Fair Value Measurements (Realized and Unrealized Gains and Losses with Level 3 Measurements) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
Recorded to income			$ 4.6
WGL Holdings, Inc.
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
Recorded to income	12.7	11.5		1.4
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	1.7	2.3	(12.5)	9.2
Unrealized Gains Losses Recorded For Level 3 Measurements	10.1	10.9	(1.8)	25.0
WGL Holdings, Inc. | Operating Revenues Non Utility [Member]
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
Recorded to income	(3.8)	(14.1)	(12.3)	4.1
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	1.8	(5.1)	2.8	16.1
WGL Holdings, Inc. | Utility Cost Of Gas [Member]
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
Recorded to income	1.4	1.9	(3.7)	12.1
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	1.2	1.8	(1.6)	12.3
WGL Holdings, Inc. | Non Utility Cost Of Energy Related Sales [Member]
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
Recorded to income	15.1	23.7	23.4	(15.8)
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	5.4	11.9	9.5	(13.6)
WGL Holdings, Inc. | Operation And Maintenance Expense [Member]
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
Recorded to income	0	0	(2.8)	1.0
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	0	0	0	1.0
Washington Gas Light Company
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
Recorded to income	1.4	1.9	(6.5)	13.1
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to regulatory assets - gas costs	1.7	2.3	(12.5)	9.2
Unrealized Gains Losses Recorded For Level 3 Measurements	2.9	4.1	(14.1)	21.5
Washington Gas Light Company | Utility Cost Of Gas [Member]
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
Recorded to income	1.4	1.9	(3.7)	12.1
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	1.2	1.8	(1.6)	12.3
Washington Gas Light Company | Operation And Maintenance Expense [Member]
Realized and Unrealized Gains (Losses) Recorded to Income for Level 3 Measurements
Recorded to income	0	0	(2.8)	1.0
Unrealized Gains (Losses) Recorded for Level 3 Measurements
Recorded to income	$ 0	$ 0	$ 0	$ 0