Accounting Policies (Narrative) (Details)			3 Months Ended	12 Months Ended
Accounting Policies (Narrative) (Details)	Jul. 06, 2018	Jan. 25, 2017	Sep. 30, 2018 USD ($) subsidiary Customers	Sep. 30, 2018 USD ($) subsidiary Customers	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Oct. 01, 2017 USD ($)
Accounting Policies [Table] [Line Items]
Business Acquisition, Date of Acquisition Agreement		Jan. 25, 2017
Business Acquisition, Effective Date of Acquisition	Jul. 06, 2018
Number of subsidiaries \| subsidiary			4	4
Number of customers (more than one million) \| Customers			1,000,000	1,000,000
Variable Interest Entity, Number of Investments			6	6
Pretax AFUDC Rate				2.42%	2.73%	5.51%
Capitalized interest				$ 11,100,000	$ 2,900,000	$ 2,100,000
Composite depreciation and amortization rate				2.78%	2.80%	2.70%
Regulated revenue billing cycle				19 days
Duration of cost of gas recovered or refunded				12 months
Gross revenue taxes				$ 82,500,000	$ 75,100,000	$ 73,000,000
Insurance Proceeds-Property Plant and Equipment				3,238,000	0	0
Current Income Tax Expense (Benefit)				(1,035,000)	3,697,000	(59,673,000)
Other income (expense) —net				(15,043,000)	1,819,000	4,646,000
ASU 2018-02
Accounting Policies [Table] [Line Items]
Retained earnings			$ (1,800,000)	(1,800,000)
ASU 2016-18
Accounting Policies [Table] [Line Items]
Rabbi Trust			65,000,000	65,000,000
ASU 2016-15
Accounting Policies [Table] [Line Items]
Insurance Proceeds-Property Plant and Equipment				3,200,000
ASU 2016-09
Accounting Policies [Table] [Line Items]
Retained earnings			$ 4,300,000	4,300,000			$ 4,300,000
Current Income Tax Expense (Benefit)				3,400,000	3,600,000
ASU 2017-07
Accounting Policies [Table] [Line Items]
Other income (expense) —net				16,400	6,500
Constitution
Accounting Policies [Table] [Line Items]
Impairment				$ 34,000,000
VIE, Primary Beneficiary
Accounting Policies [Table] [Line Items]
Variable Interest Entity, Number of Investments			5	5
Washington Gas Light Company
Accounting Policies [Table] [Line Items]
Impairment of Oil and Gas Properties			$ 38,000,000
Insurance Proceeds-Property Plant and Equipment				$ 3,238,000	0	0
Current Income Tax Expense (Benefit)				1,274,000	3,005,000	(51,021,000)
Other income (expense) —net				(18,876,000)	(4,473,000)	$ (2,143,000)
Washington Gas Light Company \| ASU 2018-02
Accounting Policies [Table] [Line Items]
Retained earnings			(1,500,000)	(1,500,000)
Washington Gas Light Company \| ASU 2016-15
Accounting Policies [Table] [Line Items]
Insurance Proceeds-Property Plant and Equipment				3,200,000
Washington Gas Light Company \| ASU 2016-09
Accounting Policies [Table] [Line Items]
Retained earnings			$ 4,200,000	4,200,000			$ 4,200,000
Current Income Tax Expense (Benefit)				3,200,000	3,500,000
Washington Gas Light Company \| ASU 2017-07
Accounting Policies [Table] [Line Items]
Other income (expense) —net				$ 11,600	3,600
WGL Energy Services \| Minimum
Accounting Policies [Table] [Line Items]
Indexed or fixed rate customer contracts terms				24 months
WGL Energy Services \| Maximum
Accounting Policies [Table] [Line Items]
Indexed or fixed rate customer contracts terms				5 years
WGL
Accounting Policies [Table] [Line Items]
Lease Impairment					$ 4,100,000
Natural Gas Customers \| WGL Energy Services
Accounting Policies [Table] [Line Items]
Number of customers \| Customers			108,900	108,900
Electricity Customers \| WGL Energy Services
Accounting Policies [Table] [Line Items]
Number of customers \| Customers			101,700	101,700
Firm Customer \| Washington Gas Light Company
Accounting Policies [Table] [Line Items]
Percentage of total gas deliveries				77.00%
Interruptible Customers \| Washington Gas Light Company
Accounting Policies [Table] [Line Items]
Percentage of total gas deliveries				13.00%
Electric Generation Customers \| Washington Gas Light Company
Accounting Policies [Table] [Line Items]
Percentage of total gas deliveries				10.00%
Regulated Operation [Member]
Accounting Policies [Table] [Line Items]
Total regulated utility segment, percentage			87.30%	87.30%	87.20%

WGL HOLDINGS INC 10-K