Asset Retirement Obligations (AROs) (Reconciliation Of Changes In Asset Retirement Obligations) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	$ 101.5		$ 91.1
Revisions in estimated cash flows	0	[1]	(8.2)	[1]
Liabilities settled	(0.1)		(1.0)
Accretion expense	1.0		0.9
Balance, March 31	102.4		82.8
IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	45.5		56.2
Revisions in estimated cash flows	0	[1]	(8.2)	[1]
Liabilities settled	0		(0.9)
Accretion expense	0.5		0.5
Balance, March 31	46.0		47.6
WPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance, January 1	46.9		34.9
Revisions in estimated cash flows	0	[1]	0	[1]
Liabilities settled	(0.1)		(0.1)
Accretion expense	0.4		0.4
Balance, March 31	47.2		35.2
Sixth Street [Member] | IPL [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Revisions in estimated cash flows			$ (8.2)	[1]

[1] For the three months ended March 31, 2012, IPL recorded revisions in estimated cash flows of ($8.2) million based on revised remediation timing and cost information for asbestos remediation at its Sixth Street Generating Station.