Summary Of Significant Accounting Policies (Schedule Of Allowance For Funds Used During Construction) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	$ 12.0	$ 18.0	$ 39.7
IPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	5.8	5.5	34.0
WPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	6.2	12.5	5.7
Equity [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	7.6	11.2	28.2
Equity [Member] | IPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	3.5	3.0	24.2
Equity [Member] | WPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	4.1	8.2	4.0
Debt [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	4.4	6.8	11.5
Debt [Member] | IPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	2.3	2.5	9.8
Debt [Member] | WPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	$ 2.1	$ 4.3	$ 1.7