Asset Retirement Obligations (AROs) (Reconciliation Of Changes In Asset Retirement Obligations) (Details) (USD $) In Millions	9 Months Ended			3 Months Ended	9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010	Sep. 30, 2011 IPL [Member] Sixth Street Asbestos [Member]	Sep. 30, 2011 IPL [Member]		Sep. 30, 2010 IPL [Member]	Sep. 30, 2011 WPL [Member]		Sep. 30, 2010 WPL [Member]
Balance, Jan.1	$ 75.9		$ 63.3		$ 43.6		$ 41.9	$ 32.3		$ 21.4
Revisions in estimated cash flows	7.8	[1]		7.0	7.7	[1]		0.1	[1]
Liabilities incurred	4.0		5.3		3.1			0.9		5.3
Accretion expense	3.4		3.1		2.1		2.1	1.3		1.0
Liabilities settled	(0.5)		(0.6)		(0.4)		(0.5)	(0.1)		(0.1)
Balance, Sep. 30	$ 90.6		$ 71.1		$ 56.1		$ 43.5	$ 34.5		$ 27.6

[1] In the third quarter of 2011, IPL recorded revisions in estimated cash flows of $7.0 million based on revised remediation timing and cost information for asbestos remediation at its Sixth Street Generating Station.