Derivative and Weather Related Instruments (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended	12 Months Ended															3 Months Ended	12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 WGL Holdings, Inc.	Sep. 30, 2010 WGL Holdings, Inc.	Jun. 30, 2011 WGL Holdings, Inc. Asset Optimization [Member] therms	Jun. 30, 2011 WGL Holdings, Inc. Retail Sales [Member] therms kWh	Sep. 30, 2010 WGL Holdings, Inc. Retail Sales [Member] kWh therms	Jun. 30, 2011 WGL Holdings, Inc. Other Risk Management Activities [Member] kWh therms	Sep. 30, 2010 WGL Holdings, Inc. Other Risk Management Activities [Member] therms kWh	Jun. 30, 2011 WGL Holdings, Inc. Other Assets [Member]	Sep. 30, 2010 WGL Holdings, Inc. Other Assets [Member]	Jun. 30, 2011 WGL Holdings, Inc. Other Assets Noncurrent [Member]	Sep. 30, 2010 WGL Holdings, Inc. Other Assets Noncurrent [Member]	Jun. 30, 2011 WGL Holdings, Inc. Accounts Payable And Other Accrued Liabilities [Member]	Jun. 30, 2011 WGL Holdings, Inc. Other Liabilities [Member]	Sep. 30, 2010 WGL Holdings, Inc. Other Liabilities [Member]	Jun. 30, 2011 WGL Holdings, Inc. Other Liabilities Noncurrent [Member]	Sep. 30, 2010 WGL Holdings, Inc. Other Liabilities Noncurrent [Member]	Jun. 30, 2011 Washington Gas Light Company	Sep. 30, 2010 Washington Gas Light Company	Jun. 30, 2011 Washington Gas Light Company Asset Optimization [Member] therms	Jun. 30, 2011 Washington Gas Light Company Retail Sales [Member] kWh therms	Sep. 30, 2010 Washington Gas Light Company Retail Sales [Member] kWh therms	Jun. 30, 2011 Washington Gas Light Company Other Risk Management Activities [Member] kWh therms	Sep. 30, 2010 Washington Gas Light Company Other Risk Management Activities [Member] kWh therms	Jun. 30, 2011 Washington Gas Light Company Other Assets [Member]	Sep. 30, 2010 Washington Gas Light Company Other Assets [Member]	Jun. 30, 2011 Washington Gas Light Company Other Assets Noncurrent [Member]	Sep. 30, 2010 Washington Gas Light Company Other Assets Noncurrent [Member]	Jun. 30, 2011 Washington Gas Light Company Other Liabilities [Member]	Sep. 30, 2010 Washington Gas Light Company Other Liabilities [Member]	Jun. 30, 2011 Washington Gas Light Company Other Liabilities Noncurrent [Member]	Sep. 30, 2010 Washington Gas Light Company Other Liabilities Noncurrent [Member]
Asset Optimization [Abstract]
Gain (Loss) on Asset Optimization Derivative Instruments Net Pretax	$ 3,900,000	$ 1,100,000	$ 5,000,000	$ 22,900,000
Unrealized Loss On Asset Optimization Derivative Instruments Net Pretax	4,800,000	1,300,000	9,600,000	13,500,000
Absolute Notional Amounts of Open Positions on Derivative Instruments
Natural Gas Derivative Transaction, Volume						1,271,100	1,438,100,000	2,900,000	5,000,000	619,300,000	316,800,000											1,271,100	1,119,100,000	0	0	412,800,000	123,200,000
Electricity Derivative Transaction, Volume								686.1	1,417.0	21,011.6	13,278.0													0	0	0	0
Interest Rate Swap Notional Principal Value					0	75,000,000															0	75,000,000
Balance Sheet Classification of Derivative Instruments
Derivative Assets					89,500,000	120,900,000						27,600,000	22,700,000	45,300,000	85,100,000	1,700,000	10,200,000	12,200,000	4,700,000	900,000	58,400	93,900						6,500	12,400	39,600	74,100	8,500	7,200	3,800	200
Derivative Liabilities					(95,200,000)	(155,000,000)						(10,500,000)	(5,400,000)	(29,500,000)	(51,900,000)	(200,000)	(32,400,000)	(67,500,000)	(22,600,000)	(30,200,000)	(57,500)	(81,500)						(3,600)	(5,400)	(28,500)	(51,900)	(13,300)	(23,700)	(12,100)	(500)
Netting of Collateral					400,000	4,600,000						100,000	0	0	0	0	(1,900,000)	1,300,000	2,200,000	3,300,000	0	0						0	0	0	0	0	0	0	0
Total					$ (5,300,000)	$ (29,500,000)						$ 17,200,000	$ 17,300,000	$ 15,800,000	$ 33,200,000	$ 1,500,000	$ (24,100,000)	$ (54,000,000)	$ (15,700,000)	$ (26,000,000)	$ 900	$ 12,400						$ 2,900	$ 7,000	$ 11,100	$ 22,200	$ (4,800)	$ (16,500)	$ (8,300)	$ (300)