Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Capitalization and liabilities:
Long-term debt (including current maturities)	$ 3,149.9	$ 2,716.3
Long-term debt (including current maturities)	3,860.5
Cumulative preferred stock of subsidiaries	212.6
Cumulative preferred stock of subsidiaries	145.1	145.1
Cumulative preferred stock of subsidiaries	60.0	60.0
IPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,363.4	1,313.4
Long-term debt (including current maturities)	1,679.9
Cumulative preferred stock of subsidiaries	151.8
Cumulative preferred stock of subsidiaries	145.1	145.1
WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,339.1	1,089.1
Long-term debt (including current maturities)	1,713.3
Cumulative preferred stock of subsidiaries	60.8
Cumulative preferred stock of subsidiaries	60.0	60.0
Carrying Amount [Member]
Assets:
Derivative assets	26.2	15.7
Deferred proceeds (sales of receivables)	66.8	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	3,138.1	2,704.5
Cumulative preferred stock of subsidiaries	205.1	205.1
Derivative liabilities	40.4	78.0
Carrying Amount [Member] | IPL [Member]
Assets:
Derivative assets	17.5	10.6
Deferred proceeds (sales of receivables)	66.8	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	1,359.5	1,309.0
Cumulative preferred stock of subsidiaries	145.1	145.1
Derivative liabilities	16.1	33.6
Carrying Amount [Member] | WPL [Member]
Assets:
Derivative assets	8.7	5.1
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,331.5	1,082.2
Cumulative preferred stock of subsidiaries	60.0	60.0
Derivative liabilities	24.3	44.4
Fair Value [Member]
Assets:
Derivative assets	26.2	15.7
Deferred proceeds (sales of receivables)	66.8	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	3,860.5	3,325.3
Cumulative preferred stock of subsidiaries	212.6	222.5
Derivative liabilities	40.4	78.0
Fair Value [Member] | IPL [Member]
Assets:
Derivative assets	17.5	10.6
Deferred proceeds (sales of receivables)	66.8	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	1,679.9	1,560.4
Cumulative preferred stock of subsidiaries	151.8	164.3
Derivative liabilities	16.1	33.6
Fair Value [Member] | WPL [Member]
Assets:
Derivative assets	8.7	5.1
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,713.3	1,439.0
Cumulative preferred stock of subsidiaries	60.8	58.2
Derivative liabilities	$ 24.3	$ 44.4