Derivative and Weather Related Instruments (Gains and Losses) (Details) (USD $)	3 Months Ended		9 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Sep. 30, 2010
Potential Collateral Requirements for Derivative Liabilities with Credit-risk-Contingent Features
Maximum potential collateral requirements	$ 10,900,000		$ 10,900,000
WGL Holdings, Inc.
Gains and (Losses) on Derivative Instruments
Gains And Losses On Derivative Instruments	3,300,000	10,600,000	(2,200,000)	6,600,000
Potential Collateral Requirements for Derivative Liabilities with Credit-risk-Contingent Features
Derivative liabilities with credit-risk-contingent features	67,300,000		67,300,000		111,300,000
Maximum potential collateral requirements	22,500,000		22,500,000		67,500,000
WGL Holdings, Inc. | Operating Revenues Non Utility [Member]
Gains and (Losses) on Derivative Instruments
Recorded to income	(3,600,000)	(14,100,000)	(11,200,000)	4,100,000
WGL Holdings, Inc. | Utility Cost Of Gas [Member]
Gains and (Losses) on Derivative Instruments
Recorded to income	2,200,000	500,000	(8,700,000)	19,600,000
WGL Holdings, Inc. | Non Utility Cost Of Energy Related Sales [Member]
Gains and (Losses) on Derivative Instruments
Recorded to income	3,100,000	29,600,000	30,800,000	(19,800,000)
WGL Holdings, Inc. | Gas Costs [Member]
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets/liabilities	1,600,000	800,000	(19,300,000)	8,700,000
WGL Holdings, Inc. | Other Member
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets/liabilities	0	(6,200,000)	6,200	(6,000,000)
Washington Gas Light Company
Gains and (Losses) on Derivative Instruments
Gains And Losses On Derivative Instruments	3,800,000	(4,900,000)	(21,800,000)	22,300,000
Potential Collateral Requirements for Derivative Liabilities with Credit-risk-Contingent Features
Derivative liabilities with credit-risk-contingent features	41,400,000		41,400,000		54,600,000
Maximum potential collateral requirements	600,000		600,000		12,300,000
Washington Gas Light Company | Operating Revenues Non Utility [Member]
Gains and (Losses) on Derivative Instruments
Recorded to income	0	0	0	0
Washington Gas Light Company | Utility Cost Of Gas [Member]
Gains and (Losses) on Derivative Instruments
Recorded to income	2,200,000	500,000	(8,700,000)	19,600,000
Washington Gas Light Company | Non Utility Cost Of Energy Related Sales [Member]
Gains and (Losses) on Derivative Instruments
Recorded to income	0	0	0	0
Washington Gas Light Company | Gas Costs [Member]
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets/liabilities	1,600,000	800,000	(19,300,000)	8,700,000
Washington Gas Light Company | Other Member
Gains and (Losses) on Derivative Instruments
Recorded to regulatory assets/liabilities	0	(6,200,000)	6,200	(6,000,000)
WGEServices
Potential Collateral Requirements for Derivative Liabilities with Credit-risk-Contingent Features
Derivative liabilities with credit-risk-contingent features	100,000		100,000
Maximum potential collateral requirements	$ 3,100,000		$ 3,100,000