Common Shareholders' Equity (Details) (USD $) In Thousands, except Share data	3 Months Ended		9 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Net income applicable to common stock	$ 2,952	$ 9,681	$ 147,612	$ 136,028
WGL Holdings, Inc.
Common shareholders' equity, beginning balance			1,153,395
Shares outstanding, beginning balance			50,974,992
Net income applicable to common stock	2,952	9,681	147,612	136,028
Net income before preferred stock dividend	3,282	10,011	148,602	137,018
Post-retirement benefits adjustment, net of taxes			421
Comprehensive income	3,098	10,458	148,033	136,636
Stock-based compensation			9,773
Dividends declared:
Common Stock			(59,025)
Preferred Stock	330	330	990	990
Common shareholders' equity, ending balance	1,252,176		1,252,176
Shares outstanding, ending balance	51,292,838		51,292,838
WGL Holdings, Inc. | Common Stock
Common shareholders' equity, beginning balance			543,121
Net income applicable to common stock			0
Post-retirement benefits adjustment, net of taxes			0
Stock-based compensation			11,675
Dividends declared:
Common Stock			0
Common shareholders' equity, ending balance	554,796		554,796
WGL Holdings, Inc. | Paid-In Capital
Common shareholders' equity, beginning balance			8,889
Net income applicable to common stock			0
Post-retirement benefits adjustment, net of taxes			0
Stock-based compensation			(1,902)
Dividends declared:
Common Stock			0
Common shareholders' equity, ending balance	6,987		6,987
WGL Holdings, Inc. | Retained Earnings
Common shareholders' equity, beginning balance			609,956
Net income applicable to common stock			147,612
Post-retirement benefits adjustment, net of taxes			0
Stock-based compensation			0
Dividends declared:
Common Stock			(59,025)
Common shareholders' equity, ending balance	698,543		698,543
WGL Holdings, Inc. | Accumulated Other Comprehensive Loss, Net of Taxes
Common shareholders' equity, beginning balance			(8,571)
Net income applicable to common stock			0
Post-retirement benefits adjustment, net of taxes			421
Stock-based compensation			0
Dividends declared:
Common Stock			0
Common shareholders' equity, ending balance	(8,150)		(8,150)
Washington Gas Light Company
Common shareholders' equity, beginning balance			994,876
Shares outstanding, beginning balance			46,479,536
Net income applicable to common stock	(4,167)	(10,687)	106,953	120,609
Net income before preferred stock dividend	(3,837)	(10,357)	107,943	121,599
Post-retirement benefits adjustment, net of taxes			421
Comprehensive income	(3,691)	(9,580)	108,364	122,207
Stock-based compensation			1,499
Dividends declared:
Common Stock			(54,805)
Preferred Stock			(990)
Common shareholders' equity, ending balance	1,048,944		1,048,944
Shares outstanding, ending balance	46,479,536		46,479,536
Washington Gas Light Company | Common Stock
Common shareholders' equity, beginning balance			46,479
Net income before preferred stock dividend			0
Post-retirement benefits adjustment, net of taxes			0
Stock-based compensation			0
Dividends declared:
Common Stock			0
Preferred Stock			0
Common shareholders' equity, ending balance	46,479		46,479
Washington Gas Light Company | Paid-In Capital
Common shareholders' equity, beginning balance			470,825
Net income before preferred stock dividend			0
Post-retirement benefits adjustment, net of taxes			0
Stock-based compensation			1,499
Dividends declared:
Common Stock			0
Preferred Stock			0
Common shareholders' equity, ending balance	472,324		472,324
Washington Gas Light Company | Retained Earnings
Common shareholders' equity, beginning balance			486,143
Net income before preferred stock dividend			107,943
Post-retirement benefits adjustment, net of taxes			0
Stock-based compensation			0
Dividends declared:
Common Stock			(54,805)
Preferred Stock			(990)
Common shareholders' equity, ending balance	538,291		538,291
Washington Gas Light Company | Accumulated Other Comprehensive Loss, Net of Taxes
Common shareholders' equity, beginning balance			(8,571)
Net income before preferred stock dividend			0
Post-retirement benefits adjustment, net of taxes			421
Stock-based compensation			0
Dividends declared:
Common Stock			0
Preferred Stock			0
Common shareholders' equity, ending balance	$ (8,150)		$ (8,150)