WGL (WGL) 10-Q 8 Feb 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Dec. 31, 2017	Jan. 31, 2018
Document Information [Line Items]
Entity Registrant Name	WGL Holdings Inc.
Entity Central Index Key	1,103,601
Document Type	10-Q
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		51,359,182
Washington Gas Light Company
Document Information [Line Items]
Entity Registrant Name	Washington Gas Light Company
Entity Central Index Key	104,819
Document Type	10-Q
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--09-30
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		46,479,536

Balance Sheets (Unaudited)

Balance Sheets (Unaudited) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Property, Plant and Equipment
At original cost	$ 6,217,878	$ 6,143,841
Accumulated depreciation and amortization	(1,538,998)	(1,513,790)
Net property, plant and equipment	4,678,880	4,630,051
Current Assets
Cash and cash equivalents	64,110	8,524
Receivables
Accounts receivable	519,980	398,149
Gas costs and other regulatory assets	17,663	21,705
Unbilled revenues	278,256	165,483
Allowance for doubtful accounts	(32,451)	(32,025)
Net receivables	783,448	553,312
Materials and supplies—principally at average cost	20,456	20,172
Storage gas	206,978	243,984
Prepaid taxes	38,824	31,549
Other prepayments	74,612	86,465
Derivatives	17,665	15,327
Other	40,174	26,556
Total current assets	1,246,267	985,889
Regulatory assets
Gas costs	117,474	90,136
Pension and other post-retirement benefits	132,837	139,499
Other	117,962	104,596
Prepaid post-retirement benefits	234,982	231,577
Derivatives	32,168	38,389
Investments in unconsolidated affiliates	489,354	394,201
Other	12,650	11,671
Total deferred charges and other assets	1,137,427	1,010,069
Total Assets	7,062,574	6,626,009
Capitalization
WGL Holdings common shareholders’ equity	1,609,607	1,502,690
Non-controlling interest	3,210	6,851
Washington Gas Light Company preferred stock	28,173	28,173
Total equity	1,640,990	1,537,714
Long-term debt	1,679,893	1,430,861
Total capitalization	3,320,883	2,968,575
Current Liabilities
Current maturities of long-term debt	300,000	250,000
Notes payable and project financing	558,700	559,844
Accounts payable and other accrued liabilities	431,327	423,824
Wages payable	22,146	18,096
Accrued interest	17,669	7,806
Dividends declared	26,520	26,452
Customer deposits and advance payments	55,158	65,841
Gas costs and other regulatory liabilities	57,275	22,814
Accrued taxes	35,676	17,657
Derivatives	35,021	43,990
Other	50,002	52,664
Total current liabilities	1,589,494	1,488,988
Deferred Credits
Unamortized investment tax credits	153,502	155,007
Deferred income taxes	413,065	868,067
Accrued pensions and benefits	184,293	181,552
Asset retirement obligations	300,130	296,810
Regulatory liabilities
Accrued asset removal costs	286,234	292,173
Other post-retirement benefits	130,882	135,035
Other—principally deferred taxes	452,091	9,403
Derivatives	128,885	122,607
Other	103,115	107,792
Total deferred credits	2,152,197	2,168,446
Commitments and Contingencies (Note 13)
Total Capitalization and Liabilities	7,062,574	6,626,009
Washington Gas Light Company
Property, Plant and Equipment
At original cost	5,358,064	5,310,337
Accumulated depreciation and amortization	(1,440,276)	(1,422,622)
Net property, plant and equipment	3,917,788	3,887,715
Current Assets
Cash and cash equivalents	6,652	1
Receivables
Accounts receivable	263,001	190,740
Gas costs and other regulatory assets	17,663	21,705
Unbilled revenues	205,107	107,967
Allowance for doubtful accounts	(24,447)	(23,741)
Net receivables	461,324	296,671
Materials and supplies—principally at average cost	20,410	20,126
Storage gas	84,960	92,753
Prepaid taxes	31,899	23,350
Other prepayments	22,776	13,238
Receivables from associated companies	27,010	32,362
Derivatives	4,343	5,061
Other	120	102
Total current assets	659,494	483,664
Regulatory assets
Gas costs	117,474	90,136
Pension and other post-retirement benefits	132,082	138,573
Other	117,570	104,538
Prepaid post-retirement benefits	233,676	230,283
Derivatives	15,385	16,244
Other	4,459	3,561
Total deferred charges and other assets	620,646	583,335
Total Assets	5,197,928	4,954,714
Capitalization
WGL Holdings common shareholders’ equity	1,295,914	1,164,749
Washington Gas Light Company preferred stock	28,173	28,173
Long-term debt	1,084,619	1,134,461
Total capitalization	2,408,706	2,327,383
Current Liabilities
Current maturities of long-term debt	50,000
Notes payable and project financing	205,772	166,772
Accounts payable and other accrued liabilities	196,363	219,827
Wages payable	20,191	16,508
Accrued interest	15,593	3,967
Dividends declared	22,166	22,098
Customer deposits and advance payments	54,711	64,194
Gas costs and other regulatory liabilities	57,275	22,814
Accrued taxes	46,954	12,808
Payables to associated companies	107,597	94,844
Derivatives	24,636	30,263
Other	7,349	7,473
Total current liabilities	808,607	661,568
Deferred Credits
Unamortized investment tax credits	3,921	4,100
Deferred income taxes	463,805	888,385
Accrued pensions and benefits	182,540	179,814
Asset retirement obligations	294,939	291,871
Regulatory liabilities
Accrued asset removal costs	286,234	292,173
Other post-retirement benefits	130,056	134,181
Other—principally deferred taxes	450,409	9,403
Derivatives	119,048	112,299
Other	49,663	53,537
Total deferred credits	1,980,615	1,965,763
Commitments and Contingencies (Note 13)
Total Capitalization and Liabilities	$ 5,197,928	$ 4,954,714

Consolidated Statements of Inco

Consolidated Statements of Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
OPERATING REVENUES
Utility	$ 374,990	$ 327,063
Non-utility	277,450	282,424
Total Operating Revenues	652,440	609,487
OPERATING EXPENSES
Utility cost of gas	122,273	75,500
Non-utility cost of energy-related sales	225,502	252,886
Operation and maintenance	102,226	100,717
Depreciation and amortization	40,985	35,283
General taxes and other assessments	44,887	40,388
Total Operating Expenses	535,873	504,774
OPERATING INCOME	116,567	104,713
Equity in earnings of unconsolidated affiliates	5,892	265
Other income (expenses) — net	(780)	478
Interest expense	20,197	16,235
INCOME BEFORE INCOME TAXES	101,482	89,221
Income Tax Expense (Benefit)	(31,110)	33,454
NET INCOME	132,592	55,767
Non-controlling interest	(5,778)	(2,535)
Dividends on Washington Gas Light Company preferred stock	330	330
NET INCOME APPLICABLE TO COMMON STOCK	$ 138,040	$ 57,972
WEIGHTED AVERAGE COMMON SHARES OUTSTANDING
Basic (in shares)	51,319	51,172
Diluted (in shares)	51,549	51,445
EARNINGS PER AVERAGE COMMON SHARE
Basic (in dollars per share)	$ 2.69	$ 1.13
Diluted (in dollars per share)	2.68	1.13
DIVIDENDS DECLARED PER COMMON SHARE (in dollars per share)	$ 0.5100	$ 0.4875
Washington Gas Light Company
OPERATING REVENUES
Utility	$ 377,470	$ 333,986
OPERATING EXPENSES
Utility cost of gas	124,745	82,431
Operation and maintenance	79,551	82,166
Depreciation and amortization	33,646	30,126
General taxes and other assessments	39,983	36,255
Total Operating Expenses	277,925	230,978
OPERATING INCOME	99,545	103,008
Other income (expenses) — net	(1,792)	(779)
Interest expense	14,973	12,762
INCOME BEFORE INCOME TAXES	82,780	89,467
Income Tax Expense (Benefit)	24,854	34,006
NET INCOME	57,926	55,461
Dividends on Washington Gas Light Company preferred stock	330	330
NET INCOME APPLICABLE TO COMMON STOCK	$ 57,596	$ 55,131

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Net income		$ 132,592	$ 55,767
OTHER COMPREHENSIVE INCOME, BEFORE INCOME TAXES:
Qualified cash flow hedging instruments	[1]	52	49,455
Pension and other post-retirement benefit plans
Change in net prior service credit	[2]	(274)	(217)
Change in actuarial net loss	[2]	527	588
Total other comprehensive income before taxes		305	49,826
INCOME TAX EXPENSE RELATED TO OTHER COMPREHENSIVE INCOME		61	20,413
OTHER COMPREHENSIVE INCOME		244	29,413
COMPREHENSIVE INCOME		132,836	85,180
Non-controlling interest		(5,778)	(2,535)
Dividends on Washington Gas Light Company preferred stock		330	330
COMPREHENSIVE INCOME ATTRIBUTABLE TO WGL HOLDINGS		138,284	87,385
Washington Gas Light Company
Net income		57,926	55,461
Pension and other post-retirement benefit plans
Change in net prior service credit	[3]	(274)	(217)
Change in actuarial net loss	[3]	527	588
Total other comprehensive income before taxes		253	371
Total pension and other post-retirement benefit plans		253	371
INCOME TAX EXPENSE RELATED TO OTHER COMPREHENSIVE INCOME		61	147
OTHER COMPREHENSIVE INCOME		192	224
COMPREHENSIVE INCOME		58,118	55,685
Dividends on Washington Gas Light Company preferred stock		$ 330	$ 330
[1]	Cash flow hedging instruments represent interest rate swap agreements related to debt issuances. Refer to Note 8- Derivative and Weather-related Instruments of the Notes to Condensed Consolidated Financial Statements for further discussion of the interest rate swap agreements.
[2]	These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost. Refer to Note 14- Pension and other post-retirement benefit plans of the Notes to Condensed Consolidated Financial Statements for additional details.
[3]	These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost. Refer to Note 14-Pension and other post-retirement benefit plans of the Notes to Condensed Consolidated Financial Statements for additional details.

Consolidated Statement of Cash

Consolidated Statement of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES
Net income	$ 132,592	$ 55,767
ADJUSTMENTS TO RECONCILE NET INCOME TO NET CASH PROVIDED BY OPERATING ACTIVITIES
Depreciation and amortization	40,985	35,283
Amortization of:
Other regulatory assets and liabilities—net	1,634	823
Debt related costs	550	445
Deferred income taxes	(29,264)	34,852
Dividends received from equity method investments	5,628
Accrued/deferred pension and other post-retirement benefit cost	2,393	5,424
Earnings in equity interest	(5,892)	265
Compensation expense related to stock-based awards	5,802	4,912
Provision for doubtful accounts	3,781	2,704
Unrealized (gain) loss on derivative contracts	12,524	(30,339)
Amortization of investment tax credits	(1,845)	(1,396)
Other non-cash credits—net	(197)	2,105
Changes in operating assets and liabilities (Note 16)	(175,402)	(201,340)
Net Cash Provided by Operating Activities	(6,711)	(90,495)
FINANCING ACTIVITIES
Common stock issued		251
Long-term debt issued	300,000
Debt issuance costs	(1,482)	(375)
Notes payable issued —net	(1,144)	324,001
Contributions from non-controlling interest	1,659	7,331
Distributions from non-controlling interest	(136)
Project financing		8,877
Dividends on common stock and preferred stock	(26,520)	(23,921)
Other financing activities—net	(6,558)	(1,295)
Net Cash Provided by Financing Activities	265,819	314,869
INVESTING ACTIVITIES
Capital expenditures (excluding Allowance for Funds Used During Construction)	(99,054)	(155,758)
Investments in non-utility interests	(104,468)	(62,007)
Distributions and receipts from non-utility interests		1,565
Loans to external parties		(863)
Net Cash Used in Investing Activities	(203,522)	(217,063)
INCREASE IN CASH AND CASH EQUIVALENTS	55,586	7,311
Cash and Cash Equivalents at Beginning of Year	8,524	5,573
Cash and Cash Equivalents at End of Period	64,110	12,884
Washington Gas Light Company
OPERATING ACTIVITIES
Net income	57,926	55,461
ADJUSTMENTS TO RECONCILE NET INCOME TO NET CASH PROVIDED BY OPERATING ACTIVITIES
Depreciation and amortization	33,646	30,126
Amortization of:
Other regulatory assets and liabilities—net	1,634	823
Debt related costs	401	353
Deferred income taxes	7,386	34,145
Accrued/deferred pension and other post-retirement benefit cost	2,398	5,411
Compensation expense related to stock-based awards	4,518	4,652
Provision for doubtful accounts	3,928	2,241
Unrealized (gain) loss on derivative contracts	1,446	(15,128)
Amortization of investment tax credits	(179)
Other non-cash credits—net	(197)	2,447
Changes in operating assets and liabilities (Note 16)	(141,364)	(169,830)
Net Cash Provided by Operating Activities	(28,457)	(49,299)
FINANCING ACTIVITIES
Capital contributions from WGL Holdings, Inc.	100,000
Debt issuance costs	(236)	(375)
Notes payable issued —net	39,000	177,000
Project financing		5,200
Dividends on common stock and preferred stock	(22,166)	(21,453)
Other financing activities—net	(6,197)	(1,226)
Net Cash Provided by Financing Activities	110,401	159,146
INVESTING ACTIVITIES
Capital expenditures (excluding Allowance for Funds Used During Construction)	(75,293)	(109,847)
Net Cash Used in Investing Activities	(75,293)	(109,847)
INCREASE IN CASH AND CASH EQUIVALENTS	6,651	0
Cash and Cash Equivalents at Beginning of Year	1	1
Cash and Cash Equivalents at End of Period	$ 6,652	$ 1

Accounting Policies

Accounting Policies	3 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Accounting Policies	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, and 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 condensed 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, 2017 . 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, 2018 and 2017 of either WGL or Washington Gas. The accompanying unaudited condensed 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 Condensed Consolidated Financial Statements of the combined Annual Report on Form 10-K for WGL and Washington Gas for the fiscal year ended September 30, 2017 . Change in Accounting Principle and Storage Gas Valuation On October 1, 2017, Washington Gas and WGL Energy Services implemented a voluntary change in the application of an accounting principle with respect to accounting for natural gas, propane, and odorant inventories. Washington Gas and WGL Energy Services now apply the average cost methodology under which the cost of units carried in inventory is based on the weighted average cost per unit of inventory. Prior to this change, Washington Gas and WGL Energy Services applied the First-in First-out (FIFO) methodology of accounting for inventory under which the oldest inventory items were recorded as being sold first. We believe the new policy is preferable as it conforms to the method predominately used by our peers, better reflects the physical flow of inventory, conforms to the method used for certain of our other inventories, and will simplify recordkeeping requirements. The change in accounting principle was implemented on a prospective basis, therefore, we did not retrospectively adjust any prior periods or record a cumulative effect adjustment, as discussed below. Washington Gas implemented the change in accounting principle on a prospective basis in accordance with Accounting Standards Codification (ASC) No. 980, Regulated Operations which permits regulated entities to implement changes for financial reporting purposes in the same way those changes are implemented for regulatory reporting purposes when the change impacts allowable costs. WGL Energy Services implemented the change on a prospective basis as the impact on its financial statements for all periods presented, including the cumulative effect at October 1, 2017, was immaterial. The difference during the quarter between the prior FIFO method and the new average cost method was immaterial. WGL Midstream continues to account for its inventory using the weighted average cost method. On October 1, 2017, WGL adopted ASU 2015-11, Inventory (Topic 330) - Simplifying the Measurement of Inventory . This standard modified the previous calculation for valuing inventory. As a result of the new standard, beginning October 1, 2017, our inventory balances are stated at the lower-of-cost or net realizable value. Prior to October 1, 2017, our inventory balances were stated at the lower-of-cost or market. Interim period inventory losses attributable to lower-of-cost or net realizable value adjustments may be reversed if the net realizable value of the inventory is recovered by the end of the same fiscal year. For more information see ASU 2015-11 in the accounting standards adopted in fiscal year 2018 table below . For the three months ended December 31, 2017, WGL and Washington Gas did not record any lower-of-cost or net realizable value adjustments. For the three months ended December 31, 2016, we did not record any lower-of-cost or market adjustments. ACCOUNTING STANDARDS ADOPTED IN FISCAL YEAR 2018 Standard Description Date of adoption Effect on the financial statements or other significant matters ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting This standard simplifies several aspects of the accounting for share-based payment transactions, including accounting for income taxes, forfeitures, statutory tax withholding requirements and classification in the statement of cash flows. October 1, 2017 Forfeitures - WGL has elected to continue to estimate forfeitures for its share-based payment awards rather than account for forfeitures when they occur. Income Taxes - On October 1, 2017, WGL and Washington Gas recorded $4.3 million and $4.2 million, respectively, on a modified retrospective basis, as a cumulative effect adjustment to retained earnings. For the three months ended December 31, 2017, WGL and Washington Gas recorded $3.4 million and $3.2 million, respectively, to current tax expense for excess tax benefits related to performance shares that vested in the quarter. Cash Flows - WGL and Washington Gas reclassified $3.6 million and $3.5 million, respectively, retroactively on the statement of cash flows for the three months ended December 31, 2016 from operating to financing activities related to shares withheld to pay for employee taxes. Statutory Tax Withholding - No changes were made. ASU 2016-06, Derivatives and Hedging (Topic 815) - Contingent Put and Call Options in Debt Instruments The amendments in this update clarify the requirements for assessing whether contingent call (put) options that can accelerate the payment of principal on debt instruments are clearly and closely related to their debt hosts. The guidance states that for contingent call (put) options to be considered clearly and closely related, they can be indexed only to interest rates or credit risk. An entity is required to assess the embedded call (put) options solely in accordance with the four-step decision sequence. October 1, 2017 The implementation of this standard did not have an effect on WGL or Washington Gas' financial statements. ASU 2015-11, Inventory (Topic 330) - Simplifying the Measurement of Inventory This standard reduces the complexity in the current measurement of inventory. This ASU requires inventory to be measured at the lower of cost and net realizable value, where net realizable value is the estimated selling price in the ordinary course of business less reasonably predictable costs of completion, disposal and transportation (no change to the definition of net realizable value). The amendment eliminates the guidance that requires inventory to be stated at the lower-of-cost or market, which includes consideration of the replacement cost of inventory and the net realizable value of inventory, less an approximately normal profit margin. October 1, 2017 The implementation of this standard did not have a material effect on WGL or Washington Gas' financial statements. OTHER NEWLY ISSUED ACCOUNTING STANDARDS Standard Description Required date of adoption Effect on the financial statements or other significant matters ASU 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost This standard requires entities to report the service cost component in the same financial statement line item as other compensation costs arising from services rendered by the pertinent employees during the period. The other components of net benefit cost are to be presented separately from service cost and outside of operating income. In addition, only the service cost component of net benefit cost is eligible for capitalization. Changes to the presentation of service costs and other components of net benefit cost should be applied retrospectively. Changes in capitalization practices should be implemented prospectively. October 1, 2018 We are currently evaluating the interaction of this standard with the various regulatory provisions concerning pensions and post-retirement benefit costs. We anticipate that the change in capitalization of retirement benefits will not have a material impact on WGL or Washington Gas' financial statements. ASU 2016-15, Statement of Cash Flows (Topic 230)—Classification of Certain Cash Receipts and Cash Payments (a consensus of the FASB Emerging Issues Task Force) This update provides guidance on the classification of certain cash receipts and payments in the statement of cash flows. October 1, 2018* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. We do not anticipate that adoption of this standard will have a material effect on WGL or Washington Gas' financial statements. ASU 2014-09, Revenue from Contracts with Customers (Topic 606), including subsequent ASUs clarifying the guidance. ASU 2014-09 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. October 1, 2018 An implementation team is currently evaluating all revenue streams and reviewing contracts with customers, as well as, related financial statement disclosures to determine the impact the adoption of this standard will have on our financial statements. WGL is also monitoring final conclusions for industry specific implementation issues that could impact the timing of revenue recognition for our regulated utility tariff based sales, including the evaluation of collectability from customers if a utility has regulatory mechanisms to help assure recovery of uncollected accounts from ratepayers and accounting for contributions in aid of construction (CIAC). WGL will adopt using the modified retrospective approach. ASU 2016-01, Financial Instruments (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities The new standard amends certain disclosure requirements associated with the fair value of financial instruments, and significantly revises an entity’s accounting related to the classification and measurement of investments in equity securities and the presentation of certain fair value changes for financial liabilities measured at fair value. October 1, 2018* We performed a preliminary evaluation and the adoption of this standard will primarily impact the disclosure of our financial instruments in our Fair Value Measurements Footnote. ASU 2016-02, Leases (Topic 842) This standard requires recognition of a right-to-use asset and lease liability on the statement of financial position and disclosure of key information about leasing arrangements. The standard requires application using a modified retrospective approach. October 1, 2019* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments For credit losses on financial instruments, this standard changes the current incurred loss impairment methodology to an expected loss methodology and requires consideration of a broader range of reasonable and supportable information to determine credit loss estimates. October 1, 2020* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted improvements to Accounting for Hedging Activities The new standard amends the hedge accounting and recognition requirements by expanding an entity's ability to hedge non-financial and financial risk components and reduce the complexity in fair value hedges of interest rate risk. Additionally, this standard eliminates the requirement to separately measure and disclose the ineffective portion of the hedge with the entire change in the fair value of a hedging instrument to be presented in the same income statement line as the hedged item. October 1, 2020* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. *Subject to acceleration if the proposed merger with AltaGas is consummated due to the difference in fiscal year end between AltaGas and WGL.

Accounts Payable and Other Accr

Accounts Payable and Other Accrued Liabilities	3 Months Ended
	Dec. 31, 2017
Accounts Payable and Accrued Liabilities [Abstract]
Accounts Payable and Other Accrued Liabilities	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) December 31, 2017 September 30, 2017 Accounts payable—trade $ 383.7 $ 361.6 Employee benefits and payroll accruals 21.2 35.0 Other accrued liabilities 26.4 27.2 Total $ 431.3 $ 423.8 Washington Gas Light Company (In millions) December 31, 2017 September 30, 2017 Accounts payable—trade $ 163.5 $ 174.9 Employee benefits and payroll accruals 20.1 32.4 Other accrued liabilities 12.8 12.5 Total $ 196.4 $ 219.8

Short-Term Debt

Short-Term Debt	3 Months Ended
	Dec. 31, 2017
Short-term Debt [Abstract]
Short-Term Debt	SHORT-TERM DEBT WGL and Washington Gas satisfy their short-term financing requirements through the sale of commercial paper, financing arrangements with third-party lenders, 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 of WGL and Washington Gas is to maintain bank credit facilities in amounts equal to or greater than their expected maximum commercial paper position. The following is a summary of committed credit available at December 31, 2017 and September 30, 2017 . Committed Credit Available ($ In millions) December 31, 2017 WGL (b) Washington Gas Total Consolidated Committed credit agreements Unsecured revolving credit facility, expires December 19, 2019 (a) $ 650.0 $ 350.0 $ 1,000.0 Less: Commercial Paper (341.9 ) (162.0 ) (503.9 ) Net committed credit available $ 308.1 $ 188.0 $ 496.1 Weighted average interest rate 1.76 % 1.48 % 1.67 % September 30, 2017 Committed credit agreements Unsecured revolving credit facility, expires December 19, 2019 (a) $ 650.0 $ 350.0 $ 1,000.0 Less: Commercial Paper (382.0 ) (123.0 ) (505.0 ) Net committed credit available $ 268.0 $ 227.0 $ 495.0 Weighted average interest rate 1.52 % 1.22 % 1.45 % (a) Washington Gas has the right to request extensions with the banks’ approval. 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, Inc. and all subsidiaries other than Washington Gas. At December 31, 2017 and September 30, 2017 , there were no outstanding bank loans from WGL’s or Washington Gas’ revolving credit facilities. PROJECT FINANCING Washington Gas previously obtained third-party project financing on behalf of the federal government to provide funds during the construction of certain energy management services projects entered into under Washington Gas' area-wide contract. In connection with work completed under the area-wide contract, the construction work is performed by WGL Energy Systems on behalf of Washington Gas and an inter-company payable is recorded for work provided by WGL Energy Systems. As work is performed, Washington Gas establishes a receivable representing the government's obligation to remit principal and interest. The payable and receivable are equal to each other at the end of the construction period, but there may be timing differences in the recognition of the project related payable and receivable during the construction period. When these projects are formally “accepted” by the government and deemed complete, Washington Gas assigns the ownership of the receivable to the third party lender in satisfaction of the obligation and removes both the receivable and the obligation related to the financing from its financial statements. In March 2016 the SCC of VA denied Washington Gas' further participation in the third party financing arrangement but allowed existing debt arrangements to remain intact until the related obligations were satisfied. At December 31, 2017 there were two contracts remaining totaling $ 43.8 million on the Washington Gas balance sheet as a short-term obligation to third party lenders in "Notes payable and project financing". In December 2016, WGL Energy Systems entered into an agreement to obtain third-party financing and receive funds directly from the third-party lender during the construction period associated with the related energy management service projects. As a result, Washington Gas will no longer be liable under future third-party financing arrangements, for projects entered into under the area-wide contract. The general terms of the financing agreement are the same as the prior financing arrangements between Washington Gas and the third-party lender mentioned above. Washington Gas will continue to record a receivable representing the government’s obligation, and will record an inter-company payable to WGL Energy Systems for the construction work performed for the same amount. As of December 31, 2017 , WGL and Washington Gas recorded $ 94.6 million and $ 85.4 million , respectively, in "Unbilled revenues" on the balance sheet, and $ 54.8 million and $ 43.8 million , respectively, in a corresponding short-term obligation to third-party lenders in "Notes payable and project financing", for energy management services projects that were not complete. As of September 30, 2017 , WGL and Washington Gas recorded $85.6 million and $ 78.2 million in "Unbilled revenues" on the balance sheet and $54.8 million and $ 43.8 million , respectively, in a corresponding short-term obligation to third party lenders in "Notes payable and project financing" for energy management services projects that were not complete. Because these projects are financed for government agencies that have minimal credit risk, and with which we have previous collection experience, neither WGL nor Washington Gas recorded a corresponding reserve for bad debts related to these receivables at December 31, 2017 or September 30, 2017 .

Long-Term Debt

Long-Term Debt	3 Months Ended
	Dec. 31, 2017
Long-term Debt, by Current and Noncurrent [Abstract]
Long-Term Debt	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 December 31, 2017 and September 30, 2017 , WGL had capacity under a shelf registration statement to issue an unlimited amount of long-term debt securities. As a result of certain covenants included in the Merger Agreement among WGL, AltaGas and Wrangler, Inc., WGL may not issue debt with a term longer than two years. Refer to Note 17 — Planned Merger with AltaGas Ltd. of the Notes to Condensed Consolidated Financial Statements for a discussion of the proposed merger. At December 31, 2017 and September 30, 2017 , Washington Gas had capacity under a shelf registration statement to issue up to $150.0 million of additional Medium-Term Notes (MTNs). The following tables show our outstanding notes as of December 31, 2017 and September 30, 2017 . Long-Term Debt Outstanding ($ In millions) WGL (a) Washington Gas Total Consolidated December 31, 2017 Long-term debt (b) $ 850.0 $ 1,146.0 $ 1,996.0 Unamortized discount (1.6 ) (3.0 ) (4.6 ) Unamortized debt expense (3.1 ) (8.4 ) (11.5 ) Total Long-Term Debt $ 845.3 $ 1,134.6 $ 1,979.9 Weighted average interest rate 2.60 % 4.89 % 3.91 % September 30, 2017 Long-term debt (b) $ 550.0 $ 1,146.0 $ 1,696.0 Unamortized discount (1.5 ) (3.0 ) (4.5 ) Unamortized debt expense (2.1 ) (8.5 ) (10.6 ) Total Long-Term Debt $ 546.4 $ 1,134.5 $ 1,680.9 Weighted average interest rate 2.81 % 4.89 % 4.21 % (a) WGL includes WGL Holdings, Inc. and all subsidiaries other than Washington Gas. (b) Includes senior notes, term loans and floating rate notes for WGL and both MTNs and private placement notes for Washington Gas. Represents face value including current maturities. The following tables show long-term debt issuances and retirements for the three months ended December 31, 2017 . Long-Term Debt Issuances and Retirements ($ In millions) Principal (b) Interest Rate Effective Cost Nominal Maturity Date Three Months Ended December 31, 2017 WGL (a) Issuances: 11/29/2017 $ 300.0 1.88 % (c) 2.01 % 11/29/2019 Total consolidated issuances $ 300.0 (a) WGL includes WGL Holdings, Inc. and all subsidiaries other than Washington Gas. (b) Represents face amount of notes. (c) Floating rate per annum and reset quarterly based on terms set forth in the prospectus supplement filed by WGL pursuant to Securities Act Rule 424 on November 27, 2017. There were no issuances of long-term debt by Washington Gas for the three months ended December 31, 2017 and there were no retirements of long-term debt for WGL or Washington Gas for the three months ended December 31, 2017. There were no issuances or retirements of long-term debt by WGL or Washington Gas for the three months ended December 31, 2016.

Components of Total Equity

Components of Total Equity	3 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Common Shareholders' Equity	The tables below reflect the components of “Total equity” for WGL and Washington Gas for the three months ended December 31, 2017 and 2016 . WGL Holdings, Inc. Components of Total Equity (In thousands, except shares) Common Stock Paid-In Capital Retained Earnings Accumulated Other Comprehensive Income (Loss), Net of Taxes WGL Holdings Common Shareholders' Equity Non-controlling Interest Washington Gas Light Company Preferred Stock Total Equity Shares Amount Balance at September 30, 2017 51,219,000 $ 582,716 $ 10,149 $ 915,822 $ (5,997 ) $ 1,502,690 $ 6,851 $ 28,173 $ 1,537,714 Net income (loss) — — — 138,040 — 138,040 (5,778 ) 330 132,592 Contributions from non-controlling interest — — — — — — 1,659 — 1,659 Distributions to non-controlling interest — — — — — — (136 ) — (136 ) Business combination (a) — — — — — — 614 — 614 Other comprehensive income — — — — 244 244 — — 244 Stock-based compensation (b) 133,540 11,251 (20,534 ) 4,174 — (5,109 ) — — (5,109 ) Dividends declared: Common stock — — — (26,258 ) — (26,258 ) — — (26,258 ) Preferred stock — — — — — — — (330 ) (330 ) Balance at December 31, 2017 51,352,540 $ 593,967 $ (10,385 ) $ 1,031,778 $ (5,753 ) $ 1,609,607 $ 3,210 $ 28,173 $ 1,640,990 Balance at September 30, 2016 51,080,612 $ 574,496 $ 12,519 $ 827,085 $ (38,539 ) $ 1,375,561 $ 409 $ 28,173 $ 1,404,143 Net income (loss) — — — 57,972 — 57,972 (2,535 ) 330 55,767 Contributions from non-controlling interest — — — — — — 7,331 — 7,331 Other comprehensive income — — — — 29,413 29,413 — — 29,413 Stock-based compensation (b) 104,676 6,564 (6,567 ) (126 ) — (129 ) — — (129 ) Issuance of (c) 25,680 1,613 — — — 1,613 — — 1,613 Dividends declared: Common stock — — — (24,965 ) — (24,965 ) — — (24,965 ) Preferred stock — — — — — — — (330 ) (330 ) Balance at December 31, 2016 51,210,968 $ 582,673 $ 5,952 $ 859,966 $ (9,126 ) $ 1,439,465 $ 5,205 $ 28,173 $ 1,472,843 (a) Resulted from the consolidation of SFEE. For more information, see Note 11—Other investments of the Notes to Condensed Consolidated Financial Statements. (b) Includes dividend equivalents related to our performance shares and implementation of ASU 2016-09, see Note 1—Accounting policies of the Notes to Condensed Consolidated Financial Statements. (c) Includes dividend reinvestment and common stock purchase plans. Washington Gas Light Company Components of Total Equity (In thousands, except shares) Common Stock Paid-In Capital Retained Earnings Accumulated Other Comprehensive Income (Loss), Net of Taxes Total Shares Amount Balance at September 30, 2017 46,479,536 $ 46,479 $ 492,101 $ 630,691 $ (4,522 ) $ 1,164,749 Net income — — — 57,926 — 57,926 Other comprehensive income — — — — 192 192 Stock-based compensation (a) — — (8,916 ) 4,197 — (4,719 ) Capital contributed by WGL Holdings — — 100,000 — — 100,000 Dividends declared: Common stock — — — (21,904 ) — (21,904 ) Preferred stock — — — (330 ) — (330 ) Balance at December 31, 2017 46,479,536 $ 46,479 $ 583,185 $ 670,580 $ (4,330 ) $ 1,295,914 Balance at September 30, 2016 46,479,536 $ 46,479 $ 488,135 $ 586,662 $ (7,830 ) $ 1,113,446 Net income — — — 55,461 — 55,461 Other comprehensive income — — — — 224 224 Stock-based compensation (a) — — (6 ) — — (6 ) Dividends declared: Common stock — — — (21,134 ) — (21,134 ) Preferred stock — — — (330 ) — (330 ) Balance at December 31, 2016 46,479,536 $ 46,479 $ 488,129 $ 620,659 $ (7,606 ) $ 1,147,661 (a) Stock-based compensation is based on the stock awards of WGL that are allocated to Washington Gas Light Company for its pro-rata share and includes implementation of ASU 2016-09, see Note 1—Accounting policies of the Notes to Condensed Consolidated Financial Statements.

Earnings Per Share

Earnings Per Share	3 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Earnings Per Share	EARNINGS PER SHARE Basic earnings per share (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 months ended December 31, 2017 and 2016 . Basic and Diluted EPS (In thousands, except per share data) Net Income Applicable to Common Stock Shares Per Share Amount Three Months Ended December 31, 2017 Basic EPS $ 138,040 51,319 $ 2.69 Stock-based compensation plans — 230 Diluted EPS $ 138,040 51,549 $ 2.68 Three Months Ended December 31, 2016 Basic EPS $ 57,972 51,172 $ 1.13 Stock-based compensation plans — 273 Diluted EPS $ 57,972 51,445 $ 1.13 There were no anti-dilutive shares issued for the three months ended December 31, 2017 and 2016 .

Income Taxes

Income Taxes	3 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES As of December 31, 2017 and September 30, 2017 , our uncertain tax positions were approximately $37.5 million and $ 48.1 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. At this time, however, an estimate of the range of reasonably possible outcomes cannot be determined. Under Accounting Standards Codification (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 December 31, 2017 and September 30, 2017 , we did not have an accrual of interest expense or penalties related to uncertain tax positions. 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, 2014. Substantially all state income tax years in major jurisdictions are closed for years ended prior to September 30, 2014. The Tax Cuts and Jobs Act (“the Act”) was enacted on December 22, 2017. The Act substantially reforms the Tax Code. Among other things, the Act reduced the federal corporate income tax rate from a top marginal rate of 35% to a flat rate of 21% . The Act also denies bonus depreciation to regulated public utilities and imposes limitations on the amount of interest expense which may be deducted by unregulated operations. ASC Topic 740 requires companies to recognize the impacts of a change in tax law or tax rates in the period of enactment. On December 22, 2017, after enactment of the Act, the Securities and Exchange Commission ("SEC") staff issued Staff Accounting Bulletin No. 118 (“SAB 118”) to address the application of US GAAP when a registrant does not have the necessary information available, prepared, or analyzed in reasonable detail to complete the necessary accounting. At December 31, 2017, we have not completed our accounting for the tax effects of enactment of the Act and have followed the guidance issued in SAB 118. SAB 118 expresses the Staff’s views on how ASC Topic 740 should be applied in the context of the Act. It allows entities to take a reasonable period of time to measure and recognize the effects of the Act, while requiring robust disclosures during that period. Under this guidance, registrants record the effects of all items for which the accounting is complete. To the extent a company has not completed the accounting for the effects of the Act, it may record reasonable estimates as provisional amounts. The recorded provisional amounts remain subject to adjustment within the measurement period. The measurement period begins in the reporting period that includes the Act’s enactment date and continues until such time as the accounting can be completed, not to exceed one year from the date of enactment. To the extent a company cannot reasonably estimate the effects of the Act, it should apply the provisions of the tax law that were in effect immediately prior to enactment. SAB 118 requires disclosure of any measurement period adjustments and the effect of measurement period adjustments on the effective tax rate. There are no measurement period adjustments in the first quarter of the fiscal year since the amounts recorded are our initial, provisional amounts. Qualitative disclosures of items having no impact on provision for tax expense at December 31, 2017: Bonus depreciation . Under the Act, regulated public utility property is generally ineligible for bonus depreciation and is depreciated under MACRS while non-utility property is generally subject to 100% bonus depreciation. We have considered all public utility property placed in service in this quarter to be ineligible for bonus depreciation. The impact of this provision in the current fiscal year is reasonably estimated to be a reduction in the amount of $ 88 million in tax depreciation expense based on current assumptions of assets to be placed in service, when compared to the amount of deprecation expense under the previous rules, and a decrease to the forecasted taxable loss. A deferred tax amount related to this depreciation was not recorded; therefore, there is no associated deferred tax amount that was required to be re-measured. Decrease in regulated revenues : As described above, the re-measurement of deferred tax assets and liabilities for the regulated public utility gives rise to amounts probable of being collected from or refunded to customers in future periods. In addition, the federal tax expense included in current base rates needs to be adjusted to the new federal rate of 21% effective January 1, 2018. We have reasonably estimated the reduction in current base rates (revenue) and the annual amortization of amounts resulting from the re-measurement of certain deferred tax assets and liabilities beginning January 1, 2018. In January 2018, Washington Gas filed applications for approval to reduce its distribution rates it charges customers to reflect the Tax Cuts and Jobs Act in Maryland, Virginia and the District of Columbia. These applications propose to reduce rates in these jurisdictions by a combined amount of $ 39.5 million annually, until base rates are reestablished in a general rate case. As described further below, the portion of this reduction in regulated revenues represented by the re-measurement of deferred tax assets and liabilities was recorded as a net regulatory liability under ASC 980, Regulated Operations. The net regulatory liability recorded is the amount we consider probable of regulatory treatment. In addition, amounts collected under other revenue mechanisms remain subject to adjustment for the decrease in the federal tax rate beginning January 1, 2018. The estimated decrease in regulated revenues is prospective only and had no impact on the accounting for the quarter ended December 31, 2017. Items for which the accounting is not yet complete as of the end of the quarter but for which we have recorded provisional amounts at December 31, 2017 which are subject to adjustment during the measurement period: Re-measurement of deferred tax assets and liabilities . Under ASC 740, the tax rate or rates that are used to measure deferred tax liabilities and deferred tax assets are the enacted tax rates expected to apply to taxable income in the years that the liability is expected to be settled or the asset recovered. As a result of using a 21% tax rate for our Q1 income tax provision, we did not create new amounts requiring re-measurement in Q1. Therefore, we re-measured the deferred tax balances at September 30, 2017. The re-measurement of our deferred tax assets and liabilities includes the re-measurement of our cumulative net operating loss deferred tax asset (NOL DTA) as of September 30, 2017. The effect of the re-measurement at a federal tax rate of 21% which resulted in a net decrease in consolidated deferred tax liabilities in the amount of $ 476.4 million , including net tax gross-up. Of this amount, $ 417.3 million is attributable to the regulated utility and $ 59.1 million is attributable to non-utility operations. Of the amount related to utility operations, the net decrease in plant-related deferred tax liabilities was $ 317.8 million before tax gross-up. In addition an increase in income tax expense of, $ 6.2 million attributable to the regulated utility was recorded as a discrete item. Of the amount related to non-utility operations, $ 59.1 million was recorded as a benefit to the tax expense recorded for the period, as a discrete item as a result of the re-measurement of deferred tax liabilities. In addition, the re-measurement of the deferred tax liability associated with the Company’s uncertain tax position was a decrease in the amount of $ 14.8 million . A reserve for uncertain tax benefits related to prior years would generally not be re-measured if the tax exposure related to prior years. However, WGLH’s reserve for uncertain tax benefits relates solely to repair deductions for which the tax consequence of any disallowed deductions related to these timing differences is expected to be prospective only. Consolidated deferred tax assets and liabilities were re-measured at a federal rate of 21%, based on the expectation that we will experience a taxable loss in the current fiscal year, increasing our net operating loss carryforward from the prior year. In the event taxable income is reported for the current fiscal year, we would need to apply a blended federal tax rate of 24.5% to any tax owed in the current period, before the utilization of any net operating loss carryforward to offset all or a portion of the taxable income on which federal tax would otherwise be owed. Additionally, the portion of the temporary differences settling or reversing in the first quarter of our fiscal year 2018 would be re-measured at the rate of 24.5% instead of 21%. We have recorded provisional estimates based on current information as the Company continues to work towards refinement and completion of the income tax provision for the current period. Under ASC 980, we recorded a net increase to the regulatory liability for excess deferred income taxes (EDIT) in the amount of $ 429.3 million including tax gross-up, and a net increase in regulatory assets in the amount of $ 5.8 million , for a net increase in regulatory liabilities in the amount of $ 423.5 million . We recorded a decrease in the regulatory asset for flow-through in the amount and $ 22.4 million , including tax gross-up; and a regulatory asset for the re-measurement of non-plant excess deferred taxes and the net operating loss carryforward deferred tax asset in the amount of $ 28.2 million , including tax gross-up, for a net increase in regulatory assets in the amount of $5.8 million. All provisional amounts recorded for re-measurement remain subject to adjustment within the measurement period pending the preparation and analysis of additional information. In addition, we have recorded a provisional estimate but have not fully completed the accounting for the tax sharing impacts of the re-measurement of the deferred tax assets and deferred tax liabilities. Existing current or deferred tax amounts for which the income tax effects of the Tax Act have not been completed: Accounting for the tax basis adjustment for ITC. Under prior tax law and under the Act, the basis of property must be reduced by one-half of the investment tax credits claimed thereon. Under GAAP, we gross up the book basis and amortize the investment tax credits over the book life of the associated assets. We have not yet completed the accounting for the effects of the Act on the gross-up of book basis and have not recast the amortization pending additional analysis.

Derivative and Weather Related

Derivative and Weather Related Instruments	3 Months Ended
	Dec. 31, 2017
Derivative Instrument Detail [Abstract]
Derivative and Weather-Related Instruments	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 sheets and Washington Gas does not currently 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 securing operating margins that Washington Gas will ultimately realize. The derivative transactions entered into under this program are subject to mark-to-market accounting treatment under ASC 820. Regulatory sharing mechanisms provide for the annual realized profit from these transactions to be shared between Washington Gas' shareholders and customers; therefore, changes in fair value are recorded through earnings, or as regulatory assets or liabilities to the extent that it is probable that realized gains and losses associated with these derivative transactions will be included in the rates charged to customers when they are realized. Unrealized gains and losses recorded to earnings may cause significant period-to-period volatility; this volatility does not change the 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 December 31, 2017 was a net gain of $5.8 million , including an unrealized loss of $1.4 million . During the three months ended December 31, 2016 , we recorded a net gain of $24.3 million , including an unrealized gain of $15.4 million . Managing Price Risk. To manage price risk associated with acquiring natural gas supply for utility customers, Washington Gas enters into physical and financial derivative transactions in the form of forward, option and other contracts, as authorized by its regulators. 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 may utilize 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 related debt. Non-Utility Operations Trading Activities. 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. WGL Midstream does 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. Managing Price Risk. WGL Energy Services enters into certain derivative contracts as part of its strategy to manage the price risk associated with the sale and purchase of natural gas and electricity. WGL Energy Services elects "normal purchases and normal sales" treatment for a portion of these physical contracts related to the purchase of natural gas and electricity to serve its customers, and, therefore, they are not subject to the fair value accounting requirements of ASC Topic 815. Derivative instruments not designated as ''normal purchases and normal sales" are recorded at fair value on our consolidated balance sheets, and changes in the fair value of these derivative instruments are reflected in the earnings of our non-utility operations, which may cause significant period-to-period volatility in earnings. WGL Energy Services does not designate derivatives as hedges under ASC Topic 815. Managing Interest-Rate Risk . WGL utilizes derivative instruments that are designed to limit the risk of interest-rate volatility associated with future debt issuances. At December 31, 2017 , WGL had $250 million of 30-year forward starting interest rate swaps which settled in January 2018. Through December 2016, WGL had designated these interest rate swaps as cash flow hedges in anticipation of a 30-year debt issuance in January 2018, and reported the effective portion of changes in fair value as a component of other comprehensive income (loss). As a result of certain covenants related to the proposed merger with AltaGas, in January 2017, WGL de-designated these hedges as it was no longer probable that the debt would be issued and any further changes in the fair value of the interest rate swaps were recorded in interest expense. WGL believes that the debt issuance is still reasonably possible to occur, therefore, the fair value of the swaps prior to the dedesignation in the amount of $ 6.4 million is recorded in accumulated other comprehensive income at December 31, 2017. In January 2018, WGL settled these swaps and realized a gain of $13.8 million , of which approximately $13.2 million will be recorded to interest expense. Refer to Note 17 —Planned Merger with AltaGas Ltd. for a discussion of the proposed merger. WGL's comprehensive income (loss) also includes amounts for settled hedges related to prior debt issuances, which are being amortized to income over the life of the outstanding debt. The amortization was minimal for the three months ended December 31, 2017 and 2016 . 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 December 31, 2017 and September 30, 2017 , respectively, the absolute notional amounts of our derivatives were as follows: Absolute Notional Amounts of Open Positions on Derivative Instruments Derivative transactions WGL Holdings, Inc. Washington Gas December 31, 2017 Notional Amounts Natural Gas (In millions of therms) Asset optimization & trading 21,960.9 11,768.0 Retail sales 143.2 — Other risk-management activities 1,597.4 1,181.0 Electricity (In millions of kWhs) Retail sales 9,279.2 — Other risk-management activities (a) 21,569.5 — Interest Rate Swaps (In millions of dollars) $ 250.0 — September 30, 2017 Natural Gas (In millions of therms) Asset optimization & trading 21,663.5 11,223.0 Retail sales 124.3 — Other risk-management activities 1,546.7 1,181.0 Electricity (In millions of kWhs) Retail sales 10,011.7 — Other risk-management activities (a) 22,962.1 — Interest Rate Swaps (In millions of dollars) $ 250.0 — (a) Comprised primarily of financial swaps, financial transmission rights and physical forward purchases. The following tables present the balance sheet classification for all derivative instruments as of December 31, 2017 and September 30, 2017 . WGL Holdings, Inc. Balance Sheet Classification of Derivative Instruments (b) (In millions) As of December 31, 2017 Gross Derivative Assets Gross Derivative Liabilities Netting of Collateral Total (a) Current Assets—Derivatives $ 29.2 $ (11.5 ) $ — $ 17.7 Deferred Charges and Other Assets—Derivatives 32.3 (0.1 ) — 32.2 Current Liabilities—Derivatives 17.8 (67.2 ) 14.4 (35.0 ) Deferred Credits—Derivatives 17.6 (154.1 ) 7.6 (128.9 ) Total $ 96.9 $ (232.9 ) $ 22.0 $ (114.0 ) As of September 30, 2017 Current Assets—Derivatives $ 26.6 $ (11.3 ) $ — $ 15.3 Deferred Charges and Other Assets—Derivatives 38.9 (0.4 ) (0.1 ) 38.4 Accounts payable 1.0 — — 1.0 Current Liabilities—Derivatives 10.9 (57.0 ) 2.1 (44.0 ) Deferred Credits—Derivatives 19.2 (148.8 ) 7.0 (122.6 ) Total $ 96.6 $ (217.5 ) $ 9.0 $ (111.9 ) Washington Gas Light Company Balance Sheet Classification of Derivative Instruments (b) (In millions) As of December 31, 2017 Gross Gross Netting of Total (a) Current Assets—Derivatives $ 9.4 $ (5.1 ) $ — $ 4.3 Deferred Charges and Other Assets—Derivatives 15.4 — — 15.4 Current Liabilities—Derivatives — (24.7 ) 0.1 (24.6 ) Deferred Credits—Derivatives — (119.0 ) — (119.0 ) Total $ 24.8 $ (148.8 ) $ 0.1 $ (123.9 ) As of September 30, 2017 Current Assets—Derivatives $ 7.5 $ (2.4 ) $ — $ 5.1 Deferred Charges and Other Assets—Derivatives 16.5 (0.3 ) — 16.2 Current Liabilities—Derivatives — (30.3 ) — (30.3 ) Deferred Credits—Derivatives — (112.3 ) — (112.3 ) Total $ 24.0 $ (145.3 ) $ — $ (121.3 ) (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) We did not have any derivative instruments outstanding that were designated as hedging instruments at December 31, 2017 or September 30, 2017 . The following table presents all gains and losses associated with derivative instruments for the three months ended December 31, 2017 and 2016 . Gains and Losses on Derivative Instruments (In millions) WGL Holdings, Inc. Washington Gas Three Months Ended December 31, 2017 2016 2017 2016 Recorded to income Operating revenues—non-utility $ (7.5 ) $ (36.7 ) $ — $ — Utility cost of gas (2.6 ) 14.7 (2.6 ) 14.7 Non-utility cost of energy-related sales 22.2 31.3 — — Interest expense (0.4 ) (0.1 ) — — Recorded to regulatory assets Gas costs (5.8 ) 19.6 (5.8 ) 19.6 Recorded to other comprehensive income 0.1 49.5 — — Total $ 6.0 $ 78.3 $ (8.4 ) $ 34.3 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 parent company 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. The table below presents collateral not offset against derivative assets and liabilities at December 31, 2017 and September 30, 2017 , respectively. Collateral Not Offset Against Derivative Assets and Liabilities (In millions) December 31, 2017 Collateral deposits posted with counterparties Cash collateral held representing an obligation Washington Gas $ 8.1 $ 0.1 WGL Energy Services 30.6 — WGL Midstream 16.8 0.4 September 30, 2017 Washington Gas $ 3.7 $ 0.1 WGL Energy Services 23.7 — WGL Midstream 44.4 1.6 Any collateral posted that is not offset against derivative assets and liabilities is included in “Other prepayments” in the accompanying condensed balance sheets. Collateral received and not offset against derivative assets and liabilities is included in “Customer deposits and advance payments” in the accompanying balance sheets. Certain derivative instruments of WGL, 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, Washington Gas, WGL Energy Services or WGL Midstream exceeds a certain level (credit-related contingent features). Due to counterparty exposure levels, at December 31, 2017 , WGL Energy Services posted $11.8 million of collateral related to its derivative liabilities that contained credit-related contingent features. At September 30, 2017 , WGL Energy Services posted $8.6 million of collateral related to these aforementioned derivative liabilities. At December 31, 2017 and September 30, 2017 , WGL was not required to post collateral related to a derivative liability that contained a credit-related contingent feature. At both December 31, 2017 and September 30, 2017 , Washington Gas and WGL Midstream were not required to post any collateral related to their respective derivative liabilities that contained credit-related 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 December 31, 2017 and September 30, 2017 , respectively. Potential Collateral Requirements for Derivative Liabilities with Credit-Risk-Contingent Features (In millions) WGL Holdings, Inc. Washington Gas December 31, 2017 Derivative liabilities with credit-risk-contingent features $ 14.5 $ 1.7 Maximum potential collateral requirements $ 8.1 $ 1.6 September 30, 2017 Derivative liabilities with credit-risk-contingent features $ 25.0 $ 2.8 Maximum potential collateral requirements $ 21.9 $ 2.8 We 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 December 31, 2017 , one counterparty represented over 10% of Washington Gas’ credit exposure to wholesale derivative counterparties for a total credit risk of $16.0 million ; three counterparties each represented over 10% of WGL Energy Services’ credit exposure to wholesale counterparties for a total credit risk of $0.9 million ; and one counterparty represented over 10% of WGL Midstream’s credit exposure to wholesale counterparties for a total credit risk of $15.9 million . WEATHER-RELATED INSTRUMENTS 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. During the three months ended December 31, 2017 and 2016 , WGL Energy Services recorded pre-tax losses of $0.8 million and $1.5 million , respectively, related to these instruments included in "Non-utility cost of energy related sales" in the accompanying condensed consolidated statements of income.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	FAIR VALUE MEASUREMENTS Recurring Basis We measure the fair value of our financial assets and liabilities using a combination of the income and market approaches 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. 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. WGL did not have any Level 1 derivatives at December 31, 2017 or September 30, 2017 . 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 December 31, 2017 and September 30, 2017 , Level 2 financial assets and liabilities included energy-related physical and financial derivative transactions such as forward, option and other contracts for deliveries at active market locations, as well as our interest rate swaps. 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 determine the fair value of 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 December 31, 2017 and September 30, 2017 , 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 majority of the life of the contract; (iii) contracts valued using historical spot price volatility assumptions and (iv) valuations using indicative broker quotes for inactive market locations. The following tables set forth financial instruments recorded at fair value as of December 31, 2017 and September 30, 2017 , 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 December 31, 2017 Assets Natural gas related derivatives $ — $ 24.6 $ 53.2 $ 77.8 Electricity related derivatives — 3.1 6.3 9.4 Interest rate derivatives — 9.7 — 9.7 Total Assets $ — $ 37.4 $ 59.5 $ 96.9 Liabilities Natural gas related derivatives $ — $ (33.0 ) $ (170.1 ) $ (203.1 ) Electricity related derivatives — (1.6 ) (19.1 ) (20.7 ) Interest rate derivatives — (9.1 ) — (9.1 ) Total Liabilities $ — $ (43.7 ) $ (189.2 ) $ (232.9 ) At September 30, 2017 Assets Natural gas related derivatives $ — $ 18.4 $ 52.8 $ 71.2 Electricity related derivatives — 0.1 15.5 $ 15.6 Interest rate derivatives — 9.8 — $ 9.8 Total Assets $ — $ 28.3 $ 68.3 $ 96.6 Liabilities Natural gas related derivatives $ — $ (15.5 ) $ (167.4 ) $ (182.9 ) Electricity related derivatives — (4.1 ) (21.7 ) (25.8 ) Interest rate derivatives — (8.8 ) — (8.8 ) Total Liabilities $ — $ (28.4 ) $ (189.1 ) $ (217.5 ) Washington Gas Light Company Fair Value Measurements Under the Fair Value Hierarchy (In millions) Level 1 Level 2 Level 3 Total At December 31, 2017 Assets Natural gas related derivatives $ — $ 8.4 $ 16.4 $ 24.8 Total Assets $ — $ 8.4 $ 16.4 $ 24.8 Liabilities Natural gas related derivatives $ — $ (7.0 ) $ (141.8 ) $ (148.8 ) Total Liabilities $ — $ (7.0 ) $ (141.8 ) $ (148.8 ) At September 30, 2017 Assets Natural gas related derivatives $ — $ 7.0 $ 17.0 $ 24.0 Total Assets $ — $ 7.0 $ 17.0 $ 24.0 Liabilities Natural gas related derivatives $ — $ (5.7 ) $ (139.6 ) $ (145.3 ) Total Liabilities $ — $ (5.7 ) $ (139.6 ) $ (145.3 ) 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 December 31, 2017 and September 30, 2017 . Quantitative Information about Level 3 Fair Value Measurements (In millions) Net Fair Value Valuation Techniques Unobservable Inputs Range WGL Holdings, Inc. Natural gas related derivatives $(113.2) Discounted Cash Flow Natural Gas Basis Price (per dekatherm) ($1.125) - $3.400 Option Model Natural Gas Basis Price (per dekatherm) ($1.030) - $3.100 $(3.7) Annualized Volatility of Spot Market Natural Gas 30.2% - 714.1% Electricity related derivatives $(12.8) Discounted Cash Flow Electricity Congestion Price (per megawatt hour) ($2.977) - $78.990 Washington Gas Light Company Natural gas related derivatives $(125.4) Discounted Cash Flow Natural Gas Basis Price (per dekatherm) ($1.125) - $2.725 (In millions) Net Fair Value WGL Holdings, Inc. Natural gas related derivatives $(112.4) Discounted Cash Flow Natural Gas Basis Price (per dekatherm) ($2.095) - $2.805 Option Model Natural Gas Basis Price (per dekatherm) ($2.095) - $2.358 $(2.2) Annualized Volatility of Spot Market Natural Gas 28.7% - 566.8% Electricity related derivatives $(6.2) Discounted Cash Flow Electricity Congestion Price (per megawatt hour) ($2.736) - $56.500 Washington Gas Light Company Natural gas related derivatives $(122.6) Discounted Cash Flow Natural Gas Basis Price (per dekatherm) ($1.928) - $2.805 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 months ended December 31, 2017 and 2016 , respectively. Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs WGL Holdings, Inc. Washington Gas Light Company (In millions) Natural Gas Related Derivatives Electricity Related Derivatives Total Total - Natural Gas Related Derivatives Three Months Ended December 31, 2017 Balance at October 1, 2017 $ (114.6 ) $ (6.2 ) $ (120.8 ) $ (122.6 ) Realized and unrealized gains (losses) Recorded to income (4.1 ) (9.2 ) (13.3 ) (3.8 ) Recorded to regulatory assets—gas costs (6.9 ) — (6.9 ) (6.9 ) Transfers into Level 3 0.3 — 0.3 0.2 Transfers out of Level 3 (0.3 ) — (0.3 ) — Purchases — 2.5 2.5 — Settlements 8.7 0.1 8.8 7.7 Balance at December 31, 2017 $ (116.9 ) $ (12.8 ) $ (129.7 ) $ (125.4 ) Three Months Ended December 31, 2016 Balance at October 1, 2016 $ (264.1 ) $ (9.1 ) $ (273.2 ) $ (251.6 ) Realized and unrealized gains (losses) Recorded to income 10.3 (3.6 ) 6.7 17.0 Recorded to regulatory assets—gas costs 22.0 — 22.0 22.0 Transfers into Level 3 (0.2 ) — (0.2 ) 0.2 Purchases — (2.8 ) (2.8 ) — Settlements 3.4 7.2 10.6 3.6 Balance at December 31, 2016 $ (228.6 ) $ (8.3 ) $ (236.9 ) $ (208.8 ) Transfers between different levels of the fair value hierarchy may occur based on fluctuations in the valuation inputs and 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 for the periods presented were due to an increase in observable market inputs, primarily reflecting a decrease in the duration of the contracts being valued. Transfers into Level 3 for the periods presented were due to an increase in unobservable market inputs, primarily pricing points. The table below sets forth the line items on the statements of income to which amounts are recorded for the three months ended December 31, 2017 and 2016 , respectively, related to fair value measurements using significant Level 3 inputs. Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements WGL Holdings, Inc. Washington Gas Light Company (In millions) Natural Gas Related Derivatives Electricity Related Derivatives Total Total - Natural Gas Related Derivatives Three Months Ended December 31, 2017 Operating revenues—non-utility $ (6.5 ) $ (16.1 ) $ (22.6 ) $ — Utility cost of gas (3.8 ) — (3.8 ) (3.8 ) Non-utility cost of energy-related sales 6.2 6.9 13.1 — Total $ (4.1 ) $ (9.2 ) $ (13.3 ) $ (3.8 ) Three Months Ended December 31, 2016 Operating revenues—non-utility $ (10.3 ) $ (10.7 ) $ (21.0 ) $ — Utility cost of gas 17.0 — 17.0 17.0 Non-utility cost of energy-related sales 3.6 7.1 10.7 — Total $ 10.3 $ (3.6 ) $ 6.7 $ 17.0 Unrealized gains (losses) attributable to derivative assets and liabilities measured using significant Level 3 inputs were recorded as follows, for the three months ended December 31, 2017 and 2016 , respectively. Unrealized Gains (Losses) Recorded for Level 3 Measurements WGL Holdings, Inc. Washington Gas Light Company (In millions) Natural Gas Related Derivatives Electricity Related Derivatives Total Total - Natural Gas Related Derivatives Three Months Ended December 31, 2017 Recorded to income Operating revenues—non-utility $ (5.5 ) $ (13.6 ) $ (19.1 ) $ — Utility cost of gas (4.6 ) — (4.6 ) (4.6 ) Non-utility cost of energy-related sales 4.2 6.2 10.4 — Recorded to regulatory assets—gas costs (7.5 ) — (7.5 ) (7.5 ) Total $ (13.4 ) $ (7.4 ) $ (20.8 ) $ (12.1 ) Three Months Ended December 31, 2016 Recorded to income Operating revenues—non-utility $ (9.7 ) $ (5.9 ) $ (15.6 ) $ — Utility cost of gas 15.6 — 15.6 15.6 Non-utility cost of energy-related sales (0.1 ) 11.7 11.6 — Recorded to regulatory assets—gas costs 19.7 — 19.7 19.7 Total $ 25.5 $ 5.8 $ 31.3 $ 35.3 The following table presents the carrying amounts and estimated fair values of our financial instruments at December 31, 2017 and September 30, 2017 . WGL Holdings, Inc. Fair Value of Financial Instruments December 31, 2017 September 30, 2017 (In millions) Carrying Amount Fair Value Carrying Amount Fair Value Money market funds (a) $ 70.1 $ 70.1 $ 11.8 $ 11.8 Commercial paper (b) $ 503.9 $ 503.9 $ 505.0 $ 505.0 Project financing (b) $ 94.6 $ 94.6 $ 54.8 $ 54.8 Long-term debt (c) $ 1,679.9 $ 1,842.8 $ 1,430.9 $ 1,577.3 Washington Gas Light Company Fair Value of Financial Instruments December 31, 2017 September 30, 2017 (In millions) Carrying Amount Fair Value Carrying Amount Fair Value Money market funds (a) $ 13.7 $ 13.7 $ 4.8 $ 4.8 Commercial paper (b) $ 162.0 $ 162.0 $ 123.0 $ 123.0 Project financing (b) $ 85.4 $ 85.4 $ 43.8 $ 43.8 Long-term debt (c) $ 1,084.6 $ 1,232.9 $ 1,134.5 $ 1,271.0 (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) Excludes current maturities. Our money market funds are Level 1 valuations and their carrying amount approximates fair value. Other short-term investments are primarily overnight investment accounts; their carrying amount approximates fair value based on Level 2 inputs. The maturity of our commercial paper outstanding at both December 31, 2017 and September 30, 2017 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. Due to the nature of our project financing arrangements, the carrying cost 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.

Operating Segment Reporting

Operating Segment Reporting	3 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Operating Segment Reporting	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 and we evaluate segment performance based on Earnings Before Interest and Taxes (EBIT). EBIT is defined as earnings before interest and taxes net of amounts attributable to non-controlling interests. 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. 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 end use 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 and other distributed generation solutions 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 specializes in the investment, management, development and optimization of natural gas storage and transportation midstream infrastructure projects. Administrative and business development activity costs associated with WGL and Washington Gas Resources and 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. Results for other activities primarily relate to costs associated with the planned merger with AltaGas. The following tables present operating segment information for the three months ended December 31, 2017 and 2016 . Operating Segment Financial Information (In thousands) Operating Revenues Depreciation and Amortization Equity in EBIT Total Assets Capital Expenditures Equity Method Investments Three Months Ended December 31, 2017 Regulated utility $ 377,470 $ 34,103 $ — $ 98,365 $ 5,227,299 $ 76,762 $ — Retail energy-marketing 248,693 282 — 3,742 533,273 — — Commercial energy systems (a) 20,063 6,584 — 5,647 1,104,588 22,292 — Midstream energy services 17,687 4 5,892 22,185 802,613 — 489,354 Other activities — — — (4,171 ) 269,719 — — Eliminations (b) (11,473 ) 12 — 1,689 (874,918 ) — — Total consolidated $ 652,440 $ 40,985 $ 5,892 $ 127,457 $ 7,062,574 $ 99,054 $ 489,354 Three Months Ended December 31, 2016 Regulated utility $ 333,986 $ 30,560 $ — $ 102,717 $ 4,891,722 $ 111,681 $ — Retail energy-marketing 298,684 305 — 29,185 518,914 404 — Commercial energy systems (a) 14,857 4,399 2,387 4,663 932,908 43,673 68,573 Midstream energy services (24,988 ) 9 (2,122 ) (28,484 ) 607,806 — 305,492 Other activities — — — (1,198 ) 417,672 — — Eliminations (b) (13,052 ) 10 — 1,108 (893,632 ) — — Total consolidated $ 609,487 $ 35,283 $ 265 $ 107,991 $ 6,475,390 $ 155,758 $ 374,065 (a) As of August 2016, Commercial Energy Systems' operating revenues include revenues from non-controlling interest. Commercial energy systems' EBIT is adjusted for the effects of non-controlling interest. (b) Intersegment eliminations include any mark-to market valuations associated with trading activities between WGL Midstream and WGL Energy Services, intercompany loans and a timing difference between Commercial Energy Systems’ recognition of revenue for the sale of Renewable Energy Credits (RECs) to Retail Energy-Marketing and Retail Energy-Marketing’s recognition of the associated expense. Retail Energy-Marketing has recorded a portion of the REC’s purchased as inventory to be used in future periods at which time they will be expensed. 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. The following table provides a reconciliation from EBIT to net income applicable to common stock. Three Months Ended December 31, (In thousands) 2017 2016 Total consolidated EBIT $ 127,457 $ 107,991 Interest expense 20,197 16,235 Income tax expense (benefit) (31,110 ) 33,454 Dividends on Washington Gas Light Company preferred stock 330 330 Net income applicable to common stock $ 138,040 $ 57,972

Other Investments

Other Investments	3 Months Ended
	Dec. 31, 2017
Other Investments [Abstract]
Other Investments	OTHER INVESTMENTS WGL has both solar and pipeline investments and accounts for its interests in legal entities as either a: (i) variable interest entity (VIE) or a (ii) voting interest entity (non-VIE). A VIE is a legal entity with one of the following characteristics: (i) has insufficient at-risk equity to fund its activities without additional subordinated financial support from any other party or parties; (ii) whose equity holders as a group lack the characteristics of a controlling financial interest; or (iii) the entity is structured with non-substantive voting rights. The determination of whether or not to consolidate a VIE under GAAP requires a significant amount of judgment. This includes, but is not limited to, consideration of our contractual relationship with the entity, the legal structure of the entity, whether or not the entity has enough equity to finance its activities without additional financial support, the voting power of the equity holders, the obligation of the equity holders to absorb losses of the entity and their rights to receive any expected residual returns. We have investments in both consolidated and unconsolidated VIEs which are described in detail below. The unconsolidated investments are accounted for under the equity method of accounting with profits and losses included in “Equity in earnings of unconsolidated affiliates” in the accompanying Condensed Consolidated Statements of Income. Under the VIE model, we have a controlling financial interest in a VIE (i.e., are the primary beneficiary) and would consolidate the entity 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 the entity is a VIE model. We also continuously evaluate whether we have a controlling financial interest in a VIE. Under the voting interest model, we consolidate an entity when we have a controlling financial interest by holding directly or indirectly, more than 50% of the voting rights or by exercising 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 relative to other investors may result in the determination that we do not have a controlling financial interest and do not consolidate the entity. We reevaluate whether we have a controlling financial interest in these entities when our voting or substantive participating rights change. Where we do not have a controlling financial interest, we apply the equity method of accounting in which investments are initially measured at cost. Investments in, and advances to, affiliated companies are presented in the caption “Investments in unconsolidated affiliates” in the accompanying Condensed Consolidated Balance Sheets. WGL uses the Hypothetical Liquidation at Book Value (HLBV) methodology to determine its earnings or losses for certain equity method investments as well as for the non-controlling interests in consolidated investments when the governing structuring agreement over the equity investment results in different liquidation rights and priorities than what is reflected by the underlying ownership interest percentage. 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 HLBV calculation may vary in its complexity depending on the capital structure and the tax considerations for the investments. When applying HLBV, WGL determines the amount that it would receive if an equity investment entity were to liquidate all of its assets at book value (as valued in accordance with GAAP) and distribute that cash to the investors based on the contractually defined liquidation priorities. The change in WGL's claim on the investee's book value at the beginning and end of the reporting period (adjusted for contributions and distributions) is WGL’s share of the earnings or losses from the equity investment for the period. Consolidated Investments Variable Interest Entities-Solar At December 31, 2017, WGL's subsidiary, WGSW, Inc. was the primary beneficiary of SFGF, SFRC, SFGF II, ASD and SFEE, because of its ability to direct the activities most significant to the economic performance of those entities. Accordingly, we have consolidated these VIE's. SFGF, SFRC, and SFGF II WGSW, along with its various tax equity partners, formed SFGF, SFRC, and SFGF II, investments to acquire, own, and operate distributed generation solar projects nationwide. WGSW is the managing member of these investments and will provide cash equal to the purchase price of the solar projects less any contributions from the tax-equity partner. WGL Energy Systems is the operations and maintenance provider, and the developer of the projects. Profits and losses are allocated between the partners under the HLBV method of accounting and the portion allocated to the tax equity partner is included in “Net income (loss) attributable to non-controlling interest” on the accompanying Condensed Consolidated Statements of Income and is recorded to Non-controlling interest on the accompanying Condensed Consolidated Balance Sheets. As of December 31, 2017, WGSW has contributed the following amounts to SFGF, SFRC, and SFGF II, respectively, $ 16.8 million , $ 20.8 million and $ 0.8 million . ASD WGSW is a limited partner in ASD Solar LP (ASD), a limited partnership formed to own and operate a portfolio of residential solar projects, primarily rooftop photovoltaic power generation systems. As a limited partner, WGSW had provided funding to the partnership but did not have power to direct the activities that most significantly affect the operations and economic performance of the entity. In January 2014, the funding commitment period ended for the partnership. Prior to July 10, 2017, ASD was being consolidated by the general partner, Solar Direct LLC (Solar Direct). Solar Direct is a wholly owned subsidiary of American Solar Direct Inc. (ASDI). In June 2017, ASDI filed for Chapter 7 bankruptcy because of financial difficulties. To ensure continuing operations of the partnership and minimal disruptions to the customers, WGSW petitioned the Bankruptcy Court to remove Solar Direct as manager of ASD operations and to approve the appointment of SF ASD, a wholly-owned subsidiary of WGL Energy Systems, which was formed to take over the management and operations of the partnership, as manager of ASD operations. On July 10, 2017, the Bankruptcy Court granted the bankruptcy trustee's emergency motion to assign management rights and control of ASD to SF ASD and WGSW consolidated ASD as a result. As of December 31, 2017 , WGSW has contributed $ 72.5 million into the tax equity partnership. SFEE On November 23, 2016, WGSW and a tax equity partner formed SFEE to acquire distributed generation solar projects that were to be developed by a third-party developer or WGL Energy Systems. New projects were to be designed and constructed under long-term power purchase agreements. On November 8, 2017, WGSW terminated the Master Purchase Agreement between SFEE, LLC and the third-party developer. The termination triggered a reassessment of the method of accounting for SFEE and as a result of the reassessment, SFEE is a VIE and consolidated by WGSW since then. As of December 31, 2017, WGSW has contributed $ 6.5 million into the tax equity partnership. The following table summarizes the fair value amounts of SFEE assets and liabilities, as well as the estimated fair value of the non-controlling interest as of the date of consolidation. Fair Value of SFEE at Date of Consolidation (in millions) Fair Value Property, plant and equipment $ 10.1 Other assets 0.1 Total assets $ 10.2 Net assets $ 10.2 Non-controlling interest $ 0.6 WGSW equity interest $ 9.6 Property, plant and equipment represents commercial solar assets for SFEE stated at cost, which was determined to equate fair value provided by a third-party appraisal. The carrying amounts and classification of the consolidated VIEs’ assets and liabilities included in our accompanying Condensed Consolidated Balance Sheets at December 31, 2017 and September 30, 2017 are as follows: WGL Holdings, Inc. Balance Sheet Location of Consolidated Investments (in millions) December 31, 2017 September 30, 2017 Current assets $ 5.8 $ 4.4 Property, Plant and Equipment 137.5 121.7 Total assets $ 143.3 $ 126.1 Current liabilities 0.2 0.2 Deferred credits 1.1 0.8 Total liabilities $ 1.3 $ 1.0 Unconsolidated Investments Variable Interest Entity-Pipelines Meade In 2014, WGL through its subsidiary WGL Midstream, entered into a limited liability company agreement and formed Meade Pipeline Co LLC (Meade), a Delaware limited liability company, with Transcontinental Gas Pipe Line Company, LLC (Williams) to invest in a regulated pipeline, a segment of Transco's Atlantic Sunrise project, called Central Penn Pipeline (Central Penn). Central Penn will be an approximately 185 -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. At December 31, 2017 and September 30, 2017 , WGL Midstream held a $ 238.6 million and $ 146.7 million , respectively, equity method investment in Meade. WGL Midstream plans to invest an estimated $410 million for a 55% interest in Meade. Although WGL Midstream holds greater than a 50% interest in Meade, Meade is accounted for under the equity method of accounting because WGL Midstream does not have the power to direct the activities most significant to the economic performance of 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 Condensed Consolidated Statements of Income and are added to or subtracted from the carrying amount of WGL’s investment balance. At September 30, 2017, this VIE was not consolidated because WGL and its subsidiaries were not the primary beneficiaries. The nature of WGL’s involvement with this investment lacks the characteristics of a controlling financial interest. WGL does not have control over activities that are economically significant to Meade. Our maximum financial exposure to loss because of our involvement with this VIE is equal to WGL Midstream's capital contributions. Non-VIE Investments-Pipelines Constitution W GL Midstream owns a 10% interest in Constitution Pipeline Company, LLC (Constitution). The Constitution Pipeline is proposed to transport natural gas from the Marcellus region in northern Pennsylvania to major northeastern markets. 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 Condensed Consolidated Statements 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. In December 2014, Constitution received approval from the FERC to construct and operate the proposed pipeline. However, on April 22, 2016, the New York State Department of Environmental Conservation (NYSDEC) denied Constitution’s application for a Section 401 Certification for the pipeline, which is necessary for the construction and operation of the pipeline. In May 2016, Constitution filed actions in both the U.S. Circuit Court of Appeals for the Second Circuit and the U.S. District Court for the Northern District of New York, appealing the decision and seeking declaratory judgment that the State of New York’s permitting authority is preempted by federal law. In May 2016, Constitution appealed the NYSDEC’s denial of the Section 401 certification to the United States Court of Appeals for the Second Circuit, and in August 2017, the court issued a decision denying in part and dismissing in part Constitution’s appeal. The court expressly declined to rule on Constitution’s argument that the NYSDEC’s decision on Constitution’s Section 401 application constitutes a waiver of the certification requirement. Constitution has filed a petition for rehearing with the Second Circuit Court's decision, but in October the court denied our petition. In January 2018, Constitution petitioned the United States Supreme Court to review the Second Circuit Court’s decision. In October 2017, Constitution filed a petition for declaratory order requesting FERC to find that, by operation of law, the Section 401 certification requirement for the New York State portion of Constitution’s pipeline project was waived due to the failure by the NYSDEC to act on Constitution’s Section 401 application within a reasonable period of time as required by the express terms of such statute. The petition was consistent with a recent decision by the District of Columbia Circuit Court in another proceeding, in which the court clarified that an applicant facing similar circumstances should present evidence of waiver to the FERC. On January 11, 2018, the FERC denied the petition, finding that Section 401 provides that a state waives certification only when it does not act on an application within one year from the date of the application. The project’s sponsors remain committed to the project and plan to seek rehearing and, if necessary, appeal of the FERC’s decision. Beginning in April 2016, we discontinued capitalization of development costs related to this project. At December 31, 2017 and September 30, 2017, WGL Midstream held a $ 38.2 million and $ 38.1 million , equity method investment in Constitution, respectively. We have evaluated our investment in Constitution for other than temporary impairment as of December 31, 2017. Our impairment assessment used income and market approaches in determining the fair value of our investment in Constitution, including consideration of the severity and duration of any decline in fair value of our investment in the project. Our key inputs included, but are not limited to, significant management judgments and estimates, including projections of the project’s cash flows, selection of a discount rate, market multipliers and probability weighting of potential outcomes of legal and regulatory proceedings. At December 31, 2017, we did not record any impairment charge to reduce the carrying value of our investment. In light of FERC’s ruling on January 11, 2018, we expect that we will record an impairment charge of up to $ 38.2 million , the current carrying amount of the investment, in the quarter ending March 31, 2018. There could also be additional expenditures beyond this impairment; however, we believe that recoveries from the sale of the inventories held by Constitution will mostly offset these expenditures. Mountain Valley Pipeline In March 2015, WGL Midstream acquired a 7% equity interest in Mountain Valley Pipeline, LLC (Mountain Valley). On October 24, 2016, WGL Midstream acquired an additional 3% equity interest in Mountain Valley by assuming all of Vega Midstream MVP LLC's (Vega Energy) interest in the joint venture. WGL Midstream now owns a 10% interest in 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 and connects with EQT Corporation's Equitrans system in Wetzel County, West Virginia to Transcontinental Gas Pipe Line Company LLC's Station 165 in Pittsylvania County, Virginia. The pipeline is scheduled to be in service by December 2018. At December 31, 2017 and September 30, 2017 , WGL Midstream held a $ 76.8 million and $ 63.0 million equity method investment in Mountain Valley, respectively. WGL Midstream expects to invest in scheduled capital contributions through the in-service date of the pipeline, its pro rata share (based on its 10% equity interest) of project costs, an estimated aggregate amount of approximately $350.0 million . The equity method is considered appropriate because Mountain Valley 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. Profits and losses are allocated under the HLBV method of accounting and are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Condensed Consolidated Statements of Income and are added to or subtracted from the carrying amount of WGL’s investment balance. Stonewall System WGL Midstream has a 30% equity interest in an entity that owns and operates certain assets known as the Stonewall Gas Gathering System (the Stonewall System). The Stonewall System has the capacity to gather up to 1.4 billion cubic feet of natural gas per day from the Marcellus production region in West Virginia, and connects with an interstate pipeline system that serves markets in the mid-Atlantic region. At December 31, 2017 and September 30, 2017 , WGL Midstream held a $135.7 million and $136.7 million equity method investment in the Stonewall System. The equity method is considered appropriate because the Stonewall System 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. Profits and losses are allocated under the HLBV method of accounting and are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Condensed Consolidated Statements of Income and are added to or subtracted from the carrying amount of WGL’s investment balance. The carrying amount of WGL Midstream's investment in the Stonewall System exceeded the amount of the underlying equity in net assets by $8.8 million as of December 31, 2017, which is being amortized over the life of the assets. The following tables present summary information about our unconsolidated VIEs and non-VIEs: WGL Holdings, Inc. Balance Sheet Location of Unconsolidated Investments Solar Investments Pipelines (in millions) Non-VIEs (a) VIEs (b) Non-VIEs (c) Total December 31, 2017 Assets Investments in unconsolidated affiliates $ — $ 238.6 $ 250.8 $ 489.4 Total assets $ — $ 238.6 $ 250.8 $ 489.4 September 30, 2017 Assets Investments in unconsolidated affiliates $ 9.6 $ 146.7 $ 237.9 $ 394.2 Total assets $ 9.6 $ 146.7 $ 237.9 $ 394.2 (a) Balance relates to interest held in SFEE on September 30, 2017 (b) Balance relates to equity method investment in Meade. (c) Balance relates to equity method investments in Constitution, Mountain Valley Pipeline and Stonewall System.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	RELATED PARTY TRANSACTIONS WGL and its subsidiaries engage in inter-company transactions in 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 outstanding, 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. Washington Gas believes that allocations based on broad measures of business activity are appropriate for allocating expenses resulting from common services. Affiliate entities are allocated a portion of common services based on a formula driven by appropriate indicators of activity, as approved by management. In connection with billing on behalf of unregulated third-party marketers, including WGL Energy Services 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 yet transferred is recorded in “Payables to associated companies” on Washington Gas’ Condensed Balance Sheets. Washington Gas previously obtained third-party project financing on behalf of the federal government to provide funds during the construction of certain energy management services projects entered into under Washington Gas' area-wide contract. In December 2016, WGL Energy Systems entered into an agreement to obtain third-party financing and receive funds directly from the third-party lender during the construction period associated with the related energy management service projects. As part of the ongoing financing arrangement, Washington Gas records a receivable representing the government’s obligation, and records an inter-company payable to WGL Energy Systems for the construction work performed for the same amount in "Payables to associated companies". Refer to Note 3—Short Term Debt of the Notes to Condensed Consolidated Financial Statements for further discussion of the project financing. The following table presents the receivables from and payables to associated companies as of December 31, 2017 and September 30, 2017 . Washington Gas Receivables From / Payables To Associated Companies (In millions) December 31, 2017 September 30, 2017 Receivables from associated companies $ 27.0 $ 32.4 Payables to associated companies $ 107.6 $ 94.8 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 related to balancing services provided to WGL Energy Services have been eliminated in the consolidated financial statements of WGL. The following table shows the amounts Washington Gas charged WGL Energy Services for balancing services which are located in "Utility Operating Revenues". Washington Gas - Gas Balancing Service Charges Three Months Ended December 31, (In millions) 2017 2016 Gas balancing service charge $ 2.5 $ 6.9 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. At December 31, 2017 and September 30, 2017 , WGL Energy Services recognized accounts receivable from Washington Gas of $4.8 million and $1.4 million 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 Condensed Consolidated Financial Statements of our combined Annual Report on Form 10-K for the fiscal year ended September 30, 2017 for further discussion of these imbalance transactions. Washington Gas participates in a purchase of receivables (POR) program as approved by the Maryland Public Service Commission (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 December 31, 2017 and September 30, 2017 , Washington Gas had balances of $6.0 million and $3.2 million , respectively, of purchased receivables from WGL Energy Services which are located in "Accounts receivable".

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES 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. Virginia Jurisdiction Virginia Rate Case. On June 30, 2016, Washington Gas filed an application with the Commonwealth of Virginia State Corporation Commission (SCC of VA) to increase its base rates for natural gas service by $ 45.6 million , which includes $ 22.3 million of revenue associated with natural gas pipeline replacement initiatives previously approved by the SCC of VA and paid by customers through a monthly rider. Additionally, the proposed rate increase included provisions designed to deliver the benefits of natural gas to more customers that include: (i) facilitating conversion to natural gas in locations already served by Washington Gas; (ii) expanding the natural gas system to high-growth communities in Virginia and (iii) research and development enabling innovations to enhance service for our customers. Interim rates went into effect, subject to refund, in the December 2016 billing cycle. Intervenors filed testimony on January 31, 2017, Staff of the SCC of VA filed testimony on February 28, 2017 and Washington Gas filed its rebuttal testimony on March 28, 2017. On April 17, 2017, Washington Gas filed with the SCC of VA a unanimous settlement as to a specific annual revenue increase, but not as to a specific return on equity, specific accounting adjustments, or specific ratemaking methodologies, except as otherwise set forth therein. The Stipulation set forth, for purposes of settlement, a base rate increase of $ 34 million (of which $ 14.1 million represents incremental base rate revenues over and above the inclusion of SAVE Plan costs which were previously recovered through monthly surcharges). For purposes of the settlement, the mid-point of the return on equity range of 9.0 - 10.0 % will be used in any application or filing, other than a change in base rates, effective December 1, 2016. On June 30, 2017, the Chief Hearing Examiner issued a report recommending that the SCC of VA approve the Stipulation. On September 8, 2017, Washington Gas received a final order from the SCC of VA accepting settlement subject to minor modifications to Washington Gas’ System Expansion Proposals. All parties agreed to a Revised Stipulation filed on September 20, 2017, reflecting the SCC of VA’s denial of one of the System Expansion Proposals and Washington Gas’ withdrawal of the second one. The SCC of VA issued its final order approving the revised stipulation on September 25, 2017. Refunds to customers, which have been accrued by Washington Gas at December 31, 2017, will be made related to the interim billings in accordance with the final order. On December 21, 2017, Washington Gas requested and was granted by the SCC of VA a 45-day extension to complete the refunds. FINANCIAL GUARANTEES WGL has guaranteed payments primarily for certain commitments on behalf of some of its subsidiaries. WGL has also guaranteed payments for certain of our external partners. At December 31, 2017 , the maximum potential amount of future payments under the guarantees for external parties totaled $0.6 million . ANTERO CONTRACT Washington Gas and WGL Midstream contracted in June 2014 with Antero Resources Corporation (Antero) to buy gas from Antero at invoiced prices based on an index, and at a delivery point, specified in the contracts. Since deliveries began, however, the index price paid has been more than the fair market value at the same physical delivery point, resulting in losses to date of $28.4 million . Accordingly, Washington Gas and WGL Midstream notified Antero that it sought to apply a provision of the contracts that would permit a new index to be established. Antero objected, claiming that the contract provisions permitting re-pricing did not apply, unless Antero itself chose to sell gas at cheaper prices at the delivery point (which Antero claimed it had not). The dispute was arbitrated in January 2017, and the arbitral tribunal ruled in favor of Antero on the applicability of the re-pricing mechanism. However, the tribunal ruled that it lacked authority to determine whether Antero was in breach of its obligation to deliver gas to Washington Gas and WGL Midstream at a point where they could obtain the higher pricing. Accordingly, Washington Gas and WGL Midstream filed suit in state court in Colorado for a determination of this issue. The state court granted Antero’s motion to dismiss the case and the case is currently on appeal. Separately, Antero has initiated suit against Washington Gas and WGL Midstream, claiming that they have failed to purchase specified daily quantities of gas and seeking alleged cover damages exceeding $80 million as of October 24, 2017, which amount continues to accumulate daily according to Antero's complaint. Washington Gas and WGL Midstream oppose both the validity and amount of Antero’s claim. WGL believes the probability that Antero could succeed in collecting these penalties is remote and therefore, no accrual was made as of December 31, 2017 . In December 2017, WGL Midstream amended its purchase contract with Antero and, effective February 1, 2018, will no longer be obligated to purchase gas at the delivery point that is the subject of these disputes. SILVER SPRING, MARYLAND INCIDENT Washington Gas continues to support the investigation by the NTSB into the August 10, 2016 explosion and fire at an apartment complex on Arliss Street in Silver Spring, Maryland, the cause of which has not been determined. Additional information will be made available by the NTSB at the appropriate time. A total of 40 civil actions related to the incident have been filed against WGL and Washington Gas in the Circuit Court for Montgomery County, Maryland. Thirty-nine of these suits seek unspecified damages for personal injury and/or property damage. The final action is a class action suit seeking total damages stated to be less than $ 5 million for, among others, property damage and various counts relating to the loss of the use of the premises. Two of the 40 cases were originally filed in the District of Columbia Superior Court, but were dismissed. Those two actions were re-filed in Maryland on November 27, 2017. We maintain excess liability insurance coverage from highly-rated insurers, subject to a nominal self-insured retention. We believe that this coverage will be sufficient to cover any significant liability to it that may result from this incident. Management is unable to determine a range of potential losses that are reasonably possible of occurring and therefore we have not recorded a reserve associated with this incident. Washington Gas was invited by the NTSB to be a party to the investigation and in that capacity, continues to work closely with the NTSB to help determine the cause of this incident.

Pension and Other Post-Retireme

Pension and Other Post-Retirement Benefit Plans	3 Months Ended
	Dec. 31, 2017
Pension and Other Postretirement Benefits Cost (Reversal of Cost) [Abstract]
Pension and Other Post-Retirement Benefit Plans	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 months ended December 31, 2017 and 2016 . Components of Net Periodic Benefit Costs (Income) Three Months Ended December 31, 2017 2016 (In millions) Pension Benefits Health and Life Benefits Pension Benefits Health and Life Benefits Service cost $ 3.7 $ 1.3 $ 4.1 $ 1.4 Interest cost 9.9 2.9 9.6 2.9 Expected return on plan assets (10.6 ) (5.9 ) (10.3 ) (5.5 ) Amortization of prior service cost (credit) 0.1 (4.4 ) 0.1 (4.4 ) Amortization of net actuarial loss 3.9 — 5.5 0.2 Net periodic benefit cost (income) 7.0 (6.1 ) 9.0 (5.4 ) Amount allocated to construction projects (1.1 ) 1.0 (1.6 ) 1.2 Amount deferred as regulatory asset/liability — net 1.7 — 1.8 (0.1 ) Amount charged (credited) to expense $ 7.6 $ (5.1 ) $ 9.2 $ (4.3 ) Amounts included in the line item “Amount deferred as 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. We expect the balances to be fully amortized by October 2019.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	3 Months Ended
	Dec. 31, 2017
Accumulated Other Comprehensive Income [Abstract]
Comprehensive Income (Loss)	ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) The following tables show the changes in accumulated other comprehensive income (loss) for WGL and Washington Gas by component for the three months ended December 31, 2017 and 2016 . WGL Holdings, Inc. Changes in Accumulated Other Comprehensive Loss by Component (In thousands) Three Months Ended December 31, 2017 2016 Beginning Balance $ (5,997 ) $ (38,539 ) Qualified cash flow hedging instruments (a) 52 49,455 Change in prior service credit (b) (274 ) (217 ) Amortization of actuarial loss (b) 527 588 Current-period other comprehensive income 305 49,826 Income tax expense related to other comprehensive income 61 20,413 Ending Balance $ (5,753 ) $ (9,126 ) (a) Cash flow hedging instruments represent interest rate swap agreements related to debt issuances. Refer to Note 8- Derivative and Weather-related Instruments of the Notes to Condensed Consolidated Financial Statements for further discussion of the interest rate swap agreements. (b) These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost. Refer to Note 14- Pension and other post-retirement benefit plans of the Notes to Condensed Consolidated Financial Statements for additional details. Washington Gas Light Company Changes in Accumulated Other Comprehensive Loss by Component (In thousands) Three Months Ended December 31, 2017 2016 Beginning Balance $ (4,522 ) $ (7,830 ) Change in prior service credit (a) (274 ) (217 ) Amortization of actuarial loss (a) 527 588 Current-period other comprehensive income 253 371 Income tax expense related to other comprehensive income 61 147 Ending Balance $ (4,330 ) $ (7,606 ) (a) These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost. Refer to Note 14-Pension and other post-retirement benefit plans of the Notes to Condensed Consolidated Financial Statements for additional details.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Dec. 31, 2017
Supplemental Cash Flow Information [Abstract]
Additional Financial Information Disclosure	SUPPLEMENTAL CASH FLOW INFORMATION The following tables detail the changes in operating assets and liabilities from operating activities, cash payments that have been included in the determination of earnings and non-cash investing and financing activities: WGL Holdings Inc. For the three months ended December 31, 2017 2016 (In thousands) CHANGES IN OPERATING ASSETS AND LIABILITIES Accounts receivable and unbilled revenues—net $ (237,959 ) $ (203,098 ) Gas costs and other regulatory assets/liabilities—net 38,503 35,400 Storage gas 37,006 (1,709 ) Prepaid taxes (7,275 ) (7,901 ) Accounts payable and other accrued liabilities 40,575 87,512 Customer deposits and advance payments (10,683 ) (12,611 ) Accrued taxes 18,019 11,149 Other current assets (15,107 ) (21,887 ) Other current liabilities (2,662 ) (22,569 ) Deferred gas costs—net (26,023 ) (42,189 ) Deferred assets—other (18,483 ) (4,431 ) Deferred liabilities—other 7,061 (16,194 ) Pension and other post-retirement benefits (291 ) (3,583 ) Other—net 1,917 771 Changes in operating assets and liabilities $ (175,402 ) $ (201,340 ) SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION Income taxes paid (refunded)—net $ (956 ) $ (4,678 ) Interest paid $ 10,392 $ 4,156 SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES: Extinguishment of project debt financing $ — $ (29,871 ) Capital expenditure accruals included in accounts payable and other accrued liabilities $ 34,371 $ 55,602 Dividends paid in common stock $ — $ 1,362 Stock based compensation $ 11,251 $ 6,564 Transfer of investments to fixed assets $ 10,054 $ 3,959 Transfer of notes receivables to investments $ — $ 10,031 Washington Gas For the three months ended December 31, 2017 2016 (In thousands) CHANGES IN OPERATING ASSETS AND LIABILITIES Accounts receivable, unbilled revenues and receivables from associated companies—net $ (167,271 ) $ (133,605 ) Gas costs and other regulatory assets/liabilities—net 38,503 35,400 Storage gas 7,793 (6,029 ) Prepaid taxes (8,549 ) (11,934 ) Accounts payable and other accrued liabilities, including payables to associated companies 15,409 49,444 Customer deposits and advance payments (9,483 ) (8,410 ) Accrued taxes 34,146 9,307 Other current assets (9,904 ) (9,456 ) Other current liabilities (124 ) (29,859 ) Deferred gas costs—net (26,023 ) (42,189 ) Deferred assets—other (17,649 ) (2,766 ) Deferred liabilities—other 539 (16,657 ) Pension and other post-retirement benefits (442 ) (3,586 ) Other—net 1,691 510 Changes in operating assets and liabilities $ (141,364 ) $ (169,830 ) SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION Income taxes paid (refunded)—net $ (958 ) $ (6,449 ) Interest paid $ 2,977 $ 683 SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES: Extinguishment of project debt financing $ — $ (29,871 ) Capital expenditure accruals included in accounts payable and other accrued liabilities $ 25,325 $ 40,601

Planned Merger with AltaGas

Planned Merger with AltaGas	3 Months Ended
	Dec. 31, 2017
Business Combination, Description [Abstract]
Planned merger with AltaGas	PLANNED MERGER WITH ALTAGAS LTD. On January 25, 2017 , WGL entered into an agreement and plan of merger (Merger Agreement) to combine with AltaGas in an all cash transaction valued at approximately $6.4 billion . The Merger Agreement provides for the merger of a newly formed indirect wholly-owned subsidiary of AltaGas with and into WGL, with WGL continuing as the surviving corporation in the merger (the Merger) and becoming an indirect wholly-owned subsidiary of AltaGas. Subject to the terms and conditions set forth in the Merger Agreement, at the Effective Time (as defined in the Merger Agreement) of the Merger, WGL shareholders will receive $88.25 in cash, without interest, for each share of WGL common stock issued and outstanding immediately prior to the Effective Time (as defined in the Merger Agreement). The Boards of Directors of each of WGL and AltaGas have unanimously approved the Merger, which is expected to close during the second calendar quarter of 2018. Consummation of the Merger is subject to the satisfaction or waiver of specified closing conditions, including, among others, the approval of the Merger by the holders of more than two-thirds of the outstanding shares of WGL common stock, which approval occurred on May 10, 2017, and approvals required from certain antitrust and other regulatory bodies. A status of each of these conditions is described below. The Merger Agreement also contains customary representations, warranties and covenants of both WGL and AltaGas. These covenants include, among others, an obligation on behalf of WGL to operate its business in the ordinary course until the Merger is consummated, subject to certain exceptions. Under the terms of the Merger Agreement, the Outside Date (as defined in the Merger Agreement) for consummation of the Merger was January 25, 2018. The Merger Agreement permits either WGL or AltaGas to extend the term of the Merger Agreement should the Merger not be consummated by such date. On January 11, 2018, WGL and AltaGas mutually agreed in writing to extend this Outside Date to July 23, 2018. The Merger Agreement also contains certain other termination rights for both AltaGas and WGL. Under specified circumstances, AltaGas would be required to pay a termination fee of $ 205 million , $ 182 million , or $ 68 million (depending on the specific circumstances of termination) to WGL. Under other specific circumstances as outlined in the Merger Agreement, WGL would be required to pay AltaGas a termination fee of $ 136 million . In connection with entering into the Merger Agreement, WGL entered into a subscription agreement with AltaGas, pursuant to which WGL agreed, upon the occurrence of certain conditions, to issue and sell to AltaGas up to an aggregate of 15,000 shares of Series A Non-Voting Non-Convertible Perpetual Preferred Stock (Non-Voting Preferred Stock) for a purchase price of $ 10,000 per share. If the consolidated debt to total capitalization ratio is forecasted to be in excess of 62% in any subsequent quarterly period, AltaGas will purchase a number of shares of Non-Voting Preferred Stock to produce a forecasted ratio equal to 62%, but not more than 5,000 shares in any single quarter or more than 15,000 shares in the aggregate. If the Merger Agreement is terminated or the Outside Date expires, no subscription will be made after the date of termination or expiration, and WGL will have six months thereafter to redeem any Non-Voting Preferred Stock previously issued. Merger Approval Proceedings District of Columbia On April 24, 2017, AltaGas, WGL and Washington Gas ("Applicants") filed an application with the PSC of DC seeking approval of the Merger Agreement. In an order issued on April 25, 2017, the PSC of DC scheduled a procedural conference on May 18, 2017 with the Staff of the PSC of DC and interested parties to consider the factors to be considered in the case to determine whether the Merger is in the public interest, identify factual issues in dispute and consider a procedural schedule for the proceeding. To approve the Merger Agreement, the PSC of DC must find that the Merger taken as a whole is in the public interest. In the April 25, 2017 order, the PSC of DC stated that in making this determination, it has balanced the interests of shareholders and investors with ratepayers and the community; determined that benefits to shareholders must not come at the expense of ratepayers; and found that to be approved, the transaction must produce a direct and tangible benefit to ratepayers. It stated further that in determining whether the public interest requirements are met, the PSC of DC has in past merger cases identified seven factors it has considered in reviewing each transaction, including the effects of the transaction on (i) ratepayers, shareholders, the financial health of the utilities standing alone and as merged, and the economy of the District; (ii) utility management and administrative operations; (iii) public safety and the safety and reliability of services; (iv) risks associated with all of the Applicants' affiliated non-jurisdictional business operations; (v) the PSC of DC's ability to regulate Washington Gas effectively; (vi) competition in the local retail and wholesale markets that impact the District and District ratepayers; and (vii) conservation of natural resources and preservation of environmental quality. The law of the District of Columbia does not impose any time limit on the PSC of DC’s review of the Merger. The District of Columbia Office of the People’s Counsel, the District of Columbia Government and other intervenors filed testimony with the PSC of DC opposing the application on September 29, 2017. The Applicants filed rebuttal testimony on October 27, 2017. Evidentiary hearings took place before the PSC of DC on December 5 through December 15, 2017. An order is expected by the second calendar quarter of 2018. Maryland On April 24, 2017, AltaGas, WGL and Washington Gas ("Applicants") filed an application with the PSC of MD seeking approval of the Merger Agreement. On April 26, 2017, the PSC of MD issued an order scheduling a pre-hearing conference on May 30, 2017, to set a procedural schedule for the proceeding, to consider any petition to intervene that have been filed, and to consider any other preliminary matters requested by the parties. Maryland law requires the PSC of MD to approve a merger subject to its review if it finds that the merger agreement is consistent with the public interest, convenience and necessity, including benefits and no harm to consumers. In making this determination, the PSC of MD is required to consider the following criteria: (i) the potential impact of the acquisition on rates and charges paid by customers and on the services and conditions of operation of the public service company; (ii) the potential impact of the acquisition on continuing investment needs for the maintenance of utility services, plant, and related infrastructure; (iii) the proposed capital structure that will result from the acquisition, including allocation of earnings from Washington Gas; (iv) the potential effects on employment; (v) the projected allocation of any savings that are expected between shareholders and rate payers; (vi) issues of reliability, quality of service, and quality of customer service; (vii) the potential impact of the acquisition on community investment; (viii) affiliate and cross-subsidization issues; (ix) the use or pledge of utility assets for the benefit of an affiliate; (x) jurisdictional and choice-of-law issues; (xi) whether it is necessary to revise the PSC of MD's ring-fencing and code of conduct regulations in light of the acquisition; and (xii) any other issues the PSC of MD considers relevant to the assessment of the acquisition in relation to the public interest, convenience, and necessity. The Staff of the PSC of MD, the Maryland Office of People’s Counsel and other intervenors filed testimony opposing the application on August 14, 2017. The Applicants filed rebuttal testimony with the PSC of MD on September 11, 2017. Evidentiary hearings were held before the PSC of MD on October 3, 2017 through October 16, 2017. Initial briefs and reply briefs were filed on November 6, 2017 and November 16, 2017, respectively. The PSC of MD is required to issue an order within 180 days of the date the application was filed, but may extend the date by 45 days for good cause. The PSC of MD issued an order extending the date for review. On December 1, 2017, AltaGas and Washington Gas entered into a settlement agreement with several major parties in the merger application proceedings pending before the PSC of MD. The PSC of MD will conduct evidentiary hearings in February 2018, to review the settlement agreement reached in December and is expected to render a final decision by April 2018. Virginia On April 24, 2017, AltaGas, WGL and Washington Gas, filed a petition with the SCC of VA seeking approval of the Merger Agreement. Virginia law provides that, if the SCC of VA determines, with or without hearing, that adequate service to the public at just and reasonable rates will not be impaired or jeopardized by granting the petition for approval, then the SCC of VA shall approve a merger with such conditions that the SCC of VA deems to be appropriate in order to satisfy this standard. On October 20, 2017, the SCC of VA issued an order approving the merger, subject to accounting, financial, and safety related requirements to which joint applicants agree. On December 21, 2017, Washington Gas filed an application with the SCC of VA for approval of a new service agreement between Washington Gas and AltaGas Services (U.S.) Inc. ("ASUS") for Washington Gas to receive centralized corporate services, and to authorize affiliate transactions for a period of five years. An order is expected on or before March 21, 2018. Committee on Foreign Investment in the United States (CFIUS) On April 24, 2017, AltaGas, WGL and Washington Gas, filed a joint voluntary notice with the CFIUS. This notice was approved on August 18, 2017. HSR On June 15, 2017, AltaGas and WGL submitted to the Federal Trade Commission and the Antitrust Division of the Department of Justice completed Premerger Notification and Report Forms with respect to the proposed acquisition by AltaGas of certain voting securities of WGL. The waiting period required by Section 7A(b)(1) of the Clayton Act, 15 U.S.C. Section 18a(b)(1) (aka the Hart-Scott Rodino Antitrust Improvements Act of 1976, as amended) expired on July 17, 2017. The expiration of the Clayton Act’s waiting period deems the Merger approved by the Federal Trade Commission and the Department of Justice. FERC On April 24, 2017, AltaGas and WGL Energy Services submitted to FERC a Joint Application for Authorization of Disposition of Jurisdictional Assets and Merger under Section 203 of the Federal Power Act. Under that section, FERC shall approve a merger if it finds that the proposed transaction will be consistent with the public interest. In making this determination, the FERC will consider the following criteria: (i) horizontal competition analysis; (ii) vertical competition issues; (iii) no adverse effect on rates; (iv) no adverse effect on regulation; and (v) no improper cross-subsidization. On July 6, 2017, the FERC issued an order authorizing the Merger, concluding that the proposed transaction is consistent with the public interest.

Subsequent Events

Subsequent Events	3 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	SUBSEQUENT EVENTS Constitution Pipeline O n January 11, 2018, the FERC denied the Section 401 Water Quality Certification for Constitution in which WGL Midstream holds a 10% membership interest. Constitution stated that it plans to appeal the decision and seek a rehearing. In light of the denial of the certification and the anticipated actions to challenge the decision, Refer to Note 11 — Other Investments of the Notes to Condensed Consolidated Financial Statements for further discussion of Constitution.

Accounting Policies (Policies)

Accounting Policies (Policies)	3 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Inventory, Policy	On October 1, 2017, Washington Gas and WGL Energy Services implemented a voluntary change in the application of an accounting principle with respect to accounting for natural gas, propane, and odorant inventories. Washington Gas and WGL Energy Services now apply the average cost methodology under which the cost of units carried in inventory is based on the weighted average cost per unit of inventory. Prior to this change, Washington Gas and WGL Energy Services applied the First-in First-out (FIFO) methodology of accounting for inventory under which the oldest inventory items were recorded as being sold first. We believe the new policy is preferable as it conforms to the method predominately used by our peers, better reflects the physical flow of inventory, conforms to the method used for certain of our other inventories, and will simplify recordkeeping requirements. The change in accounting principle was implemented on a prospective basis, therefore, we did not retrospectively adjust any prior periods or record a cumulative effect adjustment, as discussed below. Washington Gas implemented the change in accounting principle on a prospective basis in accordance with Accounting Standards Codification (ASC) No. 980, Regulated Operations which permits regulated entities to implement changes for financial reporting purposes in the same way those changes are implemented for regulatory reporting purposes when the change impacts allowable costs. beginning October 1, 2017, our inventory balances are stated at the lower-of-cost or net realizable value. Prior to October 1, 2017, our inventory balances were stated at the lower-of-cost or market. Interim period inventory losses attributable to lower-of-cost or net realizable value adjustments may be reversed if the net realizable value of the inventory is recovered by the end of the same fiscal year.
Accounting Standards Adopted Current Fiscal Year	ACCOUNTING STANDARDS ADOPTED IN FISCAL YEAR 2018 ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting This standard simplifies several aspects of the accounting for share-based payment transactions, including accounting for income taxes, forfeitures, statutory tax withholding requirements and classification in the statement of cash flows. October 1, 2017 Forfeitures - WGL has elected to continue to estimate forfeitures for its share-based payment awards rather than account for forfeitures when they occur. Income Taxes - On October 1, 2017, WGL and Washington Gas recorded $4.3 million and $4.2 million, respectively, on a modified retrospective basis, as a cumulative effect adjustment to retained earnings. For the three months ended December 31, 2017, WGL and Washington Gas recorded $3.4 million and $3.2 million, respectively, to current tax expense for excess tax benefits related to performance shares that vested in the quarter. Cash Flows - WGL and Washington Gas reclassified $3.6 million and $3.5 million, respectively, retroactively on the statement of cash flows for the three months ended December 31, 2016 from operating to financing activities related to shares withheld to pay for employee taxes. Statutory Tax Withholding - No changes were made. ______________________________________________________________________________________________________ ASU 2016-06, Derivatives and Hedging (Topic 815) - Contingent Put and Call Options in Debt Instruments The amendments in this update clarify the requirements for assessing whether contingent call (put) options that can accelerate the payment of principal on debt October 1, 2017 The implementation of this standard did not have an effect on WGL or Washington Gas' financial statements. ____________________________________________________________________________________________________ ASU 2015-11, Inventory (Topic 330) - Simplifying the Measurement of Inventory This standard reduces the complexity in the current measurement of inventory. This ASU requires inventory to be measured at the lower of cost and net realizable value, where net realizable value is the estimated selling price in the ordinary course of business less reasonably predictable costs of completion, disposal and transportation (no change to the definition of net realizable value). The amendment eliminates the guidance that requires inventory to be stated at the lower-of-cost or market, which includes consideration of the replacement cost of inventory and the net realizable value of inventory, less an approximately normal profit margin. October 1, 2017 The implementation of this standard did not have a material effect on WGL or Washington Gas' financial statements. ____________________________________________________________________________________________________
New Accounting Pronouncements	OTHER NEWLY ISSUED ACCOUNTING STANDARDS ASU 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost This standard requires entities to report the service cost component in the same financial statement line item as other compensation costs arising from services rendered by the pertinent employees during the period. The other components of net benefit cost are to be presented separately from service cost and outside of operating income. In addition, only the service cost component of net benefit cost is eligible for capitalization. Changes to the presentation of service costs and other components of net benefit cost should be applied retrospectively. Changes in capitalization practices should be implemented prospectively. October 1, 2018 We are currently evaluating the interaction of this standard with the various regulatory provisions concerning pensions and post-retirement benefit costs. We anticipate that the change in capitalization of retirement benefits will not have a material impact on WGL or Washington Gas' financial statements. _____________________________________________________________________________________________________ ASU 2016-15, Statement of Cash Flows (Topic 230)—Classification of Certain Cash Receipts and Cash Payments (a consensus of the FASB Emerging Issues Task Force) This update provides guidance on the classification of certain cash receipts and payments in the statement of cash flows. October 1, 2018* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. We do not anticipate that adoption of this standard will have a material effect on WGL or Washington Gas' financial statements. ASU 2014-09, Revenue from Contracts with Customers (Topic 606), including subsequent ASUs clarifying the guidance. ______________________________________________________________________________________________________ ASU 2014-09 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. October 1, 2018 An implementation team is currently evaluating all revenue streams and reviewing contracts with customers, as well as, related financial statement disclosures to determine the impact the adoption of this standard will have on our financial statements. WGL is also monitoring final conclusions for industry specific implementation issues that could impact the timing of revenue recognition for our regulated utility tariff based sales, including the evaluation of collectability from customers if a utility has regulatory mechanisms to help assure recovery of uncollected accounts from ratepayers and accounting for contributions in aid of construction (CIAC). WGL will adopt using the modified retrospective approach. ________________________________________________________________________________________________ ASU 2016-01, Financial Instruments (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities The new standard amends certain disclosure requirements associated with the fair value of financial instruments, and significantly revises an entity’s accounting related to the classification and measurement of investments in equity securities and the presentation of certain fair value changes for financial liabilities measured at fair value. October 1, 2018* We performed a preliminary evaluation and the adoption of this standard will primarily impact the disclosure of our financial instruments in our Fair Value Measurements Footnote. _____________________________________________________________________________________________________ ASU 2016-02, Leases (Topic 842) This standard requires recognition of a right-to-use asset and lease liability on the statement of financial position and disclosure of key information about leasing arrangements. The standard requires application using a modified retrospective approach. October 1, 2019* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. _________________________________________________________________________________________________ ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments For credit losses on financial instruments, this standard changes the current incurred loss impairment methodology to an expected loss methodology and requires consideration of a broader range of reasonable and supportable information to determine credit loss estimates. October 1, 2020* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. ____________________________________________________________________________________________________ ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted improvements to Accounting for Hedging Activities The new standard amends the hedge accounting and recognition requirements by expanding an entity's ability to hedge non-financial and financial risk components and reduce the complexity in fair value hedges of interest rate risk. Additionally, this standard eliminates the requirement to separately measure and disclose the ineffective portion of the hedge with the entire change in the fair value of a hedging instrument to be presented in the same income statement line as the hedged item. October 1, 2020* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements.
Short and Long Term Debt	WGL and Washington Gas satisfy their short-term financing requirements through the sale of commercial paper, financing arrangements with third-party lenders, 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 of WGL and Washington Gas is to maintain bank credit facilities in amounts equal to or greater than their expected maximum commercial paper position. 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.
Income Tax Policy	Under Accounting Standards Codification (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	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 sheets and Washington Gas does not currently 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. Managing Price Risk. To manage price risk associated with acquiring natural gas supply for utility customers, Washington Gas enters into physical and financial derivative transactions in the form of forward, option and other contracts, as authorized by its regulators. 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 may utilize 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 related debt. Non-Utility Operations Trading Activities. 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. WGL Midstream does 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. Managing Price Risk. WGL Energy Services enters into certain derivative contracts as part of its strategy to manage the price risk associated with the sale and purchase of natural gas and electricity. WGL Energy Services elects "normal purchases and normal sales" treatment for a portion of these physical contracts related to the purchase of natural gas and electricity to serve its customers, and, therefore, they are not subject to the fair value accounting requirements of ASC Topic 815. Derivative instruments not designated as ''normal purchases and normal sales" are recorded at fair value on our consolidated balance sheets, and changes in the fair value of these derivative instruments are reflected in the earnings of our non-utility operations, which may cause significant period-to-period volatility in earnings. WGL Energy Services does not designate derivatives as hedges under ASC Topic 815. Managing Interest-Rate Risk . WGL utilizes derivative instruments that are designed to limit the risk of interest-rate volatility associated with future debt issuances. 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 parent company 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. 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 securing operating margins that Washington Gas will ultimately realize. The derivative transactions entered into under this program are subject to mark-to-market accounting treatment under ASC 820. Regulatory sharing mechanisms provide for the annual realized profit from these transactions to be shared between Washington Gas' shareholders and customers; therefore, changes in fair value are recorded through earnings, or as regulatory assets or liabilities to the extent that it is probable that realized gains and losses associated with these derivative transactions will be included in the rates charged to customers when they are realized. Unrealized gains and losses recorded to earnings may cause significant period-to-period volatility; this volatility does not change the operating margins that Washington Gas expects to ultimately realize from these transactions through the use of its storage and transportation capacity resources.
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; their carrying amount approximates fair value based on Level 2 inputs. The maturity of our commercial paper outstanding at both December 31, 2017 and September 30, 2017 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. Due to the nature of our project financing arrangements, the carrying cost 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. Recurring Basis We measure the fair value of our financial assets and liabilities using a combination of the income and market approaches 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. 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 and we evaluate segment performance based on Earnings Before Interest and Taxes (EBIT). EBIT is defined as earnings before interest and taxes net of amounts attributable to non-controlling interests. 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. We also evaluate our operating segments based on other relevant factors, such as penetration into their respective markets and return on equity.
Other Investments Policy	WGL has both solar and pipeline investments and accounts for its interests in legal entities as either a: (i) variable interest entity (VIE) or a (ii) voting interest entity (non-VIE). A VIE is a legal entity with one of the following characteristics: (i) has insufficient at-risk equity to fund its activities without additional subordinated financial support from any other party or parties; (ii) whose equity holders as a group lack the characteristics of a controlling financial interest; or (iii) the entity is structured with non-substantive voting rights. The determination of whether or not to consolidate a VIE under GAAP requires a significant amount of judgment. This includes, but is not limited to, consideration of our contractual relationship with the entity, the legal structure of the entity, whether or not the entity has enough equity to finance its activities without additional financial support, the voting power of the equity holders, the obligation of the equity holders to absorb losses of the entity and their rights to receive any expected residual returns. We have investments in both consolidated and unconsolidated VIEs which are described in detail below. The unconsolidated investments are accounted for under the equity method of accounting with profits and losses included in “Equity in earnings of unconsolidated affiliates” in the accompanying Condensed Consolidated Statements of Income. Under the VIE model, we have a controlling financial interest in a VIE (i.e., are the primary beneficiary) and would consolidate the entity 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 the entity is a VIE model. We also continuously evaluate whether we have a controlling financial interest in a VIE. Under the voting interest model, we consolidate an entity when we have a controlling financial interest by holding directly or indirectly, more than 50% of the voting rights or by exercising 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 relative to other investors may result in the determination that we do not have a controlling financial interest and do not consolidate the entity. We reevaluate whether we have a controlling financial interest in these entities when our voting or substantive participating rights change. Where we do not have a controlling financial interest, we apply the equity method of accounting in which investments are initially measured at cost. Investments in, and advances to, affiliated companies are presented in the caption “Investments in unconsolidated affiliates” in the accompanying Condensed Consolidated Balance Sheets. WGL uses the Hypothetical Liquidation at Book Value (HLBV) methodology to determine its earnings or losses for certain equity method investments as well as for the non-controlling interests in consolidated investments when the governing structuring agreement over the equity investment results in different liquidation rights and priorities than what is reflected by the underlying ownership interest percentage. 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 HLBV calculation may vary in its complexity depending on the capital structure and the tax considerations for the investments. When applying HLBV, WGL determines the amount that it would receive if an equity investment entity were to liquidate all of its assets at book value (as valued in accordance with GAAP) and distribute that cash to the investors based on the contractually defined liquidation priorities. The change in WGL's claim on the investee's book value at the beginning and end of the reporting period (adjusted for contributions and distributions) is WGL’s share of the earnings or losses from the equity investment for the period. 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 Condensed Consolidated Statements 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. The equity method is considered appropriate because the Stonewall System 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. Profits and losses are allocated under the HLBV method of accounting and are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Condensed Consolidated Statements of Income and are added to or subtracted from the carrying amount of WGL’s investment balance. The equity method is considered appropriate because Mountain Valley 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. Profits and losses are allocated under the HLBV method of accounting and are included in “Equity in earnings of unconsolidated affiliates” in the accompanying Condensed Consolidated Statements of Income and are added to or subtracted from the carrying amount of WGL’s investment balance.
Consolidation, Variable Interest Entity, Policy	ASD WGSW is a limited partner in ASD Solar LP (ASD), a limited partnership formed to own and operate a portfolio of residential solar projects, primarily rooftop photovoltaic power generation systems. As a limited partner, WGSW had provided funding to the partnership but did not have power to direct the activities that most significantly affect the operations and economic performance of the entity. In January 2014, the funding commitment period ended for the partnership. Prior to July 10, 2017, ASD was being consolidated by the general partner, Solar Direct LLC (Solar Direct). Solar Direct is a wholly owned subsidiary of American Solar Direct Inc. (ASDI). In June 2017, ASDI filed for Chapter 7 bankruptcy because of financial difficulties. To ensure continuing operations of the partnership and minimal disruptions to the customers, WGSW petitioned the Bankruptcy Court to remove Solar Direct as manager of ASD operations and to approve the appointment of SF ASD, a wholly-owned subsidiary of WGL Energy Systems, which was formed to take over the management and operations of the partnership, as manager of ASD operations. On July 10, 2017, the Bankruptcy Court granted the bankruptcy trustee's emergency motion to assign management rights and control of ASD to SF ASD and WGSW consolidated ASD as a result. SFEE On November 23, 2016, WGSW and a tax equity partner formed SFEE to acquire distributed generation solar projects that were to be developed by a third-party developer or WGL Energy Systems. New projects were to be designed and constructed under long-term power purchase agreements. On November 8, 2017, WGSW terminated the Master Purchase Agreement between SFEE, LLC and the third-party developer. The termination triggered a reassessment of the method of accounting for SFEE and as a result of the reassessment, SFEE is a VIE and consolidated by WGSW since then. SFGF, SFRC, and SFGF II WGSW, along with its various tax equity partners, formed SFGF, SFRC, and SFGF II, investments to acquire, own, and operate distributed generation solar projects nationwide. WGSW is the managing member of these investments and will provide cash equal to the purchase price of the solar projects less any contributions from the tax-equity partner. WGL Energy Systems is the operations and maintenance provider, and the developer of the projects. Profits and losses are allocated between the partners under the HLBV method of accounting and the portion allocated to the tax equity partner is included in “Net income (loss) attributable to non-controlling interest” on the accompanying Condensed Consolidated Statements of Income and is recorded to Non-controlling interest on the accompanying Condensed Consolidated Balance Sheets. Although WGL Midstream holds greater than a 50% interest in Meade, Meade is accounted for under the equity method of accounting because WGL Midstream does not have the power to direct the activities most significant to the economic performance of 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 Condensed Consolidated Statements of Income and are added to or subtracted from the carrying amount of WGL’s investment balance. At September 30, 2017, this VIE was not consolidated because WGL and its subsidiaries were not the primary beneficiaries. The nature of WGL’s involvement with this investment lacks the characteristics of a controlling financial interest. WGL does not have control over activities that are economically significant to Meade.
Gas Balancing Arrangements, Policy	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 related to balancing services provided to WGL Energy Services have been eliminated in the consolidated financial statements of WGL.

Accounting Policies (Tables)

Accounting Policies (Tables)	3 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Accounting Standards Adopted in Current Fiscal Year	ACCOUNTING STANDARDS ADOPTED IN FISCAL YEAR 2018 Standard Description Date of adoption Effect on the financial statements or other significant matters ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting This standard simplifies several aspects of the accounting for share-based payment transactions, including accounting for income taxes, forfeitures, statutory tax withholding requirements and classification in the statement of cash flows. October 1, 2017 Forfeitures - WGL has elected to continue to estimate forfeitures for its share-based payment awards rather than account for forfeitures when they occur. Income Taxes - On October 1, 2017, WGL and Washington Gas recorded $4.3 million and $4.2 million, respectively, on a modified retrospective basis, as a cumulative effect adjustment to retained earnings. For the three months ended December 31, 2017, WGL and Washington Gas recorded $3.4 million and $3.2 million, respectively, to current tax expense for excess tax benefits related to performance shares that vested in the quarter. Cash Flows - WGL and Washington Gas reclassified $3.6 million and $3.5 million, respectively, retroactively on the statement of cash flows for the three months ended December 31, 2016 from operating to financing activities related to shares withheld to pay for employee taxes. Statutory Tax Withholding - No changes were made. ASU 2016-06, Derivatives and Hedging (Topic 815) - Contingent Put and Call Options in Debt Instruments The amendments in this update clarify the requirements for assessing whether contingent call (put) options that can accelerate the payment of principal on debt instruments are clearly and closely related to their debt hosts. The guidance states that for contingent call (put) options to be considered clearly and closely related, they can be indexed only to interest rates or credit risk. An entity is required to assess the embedded call (put) options solely in accordance with the four-step decision sequence. October 1, 2017 The implementation of this standard did not have an effect on WGL or Washington Gas' financial statements. ASU 2015-11, Inventory (Topic 330) - Simplifying the Measurement of Inventory This standard reduces the complexity in the current measurement of inventory. This ASU requires inventory to be measured at the lower of cost and net realizable value, where net realizable value is the estimated selling price in the ordinary course of business less reasonably predictable costs of completion, disposal and transportation (no change to the definition of net realizable value). The amendment eliminates the guidance that requires inventory to be stated at the lower-of-cost or market, which includes consideration of the replacement cost of inventory and the net realizable value of inventory, less an approximately normal profit margin. October 1, 2017 The implementation of this standard did not have a material effect on WGL or Washington Gas' financial statements.
Other Newly Issued Accounting Standards	OTHER NEWLY ISSUED ACCOUNTING STANDARDS Standard Description Required date of adoption Effect on the financial statements or other significant matters ASU 2017-07, Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost This standard requires entities to report the service cost component in the same financial statement line item as other compensation costs arising from services rendered by the pertinent employees during the period. The other components of net benefit cost are to be presented separately from service cost and outside of operating income. In addition, only the service cost component of net benefit cost is eligible for capitalization. Changes to the presentation of service costs and other components of net benefit cost should be applied retrospectively. Changes in capitalization practices should be implemented prospectively. October 1, 2018 We are currently evaluating the interaction of this standard with the various regulatory provisions concerning pensions and post-retirement benefit costs. We anticipate that the change in capitalization of retirement benefits will not have a material impact on WGL or Washington Gas' financial statements. ASU 2016-15, Statement of Cash Flows (Topic 230)—Classification of Certain Cash Receipts and Cash Payments (a consensus of the FASB Emerging Issues Task Force) This update provides guidance on the classification of certain cash receipts and payments in the statement of cash flows. October 1, 2018* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. We do not anticipate that adoption of this standard will have a material effect on WGL or Washington Gas' financial statements. ASU 2014-09, Revenue from Contracts with Customers (Topic 606), including subsequent ASUs clarifying the guidance. ASU 2014-09 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. October 1, 2018 An implementation team is currently evaluating all revenue streams and reviewing contracts with customers, as well as, related financial statement disclosures to determine the impact the adoption of this standard will have on our financial statements. WGL is also monitoring final conclusions for industry specific implementation issues that could impact the timing of revenue recognition for our regulated utility tariff based sales, including the evaluation of collectability from customers if a utility has regulatory mechanisms to help assure recovery of uncollected accounts from ratepayers and accounting for contributions in aid of construction (CIAC). WGL will adopt using the modified retrospective approach. ASU 2016-01, Financial Instruments (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities The new standard amends certain disclosure requirements associated with the fair value of financial instruments, and significantly revises an entity’s accounting related to the classification and measurement of investments in equity securities and the presentation of certain fair value changes for financial liabilities measured at fair value. October 1, 2018* We performed a preliminary evaluation and the adoption of this standard will primarily impact the disclosure of our financial instruments in our Fair Value Measurements Footnote. ASU 2016-02, Leases (Topic 842) This standard requires recognition of a right-to-use asset and lease liability on the statement of financial position and disclosure of key information about leasing arrangements. The standard requires application using a modified retrospective approach. October 1, 2019* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments For credit losses on financial instruments, this standard changes the current incurred loss impairment methodology to an expected loss methodology and requires consideration of a broader range of reasonable and supportable information to determine credit loss estimates. October 1, 2020* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted improvements to Accounting for Hedging Activities The new standard amends the hedge accounting and recognition requirements by expanding an entity's ability to hedge non-financial and financial risk components and reduce the complexity in fair value hedges of interest rate risk. Additionally, this standard eliminates the requirement to separately measure and disclose the ineffective portion of the hedge with the entire change in the fair value of a hedging instrument to be presented in the same income statement line as the hedged item. October 1, 2020* We are in the process of evaluating the impact the adoption of this standard will have on our financial statements. *Subject to acceleration if the proposed merger with AltaGas is consummated due to the difference in fiscal year end between AltaGas and WGL.

Accounts Payable and Other Ac26

Accounts Payable and Other Accrued Liabilities (Tables)	3 Months Ended
	Dec. 31, 2017
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) December 31, 2017 September 30, 2017 Accounts payable—trade $ 383.7 $ 361.6 Employee benefits and payroll accruals 21.2 35.0 Other accrued liabilities 26.4 27.2 Total $ 431.3 $ 423.8
Washington Gas Light Company
Accounts Payable and Other Accrued Liabilities [Line Items]
Accounts Payable and Other Accrued Liabilities	Washington Gas Light Company (In millions) December 31, 2017 September 30, 2017 Accounts payable—trade $ 163.5 $ 174.9 Employee benefits and payroll accruals 20.1 32.4 Other accrued liabilities 12.8 12.5 Total $ 196.4 $ 219.8

Short-Term Debt (Tables)

Short-Term Debt (Tables)	3 Months Ended
	Dec. 31, 2017
Short-term Debt [Abstract]
Committed Credit Available	The following is a summary of committed credit available at December 31, 2017 and September 30, 2017 . Committed Credit Available ($ In millions) December 31, 2017 WGL (b) Washington Gas Total Consolidated Committed credit agreements Unsecured revolving credit facility, expires December 19, 2019 (a) $ 650.0 $ 350.0 $ 1,000.0 Less: Commercial Paper (341.9 ) (162.0 ) (503.9 ) Net committed credit available $ 308.1 $ 188.0 $ 496.1 Weighted average interest rate 1.76 % 1.48 % 1.67 % September 30, 2017 Committed credit agreements Unsecured revolving credit facility, expires December 19, 2019 (a) $ 650.0 $ 350.0 $ 1,000.0 Less: Commercial Paper (382.0 ) (123.0 ) (505.0 ) Net committed credit available $ 268.0 $ 227.0 $ 495.0 Weighted average interest rate 1.52 % 1.22 % 1.45 % (a) Washington Gas has the right to request extensions with the banks’ approval. 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, Inc. and all subsidiaries other than Washington Gas.

Long-Term Debt (Table)

Long-Term Debt (Table)	3 Months Ended
	Dec. 31, 2017
Long-Term Debt Outstanding	The following tables show our outstanding notes as of December 31, 2017 and September 30, 2017 . Long-Term Debt Outstanding ($ In millions) WGL (a) Washington Gas Total Consolidated December 31, 2017 Long-term debt (b) $ 850.0 $ 1,146.0 $ 1,996.0 Unamortized discount (1.6 ) (3.0 ) (4.6 ) Unamortized debt expense (3.1 ) (8.4 ) (11.5 ) Total Long-Term Debt $ 845.3 $ 1,134.6 $ 1,979.9 Weighted average interest rate 2.60 % 4.89 % 3.91 % September 30, 2017 Long-term debt (b) $ 550.0 $ 1,146.0 $ 1,696.0 Unamortized discount (1.5 ) (3.0 ) (4.5 ) Unamortized debt expense (2.1 ) (8.5 ) (10.6 ) Total Long-Term Debt $ 546.4 $ 1,134.5 $ 1,680.9 Weighted average interest rate 2.81 % 4.89 % 4.21 % (a) WGL includes WGL Holdings, Inc. and all subsidiaries other than Washington Gas. (b) Includes senior notes, term loans and floating rate notes for WGL and both MTNs and private placement notes for Washington Gas. Represents face value including current maturities.
Long-Term Debt Issuances and Retirements	The following tables show long-term debt issuances and retirements for the three months ended December 31, 2017 . Long-Term Debt Issuances and Retirements ($ In millions) Principal (b) Interest Rate Effective Cost Nominal Maturity Date Three Months Ended December 31, 2017 WGL (a) Issuances: 11/29/2017 $ 300.0 1.88 % (c) 2.01 % 11/29/2019 Total consolidated issuances $ 300.0 (a) WGL includes WGL Holdings, Inc. and all subsidiaries other than Washington Gas. (b) Represents face amount of notes. (c) Floating rate per annum and reset quarterly based on terms set forth in the prospectus supplement filed by WGL pursuant to Securities Act Rule 424 on November 27, 2017.

Components of Total Equity (Tab

Components of Total Equity (Tables)	3 Months Ended
	Dec. 31, 2017
Schedule of Capitalization, Equity [Line Items]
Components of Common Shareholders' Equity	The tables below reflect the components of “Total equity” for WGL and Washington Gas for the three months ended December 31, 2017 and 2016 . WGL Holdings, Inc. Components of Total Equity (In thousands, except shares) Common Stock Paid-In Capital Retained Earnings Accumulated Other Comprehensive Income (Loss), Net of Taxes WGL Holdings Common Shareholders' Equity Non-controlling Interest Washington Gas Light Company Preferred Stock Total Equity Shares Amount Balance at September 30, 2017 51,219,000 $ 582,716 $ 10,149 $ 915,822 $ (5,997 ) $ 1,502,690 $ 6,851 $ 28,173 $ 1,537,714 Net income (loss) — — — 138,040 — 138,040 (5,778 ) 330 132,592 Contributions from non-controlling interest — — — — — — 1,659 — 1,659 Distributions to non-controlling interest — — — — — — (136 ) — (136 ) Business combination (a) — — — — — — 614 — 614 Other comprehensive income — — — — 244 244 — — 244 Stock-based compensation (b) 133,540 11,251 (20,534 ) 4,174 — (5,109 ) — — (5,109 ) Dividends declared: Common stock — — — (26,258 ) — (26,258 ) — — (26,258 ) Preferred stock — — — — — — — (330 ) (330 ) Balance at December 31, 2017 51,352,540 $ 593,967 $ (10,385 ) $ 1,031,778 $ (5,753 ) $ 1,609,607 $ 3,210 $ 28,173 $ 1,640,990 Balance at September 30, 2016 51,080,612 $ 574,496 $ 12,519 $ 827,085 $ (38,539 ) $ 1,375,561 $ 409 $ 28,173 $ 1,404,143 Net income (loss) — — — 57,972 — 57,972 (2,535 ) 330 55,767 Contributions from non-controlling interest — — — — — — 7,331 — 7,331 Other comprehensive income — — — — 29,413 29,413 — — 29,413 Stock-based compensation (b) 104,676 6,564 (6,567 ) (126 ) — (129 ) — — (129 ) Issuance of (c) 25,680 1,613 — — — 1,613 — — 1,613 Dividends declared: Common stock — — — (24,965 ) — (24,965 ) — — (24,965 ) Preferred stock — — — — — — — (330 ) (330 ) Balance at December 31, 2016 51,210,968 $ 582,673 $ 5,952 $ 859,966 $ (9,126 ) $ 1,439,465 $ 5,205 $ 28,173 $ 1,472,843 (a) Resulted from the consolidation of SFEE. For more information, see Note 11—Other investments of the Notes to Condensed Consolidated Financial Statements. (b) Includes dividend equivalents related to our performance shares and implementation of ASU 2016-09, see Note 1—Accounting policies of the Notes to Condensed Consolidated Financial Statements. (c) Includes dividend reinvestment and common stock purchase plans.
Washington Gas Light Company
Schedule of Capitalization, Equity [Line Items]
Components of Common Shareholders' Equity	Washington Gas Light Company Components of Total Equity (In thousands, except shares) Common Stock Paid-In Capital Retained Earnings Accumulated Other Comprehensive Income (Loss), Net of Taxes Total Shares Amount Balance at September 30, 2017 46,479,536 $ 46,479 $ 492,101 $ 630,691 $ (4,522 ) $ 1,164,749 Net income — — — 57,926 — 57,926 Other comprehensive income — — — — 192 192 Stock-based compensation (a) — — (8,916 ) 4,197 — (4,719 ) Capital contributed by WGL Holdings — — 100,000 — — 100,000 Dividends declared: Common stock — — — (21,904 ) — (21,904 ) Preferred stock — — — (330 ) — (330 ) Balance at December 31, 2017 46,479,536 $ 46,479 $ 583,185 $ 670,580 $ (4,330 ) $ 1,295,914 Balance at September 30, 2016 46,479,536 $ 46,479 $ 488,135 $ 586,662 $ (7,830 ) $ 1,113,446 Net income — — — 55,461 — 55,461 Other comprehensive income — — — — 224 224 Stock-based compensation (a) — — (6 ) — — (6 ) Dividends declared: Common stock — — — (21,134 ) — (21,134 ) Preferred stock — — — (330 ) — (330 ) Balance at December 31, 2016 46,479,536 $ 46,479 $ 488,129 $ 620,659 $ (7,606 ) $ 1,147,661 (a) Stock-based compensation is based on the stock awards of WGL that are allocated to Washington Gas Light Company for its pro-rata share and includes implementation of ASU 2016-09, see Note 1—Accounting policies of the Notes to Condensed Consolidated Financial Statements.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	3 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Earnings per Share	The following table reflects the computation of our basic and diluted EPS for the three months ended December 31, 2017 and 2016 . Basic and Diluted EPS (In thousands, except per share data) Net Income Applicable to Common Stock Shares Per Share Amount Three Months Ended December 31, 2017 Basic EPS $ 138,040 51,319 $ 2.69 Stock-based compensation plans — 230 Diluted EPS $ 138,040 51,549 $ 2.68 Three Months Ended December 31, 2016 Basic EPS $ 57,972 51,172 $ 1.13 Stock-based compensation plans — 273 Diluted EPS $ 57,972 51,445 $ 1.13

Derivative and Weather Relate31

Derivative and Weather Related Instruments (Tables)	3 Months Ended
	Dec. 31, 2017
Derivative [Line Items]
Absolute Notional Amounts of Open Positions on Derivative Instruments	At December 31, 2017 and September 30, 2017 , respectively, the absolute notional amounts of our derivatives were as follows: Absolute Notional Amounts of Open Positions on Derivative Instruments Derivative transactions WGL Holdings, Inc. Washington Gas December 31, 2017 Notional Amounts Natural Gas (In millions of therms) Asset optimization & trading 21,960.9 11,768.0 Retail sales 143.2 — Other risk-management activities 1,597.4 1,181.0 Electricity (In millions of kWhs) Retail sales 9,279.2 — Other risk-management activities (a) 21,569.5 — Interest Rate Swaps (In millions of dollars) $ 250.0 — September 30, 2017 Natural Gas (In millions of therms) Asset optimization & trading 21,663.5 11,223.0 Retail sales 124.3 — Other risk-management activities 1,546.7 1,181.0 Electricity (In millions of kWhs) Retail sales 10,011.7 — Other risk-management activities (a) 22,962.1 — Interest Rate Swaps (In millions of dollars) $ 250.0 — (a) Comprised primarily of financial swaps, financial transmission rights and physical forward purchases.
Balance Sheet Classification of Derivative Instruments	The following tables present the balance sheet classification for all derivative instruments as of December 31, 2017 and September 30, 2017 . WGL Holdings, Inc. Balance Sheet Classification of Derivative Instruments (b) (In millions) As of December 31, 2017 Gross Derivative Assets Gross Derivative Liabilities Netting of Collateral Total (a) Current Assets—Derivatives $ 29.2 $ (11.5 ) $ — $ 17.7 Deferred Charges and Other Assets—Derivatives 32.3 (0.1 ) — 32.2 Current Liabilities—Derivatives 17.8 (67.2 ) 14.4 (35.0 ) Deferred Credits—Derivatives 17.6 (154.1 ) 7.6 (128.9 ) Total $ 96.9 $ (232.9 ) $ 22.0 $ (114.0 ) As of September 30, 2017 Current Assets—Derivatives $ 26.6 $ (11.3 ) $ — $ 15.3 Deferred Charges and Other Assets—Derivatives 38.9 (0.4 ) (0.1 ) 38.4 Accounts payable 1.0 — — 1.0 Current Liabilities—Derivatives 10.9 (57.0 ) 2.1 (44.0 ) Deferred Credits—Derivatives 19.2 (148.8 ) 7.0 (122.6 ) Total $ 96.6 $ (217.5 ) $ 9.0 $ (111.9 )
Gains and (Losses) on Derivative Instruments	The following table presents all gains and losses associated with derivative instruments for the three months ended December 31, 2017 and 2016 . Gains and Losses on Derivative Instruments (In millions) WGL Holdings, Inc. Washington Gas Three Months Ended December 31, 2017 2016 2017 2016 Recorded to income Operating revenues—non-utility $ (7.5 ) $ (36.7 ) $ — $ — Utility cost of gas (2.6 ) 14.7 (2.6 ) 14.7 Non-utility cost of energy-related sales 22.2 31.3 — — Interest expense (0.4 ) (0.1 ) — — Recorded to regulatory assets Gas costs (5.8 ) 19.6 (5.8 ) 19.6 Recorded to other comprehensive income 0.1 49.5 — — Total $ 6.0 $ 78.3 $ (8.4 ) $ 34.3
Collateral Not Offset Against Derivative Assets and Liabilities	The table below presents collateral not offset against derivative assets and liabilities at December 31, 2017 and September 30, 2017 , respectively. Collateral Not Offset Against Derivative Assets and Liabilities (In millions) December 31, 2017 Collateral deposits posted with counterparties Cash collateral held representing an obligation Washington Gas $ 8.1 $ 0.1 WGL Energy Services 30.6 — WGL Midstream 16.8 0.4 September 30, 2017 Washington Gas $ 3.7 $ 0.1 WGL Energy Services 23.7 — WGL Midstream 44.4 1.6
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 December 31, 2017 and September 30, 2017 , respectively. Potential Collateral Requirements for Derivative Liabilities with Credit-Risk-Contingent Features (In millions) WGL Holdings, Inc. Washington Gas December 31, 2017 Derivative liabilities with credit-risk-contingent features $ 14.5 $ 1.7 Maximum potential collateral requirements $ 8.1 $ 1.6 September 30, 2017 Derivative liabilities with credit-risk-contingent features $ 25.0 $ 2.8 Maximum potential collateral requirements $ 21.9 $ 2.8
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 December 31, 2017 Gross Gross Netting of Total (a) Current Assets—Derivatives $ 9.4 $ (5.1 ) $ — $ 4.3 Deferred Charges and Other Assets—Derivatives 15.4 — — 15.4 Current Liabilities—Derivatives — (24.7 ) 0.1 (24.6 ) Deferred Credits—Derivatives — (119.0 ) — (119.0 ) Total $ 24.8 $ (148.8 ) $ 0.1 $ (123.9 ) As of September 30, 2017 Current Assets—Derivatives $ 7.5 $ (2.4 ) $ — $ 5.1 Deferred Charges and Other Assets—Derivatives 16.5 (0.3 ) — 16.2 Current Liabilities—Derivatives — (30.3 ) — (30.3 ) Deferred Credits—Derivatives — (112.3 ) — (112.3 ) Total $ 24.0 $ (145.3 ) $ — $ (121.3 ) (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) We did not have any derivative instruments outstanding that were designated as hedging instruments at December 31, 2017 or September 30, 2017 .

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Dec. 31, 2017
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 December 31, 2017 and September 30, 2017 , 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 December 31, 2017 Assets Natural gas related derivatives $ — $ 24.6 $ 53.2 $ 77.8 Electricity related derivatives — 3.1 6.3 9.4 Interest rate derivatives — 9.7 — 9.7 Total Assets $ — $ 37.4 $ 59.5 $ 96.9 Liabilities Natural gas related derivatives $ — $ (33.0 ) $ (170.1 ) $ (203.1 ) Electricity related derivatives — (1.6 ) (19.1 ) (20.7 ) Interest rate derivatives — (9.1 ) — (9.1 ) Total Liabilities $ — $ (43.7 ) $ (189.2 ) $ (232.9 ) At September 30, 2017 Assets Natural gas related derivatives $ — $ 18.4 $ 52.8 $ 71.2 Electricity related derivatives — 0.1 15.5 $ 15.6 Interest rate derivatives — 9.8 — $ 9.8 Total Assets $ — $ 28.3 $ 68.3 $ 96.6 Liabilities Natural gas related derivatives $ — $ (15.5 ) $ (167.4 ) $ (182.9 ) Electricity related derivatives — (4.1 ) (21.7 ) (25.8 ) Interest rate derivatives — (8.8 ) — (8.8 ) Total Liabilities $ — $ (28.4 ) $ (189.1 ) $ (217.5 )
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 December 31, 2017 and September 30, 2017 . Quantitative Information about Level 3 Fair Value Measurements (In millions) Net Fair Value Valuation Techniques Unobservable Inputs Range WGL Holdings, Inc. Natural gas related derivatives $(113.2) Discounted Cash Flow Natural Gas Basis Price (per dekatherm) ($1.125) - $3.400 Option Model Natural Gas Basis Price (per dekatherm) ($1.030) - $3.100 $(3.7) Annualized Volatility of Spot Market Natural Gas 30.2% - 714.1% Electricity related derivatives $(12.8) Discounted Cash Flow Electricity Congestion Price (per megawatt hour) ($2.977) - $78.990 Washington Gas Light Company Natural gas related derivatives $(125.4) Discounted Cash Flow Natural Gas Basis Price (per dekatherm) ($1.125) - $2.725 (In millions) Net Fair Value WGL Holdings, Inc. Natural gas related derivatives $(112.4) Discounted Cash Flow Natural Gas Basis Price (per dekatherm) ($2.095) - $2.805 Option Model Natural Gas Basis Price (per dekatherm) ($2.095) - $2.358 $(2.2) Annualized Volatility of Spot Market Natural Gas 28.7% - 566.8% Electricity related derivatives $(6.2) Discounted Cash Flow Electricity Congestion Price (per megawatt hour) ($2.736) - $56.500 Washington Gas Light Company Natural gas related derivatives $(122.6) Discounted Cash Flow Natural Gas Basis Price (per dekatherm) ($1.928) - $2.805
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 months ended December 31, 2017 and 2016 , respectively. Reconciliation of Fair Value Measurements Using Significant Level 3 Inputs WGL Holdings, Inc. Washington Gas Light Company (In millions) Natural Gas Related Derivatives Electricity Related Derivatives Total Total - Natural Gas Related Derivatives Three Months Ended December 31, 2017 Balance at October 1, 2017 $ (114.6 ) $ (6.2 ) $ (120.8 ) $ (122.6 ) Realized and unrealized gains (losses) Recorded to income (4.1 ) (9.2 ) (13.3 ) (3.8 ) Recorded to regulatory assets—gas costs (6.9 ) — (6.9 ) (6.9 ) Transfers into Level 3 0.3 — 0.3 0.2 Transfers out of Level 3 (0.3 ) — (0.3 ) — Purchases — 2.5 2.5 — Settlements 8.7 0.1 8.8 7.7 Balance at December 31, 2017 $ (116.9 ) $ (12.8 ) $ (129.7 ) $ (125.4 ) Three Months Ended December 31, 2016 Balance at October 1, 2016 $ (264.1 ) $ (9.1 ) $ (273.2 ) $ (251.6 ) Realized and unrealized gains (losses) Recorded to income 10.3 (3.6 ) 6.7 17.0 Recorded to regulatory assets—gas costs 22.0 — 22.0 22.0 Transfers into Level 3 (0.2 ) — (0.2 ) 0.2 Purchases — (2.8 ) (2.8 ) — Settlements 3.4 7.2 10.6 3.6 Balance at December 31, 2016 $ (228.6 ) $ (8.3 ) $ (236.9 ) $ (208.8 )
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 December 31, 2017 and 2016 , respectively, related to fair value measurements using significant Level 3 inputs. Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements WGL Holdings, Inc. Washington Gas Light Company (In millions) Natural Gas Related Derivatives Electricity Related Derivatives Total Total - Natural Gas Related Derivatives Three Months Ended December 31, 2017 Operating revenues—non-utility $ (6.5 ) $ (16.1 ) $ (22.6 ) $ — Utility cost of gas (3.8 ) — (3.8 ) (3.8 ) Non-utility cost of energy-related sales 6.2 6.9 13.1 — Total $ (4.1 ) $ (9.2 ) $ (13.3 ) $ (3.8 ) Three Months Ended December 31, 2016 Operating revenues—non-utility $ (10.3 ) $ (10.7 ) $ (21.0 ) $ — Utility cost of gas 17.0 — 17.0 17.0 Non-utility cost of energy-related sales 3.6 7.1 10.7 — Total $ 10.3 $ (3.6 ) $ 6.7 $ 17.0
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 months ended December 31, 2017 and 2016 , respectively. Unrealized Gains (Losses) Recorded for Level 3 Measurements WGL Holdings, Inc. Washington Gas Light Company (In millions) Natural Gas Related Derivatives Electricity Related Derivatives Total Total - Natural Gas Related Derivatives Three Months Ended December 31, 2017 Recorded to income Operating revenues—non-utility $ (5.5 ) $ (13.6 ) $ (19.1 ) $ — Utility cost of gas (4.6 ) — (4.6 ) (4.6 ) Non-utility cost of energy-related sales 4.2 6.2 10.4 — Recorded to regulatory assets—gas costs (7.5 ) — (7.5 ) (7.5 ) Total $ (13.4 ) $ (7.4 ) $ (20.8 ) $ (12.1 ) Three Months Ended December 31, 2016 Recorded to income Operating revenues—non-utility $ (9.7 ) $ (5.9 ) $ (15.6 ) $ — Utility cost of gas 15.6 — 15.6 15.6 Non-utility cost of energy-related sales (0.1 ) 11.7 11.6 — Recorded to regulatory assets—gas costs 19.7 — 19.7 19.7 Total $ 25.5 $ 5.8 $ 31.3 $ 35.3
Fair Value of Financial Instruments	The following table presents the carrying amounts and estimated fair values of our financial instruments at December 31, 2017 and September 30, 2017 . WGL Holdings, Inc. Fair Value of Financial Instruments December 31, 2017 September 30, 2017 (In millions) Carrying Amount Fair Value Carrying Amount Fair Value Money market funds (a) $ 70.1 $ 70.1 $ 11.8 $ 11.8 Commercial paper (b) $ 503.9 $ 503.9 $ 505.0 $ 505.0 Project financing (b) $ 94.6 $ 94.6 $ 54.8 $ 54.8 Long-term debt (c) $ 1,679.9 $ 1,842.8 $ 1,430.9 $ 1,577.3
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 December 31, 2017 Assets Natural gas related derivatives $ — $ 8.4 $ 16.4 $ 24.8 Total Assets $ — $ 8.4 $ 16.4 $ 24.8 Liabilities Natural gas related derivatives $ — $ (7.0 ) $ (141.8 ) $ (148.8 ) Total Liabilities $ — $ (7.0 ) $ (141.8 ) $ (148.8 ) At September 30, 2017 Assets Natural gas related derivatives $ — $ 7.0 $ 17.0 $ 24.0 Total Assets $ — $ 7.0 $ 17.0 $ 24.0 Liabilities Natural gas related derivatives $ — $ (5.7 ) $ (139.6 ) $ (145.3 ) Total Liabilities $ — $ (5.7 ) $ (139.6 ) $ (145.3 )
Fair Value of Financial Instruments	Washington Gas Light Company Fair Value of Financial Instruments December 31, 2017 September 30, 2017 (In millions) Carrying Amount Fair Value Carrying Amount Fair Value Money market funds (a) $ 13.7 $ 13.7 $ 4.8 $ 4.8 Commercial paper (b) $ 162.0 $ 162.0 $ 123.0 $ 123.0 Project financing (b) $ 85.4 $ 85.4 $ 43.8 $ 43.8 Long-term debt (c) $ 1,084.6 $ 1,232.9 $ 1,134.5 $ 1,271.0 (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) Excludes current maturities.

Operating Segment Reporting (Ta

Operating Segment Reporting (Tables)	3 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Operating Segment Financial Information	The following tables present operating segment information for the three months ended December 31, 2017 and 2016 . Operating Segment Financial Information (In thousands) Operating Revenues Depreciation and Amortization Equity in EBIT Total Assets Capital Expenditures Equity Method Investments Three Months Ended December 31, 2017 Regulated utility $ 377,470 $ 34,103 $ — $ 98,365 $ 5,227,299 $ 76,762 $ — Retail energy-marketing 248,693 282 — 3,742 533,273 — — Commercial energy systems (a) 20,063 6,584 — 5,647 1,104,588 22,292 — Midstream energy services 17,687 4 5,892 22,185 802,613 — 489,354 Other activities — — — (4,171 ) 269,719 — — Eliminations (b) (11,473 ) 12 — 1,689 (874,918 ) — — Total consolidated $ 652,440 $ 40,985 $ 5,892 $ 127,457 $ 7,062,574 $ 99,054 $ 489,354 Three Months Ended December 31, 2016 Regulated utility $ 333,986 $ 30,560 $ — $ 102,717 $ 4,891,722 $ 111,681 $ — Retail energy-marketing 298,684 305 — 29,185 518,914 404 — Commercial energy systems (a) 14,857 4,399 2,387 4,663 932,908 43,673 68,573 Midstream energy services (24,988 ) 9 (2,122 ) (28,484 ) 607,806 — 305,492 Other activities — — — (1,198 ) 417,672 — — Eliminations (b) (13,052 ) 10 — 1,108 (893,632 ) — — Total consolidated $ 609,487 $ 35,283 $ 265 $ 107,991 $ 6,475,390 $ 155,758 $ 374,065 (a) As of August 2016, Commercial Energy Systems' operating revenues include revenues from non-controlling interest. Commercial energy systems' EBIT is adjusted for the effects of non-controlling interest. (b) Intersegment eliminations include any mark-to market valuations associated with trading activities between WGL Midstream and WGL Energy Services, intercompany loans and a timing difference between Commercial Energy Systems’ recognition of revenue for the sale of Renewable Energy Credits (RECs) to Retail Energy-Marketing and Retail Energy-Marketing’s recognition of the associated expense. Retail Energy-Marketing has recorded a portion of the REC’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 provides a reconciliation from EBIT to net income applicable to common stock. Three Months Ended December 31, (In thousands) 2017 2016 Total consolidated EBIT $ 127,457 $ 107,991 Interest expense 20,197 16,235 Income tax expense (benefit) (31,110 ) 33,454 Dividends on Washington Gas Light Company preferred stock 330 330 Net income applicable to common stock $ 138,040 $ 57,972

Other Investment (Tables)

Other Investment (Tables)	Oct. 01, 2017	Dec. 31, 2017
Consolidated VIE's
Variable Interest Entity [Line Items]
Location of Other Investments		The carrying amounts and classification of the consolidated VIEs’ assets and liabilities included in our accompanying Condensed Consolidated Balance Sheets at December 31, 2017 and September 30, 2017 are as follows: WGL Holdings, Inc. Balance Sheet Location of Consolidated Investments (in millions) December 31, 2017 September 30, 2017 Current assets $ 5.8 $ 4.4 Property, Plant and Equipment 137.5 121.7 Total assets $ 143.3 $ 126.1 Current liabilities 0.2 0.2 Deferred credits 1.1 0.8 Total liabilities $ 1.3 $ 1.0
Unconsolidated and Non-consolidated VIE's
Variable Interest Entity [Line Items]
Location of Other Investments		The following tables present summary information about our unconsolidated VIEs and non-VIEs: WGL Holdings, Inc. Balance Sheet Location of Unconsolidated Investments Solar Investments Pipelines (in millions) Non-VIEs (a) VIEs (b) Non-VIEs (c) Total December 31, 2017 Assets Investments in unconsolidated affiliates $ — $ 238.6 $ 250.8 $ 489.4 Total assets $ — $ 238.6 $ 250.8 $ 489.4 September 30, 2017 Assets Investments in unconsolidated affiliates $ 9.6 $ 146.7 $ 237.9 $ 394.2 Total assets $ 9.6 $ 146.7 $ 237.9 $ 394.2 (a) Balance relates to interest held in SFEE on September 30, 2017 (b) Balance relates to equity method investment in Meade. (c) Balance relates to equity method investments in Constitution, Mountain Valley Pipeline and Stonewall System.
Estimated Fair Value
Variable Interest Entity [Line Items]
Location of Other Investments	Fair Value of SFEE at Date of Consolidation (in millions) Fair Value Property, plant and equipment $ 10.1 Other assets 0.1 Total assets $ 10.2 Net assets $ 10.2 Non-controlling interest $ 0.6 WGSW equity interest $ 9.6

Related Party Transactions (Tab

Related Party Transactions (Tables)	3 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	The following table presents the receivables from and payables to associated companies as of December 31, 2017 and September 30, 2017 . Washington Gas Receivables From / Payables To Associated Companies (In millions) December 31, 2017 September 30, 2017 Receivables from associated companies $ 27.0 $ 32.4 Payables to associated companies $ 107.6 $ 94.8 The following table shows the amounts Washington Gas charged WGL Energy Services for balancing services which are located in "Utility Operating Revenues". Washington Gas - Gas Balancing Service Charges Three Months Ended December 31, (In millions) 2017 2016 Gas balancing service charge $ 2.5 $ 6.9

Pension and Other Post-Retire36

Pension and Other Post-Retirement Benefit Plan (Tables)	3 Months Ended
	Dec. 31, 2017
Pension and Other Postretirement Benefits Cost (Reversal of Cost) [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 months ended December 31, 2017 and 2016 . Components of Net Periodic Benefit Costs (Income) Three Months Ended December 31, 2017 2016 (In millions) Pension Benefits Health and Life Benefits Pension Benefits Health and Life Benefits Service cost $ 3.7 $ 1.3 $ 4.1 $ 1.4 Interest cost 9.9 2.9 9.6 2.9 Expected return on plan assets (10.6 ) (5.9 ) (10.3 ) (5.5 ) Amortization of prior service cost (credit) 0.1 (4.4 ) 0.1 (4.4 ) Amortization of net actuarial loss 3.9 — 5.5 0.2 Net periodic benefit cost (income) 7.0 (6.1 ) 9.0 (5.4 ) Amount allocated to construction projects (1.1 ) 1.0 (1.6 ) 1.2 Amount deferred as regulatory asset/liability — net 1.7 — 1.8 (0.1 ) Amount charged (credited) to expense $ 7.6 $ (5.1 ) $ 9.2 $ (4.3 )

Accumulated Other Comprehensi37

Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Changes in Accumulated Other Comprehensive Income (Loss) by Component	The following tables show the changes in accumulated other comprehensive income (loss) for WGL and Washington Gas by component for the three months ended December 31, 2017 and 2016 . WGL Holdings, Inc. Changes in Accumulated Other Comprehensive Loss by Component (In thousands) Three Months Ended December 31, 2017 2016 Beginning Balance $ (5,997 ) $ (38,539 ) Qualified cash flow hedging instruments (a) 52 49,455 Change in prior service credit (b) (274 ) (217 ) Amortization of actuarial loss (b) 527 588 Current-period other comprehensive income 305 49,826 Income tax expense related to other comprehensive income 61 20,413 Ending Balance $ (5,753 ) $ (9,126 ) (a) Cash flow hedging instruments represent interest rate swap agreements related to debt issuances. Refer to Note 8- Derivative and Weather-related Instruments of the Notes to Condensed Consolidated Financial Statements for further discussion of the interest rate swap agreements. (b) These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost. Refer to Note 14- Pension and other post-retirement benefit plans of the Notes to Condensed Consolidated Financial Statements for additional details. Washington Gas Light Company Changes in Accumulated Other Comprehensive Loss by Component (In thousands) Three Months Ended December 31, 2017 2016 Beginning Balance $ (4,522 ) $ (7,830 ) Change in prior service credit (a) (274 ) (217 ) Amortization of actuarial loss (a) 527 588 Current-period other comprehensive income 253 371 Income tax expense related to other comprehensive income 61 147 Ending Balance $ (4,330 ) $ (7,606 ) (a) These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost. Refer to Note 14-Pension and other post-retirement benefit plans of the Notes to Condensed Consolidated Financial Statements for additional details.

Supplemental Cash Flow Inform38

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Dec. 31, 2017
Washington Gas Light Company
Schedule of Cash Flow, Supplemental Disclosures	Washington Gas For the three months ended December 31, 2017 2016 (In thousands) CHANGES IN OPERATING ASSETS AND LIABILITIES Accounts receivable, unbilled revenues and receivables from associated companies—net $ (167,271 ) $ (133,605 ) Gas costs and other regulatory assets/liabilities—net 38,503 35,400 Storage gas 7,793 (6,029 ) Prepaid taxes (8,549 ) (11,934 ) Accounts payable and other accrued liabilities, including payables to associated companies 15,409 49,444 Customer deposits and advance payments (9,483 ) (8,410 ) Accrued taxes 34,146 9,307 Other current assets (9,904 ) (9,456 ) Other current liabilities (124 ) (29,859 ) Deferred gas costs—net (26,023 ) (42,189 ) Deferred assets—other (17,649 ) (2,766 ) Deferred liabilities—other 539 (16,657 ) Pension and other post-retirement benefits (442 ) (3,586 ) Other—net 1,691 510 Changes in operating assets and liabilities $ (141,364 ) $ (169,830 ) SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION Income taxes paid (refunded)—net $ (958 ) $ (6,449 ) Interest paid $ 2,977 $ 683 SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES: Extinguishment of project debt financing $ — $ (29,871 ) Capital expenditure accruals included in accounts payable and other accrued liabilities $ 25,325 $ 40,601
WGL
Schedule of Cash Flow, Supplemental Disclosures	The following tables detail the changes in operating assets and liabilities from operating activities, cash payments that have been included in the determination of earnings and non-cash investing and financing activities: WGL Holdings Inc. For the three months ended December 31, 2017 2016 (In thousands) CHANGES IN OPERATING ASSETS AND LIABILITIES Accounts receivable and unbilled revenues—net $ (237,959 ) $ (203,098 ) Gas costs and other regulatory assets/liabilities—net 38,503 35,400 Storage gas 37,006 (1,709 ) Prepaid taxes (7,275 ) (7,901 ) Accounts payable and other accrued liabilities 40,575 87,512 Customer deposits and advance payments (10,683 ) (12,611 ) Accrued taxes 18,019 11,149 Other current assets (15,107 ) (21,887 ) Other current liabilities (2,662 ) (22,569 ) Deferred gas costs—net (26,023 ) (42,189 ) Deferred assets—other (18,483 ) (4,431 ) Deferred liabilities—other 7,061 (16,194 ) Pension and other post-retirement benefits (291 ) (3,583 ) Other—net 1,917 771 Changes in operating assets and liabilities $ (175,402 ) $ (201,340 ) SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION Income taxes paid (refunded)—net $ (956 ) $ (4,678 ) Interest paid $ 10,392 $ 4,156 SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES: Extinguishment of project debt financing $ — $ (29,871 ) Capital expenditure accruals included in accounts payable and other accrued liabilities $ 34,371 $ 55,602 Dividends paid in common stock $ — $ 1,362 Stock based compensation $ 11,251 $ 6,564 Transfer of investments to fixed assets $ 10,054 $ 3,959 Transfer of notes receivables to investments $ — $ 10,031

Accounting Policies (Narrative)

Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended
	Dec. 31, 2017USD ($)subsidiary	Dec. 31, 2016USD ($)
Washington Gas Light Company
Accounting Policies [Table] [Line Items]
Cumulative effect on retained earnings	$ 4.2
Current tax expense	3.2
Performance shares tax reclass		$ 3.5
WGL
Accounting Policies [Table] [Line Items]
Cumulative effect on retained earnings	4.3
Current tax expense	$ 3.4
Performance shares tax reclass		$ 3.6
Washington Gas Resources
Accounting Policies [Table] [Line Items]
Number of subsidiaries | subsidiary	4

Accounts Payable and Other Ac40

Accounts Payable and Other Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Accounts Payable and Other Accrued Liabilities [Line Items]
Accounts payable—trade	$ 383,700	$ 361,600
Employee benefits and payroll accruals	21,200	35,000
Other accrued liabilities	26,400	27,200
Total	431,327	423,824
Washington Gas Light Company
Accounts Payable and Other Accrued Liabilities [Line Items]
Accounts payable—trade	163,500	174,900
Employee benefits and payroll accruals	20,100	32,400
Other accrued liabilities	12,800	12,500
Total	$ 196,363	$ 219,827

Short-Term Debt (Details)

Short-Term Debt (Details) - USD ($)		Dec. 31, 2017	Sep. 30, 2017
Committed Credit
Short-term Debt [Line Items]
Unsecured revolving credit facility, expires December 19, 2019	[1]	$ 1,000,000,000	$ 1,000,000,000
Less: Commercial Paper		(503,900,000)	(505,000,000)
Net committed credit available		$ 496,100,000	$ 495,000,000
Weighted average interest rate		1.67%	1.45%
Washington Gas Light Company
Short-term Debt [Line Items]
Outstanding bank loans		$ 0	$ 0
Unbilled revenues		85,400,000	78,200,000
Notes payable and project financing		43,800,000	43,800,000
Reserve for bad debt		0	0
Washington Gas Light Company | Committed Credit
Short-term Debt [Line Items]
Unsecured revolving credit facility, expires December 19, 2019	[1]	350,000,000	350,000,000
Less: Commercial Paper		(162,000,000)	(123,000,000)
Net committed credit available		$ 188,000,000	$ 227,000,000
Weighted average interest rate		1.48%	1.22%
Revolving credit facility maximum borrowing capacity		$ 450,000,000
Revolving credit facility additional borrowings		100,000,000
WGL
Short-term Debt [Line Items]
Outstanding bank loans		0	$ 0
Unbilled revenues		94,600,000	85,600,000
Notes payable and project financing		54,800,000	54,800,000
Reserve for bad debt		0	0
WGL | Committed Credit
Short-term Debt [Line Items]
Unsecured revolving credit facility, expires December 19, 2019	[1],[2]	650,000,000	650,000,000
Less: Commercial Paper	[2]	(341,900,000)	(382,000,000)
Net committed credit available	[2]	$ 308,100,000	$ 268,000,000
Weighted average interest rate	[2]	1.76%	1.52%
[1]	Washington Gas has the right to request extensions with the banks’ approval. 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, Inc. and all subsidiaries other than Washington Gas.

Long-Term Debt (Details)

Long-Term Debt (Details) - USD ($) $ in Millions		3 Months Ended
		Dec. 31, 2017	Sep. 30, 2017
Total Consolidated Outstanding
Debt Instrument [Line Items]
Long-term debt	[1]	$ 1,996	$ 1,696
Unamortized discount		(4.6)	(4.5)
Unamortized debt expense		(11.5)	(10.6)
Total Long-Term Debt		$ 1,979.9	$ 1,680.9
Weighted average interest rate		3.91%	4.21%
Washington Gas Light Company
Debt Instrument [Line Items]
Long-term debt	[1]	$ 1,146	$ 1,146
Unamortized discount		(3)	(3)
Unamortized debt expense		(8.4)	(8.5)
Total Long-Term Debt		$ 1,134.6	$ 1,134.5
Weighted average interest rate		4.89%	4.89%
Washington Gas Light Company | Medium-term Notes
Debt Instrument [Line Items]
Unused Borrowing Capacity, Amount		$ 150
WGL
Debt Instrument [Line Items]
Minimum time before maturity of long-term notes		1 year
Long-term debt	[1],[2]	$ 850	$ 550
Unamortized discount	[2]	(1.6)	(1.5)
Unamortized debt expense	[2]	(3.1)	(2.1)
Total Long-Term Debt	[2]	$ 845.3	$ 546.4
Weighted average interest rate	[2]	2.60%	2.81%
WGL | Total Consolidated Issuances [Member]
Debt Instrument [Line Items]
Total Long-Term Debt	[3],[4]	$ 300
2019 Notes | WGL | Issuances
Debt Instrument [Line Items]
Long-term debt	[3],[4]	$ 300
Weighted average interest rate	[4],[5]	1.88%
Effective Cost	[4],[6]	2.01%
Nominal Maturity Date	[4]	Nov. 29, 2019
[1]	Includes senior notes, term loans and floating rate notes for WGL and both MTNs and private placement notes for Washington Gas. Represents face value including current maturities.
[2]	WGL includes WGL Holdings, Inc. and all subsidiaries other than Washington Gas.
[3]	Represents face amount of notes
[4]	WGL includes WGL Holdings, Inc. and all subsidiaries other than Washington Gas.
[5]	Floating rate per annum and reset quarterly based on terms set forth in the prospectus supplement filed by WGL pursuant to Securities Act Rule 424 on November 27, 2017.
[6]	{F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOmY3ZTA4NzgyOWIzYzRlZGJiNzUwNTY1YWJiMWU1N2EyfFRleHRTZWxlY3Rpb246RjA5RjMzMjdCNTIxNTc0N0E1QTMzRjAyMTU2RkRCNzUM}

Components of Total Equity (Det

Components of Total Equity (Details) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		$ 1,502,690
Net income (loss)		132,592	$ 55,767
Other comprehensive income		244	29,413
Common shareholders' equity, ending balance		1,609,607
Washington Gas Light Company
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		1,164,749
Net income (loss)		57,926	55,461
Other comprehensive income		192	$ 224
Common shareholders' equity, ending balance		$ 1,295,914
Ending Balance (in shares)		46,479,536	46,479,536
Washington Gas Light Company | Common Stock
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Beginning Balance (in shares)		46,479,536	46,479,536
Common shareholders' equity, beginning balance		$ 46,479	$ 46,479
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		492,101	488,135
Stock-based compensation	[1]	(8,916)	(6)
Proceeds from Contributed Capital		100,000
Common shareholders' equity, ending balance		583,185	488,129
Washington Gas Light Company | Retained Earnings
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		630,691	586,662
Net income (loss)		57,926	55,461
Stock-based compensation	[1]	4,197	0
Common shareholders' equity, ending balance		670,580	620,659
Dividends declared:
Common stock		(21,904)	(21,134)
Preferred stock		(330)	(330)
Washington Gas Light Company | Accumulated Other Comprehensive Income (Loss), Net of Taxes
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		(4,522)	(7,830)
Other comprehensive income		192	224
Common shareholders' equity, ending balance		(4,330)	(7,606)
Washington Gas Light Company | Total
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		1,164,749	1,113,446
Net income (loss)		57,926	55,461
Other comprehensive income		192	224
Stock-based compensation	[1]	(4,719)	(6)
Proceeds from Contributed Capital		100,000
Common shareholders' equity, ending balance		1,295,914	1,147,661
Dividends declared:
Common stock		(21,904)	(21,134)
Preferred stock		$ (330)	$ (330)
WGL | Common Stock
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Beginning Balance (in shares)		51,219,000	51,080,612
Common shareholders' equity, beginning balance		$ 582,716	$ 574,496
Stock-based compensation	[2]	$ 11,251	$ 6,564
Stock-based compensation plans (in shares)	[2]	133,540	104,676
Issuance of common stock	[3]		$ 1,613
Issuance of Common Stock (in shares)	[3]		25,680
Common shareholders' equity, ending balance		$ 593,967	$ 582,673
Ending Balance (in shares)		51,352,540	51,210,968
WGL | Paid-In Capital
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		$ 10,149	$ 12,519
Stock-based compensation	[2]	(20,534)	(6,567)
Common shareholders' equity, ending balance		(10,385)	5,952
WGL | Retained Earnings
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		915,822	827,085
Net income (loss)		138,040	57,972
Stock-based compensation	[2]		(126)
Stock based compensation	[2]	4,174
Common shareholders' equity, ending balance		1,031,778	859,966
Dividends declared:
Common stock		(26,258)	(24,965)
WGL | Accumulated Other Comprehensive Income (Loss), Net of Taxes
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		(5,997)	(38,539)
Other comprehensive income		244	29,413
Common shareholders' equity, ending balance		(5,753)	(9,126)
WGL | WGL Holdings Common Shareholders' Equity
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		1,502,690	1,375,561
Net income (loss)		138,040	57,972
Other comprehensive income		244	29,413
Stock-based compensation	[2]	(5,109)	(129)
Issuance of common stock	[3]		1,613
Common shareholders' equity, ending balance		1,609,607	1,439,465
Dividends declared:
Common stock		(26,258)	(24,965)
WGL | Non-controlling Interest
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		6,851	409
Net Income		(5,778)	(2,535)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		1,659	7,331
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders		(136)
Noncontrolling Interest, Increase from Business Combination	[4]	614
Common shareholders' equity, ending balance		3,210	5,205
WGL | Washington Gas Light Company Preferred Stock [Member]
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Common shareholders' equity, beginning balance		28,173	28,173
Net Income		330	330
Common shareholders' equity, ending balance		28,173	28,173
Dividends declared:
Preferred stock		(330)	(330)
WGL | Total Equity
Increase (Decrease) in Shareholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		1,640,990	1,472,843	$ 1,537,714	$ 1,404,143
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest		132,592	55,767
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		1,659	7,331
Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders		(136)
Noncontrolling Interest, Increase from Business Combination	[4]	614
Other comprehensive income		244	29,413
Stock-based compensation	[2]	(5,109)	(129)
Issuance of common stock	[3]		1,613
Dividends declared:
Common stock		(26,258)	(24,965)
Preferred stock		$ (330)	$ (330)
[1]	Stock-based compensation is based on the stock awards of WGL that are allocated to Washington Gas Light Company for its pro-rata share and includes implementation of ASU 2016-09, see Note 1—Accounting policies of the Notes to Condensed Consolidated Financial Statements.
[2]	Includes dividend equivalents related to our performance shares and implementation of ASU 2016-09, see Note 1—Accounting policies of the Notes to Condensed Consolidated Financial Statements.
[3]	Includes dividend reinvestment and common stock purchase plans.
[4]	Resulted from the consolidation of SFEE. For more information, see Note 11—Other investments of the Notes to Condensed Consolidated Financial Statements.

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net Income applicable to common stock	$ 138,040	$ 57,972
Basic EPS (in shares)	51,319,000	51,172,000
Stock-based compensation plans (in shares)	230,000	273,000
Diluted EPS (in shares)	51,549,000	51,445,000
Basic EPS (in dollars per share)	$ 2.69	$ 1.13
Diluted EPS (in dollars per share)	$ 2.68	$ 1.13
Antidilutive securities excluded from computation of earnings per share	0	0

Income Taxes-Narrative (Details

Income Taxes-Narrative (Details) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 22, 2017	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Sep. 30, 2018	Sep. 30, 2017
Income Tax Expense Benefit Details [Line Items]
Unrecognized tax benefits			$ 37,500				$ 48,100
Federal corporate income tax rate		35.00%
Public Utilities, requested rate increase (decrease)					$ 45,600
Regulatory liability			452,091				9,403
Income Tax Expense (Benefit)			(31,110)	$ 33,454
Deferred tax liabilities			413,065				868,067
Other regulatory asset			117,962				104,596
Subsequent Event
Income Tax Expense Benefit Details [Line Items]
Federal corporate income tax rate	21.00%
Public Utilities, requested rate increase (decrease)	$ (39,500)
Scenario, Forecast
Income Tax Expense Benefit Details [Line Items]
Federal corporate income tax rate						24.50%
Estimated reduction in tax depreciation expense						$ 88,000
Provisional amounts subject to adjustment
Income Tax Expense Benefit Details [Line Items]
Regulatory liability			423,500
Unrecognized Tax Benefits, Period Increase (Decrease)			(14,800)
Provisional amounts subject to adjustment | Excess deferred income tax
Income Tax Expense Benefit Details [Line Items]
Regulatory liability			429,300
Provisional amounts subject to adjustment | Consolidated deferred tax liability
Income Tax Expense Benefit Details [Line Items]
Regulatory liability			476,400
Provisional amounts subject to adjustment | Flow-through
Income Tax Expense Benefit Details [Line Items]
Decrease in regulatory assets			22,400
Provisional amounts subject to adjustment | Excess deferred income tax
Income Tax Expense Benefit Details [Line Items]
Other regulatory asset			5,800
Provisional amounts subject to adjustment | Re measurement of excess deferred taxes and net operating lass carryforward deferred tax asset [Member]
Income Tax Expense Benefit Details [Line Items]
Other regulatory asset			28,200
Regulated utility
Income Tax Expense Benefit Details [Line Items]
Regulatory liability			450,409				9,403
Income Tax Expense (Benefit)			24,854	$ 34,006
Deferred tax liabilities			463,805				888,385
Other regulatory asset			117,570				$ 104,538
Regulated utility | Provisional amounts subject to adjustment
Income Tax Expense Benefit Details [Line Items]
Regulatory liability			417,300
Regulated utility | Provisional amounts subject to adjustment | Plant-related
Income Tax Expense Benefit Details [Line Items]
Deferred tax liabilities			317,800
Regulated utility | Provisional amounts subject to adjustment | Discrete item
Income Tax Expense Benefit Details [Line Items]
Income Tax Expense (Benefit)			6,200
Non-utility operations | Provisional amounts subject to adjustment
Income Tax Expense Benefit Details [Line Items]
Regulatory liability			59,100
Non-utility operations | Provisional amounts subject to adjustment | Discrete item
Income Tax Expense Benefit Details [Line Items]
Income Tax Expense (Benefit)			$ (59,100)

Derivative and Weather Relate46

Derivative and Weather Related Instruments (Narrative) (Details) $ in Thousands		Jan. 01, 2018USD ($)	Dec. 31, 2017USD ($)Counterparties	Dec. 31, 2016USD ($)	Sep. 30, 2017USD ($)
Derivative [Line Items]
Qualified cash flow hedging instruments	[1]		$ 52	$ 49,455
Interest expense			20,197	16,235
Interest Rate Swap
Asset Optimization [Abstract]
Derivative notional amount			250,000		$ 250,000
Interest Rate Swap | Subsequent Event
Derivative [Line Items]
Gain (loss) on interest rate swap		$ 13,800
Interest expense		$ 13,200
Washington Gas Light Company
Derivative [Line Items]
Interest expense			14,973	12,762
Asset Optimization [Abstract]
Gain (loss) on asset optimization transactions, net pretax			5,800	24,300
Unrealized gain (loss) on asset optimization derivative instruments, net pretax			$ (1,400)	15,400
Concentration of Credit Risk
Number of counterparties | Counterparties			1
Percentage of credit exposure			10.00%
Obligation to counterparties			$ 16,000
WGL Energy Services
Derivative, Collateral [Abstract]
Collateral already posted, aggregate fair value			$ 11,800		$ 8,600
Concentration of Credit Risk
Number of counterparties | Counterparties			3
Percentage of credit exposure			10.00%
Obligation to counterparties			$ 900
Gain (losses) on weather related instruments, pretax			$ (800)	$ (1,500)
WGL Midstream
Concentration of Credit Risk
Number of counterparties | Counterparties			1
Percentage of credit exposure			10.00%
Obligation to counterparties			$ 15,900
WGL | Interest Rate Swap
Derivative [Line Items]
Qualified cash flow hedging instruments			$ 6,400
[1]	Cash flow hedging instruments represent interest rate swap agreements related to debt issuances. Refer to Note 8- Derivative and Weather-related Instruments of the Notes to Condensed Consolidated Financial Statements for further discussion of the interest rate swap agreements.

Derivative and Weather Relate47

Derivative and Weather Related Instruments (Details) kWh in Millions, MMBTU in Millions, $ in Millions		Dec. 31, 2017USD ($)kWhMMBTU	Sep. 30, 2017USD ($)kWhMMBTU
Balance Sheet Classification of Derivative Instruments
Netting of Collateral		$ 22	$ 9
Total	[1]	(114)	(111.9)
Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets		96.9	96.6
Gross Derivative Liabilities		(232.9)	(217.5)
Current Assets—Derivatives
Balance Sheet Classification of Derivative Instruments
Total	[1]	17.7	15.3
Current Assets—Derivatives | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets		29.2	26.6
Gross Derivative Liabilities		(11.5)	(11.3)
Deferred Charges and Other Assets—Derivatives
Balance Sheet Classification of Derivative Instruments
Total	[1]	32.2	38.4
Deferred Charges and Other Assets—Derivatives | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets		32.3	38.9
Gross Derivative Liabilities		(0.1)	(0.4)
Netting of Collateral			(0.1)
Accounts payable
Balance Sheet Classification of Derivative Instruments
Total	[1]		1
Accounts payable | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets			1
Current Liabilities—Derivatives
Balance Sheet Classification of Derivative Instruments
Netting of Collateral		14.4	2.1
Total	[1]	(35)	(44)
Current Liabilities—Derivatives | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets		17.8	10.9
Gross Derivative Liabilities		(67.2)	(57)
Deferred Credits—Derivatives
Balance Sheet Classification of Derivative Instruments
Netting of Collateral		7.6	7
Total	[1]	(128.9)	(122.6)
Deferred Credits—Derivatives | Derivative Instruments Not Designated as Hedging Instruments
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets		17.6	19.2
Gross Derivative Liabilities		$ (154.1)	$ (148.8)
Asset optimization & trading
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural gas notional amounts (in mmbtu) | MMBTU		2,196,090,000	2,166,350,000
Retail sales
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural gas notional amounts (in mmbtu) | MMBTU		14,320,000	12,430,000
Electricity notional amounts (in kWhs) | kWh		9,279.2	10,011.7
Other risk-management activities
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural gas notional amounts (in mmbtu) | MMBTU		159,740,000	154,670,000
Electricity notional amounts (in kWhs) | kWh		21,569.5	22,962.1
Interest Rate Swap
Absolute Notional Amounts of Open Positions on Derivative Instruments
Derivative notional amount		$ 250	$ 250
Washington Gas Light Company
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	[2]	24.8	24
Gross Derivative Liabilities	[2]	(148.8)	(145.3)
Netting of Collateral	[2]	0.1
Total	[1],[2]	(123.9)	(121.3)
Derivative, Collateral [Abstract]
Collateral deposits posted with counterparties		8.1	3.7
Cash collateral held representing an obligation		0.1	0.1
Washington Gas Light Company | Current Assets—Derivatives
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	[2]	9.4	7.5
Gross Derivative Liabilities	[2]	(5.1)	(2.4)
Total	[1],[2]	4.3	5.1
Washington Gas Light Company | Deferred Charges and Other Assets—Derivatives
Balance Sheet Classification of Derivative Instruments
Gross Derivative Assets	[2]	15.4	16.5
Gross Derivative Liabilities	[2]		(0.3)
Total	[1],[2]	15.4	16.2
Washington Gas Light Company | Current Liabilities—Derivatives
Balance Sheet Classification of Derivative Instruments
Gross Derivative Liabilities	[2]	(24.7)	(30.3)
Netting of Collateral	[2]	0.1
Total	[1],[2]	(24.6)	(30.3)
Washington Gas Light Company | Deferred Credits—Derivatives
Balance Sheet Classification of Derivative Instruments
Gross Derivative Liabilities	[2]	(119)	(112.3)
Total	[1],[2]	$ (119)	$ (112.3)
Washington Gas Light Company | Asset optimization & trading
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural gas notional amounts (in mmbtu) | MMBTU		1,176,800,000	1,122,300,000
Washington Gas Light Company | Other risk-management activities
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural gas notional amounts (in mmbtu) | MMBTU		118,100,000	118,100,000
WGL Energy Services
Derivative, Collateral [Abstract]
Collateral deposits posted with counterparties		$ 30.6	$ 23.7
WGL Midstream
Derivative, Collateral [Abstract]
Collateral deposits posted with counterparties		16.8	44.4
Cash collateral held representing an obligation		$ 0.4	$ 1.6
[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]	We did not have any derivative instruments outstanding that were designated as hedging instruments at December 31, 2017 or September 30, 2017.

Derivative and Weather Relate48

Derivative and Weather Related Instruments (Gains and Losses) (Details) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Gains and (Losses) on Derivative Instruments
Qualified cash flow hedging instruments	[1]	$ 52	$ 49,455
Total		6,000	78,300
Operating revenues—non-utility
Gains and (Losses) on Derivative Instruments
Recorded to income		(7,500)	(36,700)
Utility cost of gas
Gains and (Losses) on Derivative Instruments
Recorded to income		(2,600)	14,700
Non-utility cost of energy-related sales
Gains and (Losses) on Derivative Instruments
Recorded to income		22,200	31,300
Interest expense
Gains and (Losses) on Derivative Instruments
Recorded to income		(400)	(100)
Gas costs
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets		(5,800)	19,600
Recorded to other comprehensive income
Gains and (Losses) on Derivative Instruments
Qualified cash flow hedging instruments		100	49,500
Washington Gas Light Company
Gains and (Losses) on Derivative Instruments
Total		(8,400)	34,300
Washington Gas Light Company | Utility cost of gas
Gains and (Losses) on Derivative Instruments
Recorded to income		(2,600)	14,700
Washington Gas Light Company | Gas costs
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets		$ (5,800)	$ 19,600
[1]	Cash flow hedging instruments represent interest rate swap agreements related to debt issuances. Refer to Note 8- Derivative and Weather-related Instruments of the Notes to Condensed Consolidated Financial Statements for further discussion of the interest rate swap agreements.

Derivative and Weather Relate49

Derivative and Weather Related Instruments (Aggregate Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017
Potential Collateral Requirements for Derivative Liabilities with Credit-risk-Contingent Features
Derivative liabilities with credit-risk-contingent features	$ 14.5	$ 25
Maximum potential collateral requirements	8.1	21.9
Washington Gas Light Company
Potential Collateral Requirements for Derivative Liabilities with Credit-risk-Contingent Features
Derivative liabilities with credit-risk-contingent features	1.7	2.8
Maximum potential collateral requirements	$ 1.6	$ 2.8

Fair Value Measurements (Fair V

Fair Value Measurements (Fair Value Hierarchy) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	$ 96.9	$ 96.6
Liabilities	(232.9)	(217.5)
Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	77.8	71.2
Liabilities	(203.1)	(182.9)
Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	9.4	15.6
Liabilities	(20.7)	(25.8)
Interest Rate derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	9.7	9.8
Liabilities	(9.1)	(8.8)
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	37.4	28.3
Liabilities	(43.7)	(28.4)
Level 2 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	24.6	18.4
Liabilities	(33)	(15.5)
Level 2 | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	3.1	0.1
Liabilities	(1.6)	(4.1)
Level 2 | Interest Rate derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	9.7	9.8
Liabilities	(9.1)	(8.8)
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	59.5	68.3
Liabilities	(189.2)	(189.1)
Level 3 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	53.2	52.8
Liabilities	(170.1)	(167.4)
Level 3 | Electricity Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	6.3	15.5
Liabilities	(19.1)	(21.7)
Washington Gas Light Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	24.8	24
Liabilities	(148.8)	(145.3)
Washington Gas Light Company | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	24.8	24
Liabilities	(148.8)	(145.3)
Washington Gas Light Company | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	8.4	7
Liabilities	(7)	(5.7)
Washington Gas Light Company | Level 2 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	8.4	7
Liabilities	(7)	(5.7)
Washington Gas Light Company | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	16.4	17
Liabilities	(141.8)	(139.6)
Washington Gas Light Company | Level 3 | Natural Gas Related Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	16.4	17
Liabilities	$ (141.8)	$ (139.6)

Fair Value Measurements (Quanti

Fair Value Measurements (Quantitative information) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017USD ($)$ / MwH$ / dekatherm	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)$ / MwH$ / dekatherm	Dec. 31, 2016USD ($)
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Net Fair Value	$ (129.7)	$ (273.2)	$ (120.8)	$ (236.9)
Natural Gas Related Derivatives
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Net Fair Value	(116.9)	$ (264.1)	(114.6)	(228.6)
Natural Gas Related Derivatives | Discounted Cash Flow
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Net Fair Value	$ (113.2)		$ (112.4)
Natural Gas Related Derivatives | Discounted Cash Flow | Maximum | Natural Gas Basis Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Input price (usd per dekatherm/megawatt hour) | $ / dekatherm	3.400		2.805
Natural Gas Related Derivatives | Discounted Cash Flow | Minimum | Natural Gas Basis Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Input price (usd per dekatherm/megawatt hour) | $ / dekatherm	(1.125)		(2.095)
Natural Gas Related Derivatives | Option Model
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Net Fair Value	$ (3.7)		$ (2.2)
Natural Gas Related Derivatives | Option Model | Maximum | Natural Gas Basis Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Input price (usd per dekatherm/megawatt hour) | $ / dekatherm	3.100		2.358
Natural Gas Related Derivatives | Option Model | Maximum | Annualized Volatility Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Option Volatility Percentage	714.10%	566.80%
Natural Gas Related Derivatives | Option Model | Minimum | Natural Gas Basis Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Input price (usd per dekatherm/megawatt hour) | $ / dekatherm	(1.030)		(2.095)
Natural Gas Related Derivatives | Option Model | Minimum | Annualized Volatility Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Option Volatility Percentage	30.20%	28.70%
Electricity Related Derivatives
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Net Fair Value	$ (12.8)	$ (9.1)	$ (6.2)	(8.3)
Electricity Related Derivatives | Discounted Cash Flow
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Net Fair Value	$ (12.8)		$ (6.2)
Electricity Related Derivatives | Discounted Cash Flow | Maximum | Electricity Congestion Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Input price (usd per dekatherm/megawatt hour) | $ / MwH	78.990		56.500
Electricity Related Derivatives | Discounted Cash Flow | Minimum | Electricity Congestion Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Input price (usd per dekatherm/megawatt hour) | $ / MwH	(2.977)		(2.736)
Washington Gas Light Company | Natural Gas Related Derivatives
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Net Fair Value	$ (125.4)	$ (251.6)	$ (122.6)	$ (208.8)
Washington Gas Light Company | Natural Gas Related Derivatives | Discounted Cash Flow
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Net Fair Value	$ (125.4)		$ (122.6)
Washington Gas Light Company | Natural Gas Related Derivatives | Discounted Cash Flow | Maximum | Natural Gas Basis Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Input price (usd per dekatherm/megawatt hour) | $ / dekatherm	2.725		2.805
Washington Gas Light Company | Natural Gas Related Derivatives | Discounted Cash Flow | Minimum | Natural Gas Basis Price
Fair Value Inputs, Assets, Liabilities Quantitative Information [Line Items]
Input price (usd per dekatherm/megawatt hour) | $ / dekatherm	(1.125)		(1.928)

Fair Value Measurements (Reconc

Fair Value Measurements (Reconciliation with Level 3 Inputs) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	$ (120.8)	$ (273.2)
Recorded to income	(13.3)	6.7
Recorded to regulatory assets—gas costs	(6.9)	22
Transfers into Level 3	0.3	(0.2)
Transfers out of Level 3	(0.3)
Purchases	2.5	(2.8)
Settlements	8.8	10.6
Ending balance	(129.7)	(236.9)
Natural Gas Related Derivatives
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	(114.6)	(264.1)
Recorded to income	(4.1)	10.3
Recorded to regulatory assets—gas costs	(6.9)	22
Transfers into Level 3	0.3	(0.2)
Transfers out of Level 3	(0.3)
Settlements	8.7	3.4
Ending balance	(116.9)	(228.6)
Electricity Related Derivatives
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	(6.2)	(9.1)
Recorded to income	(9.2)	(3.6)
Purchases	2.5	(2.8)
Settlements	0.1	7.2
Ending balance	(12.8)	(8.3)
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	(122.6)	(251.6)
Recorded to income	(3.8)	17
Recorded to regulatory assets—gas costs	(6.9)	22
Transfers into Level 3	0.2	0.2
Settlements	7.7	3.6
Ending balance	$ (125.4)	$ (208.8)

Fair Value Measurements (Realiz

Fair Value Measurements (Realized and Unrealized Gains and Losses with Level 3 Measurements) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	$ (13.3)	$ 6.7
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Total	(20.8)	31.3
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	(4.1)	10.3
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Total	(13.4)	25.5
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	(9.2)	(3.6)
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Total	(7.4)	5.8
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	(22.6)	(21)
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	(19.1)	(15.6)
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	(6.5)	(10.3)
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	(5.5)	(9.7)
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	(16.1)	(10.7)
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	(13.6)	(5.9)
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	(3.8)	17
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	(4.6)	15.6
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	(3.8)	17
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	(4.6)	15.6
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	13.1	10.7
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	10.4	11.6
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	6.2	3.6
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	4.2	(0.1)
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	6.9	7.1
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	6.2	11.7
Recorded to regulatory assets-gas costs
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	(7.5)	19.7
Recorded to regulatory assets-gas costs | Natural Gas Related Derivatives
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	(7.5)	19.7
Washington Gas Light Company | 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	(3.8)	17
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Total	(12.1)	35.3
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	(3.8)	17
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	(4.6)	15.6
Washington Gas Light Company | Recorded to regulatory assets-gas costs | Natural Gas Related Derivatives
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	$ (7.5)	$ 19.7

Fair Value Measurements (Carryi

Fair Value Measurements (Carrying Amounts and Estimated Fair Value) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Sep. 30, 2017
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money market funds	[1]	$ 70.1	$ 11.8
Commercial paper	[2]	503.9	505
Project financing	[2]	94.6	54.8
Long-term debt	[3]	1,679.9	1,430.9
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money market funds	[1]	70.1	11.8
Commercial paper	[2]	503.9	505
Project financing	[2]	94.6	54.8
Long-term debt	[3]	1,842.8	1,577.3
Washington Gas Light Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money market funds	[1]	13.7	4.8
Commercial paper	[2]	162	123
Project financing	[2]	85.4	43.8
Long-term debt	[3]	1,084.6	1,134.5
Washington Gas Light Company | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Money market funds	[1]	13.7	4.8
Commercial paper	[2]	162	123
Project financing	[2]	85.4	43.8
Long-term debt	[3]	$ 1,232.9	$ 1,271
[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]	Excludes current maturities.

Operating Segment (Narrative) (

Operating Segment (Narrative) (Details)	3 Months Ended
	Dec. 31, 2017segment
Segment Reporting [Abstract]
Number of operating segments	4

Operating Segment Financial Inf

Operating Segment Financial Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Sep. 30, 2017
Operating Segment Financial Information [Line Items]
Operating Revenue, Regulated Utility		$ 374,990		$ 327,063
Operating Revenue, Non-Utility		277,450		282,424
Operating Revenues, Total consolidated		652,440		609,487
Depreciation and Amortization		40,985		35,283
Equity in earnings of unconsolidated affiliates		5,892		265
EBIT		127,457		107,991
Total Assets		7,062,574		6,475,390		$ 6,626,009
Capital Expenditures		99,054		155,758
Equity Method Investments		489,354		374,065
Eliminations
Operating Segment Financial Information [Line Items]
Operating Revenue, Non-Utility	[1]	(11,473)		(13,052)
Depreciation and Amortization	[1]	12		10
EBIT	[1]	1,689		1,108
Total Assets	[1]	(874,918)		(893,632)
Regulated utility | Operating Segments
Operating Segment Financial Information [Line Items]
Operating Revenue, Regulated Utility		377,470		333,986
Depreciation and Amortization		34,103		30,560
EBIT		98,365		102,717
Total Assets		5,227,299		4,891,722
Capital Expenditures		76,762		111,681
Retail energy-marketing | Operating Segments
Operating Segment Financial Information [Line Items]
Operating Revenue, Non-Utility		248,693		298,684
Depreciation and Amortization		282		305
EBIT		3,742		29,185
Total Assets		533,273		518,914
Capital Expenditures				404
Commercial energy systems | Operating Segments
Operating Segment Financial Information [Line Items]
Operating Revenue, Non-Utility	[2]	20,063		14,857
Depreciation and Amortization	[2]	6,584		4,399
Equity in earnings of unconsolidated affiliates		0	[1]	2,387	[2]
EBIT	[2]	5,647		4,663
Total Assets	[2]	1,104,588		932,908
Capital Expenditures	[2]	22,292		43,673
Equity Method Investments	[2]			68,573
Midstream energy services | Operating Segments
Operating Segment Financial Information [Line Items]
Operating Revenue, Non-Utility		17,687		(24,988)
Depreciation and Amortization		4		9
Equity in earnings of unconsolidated affiliates		5,892		(2,122)
EBIT		22,185		(28,484)
Total Assets		802,613		607,806
Equity Method Investments		489,354		305,492
Other activities
Operating Segment Financial Information [Line Items]
EBIT		(4,171)		(1,198)
Total Assets		$ 269,719		$ 417,672
[1]	(b) Intersegment eliminations include any mark-to market valuations associated with trading activities between WGL Midstream and WGL Energy Services, intercompany loans and a timing difference between Commercial Energy Systems’ recognition of revenue for the sale of Renewable Energy Credits (RECs) to Retail Energy-Marketing and Retail Energy-Marketing’s recognition of the associated expense. Retail Energy-Marketing has recorded a portion of the REC’s purchased as inventory to be used in future periods at which time they will be expensed. 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]	(a) As of August 2016, Commercial Energy Systems' operating revenues include revenues from non-controlling interest. Commercial energy systems' EBIT is adjusted for the effects of non-controlling interest.

Operating Segment - Reconcilati

Operating Segment - Reconcilation from EBIT to Net Income Applicable to Common Stock (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting [Abstract]
Total consolidated EBIT	$ 127,457	$ 107,991
Interest expense	20,197	16,235
Income tax expense	(31,110)	33,454
Dividends on Washington Gas Light Company preferred stock	330	330
Net Income applicable to common stock	$ 138,040	$ 57,972

Other Investments - Fair Value

Other Investments - Fair Value at Date of Consolidation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Oct. 01, 2017	Sep. 30, 2017
Property, plant and equipment	$ 4,678,880		$ 4,630,051
SFEE | Estimated Fair Value
Property, plant and equipment		$ 10,100
Other assets		100
Total assets		10,200
Net assets		10,200
Non-controlling interest		600
WGSW Equity interest		$ 9,600

Other Investments (Narrative) (

Other Investments (Narrative) (Details) $ in Thousands, dekatherms in Millions, MMBTU in Millions	3 Months Ended
	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)mileMMBTUdekathermsBcfe	Sep. 30, 2017USD ($)	Oct. 24, 2016	Mar. 31, 2015
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Investments in unconsolidated affiliates		$ 489,354	$ 394,201
Property, plant and equipment		6,217,878	6,143,841
WGSW | SFGF
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Contributions to the partnership		16,800
WGSW | SFRC
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Contributions to the partnership		20,800
WGSW | SFGF II
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Contributions to the partnership		800
WGSW | ASD
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Contributions to the partnership		72,500
WGSW | SFEE
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Contributions to the partnership		$ 6,500
WGL Midstream | Meade
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Pipeline length in miles | mile		185
Transportation and delivery capacity in dekatherms per day | dekatherms		1.7
Estimated Investment in Meade		$ 410,000
Ownership percentage		55.00%
Investments in unconsolidated affiliates		$ 238,600	146,700
WGL Midstream | Constitution
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership percentage		10.00%
Investments in unconsolidated affiliates		$ 38,200	38,100
WGL Midstream | Constitution | Scenario, Forecast
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Impairment	$ 38,200
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 | MMBTU		2
Ownership percentage		10.00%		3.00%	7.00%
Investments in unconsolidated affiliates		$ 76,800	63,000
WGL Midstream | Mountain Valley Pipeline | Scenario, Plan
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Pro rata share of project cost		$ 350,000
WGL Midstream | Stonewall
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Ownership percentage		30.00%
Investments in unconsolidated affiliates		$ 135,700	$ 136,700
Difference between carrying amount and underlying equity		$ 8,800
Pipeline gathering capacity | Bcfe		1.4

Other Investments Balance Sheet

Other Investments Balance Sheet Location Table (Details) - USD ($)		Dec. 31, 2017	Sep. 30, 2017
Balance Sheet Location of Unconsolidated Investments
Investments in unconsolidated affiliates		$ 489,354,000	$ 394,201,000
Property, plant and equipment		6,217,878,000	6,143,841,000
Consolidated VIE's | Current assets
Balance Sheet Location of Consolidated Investments
VIE, Consolidated, Carrying Amount, Assets		5,800	4,400
Consolidated VIE's | Property, plant and equipment
Balance Sheet Location of Consolidated Investments
VIE, Consolidated, Carrying Amount, Assets		137,500	121,700
Consolidated VIE's | Total Assets
Balance Sheet Location of Consolidated Investments
VIE, Consolidated, Carrying Amount, Assets		143,300	126,100
Consolidated VIE's | Current liabilities
Balance Sheet Location of Consolidated Investments
VIE, Consolidated, Carrying Amount, Liabilities		200	200
Consolidated VIE's | Deferred credits
Balance Sheet Location of Consolidated Investments
VIE, Consolidated, Carrying Amount, Liabilities		1,100	800
Consolidated VIE's | Total liabilities
Balance Sheet Location of Consolidated Investments
VIE, Consolidated, Carrying Amount, Liabilities		1,300	1,000
Non-consolidated VIE's | Pipelines
Balance Sheet Location of Unconsolidated Investments
Investments in unconsolidated affiliates	[1]	238,600,000	146,700,000
Total assets	[1]	238,600,000	146,700,000
Non consolidated non VIE's | Solar Investments
Balance Sheet Location of Unconsolidated Investments
Investments in unconsolidated affiliates	[2]		9,600,000
Total assets	[2]		9,600,000
Non consolidated non VIE's | Pipelines
Balance Sheet Location of Unconsolidated Investments
Investments in unconsolidated affiliates	[3]	250,800,000	237,900,000
Total assets	[3]	250,800,000	237,900,000
Total
Balance Sheet Location of Unconsolidated Investments
Investments in unconsolidated affiliates		489,400,000	394,200,000
Total assets		$ 489,400,000	$ 394,200,000
[1]	Balance relates to equity method investment in Meade.
[2]	Balance relates to interest held in SFEE on September 30, 2017
[3]	Balance relates to equity method investments in Constitution, Mountain Valley Pipeline and Stonewall System.

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017
Washington Gas Light Company
Related Party Transaction [Line Items]
Receivables from associated companies	$ 27,000		$ 32,400
Payables to associated companies	107,600		94,800
WGL Energy Services | Washington Gas Light Company
Related Party Transaction [Line Items]
Receivables from associated companies	4,800		1,400
Washington Gas Light Company
Related Party Transaction [Line Items]
Receivables from associated companies	27,010		32,362
Payables to associated companies	107,597		94,844
Washington Gas Light Company | WGL Energy Services
Related Party Transaction [Line Items]
Gas balancing service charge	2,500	$ 6,900
Receivables purchased from related party	$ 6,000		$ 3,200

Commitments and Contingencies (

Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions	Oct. 24, 2017	Apr. 17, 2017	Dec. 01, 2016	Dec. 31, 2017	Jun. 30, 2016
Commitments And Contingencies [Line Items]
Public Utilities, requested rate increase (decrease)					$ 45.6
Base rate increase		$ 34
Mid-point minimum
Commitments And Contingencies [Line Items]
Return on equity mid-point percentage range			9.00%
Mid point maximum
Commitments And Contingencies [Line Items]
Return on equity mid-point percentage range			10.00%
Pipeline replacement program
Commitments And Contingencies [Line Items]
Public Utilities, requested rate increase (decrease)					$ 22.3
Virginia SAVE program
Commitments And Contingencies [Line Items]
Base rate increase		$ 14.1
Washington Gas Light Company | Silver Spring Maryland Incident
Commitments And Contingencies [Line Items]
Loss Contingency, Damages Sought, Value				$ 5
WGL | Other External Partners | Performance guarantee
Commitments And Contingencies [Line Items]
Financial guarantees				0.6
WGL Midstream
Commitments And Contingencies [Line Items]
Losses associated with contract dispute				$ 28.4
WGL Midstream | Antero Contract
Commitments And Contingencies [Line Items]
Loss Contingency, Damages Sought, Value	$ 80

Pension and Other Post-Retire63

Pension and Other Post-Retirement Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3.7	$ 4.1
Interest cost	9.9	9.6
Expected return on plan assets	(10.6)	(10.3)
Amortization of prior service cost (credit)	0.1	0.1
Amortization of net actuarial loss	3.9	5.5
Net periodic benefit cost (income)	7	9
Amount allocated to construction projects	(1.1)	(1.6)
Amount deferred as regulatory asset/liability—net	1.7	1.8
Amount charged (credited) to expense	7.6	9.2
Health and Life Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.3	1.4
Interest cost	2.9	2.9
Expected return on plan assets	(5.9)	(5.5)
Amortization of prior service cost (credit)	(4.4)	(4.4)
Amortization of net actuarial loss		0.2
Net periodic benefit cost (income)	(6.1)	(5.4)
Amount allocated to construction projects	1	1.2
Amount deferred as regulatory asset/liability—net		(0.1)
Amount charged (credited) to expense	$ (5.1)	$ (4.3)

Accumulated Other Comprehensi64

Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
AOCI, Net of Tax [Roll Forward]
Beginning Balance		$ (5,997)	$ (38,539)
Qualified cash flow hedging instruments	[1]	52	49,455
Change in prior service credit	[2]	(274)	(217)
Amortization of actuarial loss	[2]	527	588
Current-period other comprehensive income		305	49,826
Income tax expense related to other comprehensive income		61	20,413
Ending Balance		(5,753)	(9,126)
Washington Gas Light Company
AOCI, Net of Tax [Roll Forward]
Beginning Balance		(4,522)	(7,830)
Change in prior service credit	[3]	(274)	(217)
Amortization of actuarial loss	[3]	527	588
Current-period other comprehensive income		253	371
Income tax expense related to other comprehensive income		61	147
Ending Balance		$ (4,330)	$ (7,606)
[1]	Cash flow hedging instruments represent interest rate swap agreements related to debt issuances. Refer to Note 8- Derivative and Weather-related Instruments of the Notes to Condensed Consolidated Financial Statements for further discussion of the interest rate swap agreements.
[2]	These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost. Refer to Note 14- Pension and other post-retirement benefit plans of the Notes to Condensed Consolidated Financial Statements for additional details.
[3]	These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost. Refer to Note 14-Pension and other post-retirement benefit plans of the Notes to Condensed Consolidated Financial Statements for additional details.

Supplemental Cash Flow Inform65

Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes in operating assets and liabilities	$ (175,402)	$ (201,340)
Washington Gas Light Company
Accounts receivable and unbilled revenues—net	(167,271)	(133,605)
Gas costs and other regulatory assets/liabilities—net	38,503	35,400
Storage gas	7,793	(6,029)
Prepaid taxes	(8,549)	(11,934)
Accounts payable and other accrued liabilities	15,409	49,444
Customer deposits and advance payments	(9,483)	(8,410)
Accrued taxes	34,146	9,307
Other current assets	(9,904)	(9,456)
Other current liabilities	(124)	(29,859)
Deferred gas costs—net	(26,023)	(42,189)
Deferred assets—other	(17,649)	(2,766)
Deferred liabilities—other	539	(16,657)
Pension and other post-retirement benefits	(442)	(3,586)
Other—net	1,691	510
Changes in operating assets and liabilities	(141,364)	(169,830)
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Income taxes paid (refunded)—net	(958)	(6,449)
Interest paid	2,977	683
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES:
Extinguishment of project debt financing		(29,871)
Capital expenditure accruals included in accounts payable and other accrued liabilities	25,325	40,601
WGL
Accounts receivable and unbilled revenues—net	(237,959)	(203,098)
Gas costs and other regulatory assets/liabilities—net	38,503	35,400
Storage gas	37,006	(1,709)
Prepaid taxes	(7,275)	(7,901)
Accounts payable and other accrued liabilities	40,575	87,512
Customer deposits and advance payments	(10,683)	(12,611)
Accrued taxes	18,019	11,149
Other current assets	(15,107)	(21,887)
Other current liabilities	(2,662)	(22,569)
Deferred gas costs—net	(26,023)	(42,189)
Deferred assets—other	(18,483)	(4,431)
Deferred liabilities—other	7,061	(16,194)
Pension and other post-retirement benefits	(291)	(3,583)
Other—net	1,917	771
Changes in operating assets and liabilities	(175,402)	(201,340)
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Income taxes paid (refunded)—net	(956)	(4,678)
Interest paid	10,392	4,156
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING ACTIVITIES:
Extinguishment of project debt financing		(29,871)
Capital expenditure accruals included in accounts payable and other accrued liabilities	34,371	55,602
Dividends Paid in Common Stock		1,362
Stock based compensation	11,251	6,564
Transfer of investments to fixed assets	$ 10,054	3,959
Transfer of notes receivables to investments		$ 10,031

Planned Merger With AltaGas (De

Planned Merger With AltaGas (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 25, 2017	Mar. 31, 2018
Business Acquisition [Line Items]
Business Acquisition, Date of Acquisition Agreement	Jan. 25, 2017
Business Acquisition cash to be paid by acquirer	$ 6,400
Business Acquisition, Share Price	$ 88.25
Subscription agreement | Non-voting preferred stock
Business Acquisition [Line Items]
Business Acquisition, Share Price	$ 10,000
Preferred Stock, Shares Issued	15,000
Maximum shares purchased per quarter	5,000
Shares purchased after termination or Outside Date expiration	0
Redemption period after termination or Outside Date expiration	6 months
Termination right, Circumstance 1
Business Acquisition [Line Items]
Business Acquisition, termination fee	$ 205
Termination right, Circumstance 2
Business Acquisition [Line Items]
Business Acquisition, termination fee	182
Termination right, Circumstance 3
Business Acquisition [Line Items]
Business Acquisition, termination fee	68
WGL | Termination right
Business Acquisition [Line Items]
Business Acquisition, termination fee	$ 136
Minimum | Subscription agreement | Scenario, Forecast
Business Acquisition [Line Items]
Ratio Of Indebtedness To Net Capital Percentage		62.00%

Subsequent Events - (Details)

Subsequent Events - (Details)	Jan. 11, 2018
Constitution Pipeline
Subsequent Event [Line Items]
Ownership percentage	10.00%