.7		)
Recorded to regulatory assets—gas costs	(110.8		)		—				—			(110.8		)
Settlements	28.9				(11.7		)		—			17.2
Balance at March 31, 2014	$	(432.3	)		$	(2.2	)		$	1.2		$	(433.3	)
Six Months Ended March 31, 2015
Balance at October 1, 2014	$	(294.7	)		$	(5.0	)		$	—		$	(299.7	)
Realized and unrealized gains (losses)
Recorded to income	(17.4		)		(10.5		)		—			(27.9		)
Recorded to regulatory assets—gas costs	(40.3		)		—				—			(40.3		)
Transfers into Level 3	5.4				—				—			5.4
Transfers out of Level 3	(0.3		)		—				—			(0.3		)
Purchases	—				3.4				—			3.4
Settlements	27.7				1.6				—			29.3
Balance at March 31, 2015	$	(319.6	)		$	(10.5	)		$	—		$	(330.1	)
Six Months Ended March 31, 2014
Balance at October 1, 2013	$	(155.2	)		$	2.4			$	1.1		$	(151.7	)
Realized and unrealized gains (losses)
Recorded to income	(132.6		)		3.3				0.1			(129.2		)
Recorded to regulatory assets—gas costs	(182.8		)		—				—			(182.8		)
Purchases	—				1.4				—			1.4
Settlements	38.3				(9.3		)		—			29
Balance at March 31, 2014	$	(432.3	)		$	(2.2	)		$	1.2		$	(433.3	)
Washington Gas Light Company
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
(In millions)	Natural Gas
	Related
	Derivatives
Three Months Ended March 31, 2015
Balance at January 1, 2015	$	(234.8	)
Realized and unrealized gains (losses)
Recorded to income	(37.4		)
Recorded to regulatory assets—gas costs	(53.9		)
Transfers into Level 3	5.4
Transfers out of Level 3	1.4
Settlements	19.5
Balance at March 31, 2015	$	(299.8	)
Three Months Ended March 31, 2014
Balance at January 1, 2014	$	(220.0	)
Realized and unrealized gains (losses)
Recorded to income	(81.8		)
Recorded to regulatory assets—gas costs	(110.8		)
Settlements	30
Balance at March 31, 2014	$	(382.6	)
Six Months Ended March 31, 2015
Balance at October 1, 2014	$	(270.6	)
Realized and unrealized gains (losses)
Recorded to income	(22.7		)
Recorded to regulatory assets—gas costs	(40.3		)
Transfers into Level 3	5.4
Transfers out of Level 3	(0.3		)
Settlements	28.7
Balance at March 31, 2015	$	(299.8	)
Six Months Ended March 31, 2014
Balance at October 1, 2013	$	(133.6	)
Realized and unrealized gains (losses)
Recorded to income	(105.2		)
Recorded to regulatory assets—gas costs	(182.8		)
Settlements	39
Balance at March 31, 2014	$	(382.6	)
Transfers between different levels of the fair value hierarchy may occur based on the level of observable inputs used to value the instruments from period to period. It is our policy to show both transfers into and out of the different levels of the fair value hierarchy at the fair value as of the beginning of the period. Transfers out of Level 3 during the three and six months ended March 31, 2015 were due to an increase in valuations using observable market inputs. There were no transfers in or out of Level 3 for the three and six months ended March 31, 2014.
The table below sets forth the line items on the statements of income to which amounts are recorded for the three months ended March 31, 2015 and 2014, respectively, related to fair value measurements using significant Level 3 inputs.
WGL Holdings, Inc.
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
(In millions)	Natural Gas				Electricity				Warrants			Total
	Related				Related
	Derivatives				Derivatives
Three Months Ended March 31, 2015
Operating revenues—non-utility	$	(8.9	)		$	3.2			$	—		$	(5.7	)
Utility cost of gas	(37.4		)		—				—			(37.4		)
Non-utility cost of energy-related sales	8.6				(2.8		)		—			5.8
Total	$	(37.7	)		$	0.4			$	—		$	(37.3	)
Three Months Ended March 31, 2014
Operating revenues—non-utility	$	(0.5	)		$	(14.5	)		$	—		$	(15.0	)
Utility cost of gas	(81.8		)		—				—			(81.8		)
Other income-net	—				—				—			—
Non-utility cost of energy-related sales	(3.6		)		25.7				—			22.1
Total	$	(85.9	)		$	11.2			$	—		$	(74.7	)
Six Months Ended March 31, 2015
Operating revenues—non-utility	$	1.6			$	24			$	—		$	25.6
Utility cost of gas	(22.7		)		—				—			(22.7		)
Non-utility cost of energy-related sales	3.7				(34.5		)		—			(30.8		)
Total	$	(17.4	)		$	(10.5	)		$	—		$	(27.9	)
Six Months Ended March 31, 2014
Operating revenues—non-utility	$	(26.3	)		$	(24.5	)		$	—		$	(50.8	)
Utility cost of gas	(105.2		)		—				—			(105.2		)
Other income-net	—				—				0.1			0.1
Non-utility cost of energy-related sales	(1.1		)		27.8				—			26.7
Total	$	(132.6	)		$	3.3			$	0.1		$	(129.2	)
Washington Gas Light Company
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
(In millions)	Natural Gas
	Related
	Derivatives
Three Months Ended March 31, 2015
Utility cost of gas	$	(37.4	)
Total	$	(37.4	)
Three Months Ended March 31, 2014
Utility cost of gas	$	(81.8	)
Total	$	(81.8	)
Six Months Ended March 31, 2015
Utility cost of gas	$	(22.7	)
Total	$	(22.7	)
Six Months Ended March 31, 2014
Utility cost of gas	$	(105.2	)
Total	$	(105.2	)
Unrealized gains (losses) attributable to derivative assets and liabilities measured using significant Level 3 inputs were recorded as follows, for the three and six months ended March 31, 2015 and 2014, respectively.
WGL Holdings, Inc.
Unrealized Gains (Losses) Recorded for Level 3 Measurements
(In millions)	Natural Gas				Electricity				Warrants			Total
	Related				Related
	Derivatives				Derivatives
Three Months Ended March 31, 2015
Recorded to income
Operating revenues—non-utility	$	(9.3	)		$	0.1			$	—		$	(9.2	)
Utility cost of gas	(31.7		)		—				—			(31.7		)
Non-utility cost of energy-related sales	11.8				0.6				—			12.4
Recorded to regulatory assets—gas costs	(47.7		)		—				—			(47.7		)
Total	$	(76.9	)		$	0.7			$	—		$	(76.2	)
Three Months Ended March 31, 2014
Recorded to income
Operating revenues—non-utility	$	(5.0	)		$	(28.4	)		$	—		$	(33.4	)
Utility cost of gas	(68.1		)		—				—			(68.1		)
Non-utility cost of energy-related sales	(0.8		)		27.1				—			26.3
Other income-net	—				—				—			—
Recorded to regulatory assets—gas costs	(101.1		)		—				—			(101.1		)
Total	$	(175.0	)		$	(1.3	)		$	—		$	(176.3	)
Six Months Ended March 31, 2015
Recorded to income
Operating revenues—non-utility	$	3.6			$	20.9			$	—		$	24.5
Utility cost of gas	(19.8		)		—				—			(19.8		)
Non-utility cost of energy-related sales	1.8				(26.7		)		—			(24.9		)
Recorded to regulatory assets—gas costs	(28.8		)		—				—			(28.8		)
Total	$	(43.2	)		$	(5.8	)		$	—		$	(49.0	)
Six Months Ended March 31, 2014
Recorded to income
Operating revenues—non-utility	$	(30.3	)		$	(33.4	)		$	—		$	(63.7	)
Utility cost of gas	(92.1		)		—				—			(92.1		)
Non-utility cost of energy-related sales	3.3				28.3				—			31.6
Other income-net	—				—				0.1			0.1
Recorded to regulatory assets—gas costs	(169.9		)		—				—			(169.9		)
Total	$	(289.0	)		$	(5.1	)		$	0.1		$	(294.0	)
Washington Gas Light Company
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Three Months Ended March 31, 2015
(In millions)	Natural Gas
	Related
	Derivatives
Recorded to income - utility cost of gas	$	(31.7	)
Recorded to regulatory assets—gas costs	(47.7		)
Total	$	(79.4	)
Three Months Ended March 31, 2014
Recorded to income - utility cost of gas	$	(68.1	)
Recorded to regulatory assets—gas costs	(101.1		)
Total	$	(169.2	)
Six Months Ended March 31, 2015
Recorded to income - utility cost of gas	$	(19.8	)
Recorded to regulatory assets—gas costs	(28.8		)
Total	$	(48.6	)
Six Months Ended March 31, 2014
Recorded to income - utility cost of gas	$	(92.1	)
Recorded to regulatory assets—gas costs	(169.9		)
Total	$	(262.0	)
The following table presents the carrying amounts and estimated fair values of our financial instruments at March 31, 2015 and September 30, 2014.
WGL Holdings, Inc.
Fair Value of Financial Instruments
	March 31, 2015								September 30, 2014
(In millions)	Carrying Amount				Fair Value				Carrying Amount			Fair Value
Money market funds(a)	$	14.6			$	14.6			$	9.7		$	9.7
Other short-term investments(a)	$	0.2			$	0.2			$	—		$	—
Commercial paper (b)	$	175			$	175			$	453.5		$	453.5
Long-term debt(c)	$	950.5			$	1,117.90			$	679.2		$	809.3
Washington Gas Light Company
	March 31, 2015								September 30, 2014
(In millions)	Carrying Amount				Fair Value				Carrying Amount			Fair Value
Money market funds(a)	$	10.2			$	10.2			$	4.3		$	4.3
Other short-term investments(a)	$	0.2			$	0.2			$	—		$	—
Commercial paper (b)	$	—			$	—			$	89		$	89
Long-term debt(c)	$	702.2			$	864.1			$	679.2		$	809.3
(a)	Balance is located in cash and cash equivalents in the accompanying balance sheets. These amounts may be offset by outstanding checks.
(b)	Balance is located in notes payable in the accompanying balance sheets.
(c)	Less current maturities and unamortized discounts.
Our money market funds are Level 1 valuations and their carrying amount approximates fair value. Other short-term investments are primarily overnight investment accounts; therefore, their carrying amount approximates fair value based on Level 2 inputs. The maturity of our commercial paper outstanding at both March 31, 2015 and September 30, 2014 is under 30 days. Due to the short term nature of these notes, the carrying cost of our commercial paper approximates fair value using Level 2 inputs. Neither WGL’s nor Washington Gas’ long-term debt is actively traded. The fair value of long-term debt was estimated based on the quoted market prices of the U.S. Treasury issues having a similar term to maturity, adjusted for the credit quality of the debt issuer, WGL or Washington Gas. Our long-term debt fair value measurement is classified as Level 3.
Non Recurring Basis
During the six months ended March 31, 2015, Washington Gas Resources recorded an impairment charge of its investment in ASDHI to its fair value using the income approach. The amount of the impairment was equivalent to the amount of the carrying value of $5.6 million and was due to management’s assumption of the current valuation and expected return from the investment. The fair value of this investment was a Level 3 measurement.

Operating_Segment_Reporting

Operating Segment Reporting	6 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Operating Segment Reporting
	NOTE 10. OPERATING SEGMENT REPORTING
	We have four reportable operating segments: regulated utility, retail energy-marketing, commercial energy systems and midstream energy services. The division of these segments into separate revenue generating components is based upon regulation, products and services. Our chief operating decision maker is our Chief Executive Officer. During the first quarter of 2015, our chief operating decision maker began evaluating segment performance based on Earnings Before Interest and Taxes (EBIT). EBIT is defined as earnings before interest and taxes from continuing operations. Items we do not include in EBIT are interest expense, intercompany financing activity, dividends on Washington Gas preferred stock, and income taxes. EBIT includes transactions between reportable segments. Additionally, we also evaluate our operating segments based on other relevant factors, such as penetration into their respective markets and return on equity.
	Our four segments are summarized below.
	•	Regulated Utility – The regulated utility segment is our core business. It consists of Washington Gas and Hampshire. Washington Gas provides regulated gas distribution services (including the sale and delivery of natural gas) to customers and natural gas transportation services to an unaffiliated natural gas distribution company in West Virginia under a Federal Energy Regulatory Commission (FERC) approved interstate transportation service operating agreement. Hampshire provides regulated interstate natural gas storage services to Washington Gas under a FERC approved interstate storage service tariff.
	•	Retail Energy-Marketing – The retail energy-marketing segment consists of WGL Energy Services, which sells natural gas and electricity directly to retail customers in competition with regulated utilities and unregulated gas and electricity marketers.
	•	Commercial Energy Systems – The commercial energy systems segment consists of WGL Energy Systems which provides clean and energy efficient solutions including commercial solar, energy efficiency and combined heat and power projects to government and commercial clients. In addition, this segment comprises the operations of WGSW, a holding company formed to invest in alternative energy assets.
	•	Midstream Energy Services – The midstream energy services segment consists of WGL Midstream, which engages in acquiring, investing in, managing and optimizing natural gas storage and transportation assets.
	Activities and transactions that are not significant enough on a stand-alone basis to warrant treatment as an operating segment, and that do not fit into one of our four operating segments, are aggregated as “Other Activities” in the Operating Segment Financial Information presented below. Administrative and business development activity costs associated with WGL and Washington Gas Resources are included in “Other Activities”.
	The following tables present operating segment information for the three and six months ended March 31, 2015 and 2014.
	Operating Segment Financial Information
	(In thousands)	Operating (a)			Depreciation and Amortization				Equity in			EBIT			Total			Capital		Equity Method
		Revenues							Earnings of						Assets			Expenditures		Investments
									Unconsolidated Affiliates
	Three Months Ended March 31, 2015
	Regulated utility	$	615,694		$	27,266			$	—		$	130,280		$	4,245,563		$	70,691	$	—
	Retail energy-marketing	404,601			152				—			38,426			457,847			3		—
	Commercial energy systems	11,532			2,671				502			722			553,515			10,658		65,214
	Midstream energy services	(20,288		)	31				611			(23,507		)	217,032			—		49,468
	Other activities	—			—				719			(846		)	128,624			—		—
	Eliminations(b)	(9,806		)	(17		)		—			(19		)	(452,218		)	—		—
	Total consolidated	$	1,001,733		$	30,103			$	1,832		$	145,056		$	5,150,363		$	81,352	$	114,682
	Three Months Ended March 31, 2014
	Regulated utility	$	716,808		$	25,736			$	—		$	88,535		$	4,013,934		$	50,656	$	—
	Retail energy-marketing	437,434			180				—			(1,390		)	402,675			13		—
	Commercial energy systems	8,036			1,551				479			1,545			423,803			38,497		68,517
	Midstream energy services	27,104			31				64			21,605			102,148			—		13,692
	Other activities	—			—				—			(5,175		)	232,407			—		413
	Eliminations(b)	(15,132		)	(194		)		—			498			(412,750		)	—		—
	Total consolidated	$	1,174,250		$	27,304			$	543		$	105,618		$	4,762,217		$	89,166	$	82,622
	Six Months Ended March 31, 2015
	Regulated utility	$	1,002,887		$	54,218			$	—		$	244,907		$	4,245,563		$	160,294	$	—
	Retail energy-marketing	735,090			319				—			22,531			457,847			37		—
	Commercial energy systems	21,071			4,906				1,079			981			553,515			53,975		65,214
	Midstream energy services	7,804			62				1,147			3,264			217,032			—		49,468
	Other activities	—			—				750			(7,945		)	128,624			—		—
	Eliminations(b)	(15,882		)	(42		)		—			(51		)	(452,218		)	—		—
	Total consolidated	$	1,750,970		$	59,463			$	2,976		$	263,687		$	5,150,363		$	214,306	$	114,682
	Six Months Ended March 31, 2014
	Regulated utility	$	1,107,223		$	51,105			$	—		$	153,988		$	4,013,934		$	105,749	$	—
	Retail energy-marketing	760,372			354				—			3,900			402,675			88		—
	Commercial energy systems	12,753			2,644				763			949			423,803			41,635		68,517
	Midstream energy services	(6,069		)	62				270			(12,604		)	102,148			—		13,692
	Other activities	—			—				—			(7,836		)	232,407			—		413
	Eliminations(b)	(19,732		)	(271		)		—			642			(412,750		)	—		—
	Total consolidated	$	1,854,547		$	53,894			$	1,033		$	139,039		$	4,762,217		$	147,472	$	82,622
	(a) Operating revenues are reported gross of revenue taxes. Revenue taxes of both the regulated utility and the retail energy-marketing segments include gross receipt taxes. Revenue taxes of the regulated utility segment also include PSC fees, franchise fees and energy taxes. Operating revenue amounts in the “Eliminations” row represent total intersegment revenues associated with sales from the regulated utility segment to the retail energy-marketing segment. Midstream Energy Services’ cost of energy related sales is netted with its gross revenues.
	(b) Intersegment eliminations include a timing difference between Commercial Energy Systems’ recognition of revenue for the sale of Solar Renewable Energy Credits (SRECs) to Retail Energy-Marketing and Retail Energy-Marketing’s recognition of the associated expense. Retail Energy-Marketing has recorded a portion of the SREC’s purchased as inventory to be used in future periods at which time they will be expensed.
	The following table is a reconciliation from EBIT to net income applicable to common stock.
		Three Months Ended March 31,						Six Months Ended March 31,
		2015			2014			2015			2014
	Total consolidated EBIT	$	145,056		$	105,618		$	263,687		$	139,039
	Interest expense	13,254			9,525			25,564			18,517
	Income before income taxes	131,802			96,093			238,123			120,522
	Income tax expense	50,017			34,550			92,120			40,020
	Net income	81,785			61,543			146,003			80,502
	Dividends on Washington Gas Light Company preferred stock	330			330			660			660
	Net income applicable to common stock	$	81,455		$	61,213		$	145,343		$	79,842

Other_Investments

Other Investments	6 Months Ended
	Mar. 31, 2015
Other Investments [Abstract]
Other Investments
	NOTE 11. OTHER INVESTMENTS
	When determining how to account for our interests in other legal entities, WGL first evaluates if we are required to apply the variable interest entity (VIE) model to the entity, otherwise the entity is evaluated under the voting interest model.
	Under the VIE model, we have a controlling financial interest in a VIE (i.e. are the primary beneficiary) when we have current or potential rights that give us the power to direct the activities of a VIE that most significantly impact the VIE’s economic performance combined with a variable interest that gives us the right to receive potentially significant benefits or the obligation to absorb potentially significant losses. When changes occur to the design of an entity, we reconsider whether it is subject to the VIE model. We continuously evaluate whether we have a controlling financial interest in a VIE.
	Under the voting interest model, we generally have a controlling financial interest in an entity where we currently hold, directly or indirectly, more than 50% of the voting rights or where we exercise control through substantive participating rights. However, we consider substantive rights held by other partners in determining if we hold a controlling financial interest, and in some cases, despite owning more than 50% of the common stock of an investee, an evaluation of our rights results in the determination that we do not have a controlling financial interest. We reevaluate whether we have a controlling financial interest in these entities when our voting or substantive participating rights change.
	Unconsolidated affiliates are unconsolidated VIEs and other entities evaluated under the voting interest method in which we do not have a controlling financial interest, but over which we have varying degrees of influence. Where we have significant influence, the affiliates are accounted for as equity method investments. Where we do not have significant influence, the affiliates are accounted for under the cost method. Investments in, and advances to, affiliated companies are presented on a one-line basis in the caption “Investments in unconsolidated affiliates” on our Consolidated Balance Sheet.
	WGL uses the Hypothetical Liquidation at Book Value (HLBV) methodology for certain equity method investments when the capital structure of the equity investment results in different liquidation rights and priorities than what is reflected by the underlying percentage ownership interests as defined by an equity investment agreement. For investments accounted for under the HLBV method, simply applying the percentage ownership interest to GAAP net income in order to determine earnings or losses does not accurately represent the income allocation and cash flow distributions that will ultimately be received by the investors. The equity investment agreements for ASD Solar, LP (ASD), Meade Pipeline Co LLC (Meade) and Mountain Valley Pipeline, LLC (Mountain Valley) have liquidation rights and priorities that are sufficiently different from the ownership percentages that the HLBV method was deemed appropriate. The calculation may vary in its complexity depending on the capital structure and the tax considerations for the investments.
	WGL applies HLBV using a balance sheet approach. A calculation is prepared at each balance sheet date to determine the amount that WGL would receive if an equity investment entity were to liquidate all of its assets (as valued in accordance with GAAP) and distribute that cash to the investors based on the contractually defined liquidation priorities. The difference between the calculated liquidation distribution amounts at the beginning and the end of the reporting period is WGL’s share of the earnings or losses from the equity investment for the period.
	Variable Interest Entities
	WGL has a variable interest in four investments that qualify as VIEs:
	•	Meade,
	•	SunEdison,
	•	Nextility and
	•	ASD.
	WGL previously had a variable interest through Crab Run; however, this investment was sold during the three months ended March 31, 2015. Accordingly, at March 31, 2015, the carrying amount of the investment was removed and WGL recognized a gain of $0.7 million, which was recorded in "Equity in earnings of unconsolidated affiliates" on WGL's statements of income. WGL and its subsidiaries are not the primary beneficiary for any of the above VIEs, therefore we have not consolidated any of the VIE entities. At March 31, 2015, the nature of WGL’s involvement with these investments lacks the characteristics of a controlling financial interest. WGL does not have control over any of the VIEs’ activities that are economically significant to the VIEs. In addition, WGL does not have the obligation to absorb expected losses or the right to receive expected gains that could be significant to the VIE.
	Meade
	In 2014, WGL through its subsidiary, WGL Midstream, entered into a limited liability company agreement and formed Meade, a Delaware limited liability company with COG Holdings LLC, Vega Midstream MPC LLC and River Road Interests LLC. Meade was formed to partner with Transcontinental Gas Pipeline Company, LLC (Williams) to invest in a regulated pipeline project called Central Penn Pipeline (Central Penn). The Central Penn will be an approximately 177-mile pipeline originating in Susquehanna County, Pennsylvania and extending to Lancaster County, Pennsylvania that will have the capacity to transport and deliver up to approximately 1.7 million dekatherms per day of natural gas.
	WGL Midstream plans to invest an estimated $410.0 million for a 55% interest in Meade. WGL Midstream joins COG Holdings LLC (20% share), Vega Midstream MPC LLC (15% share) and River Road Interests LLC (10% share) in Meade. Meade is accounted for under the HLBV equity method of accounting, and any profits and losses are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Consolidated Statement of Income and are added to or subtracted from the carrying amount of WGL’s investment balance. WGL Midstream held a $16.0 million equity method investment in Meade at March 31, 2015.
	Our maximum financial exposure includes contributions and guarantees on behalf of WGL Midstream. Our maximum exposure to loss at March 31, 2015 was $59.4 million, which represents the minimum funding requirements owed to Williams under the Construction and Ownership Agreement should Meade terminate its agreement with Williams early.
	SunEdison/Nextility
	WGSW is party to two agreements to fund residential and commercial retail solar energy installations with two separate, privately held companies. WGSW has a master purchase agreement and master lease agreement with SunEdison, Inc. (SunEdison), and Nextility, Inc. (Nextility) for sale/leaseback arrangements for residential and commercial solar systems.
	Our agreements with SunEdison and Nextility are accounted for as direct financing leases. WGSW records associated interest in the financing leases in “Other income (expenses)-net” line in the accompanying Consolidated Statement of Income. WGSW held a $23.5 million and $19.9 million combined investment in direct financing leases at March 31, 2015 and September 30, 2014, respectively, of which $3.3 million and $1.7 million are current receivables recorded in “Accounts Receivable” in the accompanying Consolidated Balance Sheets at March 31, 2015 and September 30, 2014, respectively.
	Minimum future lease payments receivable under direct financing leases over the next five fiscal years and thereafter are as follows:
	Minimum Payments Receivable for Direct Financing Leases
	(In millions)
	Remainder of 2015	$	2.8
	2016	2.2
	2017	2.1
	2018	2
	2019	1.9
	Thereafter	15.9
	Total	$	26.9
	Minimum payments receivable exclude $6.2 million of residual values and $2.4 million in tax credits. Associated with these investments, WGSW holds $12.0 million of unearned income on its balance sheet. The initial direct costs (incurred in FY 2012) associated with these investments was $0.7 million.
	Our maximum financial exposure from solar agreements is limited to WGSW's lease payment receivables and investment contributions made to these companies. All additional future committed contributions are contingent on the projects meeting required criteria. Our exposure is offset by the owned physical assets received as part of the transaction and the quick economic return for the investment through the investment tax credit/treasury grant proceeds and accelerated depreciation.
	ASD
	In addition to SunEdison and Nextility, WGSW is also a limited partner in ASD, a partnership formed to own and operate a portfolio of residential solar projects, primarily rooftop photovoltaic power generation systems. As a limited partner, WGSW provided funding to the partnership but is excluded from involvement in the partnership’s operations. In January 2014, the funding commitment period expired for the partnership. WGSW’s maximum financial exposure includes contributions made to the partnership.
	Our investment in ASD is accounted for under the HLBV equity method of accounting; any profits and losses are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Consolidated Statement of Income and are added to or subtracted from the carrying amount of WGSW’s investment balance. WGSW held a $65.2 million equity method investment in ASD at March 31, 2015.
	ASD is consolidated by the general partner, Solar Direct LLC. Solar Direct LLC is a wholly owned subsidiary of American Solar Direct Inc. (ASDI). At March 31, 2015, the carrying amount of WGSW’s investment in ASD exceeded the amount of the underlying equity in net assets by $35.9 million due to WGSW recording additions to its investment in ASD’s net assets at fair value of contributions in accordance with GAAP. This basis difference is being amortized over the life of the assets.
	Non-VIE Investments
	ASDHI
	At September 30, 2014, Washington Gas Resources held a $5.6 million investment in American Solar Direct Holdings Inc. (ASDHI) consisting of warrants and preferred stock. During the six months ended March 31, 2015, Washington Gas Resources impaired its entire investment in ASDHI by its carrying value of $5.6 million based on management’s assumption of the current valuation and expected return from the investment.
	Constitution
	In 2013, WGL Midstream invested in Constitution Pipeline Company, LLC (Constitution). WGL Midstream will invest an estimated $79.0 million in the project for a 10% share in the pipeline venture over the term of the agreement. WGL Midstream joins Williams Partners L.P. (41% share), Cabot Oil and Gas Corporation (25% share) and Piedmont Natural Gas (24% share) in the project. This natural gas pipeline venture will transport natural gas from the Marcellus region in northern Pennsylvania to major northeastern markets. At March 31, 2015, WGL Midstream had invested $27.6 million in Constitution. Constitution is accounted for under the equity method of accounting; any profits and losses are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Consolidated Statement of Income and are added to or subtracted from the carrying amount of WGL’s investment balance. The equity method is considered appropriate because Constitution is an LLC with specific ownership accounts and ownership between five and fifty percent resulting in WGL Midstream maintaining a more than minor influence over the partnership operating and financing policies.
	Mountain Valley Pipeline
	In March 2015, WGL Midstream acquired a 7% equity interest in Mountain Valley Pipeline, LLC (Mountain Valley).
	The proposed pipeline to be developed, constructed, owned and operated by Mountain Valley, will transport approximately 2.0 million dekatherms of natural gas per day from a new interconnect with EQT Corporation's Equitrans system, near the MarkWest Mobley plant in Smithfield, West Virginia to Transcontinental Gas Pipeline Company LLC's Station 165 in Pittsylvania County, Virginia. The pipeline is scheduled to be in service by December 2018.
	The total project investment is anticipated to be between $3.0-$3.5 billion. WGL Midstream will invest, in scheduled capital contributions through the in-service date of the pipeline, its pro rata share (based on its 7% equity interest) of project costs, which is estimated to be $230 million. In addition, WGL Midstream held a $5.8 million equity method investment in the Mountain Valley at March 31, 2015.
	Our maximum financial exposure includes guarantees on behalf of WGL Midstream and another partner in the venture. WGL's maximum exposure to loss due to the provided guarantees was $20.0 million at March 31, 2015 which represents a $14.0 million minimum funding requirement for WGL Midstream and a $6.0 million minimum funding requirement for another partner in the venture.
	In addition, WGL Midstream entered into an agreement to finance future capital commitments of one of the other partners in the venture for up to approximately $96.0 million, which represents the estimated remaining funding requirements for that partner's 3% ownership interest in the joint venture, inclusive of the minimum funding requirement. WGL Midstream has provided funding of $2.5 million as of March 31, 2015 related to this agreement.
	The balance sheet location of the investments discussed in this footnote at March 31, 2015 and September 30, 2014 are as follows:
	WGL Holdings, Inc.
	Balance Sheet Location of Other Investments
	As of March 31, 2015 (in millions)	VIEs		Non-VIEs			Total
	Assets
	Investments in unconsolidated affiliates	$	81.2	$	33.5		$	114.7
	Investments in direct financing leases, capital leases	20.2		—			20.2
	Accounts receivable	3.3		2.5		(a)	5.8
	Total assets	$	104.7	$	36		$	140.7
	As of September 30, 2014 (in millions)
	Assets
	Investments in unconsolidated affiliates	$	72.6	$	27.9		$	100.5
	Investments in direct financing leases, capital leases	18.2		—			18.2
	Accounts receivable	1.7		—			1.7
	Total assets	$	92.5	$	27.9		$	120.4
(a) Represents the financing provided to another partner in Mountain Valley to fund its capital commitment. Acquired ownership interest represents the collateral for repayment of the financing.
The income statement location of the investments discussed in this footnote for the three and six months ended March 31, 2015 and 2014 are as follows:
WGL Holdings, Inc.
Income Statement Location of Other Investments
(In millions)	VIEs		Non-VIEs			Total		VIEs		Non-VIEs			Total
	Three Months Ended							Six Months Ended
	31-Mar-15							31-Mar-15
Equity in earnings of unconsolidated affiliates	$	1	$	0.8		$	1.8	$	1.7	$	1.3		$	3
Depreciation and amortization	—		—			—		0.1		—			0.1
Other income (expenses)—net	0.8		—			0.8		1.4		(5.6		)	(4.2		)
Net income (loss)	$	1.8	$	0.8		$	2.6	$	3	$	(4.3	)	$	(1.3	)
	Three Months Ended							Six Months Ended
	31-Mar-14							31-Mar-14
Equity in earnings of unconsolidated affiliates	$	0.5	$	—		$	0.5	$	0.8	$	0.2		$	1
Depreciation and amortization	—		—			—		0.1		—			0.1
Other income—net	0.8		—			0.8		1.3		—			1.3
Net income	$	1.3	$	—		$	1.3	$	2	$	0.2		$	2.2

Related_Party_Transactions

Related Party Transactions	6 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Related Party Transactions
	NOTE 12. RELATED PARTY TRANSACTIONS
	WGL and its subsidiaries engage in transactions during the ordinary course of business. Inter-company transactions and balances have been eliminated from the consolidated financial statements of WGL, except as described below. Washington Gas provides accounting, treasury, legal and other administrative and general support to affiliates, and files consolidated tax returns that include affiliated taxable transactions. Washington Gas bills its affiliates in accordance with regulatory requirements for the actual cost of providing these services, which approximates their market value. To the extent such billings are not paid, they are reflected in “Receivables from associated companies” on Washington Gas’ balance sheets. Washington Gas assigns or allocates these costs directly to its affiliates and, therefore, does not recognize revenues or expenses associated with providing these services.
	In connection with billing for unregulated third party marketers and with other miscellaneous billing processes, Washington Gas collects cash on behalf of affiliates and transfers the cash in a reasonable time period. Cash collected by Washington Gas on behalf of its affiliates but not transferred is recorded in “Payables to associated companies” on Washington Gas’ balance sheets. The following table presents the receivables and payables from associated companies as of March 31, 2015 and September 30, 2014.
	Washington Gas Light Company Receivables / Payables from Associated Companies
	(In millions)	March 31, 2015		30-Sep-14
	Receivables from Associated Companies	$	3.4	$	4.8
	Payables to Associated Companies	$	68.9	$	54.7
	Washington Gas provides gas balancing services related to storage, injections, withdrawals and deliveries to all energy marketers participating in the sale of natural gas on an unregulated basis through the customer choice programs that operate in its service territory. These balancing services include the sale of natural gas supply commodities related to various peaking arrangements contractually supplied to Washington Gas and then partially allocated and assigned by Washington Gas to the energy marketers, including WGL Energy Services. Washington Gas records revenues for these balancing services pursuant to tariffs approved by the appropriate regulatory bodies. These related party amounts have been eliminated in the consolidated financial statements of WGL. The following table shows the amounts Washington Gas charged WGL Energy Services for balance services.
	Washington Gas Light Company-Gas Balancing Service Charges
		Three Months Ended March 31,				Six Months Ended March 31,
	(In millions)	2015		2014		2015		2014
	Gas balancing service charge	$	9.2	$	14.4	$	14.7	$	18.2
	As a result of these balancing services, an imbalance is created for volumes of natural gas received by Washington Gas that are not equal to the volumes of natural gas delivered to customers of the energy marketers. WGL Energy Services recognized an accounts payable to Washington Gas in the amount of $9.5 million and an accounts receivable from Washington Gas in the amount of $0.02 million at March 31, 2015 and September 30, 2014, respectively, related to an imbalance in gas volumes. Due to regulatory treatment, these payables and receivables are not eliminated in the consolidated financial statements of WGL. Refer to Note 1—Accounting Policies of the Notes to Consolidated Financial Statements of the combined Annual Report on Form 10-K for the fiscal year ended September 30, 2014 for further discussion of these imbalance transactions.
	Washington Gas participates in a Purchase of Receivables (POR) program as approved by the PSC of MD, whereby it purchases receivables from participating energy marketers at approved discount rates. In addition, WGL Energy Services participates in POR programs with certain Maryland and Pennsylvania utilities, whereby it sells its receivables to various utilities, including Washington Gas, at approved discount rates. The receivables purchased by Washington Gas are included in “Accounts receivable” in the accompanying balance sheet. Any activity between Washington Gas and WGL Energy Services related to the POR program has been eliminated in the accompanying financial statements for WGL. At March 31, 2015 and September 30, 2014, Washington Gas had balances of $38.3 million and $7.7 million, respectively, of purchased receivables from WGL Energy Services.

Commitments_and_Contingencies

Commitments and Contingencies	6 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies
	NOTE 13. COMMITMENTS AND CONTINGENCIES
	REGULATED UTILITY OPERATIONS
	The following table summarizes the minimum contractual payments that Washington Gas will make under its pipeline transportation, storage and peaking contracts, as well as minimum contractual payments to purchase natural gas.
	Washington Gas Contract Minimums
	(In millions)	Pipeline		Gas Purchase
		Contracts(a)		Commitments(b)
	Remainder of 2015	$	92.4	$	93.2
	2016	218.7		365.6
	2017	210.5		463.5
	2018	211.3		466.1
	2019	202.7		459.4
	Thereafter	1,172.60		5,679.00
	Total	$	2,108.20	$	7,526.80
	(a)	Represents minimum payments for natural gas transportation, storage and peaking contracts that have expiration dates through fiscal year 2031.
	(b)	Includes known and reasonably likely commitments to purchase natural gas. Cost estimates are based on market prices at March 31, 2015.
	REGULATORY CONTINGENCIES
	Certain legal and administrative proceedings incidental to our business, including regulatory contingencies, involve WGL and/or its subsidiaries. In our opinion, we have recorded an adequate provision for probable losses or refunds to customers for regulatory contingencies related to these proceedings.
	NON-UTILITY OPERATIONS
	On November 30, 2014, WGL through its subsidiary, WGL Midstream, entered into a gas sale and purchase, and capacity agreement with GAIL Global (USA) LNG LLC, a subsidiary of GAIL (India) Limited, under which WGL Midstream has agreed to sell and deliver a minimum of 340,000 dekatherms per day and up to 430,000 dekatherms per day of natural gas, for a term of 20 years from the in-service date of the export facility. The contract price is based on index pricing, but certain gas sales will occur at a market price per dekatherm of gas based on the estimate of prices prevailing at designated delivery points. WGL Midstream will make deliveries using transportation capacity released by GAIL through an asset management arrangement. On March 10, 2015, WGL Midstream entered into gas purchase commitments to buy 500,000 dekatherms of natural gas per day at index based prices for a 20-year term. We will also be a shipper on the Mountain Valley Pipeline. These commitments were executed in conjunction with the investment in Mountain Valley.
	As a result of these agreements, purchase commitments for WGL Midstream have increased as reflected in the table below. The following table summarizes the minimum commitments and contractual obligations of WGL Energy Services and WGL Midstream for the next five fiscal years and thereafter.
	Contract Minimums
		WGL Energy Services						WGL Midstream
	(In millions)	Gas Purchase		Pipeline		Electric		Gas Purchase		Pipeline		Total
		Commitments(a)		Contracts(b)		Purchase		Commitments(d)		Contracts(b)
						Commitments(c)
	Remainder of 2015	$	75.9	$	1.6	$	255	$	27.3	$	13.1	$	372.9
	2016	120.7		1.1		360.6		246.8		20.3		749.5
	2017	49.1		0.8		131.3		374.8		18.4		574.4
	2018	1.4		0.8		17.4		1,219.20		15.2		1,254.00
	2019	—		0.8		0.2		1,757.90		60.9		1,819.80
	Thereafter	—		1.6		—		36,273.80		1,097.80		37,373.20
	Total	$	247.1	$	6.7	$	764.5	$	39,899.80	$	1,225.70	$	42,143.80
	(a)	Represents fixed price commitments with city gate equivalent deliveries.
	(b)	Represents minimum payments for natural gas transportation and storage contracts that have expiration dates through fiscal year 2044.
	(c)	Represents electric purchase commitments that are based on existing fixed price and fixed volume contracts. Includes $19.1 million of commitments related to renewable energy credits.
	(d)	Includes known and reasonably likely commitments to purchase natural gas. Cost estimates are based on forward market prices as of March 31, 2015.
	FINANCIAL GUARANTEES
	WGL has guaranteed payments primarily for certain purchases of natural gas and electricity on behalf of WGL Energy Services and for certain purchase commitments on behalf of WGL Midstream. At March 31, 2015, these guarantees totaled $239.4 million and $305.9 million for WGL Energy Services and WGL Midstream, respectively. At March 31, 2015, WGL also had guarantees on behalf of other subsidiaries totaling $8.6 million. The amount of such guarantees is periodically adjusted to reflect changes in the level of financial exposure related to these commitments. For all of our financial guarantees, WGL may cancel any or all future obligations upon written notice to the counterparty, but WGL would continue to be responsible for the obligations created under the guarantees prior to the effective date of the cancellation. WGL has also guaranteed payments for certain of our external partners. At March 31, 2015, these guarantees totaled $8.1 million and the fair value of these guarantees was insignificant.

Pension_and_Other_PostRetireme

Pension and Other Post-Retirement Benefit Plans	6 Months Ended
	Mar. 31, 2015
Pension and Other Postretirement Benefit Expense [Abstract]
Pension and Other Post-Retirement Benefit Plans
	NOTE 14. PENSION AND OTHER POST-RETIREMENT BENEFIT PLANS
	The following table shows the components of net periodic benefit costs (income) recognized in our financial statements during the three and six months ended March 31, 2015 and 2014.
	Components of Net Periodic Benefit Costs (Income)
		Three Months Ended March 31,
		2015						2014
	(In millions)	Pension			Health and			Pension			Health and
		Benefits			Life Benefits			Benefits			Life Benefits
	Components of Net Periodic Benefit Costs (Income)
	Service cost	$	3.8		$	1.8		$	3.5		$	2.2
	Interest cost	9.8			3.6			10.1			5.6
	Expected return on plan assets	(11.1		)	(5.2		)	(10.2		)	(4.8		)
	Amortization of prior service cost (credit)	—			(3.8		)	—			(1.0		)
	Amortization of actuarial loss	4.7			1.1			4.2			1.2
	Net periodic benefit cost (income)	7.2			(2.5		)	7.6			3.2
	Amount allocated to construction projects	(1.1		)	0.5			(1.0		)	(0.4		)
	Amortization of regulatory asset/liability—net	1.7			—			1.8			0.1
	Amount charged (credited) to expense	$	7.8		$	(2.0	)	$	8.4		$	2.9
		Six Months Ended March 31,
		2015						2014
	Components of Net Periodic Benefit Costs (Income)
	Service cost	$	7.7		$	3.5		$	7		$	4.3
	Interest cost	19.6			7.3			20.2			11
	Expected return on plan assets	(22.3		)	(10.4		)	(20.5		)	(9.5		)
	Amortization of prior service cost (credit)	0.1			(7.6		)	0.1			(2.0		)
	Amortization of actuarial loss	9.4			2.2			8.4			1.9
	Net periodic benefit cost (income)	14.5			(5.0		)	15.2			5.7
	Amount allocated to construction projects	(2.2		)	1			(2.0		)	(0.8		)
	Amortization of regulatory asset/liability—net	3.5			(0.1		)	3.6			0.2
	Amount charged (credited) to expense	$	15.8		$	(4.1	)	$	16.8		$	5.1
	Amounts included in the line item “Amortization of regulatory asset/liability—net,” as shown in the table above, represent the amortization of previously unrecovered costs of the applicable pension benefits or the health and life benefits as approved in the District of Columbia. These balances are being amortized over a five year period.

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income	6 Months Ended
	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated Other Comprehensive Income
	NOTE 15. ACCUMULATED OTHER COMPREHENSIVE INCOME
	The following tables show the changes in accumulated other comprehensive income for WGL and Washington Gas by component for the three and six months ended March 31, 2015 and 2014.
	WGL Holdings, Inc.
	Changes in Accumulated Other Comprehensive Income by Component
		Three Months Ended March 31,						Six Months Ended March 31,
		2015			2014			2015			2014
	(In thousands)
	Beginning Balance	$	(11,966	)	$	(10,849	)	$	(7,961	)	$	(11,048	)
	Qualified cash flow hedging instruments(a)	222			—			(8,042		)	—
	Change in prior service credit (b)	(171		)	(35		)	(342		)	(69		)
	Amortization of actuarial loss (gain) (b)	491			(164		)	974			200
	Current-period other comprehensive income (loss)	542			(199		)	(7,410		)	131
	Income tax expense (benefit) related to other comprehensive income	219			(79		)	(3,728		)	52
	Ending Balance	$	(11,643	)	$	(10,969	)	$	(11,643	)	$	(10,969	)
	(a)	Cash flow hedging instruments represent interest rate swaps agreements on debt issuances. Refer to Note 8- Derivative and weather-related instruments for further discussion of the interest rate swap agreements.
	(b)	These accumulated other comprehensive income components are included in the computation of net periodic benefit cost. Refer to Note 14- Pension and other post-retirement benefit plans for additional details.
	Washington Gas Light Company
	Changes in Accumulated Other Comprehensive Income by Component
		Three Months Ended March 31,						Six Months Ended March 31,
		2015			2014			2015			2014
	(In thousands)
	Beginning Balance	$	(6,224	)	$	(10,849	)	$	(6,413	)	$	(11,048	)
	Change in prior service credit (a)	(171		)	(35		)	(342		)	(69		)
	Amortization of actuarial loss (gain) (a)	491			(164		)	974			200
	Current-period other comprehensive income (loss)	320			(199		)	632			131
	Income tax expense (benefit) related to other comprehensive income	127			(79		)	250			52
	Ending Balance	$	(6,031	)	$	(10,969	)	$	(6,031	)	$	(10,969	)
	(a)	These accumulated other comprehensive income components are included in the computation of net periodic benefit cost. Refer to Note 14-Pension and other post-retirement benefit plans for additional details.

Subsequent_Events

Subsequent Events	6 Months Ended
	Mar. 31, 2015
Subsequent Events [Abstract]
Subsequent Events
	NOTE 16. SUBSEQUENT EVENTS
	In the 2014 Session, the General Assembly of the Commonwealth of Virginia amended Title 56 of the Virginia Code. The legislative provisions are intended to encourage regulated utilities to invest in natural gas reserves, upstream pipelines and facilities that are reasonably expected to benefit customers by lowering costs, reducing volatility or lowering the utility’s supply risk. A regulated utility company can obtain the recovery through its rates charged to customers for the entire incurred cost, including the return of and the return on the investment in reserves, as well as all operating costs.
	Pursuant to the legislation, on May 6, 2015, WGL entered into a 20-year agreement with Energy Corporation of America (ECA) to acquire natural gas reserves through non-operating working interests in 25 producing wells located in Pennsylvania's Appalachian Basin for $126 million.
	The purchase of the reserves is conditional upon approval by the Commonwealth of Virginia's State Corporation Commission (SCC of VA). WGL will submit a filing to the SCC of VA for approval of the investment, with a decision due within 180 days from filing.

Accounting_Policies_Policies

Accounting Policies (Policies)	6 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation
	WGL Holdings, Inc. (WGL) is a holding company that owns all of the shares of common stock of Washington Gas Light Company (Washington Gas), a regulated natural gas utility, all of the shares of common stock of Washington Gas Resources Corporation (Washington Gas Resources) and Hampshire Gas Company (Hampshire). Washington Gas Resources owns all of the shares of common stock of four non-utility subsidiaries that include WGL Energy Services, Inc. (WGL Energy Services), WGL Energy Systems, Inc. (WGL Energy Systems), WGL Midstream, Inc. (WGL Midstream) and WGSW, Inc. (WGSW). Except where the content clearly indicates otherwise, “WGL,” “we,” “us” or “our” refers to the holding company or the consolidated entity of WGL Holdings, Inc. and all of its subsidiaries. Unless otherwise noted, these notes apply equally to WGL and Washington Gas.
	The condensed consolidated financial statements have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (SEC). Therefore, certain financial information and note disclosures accompanying annual financial statements prepared in accordance with generally accepted accounting principles in the United States of America (GAAP) are omitted in this interim report. The interim consolidated financial statements and accompanying notes should be read in conjunction with the combined Annual Report on Form 10-K for WGL and Washington Gas for the fiscal year ended September 30, 2014. Due to the seasonal nature of our businesses, the results of operations for the periods presented in this report are not necessarily indicative of actual results for the full fiscal years ending September 30, 2015 and 2014 of either WGL or Washington Gas.
	The accompanying unaudited financial statements for WGL and Washington Gas reflect all normal recurring adjustments that are necessary, in our opinion, to present fairly the results of operations in accordance with GAAP.
	For a complete description of our accounting policies, refer to Note 1 of the Notes to Consolidated Financial Statements of the combined Annual Report on Form 10-K for WGL and Washington Gas for the fiscal year ended September 30, 2014.
Storage Gas Valuation Methods	Storage Gas Valuations
	For Washington Gas and WGL Energy Services, storage gas inventory is stated at the lower-of-cost or market as determined using the first-in, first-out method. For WGL Midstream, storage gas inventory is stated at the lower-of-cost or market using the weighted average cost method. Interim period inventory losses attributable to lower-of-cost or market adjustments may be reversed if the market value of the inventory is recovered by the end of the same fiscal year.
Accounting Standards Adopted Current Period	ACCOUNTING STANDARDS ADOPTED IN FISCAL YEAR 2015
	ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (Topic 740)
	The standard requires an unrecognized tax benefit, or a portion of an unrecognized tax benefit to be presented in the financial statements as a reduction to a deferred tax asset for net operating loss carryforwards, a similar loss, or a tax
	credit carryforward.
	1-Oct-14
	As a result of the implementation of this standard, we reduced our deferred tax assets by a portion of our unrecognized tax benefits. The adoption of this standard did not have a material effect on our financial statements.
New Accounting Pronouncements	OTHER NEWLY ISSUED ACCOUNTING STANDARDS
	ASU 2015-03, Simplifying the Presentation of Debt Issuance Cost (Topic 835)
	The standard requires an entity to present debt issuance costs in the balance sheet as a direct deduction of the debt liability in a manner consistent with its accounting treatment of debt discounts.
	1-Oct-16
	We are in the process of evaluating the impact the adoption of this standard will have on our financial statements.
	ASU 2014-09, Revenue from Contracts with Customers (Topic 606)
	The standard establishes a comprehensive revenue recognition model clarifying the method used to determine the timing and requirements for revenue recognition from contracts with customers. The disclosure requirements under the new standard will enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers.
	1-Oct-17
	We are in the process of evaluating the impact the adoption of this standard will have on our financial statements.
Income Tax Policy	Under ASC Topic 740, Income Taxes, Washington Gas recognizes any accrued interest associated with uncertain tax positions in interest expense and recognizes any accrued penalties associated with uncertain tax positions in other expenses in the statements of income.
Derivatives, Reporting Of Derivative Activity	Asset Optimization. Washington Gas optimizes the value of its long-term natural gas transportation and storage capacity resources during periods when these resources are not being used to physically serve utility customers. Specifically, Washington Gas utilizes its transportation capacity assets to benefit from favorable natural gas prices between different geographic locations and utilizes its storage capacity assets to benefit from favorable natural gas prices between different time periods. As part of this asset optimization program, Washington Gas enters into physical and financial derivative transactions in the form of forward, futures and option contracts with the primary objective of locking in operating margins that Washington Gas will ultimately realize. The derivatives used under this program are subject to mark-to-market accounting treatment while the capacity and transportation resources are not.
	Regulatory sharing mechanisms allow the profit from these transactions to be shared between Washington Gas’ shareholders and customers; therefore, any changes in fair value are recorded through earnings, or as regulatory assets or liabilities to the extent that gains and losses associated with these derivative instruments will be included in the rates charged to customers when they are realized. Valuation changes for the portion of net profits to be retained for shareholders may cause significant period-to-period volatility in earnings from unrealized gains and losses. This volatility does not change the locked-in operating margins that Washington Gas expects to ultimately realize from these transactions through the use of its storage and transportation capacity resources.
	All physically and financially settled contracts under our asset optimization program are reported on a net basis in the statements of income in “Utility cost of gas.”
	ollateral
	WGL utilizes standardized master netting agreements, which facilitate the netting of cash flows into a single net exposure for a given counterparty. As part of these master netting agreements, cash, letters of credit and parental guarantees may be required to be posted or obtained from counterparties in order to mitigate credit risk related to both derivatives and non-derivative positions. Under WGL’s offsetting policy, collateral balances are offset against the related counterparties’ derivative positions to the extent the application would not result in the over-collateralization of those derivative positions on the balance sheet.
	Regulated Utility Operations
	Washington Gas enters into contracts that qualify as derivative instruments and are accounted for under ASC Topic 815. These derivative instruments are recorded at fair value on our balance sheet and Washington Gas does not designate any derivatives as hedges under ASC Topic 815.
	Any collateral posted that is not offset against open and settled derivative contracts is included in “Other prepayments” in the accompanying balance sheet. Collateral received and not offset against open and settled derivative contracts is included in “Customer deposits and advance payments” in the accompanying balance sheet.
	Managing Price Risk. To manage price risk associated with acquiring natural gas supply for utility customers, Washington Gas enters into forward contracts, option contracts, financial contracts and other contracts, as authorized by its regulators. These instruments are accounted for as derivative instruments. Any gains and losses associated with these derivatives are recorded as regulatory liabilities or assets, respectively, to reflect the rate treatment for these economic hedging activities.
	Managing Interest-Rate Risk. Washington Gas utilizes derivative instruments that are designed to minimize the risk of interest-rate volatility associated with planned issuances of debt securities. Any gains and losses associated with these types of derivatives are recorded as regulatory liabilities or assets, respectively, and amortized in accordance with regulatory requirements, typically over the life of the newly issued debt.
	Non-Utility Operations
	Managing Price Risk. WGL Energy Services enters into certain derivative contracts as part of managing the price risk associated with the sale and purchase of natural gas and electricity. WGL Midstream enters into derivative contracts for the purpose of optimizing its storage and transportation capacity as well as managing the transportation and storage assets on behalf of third parties. Derivative instruments are recorded at fair value on our consolidated balance sheets. WGL Energy Services and WGL Midstream do not designate these derivatives as hedges under ASC Topic 815; therefore, changes in the fair value of these derivative instruments are reflected in the earnings of our non-utility operations and may cause significant period-to-period volatility in earnings. The storage and transportation capacities utilized by WGL Midstream are not considered to be derivative instruments, and thus they are not recorded at fair value on our consolidated balance sheets.
	Managing Interest-Rate Risk. WGL utilizes derivative instruments that are designed to minimize the risk of interest-rate volatility associated with future debt issuances. WGL elected cash flow hedge accounting for its interest rate derivative instruments, which settled with the issuance of the related debt issuance in the first quarter of 2015. The effective portion of the gains and losses on the hedge were recorded within other comprehensive income and are being amortized over the life of the debt (through 2044).
Fair Value Measurement Policy	Our money market funds are Level 1 valuations and their carrying amount approximates fair value. Other short-term investments are primarily overnight investment accounts; therefore, their carrying amount approximates fair value based on Level 2 inputs. The maturity of our commercial paper outstanding at both March 31, 2015 and September 30, 2014 is under 30 days. Due to the short term nature of these notes, the carrying cost of our commercial paper approximates fair value using Level 2 inputs. Neither WGL’s nor Washington Gas’ long-term debt is actively traded. The fair value of long-term debt was estimated based on the quoted market prices of the U.S. Treasury issues having a similar term to maturity, adjusted for the credit quality of the debt issuer, WGL or Washington Gas. Our long-term debt fair value measurement is classified as Level 3.
	Non Recurring Basis
	During the six months ended March 31, 2015, Washington Gas Resources recorded an impairment charge of its investment in ASDHI to its fair value using the income approach. The amount of the impairment was equivalent to the amount of the carrying value of $5.6 million and was due to management’s assumption of the current valuation and expected return from the investment. The fair value of this investment was a Level 3 measurement.
	We measure the fair value of our financial assets and liabilities using a combination of the income and market approach in accordance with ASC Topic 820. These financial assets and liabilities primarily consist of derivatives recorded on our balance sheet under ASC Topic 815 and short-term investments, commercial paper and long-term debt outstanding required to be disclosed at fair value. Under ASC Topic 820, fair value is defined as the exit price, representing the amount that would be received in the sale of an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. To value our financial instruments, we use market data or assumptions that market participants would use, including assumptions about credit risk (both our own credit risk and the counterparty’s credit risk) and the risks inherent in the inputs to valuation.
	We enter into derivative contracts in the futures and over-the-counter (OTC) wholesale and retail markets. These markets are the principal markets for the respective wholesale and retail contracts. Our relevant market participants are our existing counterparties and others who have participated in energy transactions at our delivery points. These participants have access to the same market data as WGL. We value our derivative contracts based on an “in-exchange” premise, and valuations are generally based on pricing service data or indicative broker quotes depending on the market location. We measure the net credit exposure at the counterparty level where the right to set-off exists. The net exposure is determined using the mark-to-market exposure adjusted for collateral, letters of credit and parent guarantees. We use published default rates from Standard & Poor’s Ratings Services and Moody’s Investors Service as inputs for determining credit adjustments.
Segment Reporting Policy	We have four reportable operating segments: regulated utility, retail energy-marketing, commercial energy systems and midstream energy services. The division of these segments into separate revenue generating components is based upon regulation, products and services. Our chief operating decision maker is our Chief Executive Officer. During the first quarter of 2015, our chief operating decision maker began evaluating segment performance based on Earnings Before Interest and Taxes (EBIT). EBIT is defined as earnings before interest and taxes from continuing operations. Items we do not include in EBIT are interest expense, intercompany financing activity, dividends on Washington Gas preferred stock, and income taxes. EBIT includes transactions between reportable segments. Additionally, we also evaluate our operating segments based on other relevant factors, such as penetration into their respective markets and return on equity.
Investment Policy	Meade is accounted for under the HLBV equity method of accounting, and any profits and losses are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Consolidated Statement of Income and are added to or subtracted from the carrying amount of WGL’s investment balance.
	Constitution is accounted for under the equity method of accounting; any profits and losses are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Consolidated Statement of Income and are added to or subtracted from the carrying amount of WGL’s investment balance. The equity method is considered appropriate because Constitution is an LLC with specific ownership accounts and ownership between five and fifty percent resulting in WGL Midstream maintaining a more than minor influence over the partnership operating and financing policies.
	Our investment in ASD is accounted for under the HLBV equity method of accounting; any profits and losses are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Consolidated Statement of Income and are added to or subtracted from the carrying amount of WGSW’s investment balance.
	When determining how to account for our interests in other legal entities, WGL first evaluates if we are required to apply the variable interest entity (VIE) model to the entity, otherwise the entity is evaluated under the voting interest model.
	Under the VIE model, we have a controlling financial interest in a VIE (i.e. are the primary beneficiary) when we have current or potential rights that give us the power to direct the activities of a VIE that most significantly impact the VIE’s economic performance combined with a variable interest that gives us the right to receive potentially significant benefits or the obligation to absorb potentially significant losses. When changes occur to the design of an entity, we reconsider whether it is subject to the VIE model. We continuously evaluate whether we have a controlling financial interest in a VIE.
	Under the voting interest model, we generally have a controlling financial interest in an entity where we currently hold, directly or indirectly, more than 50% of the voting rights or where we exercise control through substantive participating rights. However, we consider substantive rights held by other partners in determining if we hold a controlling financial interest, and in some cases, despite owning more than 50% of the common stock of an investee, an evaluation of our rights results in the determination that we do not have a controlling financial interest. We reevaluate whether we have a controlling financial interest in these entities when our voting or substantive participating rights change.
	Unconsolidated affiliates are unconsolidated VIEs and other entities evaluated under the voting interest method in which we do not have a controlling financial interest, but over which we have varying degrees of influence. Where we have significant influence, the affiliates are accounted for as equity method investments. Where we do not have significant influence, the affiliates are accounted for under the cost method. Investments in, and advances to, affiliated companies are presented on a one-line basis in the caption “Investments in unconsolidated affiliates” on our Consolidated Balance Sheet.
	WGL uses the Hypothetical Liquidation at Book Value (HLBV) methodology for certain equity method investments when the capital structure of the equity investment results in different liquidation rights and priorities than what is reflected by the underlying percentage ownership interests as defined by an equity investment agreement. For investments accounted for under the HLBV method, simply applying the percentage ownership interest to GAAP net income in order to determine earnings or losses does not accurately represent the income allocation and cash flow distributions that will ultimately be received by the investors. The equity investment agreements for ASD Solar, LP (ASD), Meade Pipeline Co LLC (Meade) and Mountain Valley Pipeline, LLC (Mountain Valley) have liquidation rights and priorities that are sufficiently different from the ownership percentages that the HLBV method was deemed appropriate. The calculation may vary in its complexity depending on the capital structure and the tax considerations for the investments.
	WGL applies HLBV using a balance sheet approach. A calculation is prepared at each balance sheet date to determine the amount that WGL would receive if an equity investment entity were to liquidate all of its assets (as valued in accordance with GAAP) and distribute that cash to the investors based on the contractually defined liquidation priorities. The difference between the calculated liquidation distribution amounts at the beginning and the end of the reporting period is WGL’s share of the earnings or losses from the equity investment for the period.
	Our agreements with SunEdison and Nextility are accounted for as direct financing leases. WGSW records associated interest in the financing leases in “Other income (expenses)-net” line in the accompanying Consolidated Statement of Income.

Accounting_Policies_Tables

Accounting Policies (Tables)	6 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Schedule of Accounting Policies
	ACCOUNTING STANDARDS ADOPTED IN FISCAL YEAR 2015
	Standard	Description	Date of adoption	Effect on the financial statements or other significant matters
	ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (Topic 740)	The standard requires an unrecognized tax benefit, or a portion of an unrecognized tax benefit to be presented in the financial statements as a reduction to a deferred tax asset for net operating loss carryforwards, a similar loss, or a tax credit carryforward.	1-Oct-14	As a result of the implementation of this standard, we reduced our deferred tax assets by a portion of our unrecognized tax benefits. The adoption of this standard did not have a material effect on our financial statements.
	OTHER NEWLY ISSUED ACCOUNTING STANDARDS
	Standard	Description	Date of adoption	Effect on the financial statements or other significant matters
	ASU 2015-03, Simplifying the Presentation of Debt Issuance Cost (Topic 835)	The standard requires an entity to present debt issuance costs in the balance sheet as a direct deduction of the debt liability in a manner consistent with its accounting treatment of debt discounts.	1-Oct-16	We are in the process of evaluating the impact the adoption of this standard will have on our financial statements.
	ASU 2014-09, Revenue from Contracts with Customers (Topic 606)	The standard establishes a comprehensive revenue recognition model clarifying the method used to determine the timing and requirements for revenue recognition from contracts with customers. The disclosure requirements under the new standard will enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers.	1-Oct-17	We are in the process of evaluating the impact the adoption of this standard will have on our financial statements.

Accounts_Payable_and_Other_Acc1

Accounts Payable and Other Accrued Liabilities (Tables)	6 Months Ended
	Mar. 31, 2015
WGL Holdings, Inc.
Accounts Payable and Other Accrued Liabilities [Line Items]
Accounts Payable and Other Accrued Liabilities	The tables below provide details for the amounts included in “Accounts payable and other accrued liabilities” on the balance sheets for both WGL and Washington Gas.
	WGL Holdings, Inc.
	(In millions)	31-Mar-15			30-Sep-14
	Accounts payable—trade	$	274.4		$	278.8
	Employee benefits and payroll accruals	21			19.8
	Other accrued liabilities	28.6			14.6
	Total	$	324		$	313.2
Washington Gas Light Company
Accounts Payable and Other Accrued Liabilities [Line Items]
Accounts Payable and Other Accrued Liabilities
	Washington Gas Light Company
	(In millions)	March 31, 2015		September 30, 2014
	Accounts payable—trade	126.4		146.4
	Employee benefits and payroll accruals	19.7		18.2
	Other accrued liabilities	20.8		11.9
	Total	166.9		176.5

ShortTerm_Debt_Tables

Short-Term Debt (Tables)	6 Months Ended
	Mar. 31, 2015
Short-term Debt [Abstract]
Commited Credit Available	The following is a summary of committed credit available at March 31, 2015 and September 30, 2014.
	Committed Credit Available (In millions)
	March 31, 2015	WGL(b)			Washington Gas			Total Consolidated
	Committed credit agreements
	Unsecured revolving credit facility, expires December 19, 2019(a)	$	450		$	350		$	800
	Less: Commercial Paper	(175.0		)	—			(175.0		)
	Net committed credit available	$	275		$	350		$	625
	Weighted average interest rate	0.26		%	—		%	0.26		%
	September 30, 2014
	Committed credit agreements
	Unsecured revolving credit facility, expires April 3, 2017(a)	$	450		$	350		$	800
	Less: Commercial Paper	(364.5		)	(89.0		)	(453.5		)
	Net committed credit available	$	85.5		$	261		$	346.5
	Weighted average interest rate	0.2		%	0.13		%	0.19		%
	(a)	Both WGL and Washington Gas have the right to request extensions with the banks’ approval. WGL’s revolving credit facility permits it to borrow an additional $100 million, with the banks’ approval, for a total of $550 million. Washington Gas’ revolving credit facility permits it to borrow an additional $100 million, with the banks’ approval, for a total of $450 million.
	(b)	WGL includes WGL Holdings and all subsidiaries other than Washington Gas

LongTerm_Debt_Table

Long-Term Debt (Table)	6 Months Ended
	Mar. 31, 2015
Long-term Debt, by Current and Noncurrent [Abstract]
Senior Notes, MTN And Private Placement Notes Outstanding	The following tables show the outstanding notes as of March 31, 2015 and September 30, 2014.
	Senior Notes, MTNs and Private Placement Notes Outstanding
	($ In millions)	WGL(a)			Washington Gas				Total Consolidated
	March 31, 2015
	Long-term notes(b)	$	250		$	741			$	991
	Weighted average interest rate	3.66		%	5.56		%		5.06		%
	September 30, 2014
	Long-term notes(b)	$	—		$	691			$	691
	Weighted average interest rate	n/a			5.65		%		5.65		%
	(a)	WGL includes WGL Holdings and all subsidiaries other than Washington Gas.
	(b)	Includes Senior Notes for WGL and both MTNs and private placement notes for Washington Gas. Represents face value including current maturities.
Senior Notes, MTN and Private Placement Issuances and Retirements	The following tables show senior notes, MTN and private placement issuances and retirements for the six months ended March 31, 2015 and 2014.
	Senior Notes, MTNs and Private Placement Issuances and Retirements
	($ In millions)	Principal(b)			Interest			Effective			Nominal
					Rate			Cost(c)			Maturity Date
	Six Months Ended March 31, 2015
	WGL(a)
	Issuances:
	10/24/14	$	100		2.25	%		2.42	%		11/1/19
	10/24/14	125			4.6	%		5.11	%		11/1/44
	12/16/14	25			4.6	%		5.53	%		11/1/44
	Total	$	250
	Washington Gas
	Issuances:
	12/15/14	$	50		4.24	%		4.41	%		12/15/44
	Total	$	50
	Total consolidated issuances	$	300
	Six Months Ended March 31, 2014
	Washington Gas
	Issuances:
	12/5/13	$	75		5	%		4.95	%		12/15/43
	Total	$	75
	Retirements:
	11/7/13	$	37		4.88	%		n/a			11/7/13
	Total	$	37
	(a)	WGL includes WGL Holdings and all subsidiaries other than Washington Gas.
	(b)	Represents face amount.
	(c)	The estimated effective cost of the issued notes, including consideration of issuance fees and hedge costs.

Common_Shareholders_Equity_Tab

Common Shareholders' Equity (Tables)	6 Months Ended
	Mar. 31, 2015
WGL Holdings, Inc.
Schedule of Capitalization, Equity [Line Items]
Components of Common Shareholders' Equity	The tables below reflect the components of “Common shareholders’ equity” for WGL and “Common shareholder’s equity” for Washington Gas for the six months ended March 31, 2015.
	WGL Holdings, Inc.
	Components of Common Shareholders’ Equity
	(In thousands)	Common Stock			Paid-In		Retained			Accumulated Other Comprehensive Loss, Net of Taxes			Total
		Amount			Capital		Earnings
	Balance at September 30, 2014	$	525,932		$	11,847	$	716,758		$	(7,961	)	$	1,246,576
	Net income	—			—		146,003			—			146,003
	Other comprehensive loss	—			—		—			(3,682		)	(3,682		)
	Repurchase of common stock	(41,485		)	—		—			—			(41,485		)
	Stock-based compensation	1,009			800		—			—			1,809
	Dividends declared:
	Common stock	—			—		(44,877		)	—			(44,877		)
	Preferred stock	—			—		(660		)	—			(660		)
	Balance at March 31, 2015	$	485,456		$	12,647	$	817,224		$	(11,643	)	$	1,303,684
Washington Gas Light Company
Schedule of Capitalization, Equity [Line Items]
Components of Common Shareholders' Equity
	Washington Gas Light Company
	Components of Common Shareholder’s Equity
	(In thousands)	Common Stock			Paid-In		Retained			Accumulated Other			Total
		Amount			Capital		Earnings			Comprehensive
										Loss, Net of Taxes
	Balance at September 30, 2014	$	46,479		$	480,620	$	529,480		$	(6,413	)	$	1,050,166
	Net income	—			—		139,645			—			139,645
	Other comprehensive income	—			—					382			382
	Stock-based compensation	—			715		—			—			715
	Dividends declared:
	Common stock	—			—		(39,160		)	—			(39,160		)
	Preferred stock	—			—		(660		)	—			(660		)
	Balance at March 31, 2015	$	46,479		$	481,335	$	629,305		$	(6,031	)	$	1,151,088

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	6 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Earnings per Share	The following table reflects the computation of our basic and diluted EPS for the three and six months ended March 31, 2015 and 2014.
	Basic and Diluted EPS
	(in thousands, except per share data)	Net Income		Shares	Per Share
		Applicable to			Amount
		Common Stock
	Three Months Ended March 31, 2015
	Basic EPS	$	81,455	49,720	$	1.64
	Stock-based compensation plans	—		263
	Diluted EPS	$	81,455	49,983	$	1.63
	Three Months Ended March 31, 2014
	Basic EPS	$	61,213	51,875	$	1.18
	Stock-based compensation plans	—		24
	Diluted EPS	$	61,213	51,899	$	1.18
	Six Months Ended March 31, 2015
	Basic EPS	$	145,343	49,851	$	2.92
	Stock-based compensation plans	—		204
	Diluted EPS	$	145,343	50,055	$	2.9
	Six Months Ended March 31, 2014
	Basic EPS	$	79,842	51,846	$	1.54
	Stock-based compensation plans	—		18
	Diluted EPS	$	79,842	51,864	$	1.54

Derivative_and_Weather_Related1

Derivative and Weather Related Instruments (Tables)	6 Months Ended
	Mar. 31, 2015
Derivative [Line Items]
Absolute Notional Amounts of Open Positions on Derivative Instruments	At March 31, 2015 and September 30, 2014, respectively, the absolute notional amounts of our derivatives were as follows:
	Absolute Notional Amounts
	of Open Positions on Derivative Instruments
	March 31, 2015	Notional Amounts
	Derivative transactions	WGL Holdings			Washington Gas
	Natural Gas (In millions of therms)
	Asset Optimization	21,936.90			14,409.40
	Retail sales	9.6			—
	Other risk-management activities	1,745.70			1,389.10
	Electricity (In millions of kWhs)
	Retail sales	4,126.90			—
	Other risk-management activities	15,842.90			—
	30-Sep-14
	Derivative transactions
	Natural Gas (In millions of therms)
	Asset Optimization	20,593.30			13,740.90
	Retail sales	44.7			—
	Other risk-management activities	1,641.30			1,398.20
	Electricity (In millions of kWhs)
	Retail sales	3,831.40			—
	Other risk-management activities	16,734.10			—
	Warrants (In millions of shares)	4.6			—
	Interest Rate Swaps (In millions of dollars)	150			—
Gains and (Losses) on Derivative Instruments	The following table presents all gains and losses associated with derivative instruments for the three and six months ended March 31, 2015 and 2014.
	Gains and Losses on Derivative Instruments
	(In millions)	WGL Holdings, Inc.							Washington Gas
									Light Company
	Three Months Ended March 31,	2015				2014			2015			2014
	Recorded to income
	Operating revenues—non-utility	$	(1.1	)		$	(35.8	)	$	—		$	—
	Utility cost of gas	(35.8		)		(96.4		)	(35.8		)	(96.4		)
	Non-utility cost of energy-related sales	10.5				27.6			—			—
	Other income-net	—				—			—			—
	Interest expense(a)	(0.1		)		—			—			—
	Recorded to regulatory assets
	Gas costs	(52.1		)		(133.7		)	(52.1		)	(133.7		)
	Other	—				1.2			—			1.2
	Recorded to other comprehensive income	0.2				—			—			—
	Total	$	(78.4	)		$	(237.1	)	$	(87.9	)	$	(228.9	)
	Six Months Ended March 31,	2015				2014			2015			2014
	Recorded to income
	Operating revenues—non-utility	$	74.3			$	(84.3	)	$	—		$	—
	Utility cost of gas	(10.0		)		(123.7		)	(10.0		)	(123.7		)
	Non-utility cost of energy-related sales	(39.1		)		43.9			—			—
	Other income-net	—				0.1			—			—
	Interest expense(b)	(0.5		)		—			—			—
	Recorded to regulatory assets
	Gas costs	(23.9		)		(212.0		)	(23.9		)	(212.0		)
	Other	—				1.2			—			1.2
	Recorded to other comprehensive income(c)	(8.0		)		—			—			—
	Total	$	(7.2	)		$	(374.8	)	$	(33.9	)	$	(334.5	)
	(a) Represents $(0.1) million of amortization of amounts previously recorded to Accumulated Other Comprehensive Income.
	(b) Represents $(0.4) million of ineffectiveness for our cash flow hedge and $(0.1) million of amortization of amounts previously recorded to Accumulated Other Comprehensive Income.
	(c) Represents the effective portion of our cash flow hedge of $(8.1) million less $(0.1) million of amortization that was reclassified to interest expense.
Potential Collateral Requirements for Derivative Liabilities with Credit-risk-Contingent Features	The following table shows the aggregate fair value of all derivative instruments with credit-related contingent features that are in a liability position, as well as the maximum amount of collateral that would be required if the most intrusive credit-risk-related contingent features underlying these agreements were triggered on March 31, 2015 and September 30, 2014, respectively.
	Potential Collateral Requirements for Derivative Liabilities
	with Credit-Risk-Contingent Features
	(In millions)	WGL Holdings, Inc.				Washington Gas
	March 31, 2015
	Derivative liabilities with credit-risk-contingent features	$	40.8			$	7.3
	Maximum potential collateral requirements	32.3				7.3
	September 30, 2014
	Derivative liabilities with credit-risk-contingent features	$	28.8			$	20.6
	Maximum potential collateral requirements	16.5				16.1
WGL Holdings, Inc.
Derivative [Line Items]
Balance Sheet Classification of Derivative Instruments	The following tables present the balance sheet classification for all derivative instruments as of March 31, 2015 and September 30, 2014.
	WGL Holdings, Inc.
	Balance Sheet Classification of Derivative Instruments
	(In millions)	Derivative Instruments Not Designated as Hedging Instruments							Derivative Instruments Designated as Hedging Instruments
	As of March 31, 2015	Gross				Gross			Gross			Gross			Netting of			Total(a)
		Derivative				Derivative			Derivative			Derivative			Collateral
		Assets				Liabilities			Assets			Liabilities
	Current Assets—Derivatives	$	42.1			$	(16.5	)	$	—		$	—		$	(2.1	)	$	23.5
	Deferred Charges and Other Assets—Derivatives	35.1				—			—			—			—			35.1
	Current Liabilities—Derivatives	4.3				(58.0		)	—			—			6			(47.7		)
	Deferred Credits—Derivatives	12				(352.7		)	—			—			2.9			(337.8		)
	Total	$	93.5			$	(427.2	)	$	—		$	—		$	6.8		$	(326.9	)
	As of September 30, 2014
	Current Assets—Derivatives	$	20.8			$	(2.5	)	$	—		$	—		$	—		$	18.3
	Deferred Charges and Other Assets—Derivatives	18.7				—			—			—			—			18.7
	Current Liabilities—Derivatives	15.4				(70.3		)	—			(1.7		)	8			(48.6		)
	Deferred Credits—Derivatives	17.7				(316.4		)	—			—			4			(294.7		)
	Total	$	72.6			$	(389.2	)	$	—		$	(1.7	)	$	12		$	(306.3	)
Washington Gas Light Company
Derivative [Line Items]
Balance Sheet Classification of Derivative Instruments
	Washington Gas Light Company
	Balance Sheet Classification of Derivative Instruments(b)
	(In millions)
	As of March 31, 2015	Gross				Gross			Netting of			Total(a)
		Derivative				Derivative			Collateral
		Assets				Liabilities
	Current Assets—Derivatives	$	21.9			$	(5.9	)	$	(2.1	)	$	13.9
	Deferred Charges and Other Assets—Derivatives	14.9				—			—			14.9
	Current Liabilities—Derivatives	—				(31.5		)	—			(31.5		)
	Deferred Credits—Derivatives	0.9				(289.5		)	—			(288.6		)
	Total	$	37.7			$	(326.9	)	$	(2.1	)	$	(291.3	)
	As of September 30, 2014
	Current Assets—Derivatives	$	3.9			$	—		$	—		$	3.9
	Deferred Charges and Other Assets—Derivatives	9.5				—			—			9.5
	Current Liabilities—Derivatives	8.6				(43.2		)	0.7			(33.9		)
	Deferred Credits—Derivatives	4.2				(265.2		)	0.2			(260.8		)
	Total	$	26.2			$	(308.4	)	$	0.9		$	(281.3	)
	(a) WGL has elected to offset the fair value of recognized derivative instruments against the right to reclaim or the obligation to return collateral for derivative instruments executed under the same master netting arrangement in accordance with ASC 815. All recognized derivative contracts and associated financial collateral subject to a master netting arrangement or similar that is eligible for offset under ASC 815 have been presented net in the balance sheet.
	(b) Washington Gas did not have any derivative instruments outstanding that were designated as hedging instruments at March 31, 2015 or September 30, 2014.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	6 Months Ended
	Mar. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quantitative Information about Level 3 Fair Value Measurements	The following table includes quantitative information about the significant unobservable inputs used in the fair value measurement of our Level 3 financial instruments and the respective fair values of the net derivative asset and liability positions, by contract type, as of March 31, 2015 and September 30, 2014.
	Quantitative Information about Level 3 Fair Value Measurements
			Net Fair Value		Valuation Techniques		Unobservable Inputs		Range
			March 31, 2015
	WGL Holdings, Inc.		(In millions)
	Natural gas related derivatives		($319.60)		Discounted Cash Flow		Natural Gas Basis Price		($1.630) - $4.760
							(per dekatherm)
					Option Model		Natural Gas Basis Price		($0.215) - $0.039
							(per dekatherm)
							Annualized Volatility of Spot Market Natural Gas		30.5% - 51.1%
	Electricity related derivatives		($10.50)		Discounted Cash Flow		Electricity Congestion Price		($3.103) - $82.600
							(per megawatt hour)
	Washington Gas Light Company
	Natural gas related derivatives		($299.80)		Discounted Cash Flow		Natural Gas Basis Price		($1.630) - $4.268
							(per dekatherm)
			Net Fair Value
			September 30, 2014
	WGL Holdings, Inc.
	Natural gas related derivatives		($294.70)		Discounted Cash Flow		Natural Gas Basis Price		($2.101) - $6.154
							(per dekatherm)
					Option Model		Natural Gas Basis Price		($1.675) - $6.154
							(per dekatherm)
							Annualized Volatility of Spot Market Natural Gas		30.9% - 589.6%
	Electricity related derivatives		($5.00)		Discounted Cash Flow		Electricity Congestion Price		($2.85) - $90.95
							(per megawatt hour)
	Washington Gas Light Company
	Natural gas related derivatives		($270.60)		Discounted Cash Flow		Natural Gas Basis Price		($2.101) - $6.154
							(per dekatherm)
WGL Holdings, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements Under the Fair Value Hierarchy	The following tables set forth financial instruments recorded at fair value as of March 31, 2015 and September 30, 2014, respectively. A financial instrument’s classification within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Our assessment of the significance of a particular input to the fair value measurement requires judgment, and may affect the valuation of fair value assets and liabilities and their placement within the fair value hierarchy.
	WGL Holdings, Inc.
	Fair Value Measurements Under the Fair Value Hierarchy
	(In millions)	Level 1				Level 2				Level 3			Total
	At March 31, 2015
	Assets
	Natural gas related derivatives	$	—			$	32			$	39.9		$	71.9
	Electricity related derivatives	—				2.9				18.7			21.6
	Total Assets	$	—			$	34.9			$	58.6		$	93.5
	Liabilities
	Natural gas related derivatives	$	—			$	(35.2	)		$	(359.5	)	$	(394.7	)
	Electricity related derivatives	—				(3.3		)		(29.2		)	(32.5		)
	Total Liabilities	$	—			$	(38.5	)		$	(388.7	)	$	(427.2	)
	At September 30, 2014
	Assets
	Natural gas related derivatives	$	—			$	22.7			$	33.7		$	56.4
	Electricity related derivatives	—				0.3				15.9			16.2
	Total Assets	$	—			$	23			$	49.6		$	72.6
	Liabilities
	Natural gas related derivatives	$	—			$	(39.8	)		$	(328.4	)	$	(368.2	)
	Electricity related derivatives	—				(0.1		)		(20.9		)	(21.0		)
	Interest rate derivatives	—				(1.7		)		—			(1.7		)
	Total Liabilities	$	—			$	(41.6	)		$	(349.3	)	$	(390.9	)
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs	The following tables are a summary of the changes in the fair value of our derivative instruments that are measured at net fair value on a recurring basis in accordance with ASC Topic 820 using significant Level 3 inputs during the three and six months ended March 31, 2015 and 2014, respectively.
	WGL Holdings, Inc.
	Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
	(In millions)	Natural Gas				Electricity				Warrants			Total
		Related				Related
		Derivatives				Derivatives
	Three Months Ended March 31, 2015
	Balance at January 1, 2015	$	(252.6	)		$	(11.5	)		$	—		$	(264.1	)
	Realized and unrealized gains (losses)
	Recorded to income	(37.7		)		0.4				—			(37.3		)
	Recorded to regulatory assets—gas costs	(53.9		)		—				—			(53.9		)
	Transfers into Level 3	5.4				—				—			5.4
	Transfers out of Level 3	1.4				—				—			1.4
	Purchases	—				0.2				—			0.2
	Settlements	17.8				0.4				—			18.2
	Balance at March 31, 2015	$	(319.6	)		$	(10.5	)		$	—		$	(330.1	)
	Three Months Ended March 31, 2014
	Balance at January 1, 2014	$	(264.5	)		$	(1.7	)		$	1.2		$	(265.0	)
	Realized and unrealized gains (losses)
	Recorded to income	(85.9		)		11.2				—			(74.7		)
	Recorded to regulatory assets—gas costs	(110.8		)		—				—			(110.8		)
	Settlements	28.9				(11.7		)		—			17.2
	Balance at March 31, 2014	$	(432.3	)		$	(2.2	)		$	1.2		$	(433.3	)
	Six Months Ended March 31, 2015
	Balance at October 1, 2014	$	(294.7	)		$	(5.0	)		$	—		$	(299.7	)
	Realized and unrealized gains (losses)
	Recorded to income	(17.4		)		(10.5		)		—			(27.9		)
	Recorded to regulatory assets—gas costs	(40.3		)		—				—			(40.3		)
	Transfers into Level 3	5.4				—				—			5.4
	Transfers out of Level 3	(0.3		)		—				—			(0.3		)
	Purchases	—				3.4				—			3.4
	Settlements	27.7				1.6				—			29.3
	Balance at March 31, 2015	$	(319.6	)		$	(10.5	)		$	—		$	(330.1	)
	Six Months Ended March 31, 2014
	Balance at October 1, 2013	$	(155.2	)		$	2.4			$	1.1		$	(151.7	)
	Realized and unrealized gains (losses)
	Recorded to income	(132.6		)		3.3				0.1			(129.2		)
	Recorded to regulatory assets—gas costs	(182.8		)		—				—			(182.8		)
	Purchases	—				1.4				—			1.4
	Settlements	38.3				(9.3		)		—			29
	Balance at March 31, 2014	$	(432.3	)		$	(2.2	)		$	1.2		$	(433.3	)
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements	The table below sets forth the line items on the statements of income to which amounts are recorded for the three months ended March 31, 2015 and 2014, respectively, related to fair value measurements using significant Level 3 inputs.
	WGL Holdings, Inc.
	Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
	(In millions)	Natural Gas				Electricity				Warrants			Total
		Related				Related
		Derivatives				Derivatives
	Three Months Ended March 31, 2015
	Operating revenues—non-utility	$	(8.9	)		$	3.2			$	—		$	(5.7	)
	Utility cost of gas	(37.4		)		—				—			(37.4		)
	Non-utility cost of energy-related sales	8.6				(2.8		)		—			5.8
	Total	$	(37.7	)		$	0.4			$	—		$	(37.3	)
	Three Months Ended March 31, 2014
	Operating revenues—non-utility	$	(0.5	)		$	(14.5	)		$	—		$	(15.0	)
	Utility cost of gas	(81.8		)		—				—			(81.8		)
	Other income-net	—				—				—			—
	Non-utility cost of energy-related sales	(3.6		)		25.7				—			22.1
	Total	$	(85.9	)		$	11.2			$	—		$	(74.7	)
	Six Months Ended March 31, 2015
	Operating revenues—non-utility	$	1.6			$	24			$	—		$	25.6
	Utility cost of gas	(22.7		)		—				—			(22.7		)
	Non-utility cost of energy-related sales	3.7				(34.5		)		—			(30.8		)
	Total	$	(17.4	)		$	(10.5	)		$	—		$	(27.9	)
	Six Months Ended March 31, 2014
	Operating revenues—non-utility	$	(26.3	)		$	(24.5	)		$	—		$	(50.8	)
	Utility cost of gas	(105.2		)		—				—			(105.2		)
	Other income-net	—				—				0.1			0.1
	Non-utility cost of energy-related sales	(1.1		)		27.8				—			26.7
	Total	$	(132.6	)		$	3.3			$	0.1		$	(129.2	)
Unrealized Gains (Losses) Recorded for Level 3 Measurements	Unrealized gains (losses) attributable to derivative assets and liabilities measured using significant Level 3 inputs were recorded as follows, for the three and six months ended March 31, 2015 and 2014, respectively.
	WGL Holdings, Inc.
	Unrealized Gains (Losses) Recorded for Level 3 Measurements
	(In millions)	Natural Gas				Electricity				Warrants			Total
		Related				Related
		Derivatives				Derivatives
	Three Months Ended March 31, 2015
	Recorded to income
	Operating revenues—non-utility	$	(9.3	)		$	0.1			$	—		$	(9.2	)
	Utility cost of gas	(31.7		)		—				—			(31.7		)
	Non-utility cost of energy-related sales	11.8				0.6				—			12.4
	Recorded to regulatory assets—gas costs	(47.7		)		—				—			(47.7		)
	Total	$	(76.9	)		$	0.7			$	—		$	(76.2	)
	Three Months Ended March 31, 2014
	Recorded to income
	Operating revenues—non-utility	$	(5.0	)		$	(28.4	)		$	—		$	(33.4	)
	Utility cost of gas	(68.1		)		—				—			(68.1		)
	Non-utility cost of energy-related sales	(0.8		)		27.1				—			26.3
	Other income-net	—				—				—			—
	Recorded to regulatory assets—gas costs	(101.1		)		—				—			(101.1		)
	Total	$	(175.0	)		$	(1.3	)		$	—		$	(176.3	)
	Six Months Ended March 31, 2015
	Recorded to income
	Operating revenues—non-utility	$	3.6			$	20.9			$	—		$	24.5
	Utility cost of gas	(19.8		)		—				—			(19.8		)
	Non-utility cost of energy-related sales	1.8				(26.7		)		—			(24.9		)
	Recorded to regulatory assets—gas costs	(28.8		)		—				—			(28.8		)
	Total	$	(43.2	)		$	(5.8	)		$	—		$	(49.0	)
	Six Months Ended March 31, 2014
	Recorded to income
	Operating revenues—non-utility	$	(30.3	)		$	(33.4	)		$	—		$	(63.7	)
	Utility cost of gas	(92.1		)		—				—			(92.1		)
	Non-utility cost of energy-related sales	3.3				28.3				—			31.6
	Other income-net	—				—				0.1			0.1
	Recorded to regulatory assets—gas costs	(169.9		)		—				—			(169.9		)
	Total	$	(289.0	)		$	(5.1	)		$	0.1		$	(294.0	)
Fair Value of Financial Instruments	The following table presents the carrying amounts and estimated fair values of our financial instruments at March 31, 2015 and September 30, 2014.
	WGL Holdings, Inc.
	Fair Value of Financial Instruments
		March 31, 2015								September 30, 2014
	(In millions)	Carrying Amount				Fair Value				Carrying Amount			Fair Value
	Money market funds(a)	$	14.6			$	14.6			$	9.7		$	9.7
	Other short-term investments(a)	$	0.2			$	0.2			$	—		$	—
	Commercial paper (b)	$	175			$	175			$	453.5		$	453.5
	Long-term debt(c)	$	950.5			$	1,117.90			$	679.2		$	809.3
Washington Gas Light Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurements Under the Fair Value Hierarchy
	Washington Gas Light Company
	Fair Value Measurements Under the Fair Value Hierarchy
	(In millions)	Level 1				Level 2				Level 3			Total
	At March 31, 2015
	Assets
	Natural gas related derivatives	$	—			$	22.6			$	15.1		$	37.7
	Total Assets	$	—			$	22.6			$	15.1		$	37.7
	Liabilities
	Natural gas related derivatives	$	—			$	(12.0	)		$	(314.9	)	$	(326.9	)
	Total Liabilities	$	—			$	(12.0	)		$	(314.9	)	$	(326.9	)
	At September 30, 2014
	Assets
	Natural gas related derivatives	$	—			$	13.5			$	12.7		$	26.2
	Total Assets	$	—			$	13.5			$	12.7		$	26.2
	Liabilities
	Natural gas related derivatives	$	—			$	(25.1	)		$	(283.3	)	$	(308.4	)
	Total Liabilities	$	—			$	(25.1	)		$	(283.3	)	$	(308.4	)
Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
	Washington Gas Light Company
	Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs
	(In millions)	Natural Gas
		Related
		Derivatives
	Three Months Ended March 31, 2015
	Balance at January 1, 2015	$	(234.8	)
	Realized and unrealized gains (losses)
	Recorded to income	(37.4		)
	Recorded to regulatory assets—gas costs	(53.9		)
	Transfers into Level 3	5.4
	Transfers out of Level 3	1.4
	Settlements	19.5
	Balance at March 31, 2015	$	(299.8	)
	Three Months Ended March 31, 2014
	Balance at January 1, 2014	$	(220.0	)
	Realized and unrealized gains (losses)
	Recorded to income	(81.8		)
	Recorded to regulatory assets—gas costs	(110.8		)
	Settlements	30
	Balance at March 31, 2014	$	(382.6	)
	Six Months Ended March 31, 2015
	Balance at October 1, 2014	$	(270.6	)
	Realized and unrealized gains (losses)
	Recorded to income	(22.7		)
	Recorded to regulatory assets—gas costs	(40.3		)
	Transfers into Level 3	5.4
	Transfers out of Level 3	(0.3		)
	Settlements	28.7
	Balance at March 31, 2015	$	(299.8	)
	Six Months Ended March 31, 2014
	Balance at October 1, 2013	$	(133.6	)
	Realized and unrealized gains (losses)
	Recorded to income	(105.2		)
	Recorded to regulatory assets—gas costs	(182.8		)
	Settlements	39
	Balance at March 31, 2014	$	(382.6	)
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
	Washington Gas Light Company
	Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
	(In millions)	Natural Gas
		Related
		Derivatives
	Three Months Ended March 31, 2015
	Utility cost of gas	$	(37.4	)
	Total	$	(37.4	)
	Three Months Ended March 31, 2014
	Utility cost of gas	$	(81.8	)
	Total	$	(81.8	)
	Six Months Ended March 31, 2015
	Utility cost of gas	$	(22.7	)
	Total	$	(22.7	)
	Six Months Ended March 31, 2014
	Utility cost of gas	$	(105.2	)
	Total	$	(105.2	)
Unrealized Gains (Losses) Recorded for Level 3 Measurements
	Washington Gas Light Company
	Unrealized Gains (Losses) Recorded for Level 3 Measurements
	Three Months Ended March 31, 2015
	(In millions)	Natural Gas
		Related
		Derivatives
	Recorded to income - utility cost of gas	$	(31.7	)
	Recorded to regulatory assets—gas costs	(47.7		)
	Total	$	(79.4	)
	Three Months Ended March 31, 2014
	Recorded to income - utility cost of gas	$	(68.1	)
	Recorded to regulatory assets—gas costs	(101.1		)
	Total	$	(169.2	)
	Six Months Ended March 31, 2015
	Recorded to income - utility cost of gas	$	(19.8	)
	Recorded to regulatory assets—gas costs	(28.8		)
	Total	$	(48.6	)
	Six Months Ended March 31, 2014
	Recorded to income - utility cost of gas	$	(92.1	)
	Recorded to regulatory assets—gas costs	(169.9		)
	Total	$	(262.0	)
Fair Value of Financial Instruments
	Washington Gas Light Company
		March 31, 2015								September 30, 2014
	(In millions)	Carrying Amount				Fair Value				Carrying Amount			Fair Value
	Money market funds(a)	$	10.2			$	10.2			$	4.3		$	4.3
	Other short-term investments(a)	$	0.2			$	0.2			$	—		$	—
	Commercial paper (b)	$	—			$	—			$	89		$	89
	Long-term debt(c)	$	702.2			$	864.1			$	679.2		$	809.3
	(a)	Balance is located in cash and cash equivalents in the accompanying balance sheets. These amounts may be offset by outstanding checks.
	(b)	Balance is located in notes payable in the accompanying balance sheets.
	(c)	Less current maturities and unamortized discounts.

Operating_Segment_Reporting_Ta

Operating Segment Reporting (Tables)	6 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Operating Segment Financial Information	The following tables present operating segment information for the three and six months ended March 31, 2015 and 2014.
	Operating Segment Financial Information
	(In thousands)	Operating (a)			Depreciation and Amortization				Equity in			EBIT			Total			Capital		Equity Method
		Revenues							Earnings of						Assets			Expenditures		Investments
									Unconsolidated Affiliates
	Three Months Ended March 31, 2015
	Regulated utility	$	615,694		$	27,266			$	—		$	130,280		$	4,245,563		$	70,691	$	—
	Retail energy-marketing	404,601			152				—			38,426			457,847			3		—
	Commercial energy systems	11,532			2,671				502			722			553,515			10,658		65,214
	Midstream energy services	(20,288		)	31				611			(23,507		)	217,032			—		49,468
	Other activities	—			—				719			(846		)	128,624			—		—
	Eliminations(b)	(9,806		)	(17		)		—			(19		)	(452,218		)	—		—
	Total consolidated	$	1,001,733		$	30,103			$	1,832		$	145,056		$	5,150,363		$	81,352	$	114,682
	Three Months Ended March 31, 2014
	Regulated utility	$	716,808		$	25,736			$	—		$	88,535		$	4,013,934		$	50,656	$	—
	Retail energy-marketing	437,434			180				—			(1,390		)	402,675			13		—
	Commercial energy systems	8,036			1,551				479			1,545			423,803			38,497		68,517
	Midstream energy services	27,104			31				64			21,605			102,148			—		13,692
	Other activities	—			—				—			(5,175		)	232,407			—		413
	Eliminations(b)	(15,132		)	(194		)		—			498			(412,750		)	—		—
	Total consolidated	$	1,174,250		$	27,304			$	543		$	105,618		$	4,762,217		$	89,166	$	82,622
	Six Months Ended March 31, 2015
	Regulated utility	$	1,002,887		$	54,218			$	—		$	244,907		$	4,245,563		$	160,294	$	—
	Retail energy-marketing	735,090			319				—			22,531			457,847			37		—
	Commercial energy systems	21,071			4,906				1,079			981			553,515			53,975		65,214
	Midstream energy services	7,804			62				1,147			3,264			217,032			—		49,468
	Other activities	—			—				750			(7,945		)	128,624			—		—
	Eliminations(b)	(15,882		)	(42		)		—			(51		)	(452,218		)	—		—
	Total consolidated	$	1,750,970		$	59,463			$	2,976		$	263,687		$	5,150,363		$	214,306	$	114,682
	Six Months Ended March 31, 2014
	Regulated utility	$	1,107,223		$	51,105			$	—		$	153,988		$	4,013,934		$	105,749	$	—
	Retail energy-marketing	760,372			354				—			3,900			402,675			88		—
	Commercial energy systems	12,753			2,644				763			949			423,803			41,635		68,517
	Midstream energy services	(6,069		)	62				270			(12,604		)	102,148			—		13,692
	Other activities	—			—				—			(7,836		)	232,407			—		413
	Eliminations(b)	(19,732		)	(271		)		—			642			(412,750		)	—		—
	Total consolidated	$	1,854,547		$	53,894			$	1,033		$	139,039		$	4,762,217		$	147,472	$	82,622
	(a) Operating revenues are reported gross of revenue taxes. Revenue taxes of both the regulated utility and the retail energy-marketing segments include gross receipt taxes. Revenue taxes of the regulated utility segment also include PSC fees, franchise fees and energy taxes. Operating revenue amounts in the “Eliminations” row represent total intersegment revenues associated with sales from the regulated utility segment to the retail energy-marketing segment. Midstream Energy Services’ cost of energy related sales is netted with its gross revenues.
	(b) Intersegment eliminations include a timing difference between Commercial Energy Systems’ recognition of revenue for the sale of Solar Renewable Energy Credits (SRECs) to Retail Energy-Marketing and Retail Energy-Marketing’s recognition of the associated expense. Retail Energy-Marketing has recorded a portion of the SREC’s purchased as inventory to be used in future periods at which time they will be expensed.
Reconciliation from EBIT to net income applicable to common stock	The following table is a reconciliation from EBIT to net income applicable to common stock.
		Three Months Ended March 31,						Six Months Ended March 31,
		2015			2014			2015			2014
	Total consolidated EBIT	$	145,056		$	105,618		$	263,687		$	139,039
	Interest expense	13,254			9,525			25,564			18,517
	Income before income taxes	131,802			96,093			238,123			120,522
	Income tax expense	50,017			34,550			92,120			40,020
	Net income	81,785			61,543			146,003			80,502
	Dividends on Washington Gas Light Company preferred stock	330			330			660			660
	Net income applicable to common stock	$	81,455		$	61,213		$	145,343		$	79,842

Other_Investment_Tables

Other Investment (Tables)	6 Months Ended
	Mar. 31, 2015
Other Investments [Abstract]
Minimum Payments Receivable for Direct Financing Leases	Minimum future lease payments receivable under direct financing leases over the next five fiscal years and thereafter are as follows:
	Minimum Payments Receivable for Direct Financing Leases
	(In millions)
	Remainder of 2015	$	2.8
	2016	2.2
	2017	2.1
	2018	2
	2019	1.9
	Thereafter	15.9
	Total	$	26.9
Location of Investments	The income statement location of the investments discussed in this footnote for the three and six months ended March 31, 2015 and 2014 are as follows:
	WGL Holdings, Inc.
	Income Statement Location of Other Investments
	(In millions)	VIEs		Non-VIEs			Total		VIEs		Non-VIEs			Total
		Three Months Ended							Six Months Ended
		31-Mar-15							31-Mar-15
	Equity in earnings of unconsolidated affiliates	$	1	$	0.8		$	1.8	$	1.7	$	1.3		$	3
	Depreciation and amortization	—		—			—		0.1		—			0.1
	Other income (expenses)—net	0.8		—			0.8		1.4		(5.6		)	(4.2		)
	Net income (loss)	$	1.8	$	0.8		$	2.6	$	3	$	(4.3	)	$	(1.3	)
		Three Months Ended							Six Months Ended
		31-Mar-14							31-Mar-14
	Equity in earnings of unconsolidated affiliates	$	0.5	$	—		$	0.5	$	0.8	$	0.2		$	1
	Depreciation and amortization	—		—			—		0.1		—			0.1
	Other income—net	0.8		—			0.8		1.3		—			1.3
	Net income	$	1.3	$	—		$	1.3	$	2	$	0.2		$	2.2
	The balance sheet location of the investments discussed in this footnote at March 31, 2015 and September 30, 2014 are as follows:
	WGL Holdings, Inc.
	Balance Sheet Location of Other Investments
	As of March 31, 2015 (in millions)	VIEs		Non-VIEs			Total
	Assets
	Investments in unconsolidated affiliates	$	81.2	$	33.5		$	114.7
	Investments in direct financing leases, capital leases	20.2		—			20.2
	Accounts receivable	3.3		2.5		(a)	5.8
	Total assets	$	104.7	$	36		$	140.7
	As of September 30, 2014 (in millions)
	Assets
	Investments in unconsolidated affiliates	$	72.6	$	27.9		$	100.5
	Investments in direct financing leases, capital leases	18.2		—			18.2
	Accounts receivable	1.7		—			1.7
	Total assets	$	92.5	$	27.9		$	120.4
	(a) Represents the financing provided to another partner in Mountain Valley to fund its capital commitment. Acquired ownership interest represents the collateral for repayment of the financing.

Related_Party_Transactions_Tab

Related Party Transactions (Tables)	6 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Related Party Transactions	The following table shows the amounts Washington Gas charged WGL Energy Services for balance services.
	Washington Gas Light Company-Gas Balancing Service Charges
		Three Months Ended March 31,				Six Months Ended March 31,
	(In millions)	2015		2014		2015		2014
	Gas balancing service charge	$	9.2	$	14.4	$	14.7	$	18.2
	The following table presents the receivables and payables from associated companies as of March 31, 2015 and September 30, 2014.
	Washington Gas Light Company Receivables / Payables from Associated Companies
	(In millions)	March 31, 2015		30-Sep-14
	Receivables from Associated Companies	$	3.4	$	4.8
	Payables to Associated Companies	$	68.9	$	54.7

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	6 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Minimum Commitments and Contractual Obligations	The following table summarizes the minimum contractual payments that Washington Gas will make under its pipeline transportation, storage and peaking contracts, as well as minimum contractual payments to purchase natural gas.
	Washington Gas Contract Minimums
	(In millions)	Pipeline		Gas Purchase
		Contracts(a)		Commitments(b)
	Remainder of 2015	$	92.4	$	93.2
	2016	218.7		365.6
	2017	210.5		463.5
	2018	211.3		466.1
	2019	202.7		459.4
	Thereafter	1,172.60		5,679.00
	Total	$	2,108.20	$	7,526.80
	(a)	Represents minimum payments for natural gas transportation, storage and peaking contracts that have expiration dates through fiscal year 2031.
	(b)	Includes known and reasonably likely commitments to purchase natural gas. Cost estimates are based on market prices at March 31, 2015.
	The following table summarizes the minimum commitments and contractual obligations of WGL Energy Services and WGL Midstream for the next five fiscal years and thereafter.
	Contract Minimums
		WGL Energy Services						WGL Midstream
	(In millions)	Gas Purchase		Pipeline		Electric		Gas Purchase		Pipeline		Total
		Commitments(a)		Contracts(b)		Purchase		Commitments(d)		Contracts(b)
						Commitments(c)
	Remainder of 2015	$	75.9	$	1.6	$	255	$	27.3	$	13.1	$	372.9
	2016	120.7		1.1		360.6		246.8		20.3		749.5
	2017	49.1		0.8		131.3		374.8		18.4		574.4
	2018	1.4		0.8		17.4		1,219.20		15.2		1,254.00
	2019	—		0.8		0.2		1,757.90		60.9		1,819.80
	Thereafter	—		1.6		—		36,273.80		1,097.80		37,373.20
	Total	$	247.1	$	6.7	$	764.5	$	39,899.80	$	1,225.70	$	42,143.80
	(a)	Represents fixed price commitments with city gate equivalent deliveries.
	(b)	Represents minimum payments for natural gas transportation and storage contracts that have expiration dates through fiscal year 2044.
	(c)	Represents electric purchase commitments that are based on existing fixed price and fixed volume contracts. Includes $19.1 million of commitments related to renewable energy credits.
	(d)	Includes known and reasonably likely commitments to purchase natural gas. Cost estimates are based on forward market prices as of March 31, 2015.

Pension_and_Other_PostRetireme1

Pension and Other Post-Retirement Benefit Plan (Tables)	6 Months Ended
	Mar. 31, 2015
Pension and Other Postretirement Benefit Expense [Abstract]
Components of Net Periodic Benefit Costs (Income)	The following table shows the components of net periodic benefit costs (income) recognized in our financial statements during the three and six months ended March 31, 2015 and 2014.
	Components of Net Periodic Benefit Costs (Income)
		Three Months Ended March 31,
		2015						2014
	(In millions)	Pension			Health and			Pension			Health and
		Benefits			Life Benefits			Benefits			Life Benefits
	Components of Net Periodic Benefit Costs (Income)
	Service cost	$	3.8		$	1.8		$	3.5		$	2.2
	Interest cost	9.8			3.6			10.1			5.6
	Expected return on plan assets	(11.1		)	(5.2		)	(10.2		)	(4.8		)
	Amortization of prior service cost (credit)	—			(3.8		)	—			(1.0		)
	Amortization of actuarial loss	4.7			1.1			4.2			1.2
	Net periodic benefit cost (income)	7.2			(2.5		)	7.6			3.2
	Amount allocated to construction projects	(1.1		)	0.5			(1.0		)	(0.4		)
	Amortization of regulatory asset/liability—net	1.7			—			1.8			0.1
	Amount charged (credited) to expense	$	7.8		$	(2.0	)	$	8.4		$	2.9
		Six Months Ended March 31,
		2015						2014
	Components of Net Periodic Benefit Costs (Income)
	Service cost	$	7.7		$	3.5		$	7		$	4.3
	Interest cost	19.6			7.3			20.2			11
	Expected return on plan assets	(22.3		)	(10.4		)	(20.5		)	(9.5		)
	Amortization of prior service cost (credit)	0.1			(7.6		)	0.1			(2.0		)
	Amortization of actuarial loss	9.4			2.2			8.4			1.9
	Net periodic benefit cost (income)	14.5			(5.0		)	15.2			5.7
	Amount allocated to construction projects	(2.2		)	1			(2.0		)	(0.8		)
	Amortization of regulatory asset/liability—net	3.5			(0.1		)	3.6			0.2
	Amount charged (credited) to expense	$	15.8		$	(4.1	)	$	16.8		$	5.1

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Tables)	6 Months Ended
	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Changes in Accumulated Other Comprehensive Income by Component	The following tables show the changes in accumulated other comprehensive income for WGL and Washington Gas by component for the three and six months ended March 31, 2015 and 2014.
	WGL Holdings, Inc.
	Changes in Accumulated Other Comprehensive Income by Component
		Three Months Ended March 31,						Six Months Ended March 31,
		2015			2014			2015			2014
	(In thousands)
	Beginning Balance	$	(11,966	)	$	(10,849	)	$	(7,961	)	$	(11,048	)
	Qualified cash flow hedging instruments(a)	222			—			(8,042		)	—
	Change in prior service credit (b)	(171		)	(35		)	(342		)	(69		)
	Amortization of actuarial loss (gain) (b)	491			(164		)	974			200
	Current-period other comprehensive income (loss)	542			(199		)	(7,410		)	131
	Income tax expense (benefit) related to other comprehensive income	219			(79		)	(3,728		)	52
	Ending Balance	$	(11,643	)	$	(10,969	)	$	(11,643	)	$	(10,969	)
	(a)	Cash flow hedging instruments represent interest rate swaps agreements on debt issuances. Refer to Note 8- Derivative and weather-related instruments for further discussion of the interest rate swap agreements.
	(b)	These accumulated other comprehensive income components are included in the computation of net periodic benefit cost. Refer to Note 14- Pension and other post-retirement benefit plans for additional details.
	Washington Gas Light Company
	Changes in Accumulated Other Comprehensive Income by Component
		Three Months Ended March 31,						Six Months Ended March 31,
		2015			2014			2015			2014
	(In thousands)
	Beginning Balance	$	(6,224	)	$	(10,849	)	$	(6,413	)	$	(11,048	)
	Change in prior service credit (a)	(171		)	(35		)	(342		)	(69		)
	Amortization of actuarial loss (gain) (a)	491			(164		)	974			200
	Current-period other comprehensive income (loss)	320			(199		)	632			131
	Income tax expense (benefit) related to other comprehensive income	127			(79		)	250			52
	Ending Balance	$	(6,031	)	$	(10,969	)	$	(6,031	)	$	(10,969	)
	(a)	These accumulated other comprehensive income components are included in the computation of net periodic benefit cost. Refer to Note 14-Pension and other post-retirement benefit plans for additional details.

Accounting_Policies_Details

Accounting Policies (Details) (USD $)	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Washington Gas Light Company
Inventory [Line Items]
Decrease to net income due to lower-of-cost or market adjustment	$0	$400,000	$700,000	$400,000
WGL Energy Services
Inventory [Line Items]
Decrease to net income due to lower-of-cost or market adjustment	0	0	0	0
WGL Midstream
Inventory [Line Items]
Decrease to net income due to lower-of-cost or market adjustment	$3,000,000	$0	$20,500,000	$0

Accounts_Payable_and_Other_Acc2

Accounts Payable and Other Accrued Liabilities (Details) (USD $)	Mar. 31, 2015	Sep. 30, 2014
WGL Holdings, Inc.
Accounts Payable and Other Accrued Liabilities [Line Items]
Accounts payable - trade	$274,400,000	$278,800,000
Employee benefits and payroll accruals	21,000,000	19,800,000
Other accrued liabilities	28,600,000	14,600,000
Accounts Payable and Accrued Liabilities, Current, Total	324,046,000	313,221,000
Washington Gas Light Company
Accounts Payable and Other Accrued Liabilities [Line Items]
Accounts payable - trade	126,400,000	146,400,000
Employee benefits and payroll accruals	19,700,000	18,200,000
Other accrued liabilities	20,800,000	11,900,000
Accounts Payable and Accrued Liabilities, Current, Total	$166,859,000	$176,467,000

ShortTerm_Debt_Details

Short-Term Debt (Details) (USD $)	Mar. 31, 2015		Sep. 30, 2014
Committed Credit
Short-term Debt [Line Items]
Unsecured revolving credit facility	$800,000,000	[1]	$800,000,000	[1]
Less: Commercial Paper	-175,000,000		-453,500,000
Net committed credit available	625,000,000		346,500,000
Weighted average interest rate	0.26%		0.19%
WGL Holdings, Inc.
Short-term Debt [Line Items]
Less: Commercial Paper	-175,000,000		-453,500,000
Footnote Details [Abstract]
Outstanding bank loans	0		0
WGL Holdings, Inc. | Committed Credit
Short-term Debt [Line Items]
Unsecured revolving credit facility	450,000,000	[1],[2]	450,000,000	[1],[2]
Less: Commercial Paper	-175,000,000	[2]	-364,500,000	[2]
Net committed credit available	275,000,000	[2]	85,500,000	[2]
Weighted average interest rate	0.26%	[2]	0.20%	[2]
Footnote Details [Abstract]
Revolving credit facility additional borrowings	100,000,000
Revolving credit facility maximum borrowing capacity	550,000,000
Number of one-year extension options	2
Washington Gas Light Company
Short-term Debt [Line Items]
Less: Commercial Paper	0		-89,000,000
Footnote Details [Abstract]
Outstanding bank loans	0		0
Washington Gas Light Company | Committed Credit
Short-term Debt [Line Items]
Unsecured revolving credit facility	350,000,000	[1]	350,000,000	[1]
Less: Commercial Paper	0		-89,000,000
Net committed credit available	350,000,000		261,000,000
Weighted average interest rate	0.00%		0.13%
Footnote Details [Abstract]
Revolving credit facility additional borrowings	100,000,000
Revolving credit facility maximum borrowing capacity	$450,000,000
Number of one-year extension options	2
[1]	Both WGL and Washington Gas have the right to request extensions with the banks’ approval. WGL’s revolving credit facility permits it to borrow an additional $100 million, with the banks’ approval, for a total of $550 million. Washington Gas’ revolving credit facility permits it to borrow an additional $100 million, with the banks’ approval, for a total of $450 million.
[2]	WGL includes WGL Holdings and all subsidiaries other than Washington Gas

LongTerm_Debt_Details

Long-Term Debt (Details) (USD $)	6 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Sep. 30, 2014
Total consolidated issuances
Debt Instrument [Line Items]
Long-term notes	$300,000,000	[1]
Issuances | WGL Holdings, Inc.
Debt Instrument [Line Items]
Long-term notes	250,000,000	[1],[2]
Issuances | Washington Gas Light Company
Debt Instrument [Line Items]
Long-term notes	50,000,000	[1]	75,000,000	[1]
Interest rate	4.24%		5.00%
Issuance Date	15-Dec-14		5-Dec-13
Principal	50,000,000	[1]	75,000,000	[1]
Estimated effective interest rate	4.41%	[3]	4.95%	[3]
Nominal Maturity Date	15-Dec-44		15-Dec-43
Retirements | Washington Gas Light Company
Debt Instrument [Line Items]
Long-term notes			37,000,000	[1]
Interest rate			4.88%
Principal			37,000,000	[1]
Nominal Maturity Date			7-Nov-13
Retirement Date			7-Nov-13
Total Consolidated
Debt Instrument [Line Items]
Long-term notes	991,000,000	[4]			691,000,000	[4]
Interest rate	5.06%				5.65%
Total Consolidated | WGL Holdings, Inc.
Debt Instrument [Line Items]
Long-term notes	250,000,000	[2],[4]			0	[2],[4]
Interest rate	3.66%	[2]
Total Consolidated | Washington Gas Light Company
Debt Instrument [Line Items]
Long-term notes	741,000,000	[4]			691,000,000	[4]
Interest rate	5.56%				5.65%
Maturity-2019 Notes | Issuances | WGL Holdings, Inc.
Debt Instrument [Line Items]
Interest rate	2.25%	[2]
Issuance Date	24-Oct-14	[2]
Principal	100,000,000	[1],[2]
Estimated effective interest rate	2.42%	[2],[3]
Nominal Maturity Date	1-Nov-19	[2]
Maturity-2044 Notes | Issuances | WGL Holdings, Inc.
Debt Instrument [Line Items]
Interest rate	4.60%	[2]
Issuance Date	24-Oct-14	[2]
Principal	125,000,000	[1],[2]
Estimated effective interest rate	5.11%	[2],[3]
Nominal Maturity Date	1-Nov-44	[2]
December 16, 2014 | Issuances | WGL Holdings, Inc.
Debt Instrument [Line Items]
Interest rate	4.60%	[2]
Issuance Date	16-Dec-14	[2]
Principal	25,000,000	[1],[2]
Estimated effective interest rate	5.53%	[2],[3]
Nominal Maturity Date	1-Nov-44	[2]
Medium-Term Notes | Washington Gas Light Company
Debt Instrument [Line Items]
Unused borrowing capacity amount	$275,000,000				$275,000,000
[1]	Represents face amount.
[2]	WGL includes WGL Holdings and all subsidiaries other than Washington Gas.
[3]	The estimated effective cost of the issued notes, including consideration of issuance fees and hedge costs.
[4]	Includes Senior Notes for WGL and both MTNs and private placement notes for Washington Gas. Represents face value including current maturities.

Common_Shareholders_Equity_Det

Common Shareholders' Equity (Details) (USD $)	3 Months Ended		6 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2014
WGL Holdings, Inc.
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			$1,246,576
Net income	81,785	61,543	146,003	80,502
Other comprehensive income (loss)	323	-120	-3,682	79
Repurchase of common stock			-41,485
Stock-based compensation			1,809
Dividends declared:
Common stock			-44,877
Preferred stock			-660
Common shareholders' equity, ending balance	1,303,684		1,303,684
Shares outstanding	49,728,662		49,728,662		50,656,553
WGL Holdings, Inc. | Common Stock
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			525,932
Net income			0
Other comprehensive income (loss)			0
Repurchase of common stock			-41,485
Stock-based compensation			1,009
Dividends declared:
Common stock			0
Preferred stock			0
Common shareholders' equity, ending balance	485,456		485,456
WGL Holdings, Inc. | Paid-In Capital
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			11,847
Net income			0
Other comprehensive income (loss)			0
Repurchase of common stock			0
Stock-based compensation			800
Dividends declared:
Common stock			0
Preferred stock			0
Common shareholders' equity, ending balance	12,647		12,647
WGL Holdings, Inc. | Retained Earnings
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			716,758
Net income			146,003
Other comprehensive income (loss)			0
Repurchase of common stock			0
Stock-based compensation			0
Dividends declared:
Common stock			-44,877
Preferred stock			-660
Common shareholders' equity, ending balance	817,224		817,224
WGL Holdings, Inc. | Accumulated Other Comprehensive Loss, Net of Taxes
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			-7,961
Net income			0
Other comprehensive income (loss)			-3,682
Repurchase of common stock			0
Stock-based compensation			0
Dividends declared:
Common stock			0
Preferred stock			0
Common shareholders' equity, ending balance	-11,643		-11,643
Washington Gas Light Company
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			1,050,166
Net income	74,694	49,506	139,645	88,313
Other comprehensive income (loss)	193	-120	382	79
Stock-based compensation			715
Dividends declared:
Common stock			-39,160
Preferred stock			-660
Common shareholders' equity, ending balance	1,151,088		1,151,088
Shares outstanding	46,479,536		46,479,536		46,479,536
Washington Gas Light Company | Common Stock
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			46,479
Net income			0
Other comprehensive income (loss)			0
Stock-based compensation			0
Dividends declared:
Common stock			0
Preferred stock			0
Common shareholders' equity, ending balance	46,479		46,479
Washington Gas Light Company | Paid-In Capital
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			480,620
Net income			0
Other comprehensive income (loss)			0
Stock-based compensation			715
Dividends declared:
Common stock			0
Preferred stock			0
Common shareholders' equity, ending balance	481,335		481,335
Washington Gas Light Company | Retained Earnings
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			529,480
Net income			139,645
Other comprehensive income (loss)
Stock-based compensation			0
Dividends declared:
Common stock			-39,160
Preferred stock			-660
Common shareholders' equity, ending balance	629,305		629,305
Washington Gas Light Company | Accumulated Other Comprehensive Loss, Net of Taxes
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance			-6,413
Net income			0
Other comprehensive income (loss)			382
Stock-based compensation			0
Dividends declared:
Common stock			0
Preferred stock			0
Common shareholders' equity, ending balance	($6,031)		($6,031)

Earnings_Per_Share_Details

Earnings Per Share (Details) (USD $)	3 Months Ended		6 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Schedule of Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive securities excluded from computation of earnings per share	0	0	0	0
WGL Holdings, Inc.
Schedule of Earnings Per Share, Basic and Diluted [Line Items]
Net Income (Loss) applicable to common stock	81,455	61,213	145,343	79,842
Basic EPS (in shares)	49,720,000	51,875,000	49,851,000	51,846,000
Stock-based compensation plans (in shares)	263,000	24,000	204,000	18,000
Diluted EPS (in shares)	49,983,000	51,899,000	50,055,000	51,864,000
Basic EPS (in dollars per share)	1.64	1.18	2.92	1.54
Diluted EPS (in dollars per share)	1.63	1.18	2.9	1.54

Income_TaxesBalance_Sheet_Deta

Income Taxes-Balance Sheet (Details) (USD $)	Mar. 31, 2015	Sep. 30, 2014
Income Tax Expense Benefit Details [Line Items]
Unrecognized tax benefits	$30,200,000	$32,600,000
Washington Gas Light Company
Income Tax Expense Benefit Details [Line Items]
Accrued interest related to uncertain tax positions	$0	$100,000

Derivative_and_Weather_Related2

Derivative and Weather Related Instruments Narrative (Details) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2014
	Counterparties		Counterparties
WGL Energy Services
Derivative, Collateral [Abstract]
Right to reclaim cash	$16.70		$16.70		$5.70
Collateral already posted, aggregate fair value	9		9		5.3
Concentration of Credit Risk
Number of counterparties	2		2
Percentage of credit exposure	10.00%		10.00%
Obligation to counterparties	1.2		1.2
Gain (losses) on weather related instruments, pretax	4.9	6.4	3.2	5.4
WGL Midstream
Derivative, Collateral [Abstract]
Right to reclaim cash	8.6		8.6		11.4
Concentration of Credit Risk
Number of counterparties	2		2
Percentage of credit exposure	10.00%		10.00%
Obligation to counterparties	15.1		15.1
Washington Gas Light Company
Asset Optimization [Abstract]
Unrealized gain (loss) on asset optimization derivative instruments, net pretax	-28	-77.9	-2.9	-104.1
Gain (loss) on asset optimization transactions, net pretax	-14	-60.4	17.1	-80.2
Derivative, Collateral [Abstract]
Right to reclaim cash	3		3		8.2
Obligation to return cash	0.4		0.4		2.5
Concentration of Credit Risk
Number of counterparties	3		3
Percentage of credit exposure	10.00%		10.00%
Obligation to counterparties	$22.50		$22.50

Derivative_and_Weather_Related3

Derivative and Weather Related Instruments (Details) (USD $)	Mar. 31, 2015		Sep. 30, 2014
In Millions, unless otherwise specified
WGL Holdings, Inc.
Absolute Notional Amounts of Open Positions on Derivative Instruments
Warrant Transaction Volume (in shares)			4.6
Interest rate swap notional principal value			$150
Balance Sheet Classification of Derivative Instruments
Netting of Collateral	6.8		12
Total	-326.9	[1]	-306.3	[1]
WGL Holdings, Inc. | Derivative Instruments Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Liabilities			-1.7
WGL Holdings, Inc. | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	93.5		72.6
Gross Derivative Liabilities	-427.2		-389.2
WGL Holdings, Inc. | Current assets- derivatives
Balance Sheet Classification of Derivative Instruments
Netting of Collateral	-2.1		0
Total	23.5	[1]	18.3	[1]
WGL Holdings, Inc. | Current assets- derivatives | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	42.1		20.8
Gross Derivative Liabilities	-16.5		-2.5
WGL Holdings, Inc. | Deferred Charges and Other Assets-Derivatives
Balance Sheet Classification of Derivative Instruments
Netting of Collateral	0		0
Total	35.1	[1]	18.7	[1]
WGL Holdings, Inc. | Deferred Charges and Other Assets-Derivatives | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	35.1		18.7
Gross Derivative Liabilities	0		0
WGL Holdings, Inc. | Current liabilities- Derivatives
Balance Sheet Classification of Derivative Instruments
Netting of Collateral	6		8
Total	-47.7	[1]	-48.6	[1]
WGL Holdings, Inc. | Current liabilities- Derivatives | Derivative Instruments Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Liabilities			-1.7
WGL Holdings, Inc. | Current liabilities- Derivatives | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	4.3		15.4
Gross Derivative Liabilities	-58		-70.3
WGL Holdings, Inc. | Deferred Credits-Derivatives
Balance Sheet Classification of Derivative Instruments
Netting of Collateral	2.9		4
Total	-337.8	[1]	-294.7	[1]
WGL Holdings, Inc. | Deferred Credits-Derivatives | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	12		17.7
Gross Derivative Liabilities	-352.7		-316.4
WGL Holdings, Inc. | Asset Optimization
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural Gas Derivative Transaction, Volume (in mmbtu)	2,193,690,000		2,059,330,000
WGL Holdings, Inc. | Retail sales
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural Gas Derivative Transaction, Volume (in mmbtu)	960,000		4,470,000
Electricity Derivative Transaction, Volume (in KWh)	4,126,900,000		3,831,400,000
WGL Holdings, Inc. | Other risk-management activities
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural Gas Derivative Transaction, Volume (in mmbtu)	174,570,000		164,130,000
Electricity Derivative Transaction, Volume (in KWh)	15,842,900,000		16,734,100,000
Washington Gas Light Company
Absolute Notional Amounts of Open Positions on Derivative Instruments
Warrant Transaction Volume (in shares)			0
Interest rate swap notional principal value			0
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	37.7	[2]	26.2	[2]
Gross Derivative Liabilities	-326.9	[2]	-308.4	[2]
Netting of Collateral	-2.1	[2]	0.9	[2]
Total	-291.3	[1],[2]	-281.3	[1],[2]
Washington Gas Light Company | Current assets- derivatives
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	21.9	[2]	3.9	[2]
Gross Derivative Liabilities	-5.9	[2]	0	[2]
Netting of Collateral	-2.1	[2]	0	[2]
Total	13.9	[1],[2]	3.9	[1],[2]
Washington Gas Light Company | Deferred Charges and Other Assets-Derivatives
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	14.9	[2]	9.5	[2]
Gross Derivative Liabilities	0	[2]	0	[2]
Netting of Collateral	0	[2]	0	[2]
Total	14.9	[1],[2]	9.5	[1],[2]
Washington Gas Light Company | Current liabilities- Derivatives
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	0	[2]	8.6	[2]
Gross Derivative Liabilities	-31.5	[2]	-43.2	[2]
Netting of Collateral	0	[2]	0.7	[2]
Total	-31.5	[1],[2]	-33.9	[1],[2]
Washington Gas Light Company | Deferred Credits-Derivatives
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	0.9	[2]	4.2	[2]
Gross Derivative Liabilities	-289.5	[2]	-265.2	[2]
Netting of Collateral	0	[2]	0.2	[2]
Total	($288.60)	[1],[2]	($260.80)	[1],[2]
Washington Gas Light Company | Asset Optimization
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural Gas Derivative Transaction, Volume (in mmbtu)	1,440,940,000		1,374,090,000
Washington Gas Light Company | Retail sales
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural Gas Derivative Transaction, Volume (in mmbtu)	0		0
Electricity Derivative Transaction, Volume (in KWh)	0		0
Washington Gas Light Company | Other risk-management activities
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural Gas Derivative Transaction, Volume (in mmbtu)	138,910,000		139,820,000
Electricity Derivative Transaction, Volume (in KWh)	0		0
[1]	WGL has elected to offset the fair value of recognized derivative instruments against the right to reclaim or the obligation to return collateral for derivative instruments executed under the same master netting arrangement in accordance with ASC 815. All recognized derivative contracts and associated financial collateral subject to a master netting arrangement or similar that is eligible for offset under ASC 815 have been presented net in the balance sheet.
[2]	Represents $(0.4) million of ineffectiveness for our cash flow hedge and $(0.1) million of amortization of amounts previously recorded to Accumulated Other Comprehensive Income.

Derivative_and_Weather_Related4

Derivative and Weather Related Instruments (Gains and Losses) (Details) (USD $)	3 Months Ended				6 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Mar. 31, 2015		Mar. 31, 2014
WGL Holdings, Inc.
Gains and (Losses) on Derivative Instruments
Gains (Losses) On Derivative Instruments	($78,400,000)		($237,100,000)		($7,200,000)		($374,800,000)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax	222,000	[1]	0	[1]	-8,042,000	[1]	0	[1]
WGL Holdings, Inc. | Operating revenues—non-utility
Gains and (Losses) on Derivative Instruments
Recorded to income	-1,100,000		-35,800,000		74,300,000		-84,300,000
WGL Holdings, Inc. | Utility cost of gas
Gains and (Losses) on Derivative Instruments
Recorded to income	-35,800,000		-96,400,000		-10,000,000		-123,700,000
WGL Holdings, Inc. | Non-utility cost of energy-related sales
Gains and (Losses) on Derivative Instruments
Recorded to income	10,500,000		27,600,000		-39,100,000		43,900,000
WGL Holdings, Inc. | Other income-net
Gains and (Losses) on Derivative Instruments
Recorded to income	0		0		0		100,000
WGL Holdings, Inc. | Gas costs
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets/liabilities	-52,100,000		-133,700,000		-23,900,000		-212,000,000
WGL Holdings, Inc. | Other
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets/liabilities	0		1,200,000		0		1,200,000
WGL Holdings, Inc. | Interest expense
Gains and (Losses) on Derivative Instruments
Recorded to income	-100,000	[2]	0	[2]	-500,000	[3]	0	[3]
Amortization of amounts recorded to AOCI					-100,000	[2]
Ineffectiveness of cash flow hedge					-400,000
WGL Holdings, Inc. | Recorded to other comprehensive income
Gains and (Losses) on Derivative Instruments
Gains (Losses) On Derivative Instruments	200,000		0		-8,100,000		0	[4]
Amortization of amounts recorded to AOCI	100,000	[2]			100,000	[2]
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax					-8,000,000	[4]
Washington Gas Light Company
Gains and (Losses) on Derivative Instruments
Gains (Losses) On Derivative Instruments	-87,900,000		-228,900,000		-33,900,000		-334,500,000
Washington Gas Light Company | Operating revenues—non-utility
Gains and (Losses) on Derivative Instruments
Recorded to income	0		0		0		0
Washington Gas Light Company | Utility cost of gas
Gains and (Losses) on Derivative Instruments
Recorded to income	-35,800,000		-96,400,000		-10,000,000		-123,700,000
Washington Gas Light Company | Non-utility cost of energy-related sales
Gains and (Losses) on Derivative Instruments
Recorded to income	0		0		0		0
Washington Gas Light Company | Other income-net
Gains and (Losses) on Derivative Instruments
Recorded to income	0		0		0		0
Washington Gas Light Company | Gas costs
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets/liabilities	-52,100,000		-133,700,000		-23,900,000		-212,000,000
Washington Gas Light Company | Other
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets/liabilities	0		1,200,000		0		1,200,000
Washington Gas Light Company | Interest expense
Gains and (Losses) on Derivative Instruments
Recorded to income	0	[2]	0	[2]	0	[3]	0	[3]
Washington Gas Light Company | Recorded to other comprehensive income
Gains and (Losses) on Derivative Instruments
Gains (Losses) On Derivative Instruments	$0		$0		$0	[4]	$0	[4]
[1]	Cash flow hedging instruments represent interest rate swaps agreements on debt issuances. Refer to Note 8- Derivative and weather-related instruments for further discussion of the interest rate swap agreements.
[2]	Represents $(0.1) million of amortization of amounts previously recorded to Accumulated Other Comprehensive Income.
[3]	Represents $(0.4) million of ineffectiveness for our cash flow hedge and $(0.1) million of amortization of amounts previously recorded to Accumulated Other Comprehensive Income.
[4]	Represents the effective portion of our cash flow hedge of $(8.1) million less $(0.1) million of amortization that was reclassified to interest expense.

Derivative_and_Weather_Related5

Derivative and Weather Related Instruments (Aggregate Fair Value) (Details) (USD $)	Mar. 31, 2015	Sep. 30, 2014
In Millions, unless otherwise specified
WGL Holdings, Inc.
Potential Collateral Requirements for Derivative Liabilities with Credit-risk-Contingent Features
Derivative liabilities with credit-risk-contingent features	$40.80	$28.80
Maximum potential collateral requirements	32.3	16.5
Washington Gas Light Company
Potential Collateral Requirements for Derivative Liabilities with Credit-risk-Contingent Features
Derivative liabilities with credit-risk-contingent features	7.3	20.6
Maximum potential collateral requirements	$7.30	$16.10

Fair_Value_Measurements_Fair_V

Fair Value Measurements (Fair Value Hierarchy) (Details) (USD $)	Mar. 31, 2015	Sep. 30, 2014
In Millions, unless otherwise specified
WGL Holdings, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	$93.50	$72.60
Liabilities	-427.2	-390.9
WGL Holdings, Inc. | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	71.9	56.4
Liabilities	-394.7	-368.2
WGL Holdings, Inc. | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	21.6	16.2
Liabilities	-32.5	-21
WGL Holdings, Inc. | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Liabilities		-1.7
WGL Holdings, Inc. | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	0	0
Liabilities	0	0
WGL Holdings, Inc. | Level 1 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	0	0
Liabilities	0	0
WGL Holdings, Inc. | Level 1 | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	0	0
Liabilities	0	0
WGL Holdings, Inc. | Level 1 | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Liabilities		0
WGL Holdings, Inc. | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	34.9	23
Liabilities	-38.5	-41.6
WGL Holdings, Inc. | Level 2 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	32	22.7
Liabilities	-35.2	-39.8
WGL Holdings, Inc. | Level 2 | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	2.9	0.3
Liabilities	-3.3	-0.1
WGL Holdings, Inc. | Level 2 | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Liabilities		-1.7
WGL Holdings, Inc. | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	58.6	49.6
Liabilities	-388.7	-349.3
WGL Holdings, Inc. | Level 3 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	39.9	33.7
Liabilities	-359.5	-328.4
WGL Holdings, Inc. | Level 3 | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	18.7	15.9
Liabilities	-29.2	-20.9
WGL Holdings, Inc. | Level 3 | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Liabilities		0
Washington Gas Light Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	37.7	26.2
Liabilities	-326.9	-308.4
Washington Gas Light Company | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	37.7	26.2
Liabilities	-326.9	-308.4
Washington Gas Light Company | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	0	0
Liabilities	0	0
Washington Gas Light Company | Level 1 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	0	0
Liabilities	0	0
Washington Gas Light Company | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	22.6	13.5
Liabilities	-12	-25.1
Washington Gas Light Company | Level 2 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	22.6	13.5
Liabilities	-12	-25.1
Washington Gas Light Company | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	15.1	12.7
Liabilities	-314.9	-283.3
Washington Gas Light Company | Level 3 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	15.1	12.7
Liabilities	($314.90)	($283.30)

Fair_Value_Measurements_Quanti

Fair Value Measurements (Quantitative information) (Details) (USD $)	6 Months Ended	12 Months Ended
	Mar. 31, 2015	Sep. 30, 2014	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013
WGL Holdings, Inc.
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net Fair Value	-330,100,000	-299,700,000	($264,100,000)	($433,300,000)	($265,000,000)	($151,700,000)
WGL Holdings, Inc. | Natural Gas Related Derivatives
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net Fair Value	-319,600,000	-294,700,000	-252,600,000	-432,300,000	-264,500,000	-155,200,000
WGL Holdings, Inc. | Natural Gas Related Derivatives | Discounted Cash Flow | Maximum | Natural Gas Basis Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Input Price	4.76	6.154
WGL Holdings, Inc. | Natural Gas Related Derivatives | Discounted Cash Flow | Minimum | Natural Gas Basis Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Input Price	-1.63	-2.101
WGL Holdings, Inc. | Natural Gas Related Derivatives | Option Model | Maximum | Natural Gas Basis Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Input Price	0.039	6.154
WGL Holdings, Inc. | Natural Gas Related Derivatives | Option Model | Maximum | Annualized Volatility Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Option Volatility Percentage	51.10%	589.60%
WGL Holdings, Inc. | Natural Gas Related Derivatives | Option Model | Minimum | Natural Gas Basis Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Input Price	-0.215	-1.675
WGL Holdings, Inc. | Natural Gas Related Derivatives | Option Model | Minimum | Annualized Volatility Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Option Volatility Percentage	30.50%	30.90%
WGL Holdings, Inc. | Electricity Related Derivatives
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net Fair Value	-10,500,000	-5,000,000	-11,500,000	-2,200,000	-1,700,000	2,400,000
WGL Holdings, Inc. | Electricity Related Derivatives | Discounted Cash Flow | Maximum | Electricity Congestion Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Input Price	82.6	90.95
WGL Holdings, Inc. | Electricity Related Derivatives | Discounted Cash Flow | Minimum | Electricity Congestion Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Input Price	-3.103	-2.85
Washington Gas Light Company | Discounted Cash Flow | Maximum | Natural Gas Basis Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Input Price	4.268	6.154
Washington Gas Light Company | Discounted Cash Flow | Minimum | Natural Gas Basis Price
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Input Price	-1.63	-2.101
Washington Gas Light Company | Natural Gas Related Derivatives
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net Fair Value	-299,800,000	-270,600,000	($234,800,000)	($382,600,000)	($220,000,000)	($133,600,000)

Fair_Value_Measurements_Reconc

Fair Value Measurements (Reconciliation with Level 3 Inputs) (Details) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
WGL Holdings, Inc.
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	($264.10)	($265)	($299.70)	($151.70)
Recorded to income	-37.3	-74.7	-27.9	-129.2
Recorded to regulatory assets—gas costs	-53.9	-110.8	-40.3	-182.8
Transfers into Level 3	5.4		5.4
Transfers out of Level 3	1.4		-0.3
Purchases	0.2		3.4	1.4
Settlements	18.2	17.2	29.3	29
Ending balance	-330.1	-433.3	-330.1	-433.3
WGL Holdings, Inc. | Natural Gas Related Derivatives
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	-252.6	-264.5	-294.7	-155.2
Recorded to income	-37.7	-85.9	-17.4	-132.6
Recorded to regulatory assets—gas costs	-53.9	-110.8	-40.3	-182.8
Transfers into Level 3	5.4		5.4
Transfers out of Level 3	1.4		-0.3
Purchases	0		0	0
Settlements	17.8	28.9	27.7	38.3
Ending balance	-319.6	-432.3	-319.6	-432.3
WGL Holdings, Inc. | Electricity Related Derivatives
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	-11.5	-1.7	-5	2.4
Recorded to income	0.4	11.2	-10.5	3.3
Recorded to regulatory assets—gas costs	0	0	0	0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Purchases	0.2		3.4	1.4
Settlements	0.4	-11.7	1.6	-9.3
Ending balance	-10.5	-2.2	-10.5	-2.2
WGL Holdings, Inc. | Warrant
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	0	1.2	0	1.1
Recorded to income	0	0	0	0.1
Recorded to regulatory assets—gas costs	0	0	0	0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Purchases	0		0	0
Settlements	0	0	0	0
Ending balance	0	1.2	0	1.2
Washington Gas Light Company | Natural Gas Related Derivatives
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	-234.8	-220	-270.6	-133.6
Recorded to income	-37.4	-81.8	-22.7	-105.2
Recorded to regulatory assets—gas costs	-53.9	-110.8	-40.3	-182.8
Transfers into Level 3	5.4		5.4
Transfers out of Level 3	1.4		-0.3
Settlements	19.5	30	28.7	39
Ending balance	($299.80)	($382.60)	($299.80)	($382.60)

Fair_Value_Measurements_Realiz

Fair Value Measurements (Realized and Unrealized Gains and Losses with Level 3 Measurements) (Details) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
WGL Holdings, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recorded to regulatory assets—gas costs	($53.90)	($110.80)	($40.30)	($182.80)
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	-37.3	-74.7	-27.9	-129.2
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	-47.7	-101.1	-28.8	-169.9
Total	-76.2	-176.3	-49	-294
WGL Holdings, Inc. | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recorded to regulatory assets—gas costs	-53.9	-110.8	-40.3	-182.8
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	-37.7	-85.9	-17.4	-132.6
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	-47.7	-101.1	-28.8	-169.9
Total	-76.9	-175	-43.2	-289
WGL Holdings, Inc. | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recorded to regulatory assets—gas costs	0	0	0	0
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	0.4	11.2	-10.5	3.3
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	0	0	0	0
Total	0.7	-1.3	-5.8	-5.1
WGL Holdings, Inc. | Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recorded to regulatory assets—gas costs	0	0	0	0
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	0	0	0	0.1
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	0	0	0	0
Total	0	0	0	0.1
WGL Holdings, Inc. | Operating revenues—non-utility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	-5.7	-15	25.6	-50.8
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	-9.2	-33.4	24.5	-63.7
WGL Holdings, Inc. | Operating revenues—non-utility | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	-8.9	-0.5	1.6	-26.3
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	-9.3	-5	3.6	-30.3
WGL Holdings, Inc. | Operating revenues—non-utility | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	3.2	-14.5	24	-24.5
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	0.1	-28.4	20.9	-33.4
WGL Holdings, Inc. | Operating revenues—non-utility | Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	0	0	0	0
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	0	0	0	0
WGL Holdings, Inc. | Utility cost of gas
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	-37.4	-81.8	-22.7	-105.2
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	-31.7	-68.1	-19.8	-92.1
WGL Holdings, Inc. | Utility cost of gas | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	-37.4	-81.8	-22.7	-105.2
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	-31.7	-68.1	-19.8	-92.1
WGL Holdings, Inc. | Utility cost of gas | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	0	0	0	0
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	0	0	0	0
WGL Holdings, Inc. | Utility cost of gas | Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	0	0	0	0
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	0	0	0	0
WGL Holdings, Inc. | Non-utility cost of energy-related sales
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	5.8	22.1	-30.8	26.7
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	12.4	26.3	-24.9	31.6
WGL Holdings, Inc. | Non-utility cost of energy-related sales | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	8.6	-3.6	3.7	-1.1
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	11.8	-0.8	1.8	3.3
WGL Holdings, Inc. | Non-utility cost of energy-related sales | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	-2.8	25.7	-34.5	27.8
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	0.6	27.1	-26.7	28.3
WGL Holdings, Inc. | Non-utility cost of energy-related sales | Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	0	0	0	0
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	0	0	0	0
WGL Holdings, Inc. | Other income-net
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements		0		0.1
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income		0		0.1
WGL Holdings, Inc. | Other income-net | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements		0		0
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income		0		0
WGL Holdings, Inc. | Other income-net | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements		0		0
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income		0		0
WGL Holdings, Inc. | Other income-net | Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements		0		0.1
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income		0		0.1
Washington Gas Light Company | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recorded to regulatory assets—gas costs	-53.9	-110.8	-40.3	-182.8
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	-37.4	-81.8	-22.7	-105.2
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	-47.7	-101.1	-28.8	-169.9
Total	-79.4	-169.2	-48.6	-262
Washington Gas Light Company | Utility cost of gas | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Realized and unrealized gains (losses) recorded to income for Level 3 measurements	-37.4	-81.8	-22.7	-105.2
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	($31.70)	($68.10)	($19.80)	($92.10)

Fair_Value_Measurements_Carryi

Fair Value Measurements (Carrying Amounts and Estimated Fair Value) (Details) (USD $)	Mar. 31, 2015		Sep. 30, 2014
WGL Holdings, Inc.
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial Paper	$175,000,000		$453,500,000
Washington Gas Light Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commercial Paper	0		89,000,000
Carrying Amount | WGL Holdings, Inc.
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money Market Funds	14,600,000	[1]	9,700,000	[1]
Other Short-term Investments	200,000	[1]	0	[1]
Commercial Paper	175,000,000	[2]	453,500,000	[2]
Long-term debt	950,500,000	[3]	679,200,000	[3]
Carrying Amount | Washington Gas Light Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money Market Funds	10,200,000	[1]	4,300,000	[1]
Other Short-term Investments	200,000	[1]	0	[1]
Commercial Paper	0	[2]	89,000,000	[2]
Long-term debt	702,200,000	[3]	679,200,000	[3]
Fair Value | WGL Holdings, Inc.
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money Market Funds	14,600,000	[1]	9,700,000	[1]
Other Short-term Investments	200,000	[1]	0	[1]
Commercial Paper	175,000,000	[2]	453,500,000	[2]
Long-term debt	1,117,900,000	[3]	809,300,000	[3]
Fair Value | Washington Gas Light Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money Market Funds	10,200,000	[1]	4,300,000	[1]
Other Short-term Investments	200,000	[1]	0	[1]
Commercial Paper	0	[2]	89,000,000	[2]
Long-term debt	$864,100,000	[3]	$809,300,000	[3]
[1]	Balance is located in cash and cash equivalents in the accompanying balance sheets. These amounts may be offset by outstanding checks.
[2]	Balance is located in notes payable in the accompanying balance sheets.
[3]	Less current maturities and unamortized discounts.

Fair_Value_Measurements_Narrat

Fair Value Measurements (Narrative) (Details) (USD $)	6 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Impairment loss	$5.60

Operating_Segment_Narrative_De

Operating Segment (Narrative) (Details)	6 Months Ended
	Mar. 31, 2015
	segment
Segment Reporting [Abstract]
Number of Operating Segments	4

Operating_Segment_Details

Operating Segment (Details) (USD $)	3 Months Ended				6 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014		Mar. 31, 2015		Mar. 31, 2014
Regulated utility
Operating Segment Financial Information [Line Items]
Operating Revenues	$615,694	[1]	$716,808	[1]	$1,002,887	[1]	$1,107,223	[1]
Depreciation and Amortization	27,266		25,736		54,218		51,105
Equity in Earnings of Unconsolidated Affiliates	0		0		0		0
EBIT	130,280		88,535		244,907		153,988
Total Assets	4,245,563		4,013,934		4,245,563		4,013,934
Capital Expenditures	70,691		50,656		160,294		105,749
Equity Method Investments	0		0		0		0
Retail energy-marketing
Operating Segment Financial Information [Line Items]
Operating Revenues	404,601	[1]	437,434	[1]	735,090	[1]	760,372	[1]
Depreciation and Amortization	152		180		319		354
Equity in Earnings of Unconsolidated Affiliates	0		0		0		0
EBIT	38,426		-1,390		22,531		3,900
Total Assets	457,847		402,675		457,847		402,675
Capital Expenditures	3		13		37		88
Equity Method Investments	0		0		0		0
Commercial energy systems
Operating Segment Financial Information [Line Items]
Operating Revenues	11,532	[1]	8,036	[1]	21,071	[1]	12,753	[1]
Depreciation and Amortization	2,671		1,551		4,906		2,644
Equity in Earnings of Unconsolidated Affiliates	502		479		1,079		763
EBIT	722		1,545		981		949
Total Assets	553,515		423,803		553,515		423,803
Capital Expenditures	10,658		38,497		53,975		41,635
Equity Method Investments	65,214		68,517		65,214		68,517
Midstream energy services
Operating Segment Financial Information [Line Items]
Operating Revenues	-20,288	[1]	27,104	[1]	7,804	[1]	-6,069	[1]
Depreciation and Amortization	31		31		62		62
Equity in Earnings of Unconsolidated Affiliates	611		64		1,147		270
EBIT	-23,507		21,605		3,264		-12,604
Total Assets	217,032		102,148		217,032		102,148
Capital Expenditures	0		0		0		0
Equity Method Investments	49,468		13,692		49,468		13,692
Other activities
Operating Segment Financial Information [Line Items]
Operating Revenues	0	[1]	0	[1]	0	[1]	0	[1]
Depreciation and Amortization	0		0		0		0
Equity in Earnings of Unconsolidated Affiliates	719		0		750		0
EBIT	-846		-5,175		-7,945		-7,836
Total Assets	128,624		232,407		128,624		232,407
Capital Expenditures	0		0		0		0
Equity Method Investments	0		413		0		413
Eliminations
Operating Segment Financial Information [Line Items]
Operating Revenues	-9,806	[1],[2]	-15,132	[1],[2]	-15,882	[1],[2]	-19,732	[1],[2]
Depreciation and Amortization	-17	[2]	-194	[2]	-42	[2]	-271	[2]
Equity in Earnings of Unconsolidated Affiliates	0	[2]	0	[2]	0	[2]	0	[2]
EBIT	-19	[2]	498	[2]	-51	[2]	642	[2]
Total Assets	-452,218	[2]	-412,750	[2]	-452,218	[2]	-412,750	[2]
Capital Expenditures	0	[2]	0	[2]	0	[2]	0	[2]
Equity Method Investments	0	[2]	0	[2]	0	[2]	0	[2]
Total consolidated
Operating Segment Financial Information [Line Items]
Operating Revenues	1,001,733	[1]	1,174,250	[1]	1,750,970	[1]	1,854,547	[1]
Depreciation and Amortization	30,103		27,304		59,463		53,894
Equity in Earnings of Unconsolidated Affiliates	1,832		543		2,976		1,033
EBIT	145,056		105,618		263,687		139,039
Total Assets	5,150,363		4,762,217		5,150,363		4,762,217
Capital Expenditures	81,352		89,166		214,306		147,472
Equity Method Investments	$114,682		$82,622		$114,682		$82,622
[1]	Operating revenues are reported gross of revenue taxes. Revenue taxes of both the regulated utility and the retail energy-marketing segments include gross receipt taxes. Revenue taxes of the regulated utility segment also include PSC fees, franchise fees and energy taxes. Operating revenue amounts in the “Eliminations” row represent total intersegment revenues associated with sales from the regulated utility segment to the retail energy-marketing segment. Midstream Energy Services’ cost of energy related sales is netted with its gross revenues.
[2]	Intersegment eliminations include a timing difference between Commercial Energy Systems’ recognition of revenue for the sale of Solar Renewable Energy Credits (SRECs) to Retail Energy-Marketing and Retail Energy-Marketing’s recognition of the associated expense. Retail Energy-Marketing has recorded a portion of the SREC’s purchased as inventory to be used in future periods at which time they will be expensed.

Operating_Segment_Details_2

Operating Segment (Details 2) (Total consolidated, USD $)	3 Months Ended		6 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Total consolidated
Reconciliation from EBIT to Net Income Applicable to Common Stock [Line Items]
Total consolidated EBIT	$145,056	$105,618	$263,687	$139,039
Interest expense	13,254	9,525	25,564	18,517
INCOME BEFORE INCOME TAXES	131,802	96,093	238,123	120,522
Income tax expense	50,017	34,550	92,120	40,020
NET INCOME	81,785	61,543	146,003	80,502
Dividends on Washington Gas preferred stock	330	330	660	660
NET INCOME APPLICABLE TO COMMON STOCK	$81,455	$61,213	$145,343	$79,842

Other_Investments_Narrative_De

Other Investments Narrative (Details) (USD $)	6 Months Ended
	Mar. 31, 2015	Sep. 30, 2014	Mar. 10, 2015
			MMBTU
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Impairment loss	$5,600,000
WGL Holdings, Inc.
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Gain on sale of investments	700,000
Investments in unconsolidated affiliates	114,682,000	100,528,000
Investments in direct financing leases, capital leases	20,227,000	18,159,000
WGL Holdings, Inc. | Meade
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Pipeline length in miles	177
Transportation and delivery capacity in dekatherms per day	1,700,000
Estimated Investment in Meade	410,000,000
Ownership percentage	55.00%
Investments in unconsolidated affiliates	16,000,000
Maximum financial exposure	59,400,000
WGL Holdings, Inc. | Constitution
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership percentage	10.00%
Investments in unconsolidated affiliates	27,600,000
Estimated investment In constitution	79,000,000
Willams Partners L.P. | Constitution
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership percentage	41.00%
Cabot Oil And Gas Corporation | Constitution
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership percentage	25.00%
Piedmont Natural Gas | Constitution
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership percentage	24.00%
COG Holdings LLC | Meade
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership percentage	20.00%
Vega Midstream MPC LLC | Meade
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership percentage	15.00%
River Road Interests LLC | Meade
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership percentage	10.00%
Washington Gas Resources Corp. | American Solar Direct Holdings Inc. (ASDHI)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost method investments		5,600,000
Impairment loss	5,600,000
WGSW
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Investments in direct financing leases, capital leases	26,900,000
Residual value	6,200,000
Tax credits	2,400,000
Unearned income	12,000,000
Initial direct costs	700,000
WGSW | Sun Edison/Nextility
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Investments in direct financing leases, capital leases	23,500,000	19,900,000
Current investment recorded in accounts receivable	3,300,000	1,700,000
WGSW | ASD Solar, LP
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Investments in unconsolidated affiliates	65,200,000
Difference between carrying amount and underlying equity	35,900,000
WGL Midstream | Mountain Valley Pipeline
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Transportation and delivery capacity in dekatherms per day	2,000,000
Ownership percentage	7.00%
Pro rata share of project cost	2,500,000
Investments in unconsolidated affiliates	5,800,000
Mountain Valley Gas Purchase Commitment | WGL Midstream
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Transportation and delivery capacity in dekatherms per day			500,000
External Partners | WGL Midstream | Mountain Valley Pipeline
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Maximum financial exposure	96,000,000
External Partners | WGL Midstream | Mountain Valley Pipeline
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership interest in joint venture after funding requirements are met	3.00%
Scenario, Plan | WGL Midstream | Mountain Valley Pipeline
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Pro rata share of project cost	230,000,000
Scenario, Plan | Minimum | WGL Midstream | Mountain Valley Pipeline
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Estimated investment in Mountain Valley Pipeline	3,000,000,000
Scenario, Plan | Maximum | WGL Midstream | Mountain Valley Pipeline
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Estimated investment in Mountain Valley Pipeline	3,500,000,000
Performance guarantee | WGL Midstream | Mountain Valley Pipeline
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Maximum financial exposure	20,000,000
Performance guarantee | External Partners | WGL Midstream | Mountain Valley Pipeline
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Maximum financial exposure	6,000,000
Performance guarantee | WGL Midstream | WGL Midstream | Mountain Valley Pipeline
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Maximum financial exposure	$14,000,000

Other_Investments_Financing_Le

Other Investments Financing Leases Table (Details) (WGSW, USD $)	Mar. 31, 2015
WGSW
Minimum Payments Receivable for Direct Financing Leases
Remainder of 2015	$2,800,000
2016	2,200,000
2017	2,100,000
2018	2,000,000
2019	1,900,000
Thereafter	15,900,000
Total	$26,900,000

Other_Investments_Balance_Shee

Other Investments Balance Sheet Location Table (Details) (USD $)	Mar. 31, 2015		Sep. 30, 2014
VIEs
Assets [Abstract]
Investments in unconsolidated affiliates	$81,200,000		$72,600,000
Investments in direct financing leases, capital leases	20,200,000		18,200,000
Accounts receivable	3,300,000		1,700,000
Total assets	104,700,000		92,500,000
Non VIEs
Assets [Abstract]
Investments in unconsolidated affiliates	33,500,000		27,900,000
Investments in direct financing leases, capital leases	0		0
Accounts receivable	2,500,000	[1]	0
Total assets	36,000,000		27,900,000
Total
Assets [Abstract]
Investments in unconsolidated affiliates	114,700,000		100,500,000
Investments in direct financing leases, capital leases	20,200,000		18,200,000
Accounts receivable	5,800,000		1,700,000
Total assets	$140,700,000		$120,400,000
[1]	Represents the financing provided to another partner in Mountain Valley to fund its capital commitment. Acquired ownership interest represents the collateral for repayment of the financing.

Other_Investments_Income_State

Other Investments Income Statement Location Table (Details) (USD $)	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
VIEs
Net Income (Loss) Attributable to Parent [Abstract]
Equity in earnings of unconsolidated affiliates	$1,000,000	$500,000	$1,700,000	$800,000
Depreciation and amortization	0	0	100,000	100,000
Other income (expenses) - net	800,000	800,000	1,400,000	1,300,000
NET INCOME	1,800,000	1,300,000	3,000,000	2,000,000
Non VIEs
Net Income (Loss) Attributable to Parent [Abstract]
Equity in earnings of unconsolidated affiliates	800,000	0	1,300,000	200,000
Depreciation and amortization	0	0	0	0
Other income (expenses) - net	0	0	-5,600,000	0
NET INCOME	800,000	0	-4,300,000	200,000
Total Other Investments
Net Income (Loss) Attributable to Parent [Abstract]
Equity in earnings of unconsolidated affiliates	1,800,000	500,000	3,000,000	1,000,000
Depreciation and amortization	0	0	100,000	100,000
Other income (expenses) - net	800,000	800,000	-4,200,000	1,300,000
NET INCOME	$2,600,000	$1,300,000	($1,300,000)	$2,200,000

Related_Party_Transactions_Det

Related Party Transactions (Details) (USD $)	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2014
Washington Gas Light Company
Related Party Transaction [Line Items]
Receivables from associated companies	$3,400,000		$3,400,000		$4,800,000
Payables to associated companies	68,900,000		68,900,000		54,700,000
WGL Energy Services | Washington Gas Light Company
Related Party Transaction [Line Items]
Accounts payable, related parties	9,500,000		9,500,000
Accounts receivable, related parties					20,000
Washington Gas Light Company
Related Party Transaction [Line Items]
Receivables from associated companies	3,387,000		3,387,000		4,821,000
Payables to associated companies	68,942,000		68,942,000		54,685,000
Washington Gas Light Company | WGL Energy Services
Related Party Transaction [Line Items]
Gas balancing service charge	9,200,000	14,400,000	14,700,000	18,200,000
Receivables purchased from related party	$38,300,000		$38,300,000		$7,700,000

Commitments_and_Contingencies_1

Commitments and Contingencies (Narrative) (Details) (USD $)	6 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Nov. 30, 2014	Mar. 10, 2015
		MMBTU	MMBTU
WGL Holdings, Inc. | GAIL Global (USA) LNG, LLC
Commitments And Contingencies [Line Items]
Gas sale and purchase and capacity agreement terms	20 years
WGL Holdings, Inc. | WGL Energy Services
Commitments And Contingencies [Line Items]
Recorded unconditional purchase obligation	239.4
WGL Holdings, Inc. | WGL Midstream
Commitments And Contingencies [Line Items]
Recorded unconditional purchase obligation	305.9
WGL Holdings, Inc. | Other Subsidiaries
Commitments And Contingencies [Line Items]
Recorded unconditional purchase obligation	8.6
WGL Holdings, Inc. | External Partners
Commitments And Contingencies [Line Items]
Recorded unconditional purchase obligation	8.1
WGL Holdings, Inc. | Minimum | GAIL Global (USA) LNG, LLC
Commitments And Contingencies [Line Items]
Transportation and delivery capacity in dekatherms per day		340,000
WGL Holdings, Inc. | Maximum | GAIL Global (USA) LNG, LLC
Commitments And Contingencies [Line Items]
Transportation and delivery capacity in dekatherms per day		430,000
WGL Midstream | Mountain Valley Gas Purchase Commitment
Commitments And Contingencies [Line Items]
Transportation and delivery capacity in dekatherms per day			500,000
Gas sale and purchase and capacity agreement terms	20 years

Commitments_and_Contingencies_2

Commitments and Contingencies (Commitments) (Details) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Remainder of 2015	$372.90
2016	749.5
2017	574.4
2018	1,254
2019	1,819.80
Thereafter	37,373.20
Total	42,143.80
Washington Gas Light Company | Gas Purchase Commitments
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Remainder of 2015	93.2	[1]
2016	365.6	[1]
2017	463.5	[1]
2018	466.1	[1]
2019	459.4	[1]
Thereafter	5,679	[1]
Total	7,526.80	[1]
Washington Gas Light Company | Pipeline Contracts
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Remainder of 2015	92.4	[2]
2016	218.7	[2]
2017	210.5	[2]
2018	211.3	[2]
2019	202.7	[2]
Thereafter	1,172.60	[2]
Total	2,108.20	[2]
WGL Energy Services | Gas Purchase Commitments
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Remainder of 2015	75.9	[3]
2016	120.7	[3]
2017	49.1	[3]
2018	1.4	[3]
2019	0	[3]
Thereafter	0	[3]
Total	247.1	[3]
WGL Energy Services | Pipeline Contracts
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Remainder of 2015	1.6	[4]
2016	1.1	[4]
2017	0.8	[4]
2018	0.8	[4]
2019	0.8	[4]
Thereafter	1.6	[4]
Total	6.7	[4]
WGL Energy Services | Electric Purchase Commitments
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Remainder of 2015	255	[5]
2016	360.6	[5]
2017	131.3	[5]
2018	17.4	[5]
2019	0.2	[5]
Thereafter	0	[5]
Total	764.5	[5]
Commitments related to renewable energy credits	19.1
WGL Midstream | Gas Purchase Commitments
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Remainder of 2015	27.3	[6]
2016	246.8	[6]
2017	374.8	[6]
2018	1,219.20	[6]
2019	1,757.90	[6]
Thereafter	36,273.80	[6]
Total	39,899.80	[6]
WGL Midstream | Pipeline Contracts
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Remainder of 2015	13.1	[4]
2016	20.3	[4]
2017	18.4	[4]
2018	15.2	[4]
2019	60.9	[4]
Thereafter	1,097.80	[4]
Total	$1,225.70	[4]
[1]	Includes known and reasonably likely commitments to purchase natural gas. Cost estimates are based on market prices at March 31, 2015.
[2]	Represents minimum payments for natural gas transportation, storage and peaking contracts that have expiration dates through fiscal year 2031.
[3]	Represents fixed price commitments with city gate equivalent deliveries.
[4]	Represents minimum payments for natural gas transportation and storage contracts that have expiration dates through fiscal year 2044.
[5]	Represents electric purchase commitments that are based on existing fixed price and fixed volume contracts. Includes $19.1 million of commitments related to renewable energy credits.
[6]	Includes known and reasonably likely commitments to purchase natural gas. Cost estimates are based on forward market prices as of March 31, 2015.

Pension_and_Other_PostRetireme2

Pension and Other Post-Retirement Benefit Plans (Details) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$3.80	$3.50	$7.70	$7
Interest cost	9.8	10.1	19.6	20.2
Expected return on plan assets	-11.1	-10.2	-22.3	-20.5
Amortization of prior service cost (credit)	0	0	0.1	0.1
Amortization of actuarial loss	4.7	4.2	9.4	8.4
Net periodic benefit cost	7.2	7.6	14.5	15.2
Amount allocated to construction projects	-1.1	-1	-2.2	-2
Amount deferred as regulatory asset (liability)-net	1.7	1.8	3.5	3.6
Amount charged (credited) to expense	7.8	8.4	15.8	16.8
Health and Life Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.8	2.2	3.5	4.3
Interest cost	3.6	5.6	7.3	11
Expected return on plan assets	-5.2	-4.8	-10.4	-9.5
Amortization of prior service cost (credit)	-3.8	-1	-7.6	-2
Amortization of actuarial loss	1.1	1.2	2.2	1.9
Net periodic benefit cost	-2.5	3.2	-5	5.7
Amount allocated to construction projects	0.5	-0.4	1	-0.8
Amount deferred as regulatory asset (liability)-net	0	0.1	-0.1	0.2
Amount charged (credited) to expense	($2)	$2.90	($4.10)	$5.10

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Details) (USD $)	3 Months Ended				6 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014		Mar. 31, 2015		Mar. 31, 2014
WGL Holdings, Inc.
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	($11,966)		($10,849)		($7,961)		($11,048)
Qualified cash flow hedging instruments	222	[1]	0	[1]	-8,042	[1]	0	[1]
Change in prior service credit	-171	[2]	-35	[2]	-342	[2]	-69	[2]
Change in actuarial net loss (gain)	491	[2]	-164	[2]	974	[2]	200	[2]
Current-period other comprehensive income (loss)	542		-199		-7,410		131
INCOME TAX EXPENSE (BENEFIT) RELATED TO OTHER COMPREHENSIVE INCOME	219		-79		-3,728		52
Ending Balance	-11,643		-10,969		-11,643		-10,969
Washington Gas Light Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	-6,224		-10,849		-6,413		-11,048
Change in prior service credit	-171	[3]	-35	[3]	-342	[3]	-69	[3]
Change in actuarial net loss (gain)	491	[3]	-164	[3]	974	[3]	200	[3]
Current-period other comprehensive income (loss)	320		-199		632		131
INCOME TAX EXPENSE (BENEFIT) RELATED TO OTHER COMPREHENSIVE INCOME	127		-79		250		52
Ending Balance	($6,031)		($10,969)		($6,031)		($10,969)
[1]	Cash flow hedging instruments represent interest rate swaps agreements on debt issuances. Refer to Note 8- Derivative and weather-related instruments for further discussion of the interest rate swap agreements.
[2]	These accumulated other comprehensive income components are included in the computation of net periodic benefit cost. Refer to Note 14- Pension and other post-retirement benefit plans for additional details.
[3]	These accumulated other comprehensive income components are included in the computation of net periodic benefit cost. Refer to Note 14-Pension and other post-retirement benefit plans for additional details.

Subsequent_Events_Details

Subsequent Events (Details) (Subsequent event, USD $)	0 Months Ended
In Millions, unless otherwise specified	6-May-15	6-May-15
		well
Subsequent event
Subsequent Event [Line Items]
Term of agreement	20 years
Number of wells	25	25
Cost of acquired non-operating working interest	$126
Decision period	180 days