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, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	$ 21.9	$ 12.0	$ 18.0
IPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	8.4	5.8	5.5
WPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	13.5	6.2	12.5
Debt [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	7.8	4.4	6.8
Debt [Member] | IPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	3.2	2.3	2.5
Debt [Member] | WPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	4.6	2.1	4.3
Equity [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	14.1	7.6	11.2
Equity [Member] | IPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	5.2	3.5	3.0
Equity [Member] | WPL [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction	$ 8.9	$ 4.1	$ 8.2