Short-Term Debt (Details) (USD $)	Jun. 30, 2011	Sep. 30, 2010	Jun. 30, 2010
Line Of Credit Facility [Line Items]
Unsecured revolving credit facility, expires August 3, 2012	$ 700,000,000	$ 700,000,000
Less: Commercial Paper	(13,000,000)	(100,400,000)
Net committed credit available	687,000,000	599,600,000
Short-term debt weighted average interest rate	0.20%		0.31%
WGL Holdings, Inc.
Line Of Credit Facility [Line Items]
Unsecured revolving credit facility, expires August 3, 2012	400,000,000	400,000,000
Less: Commercial Paper	(13,000,000)	(57,000,000)
Net committed credit available	387,000,000	343,000,000
Revolving credit facility maximum borrowing capacity	450,000
Revolving credit facility additional borrowings	50,000
Washington Gas Light Company
Line Of Credit Facility [Line Items]
Unsecured revolving credit facility, expires August 3, 2012	300,000,000	300,000,000
Less: Commercial Paper	0	(43,400,000)
Net committed credit available	300,000,000	256,600,000
Revolving credit facility maximum borrowing capacity	400,000
Revolving credit facility additional borrowings	$ 100,000