Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets:
Derivative assets	$ 23.6	$ 26.2
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,806.5	3,860.5
Cumulative preferred stock	200.0	145.1
Cumulative preferred stock	201.8
Cumulative preferred stock	0	60.0
Cumulative preferred stock		212.6
Derivative liabilities	17.1	40.4
IPL [Member]
Assets:
Derivative assets	12.5	17.5
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,643.8	1,679.9
Cumulative preferred stock	200.0	145.1
Cumulative preferred stock	201.8	151.8
Derivative liabilities	6.1	16.1
WPL [Member]
Assets:
Derivative assets	11.1	8.7
Capitalization and liabilities:
Long-term debt (including current maturities)	1,691.6	1,713.3
Cumulative preferred stock	0	60.0
Cumulative preferred stock	0	60.8
Derivative liabilities	11.0	24.3
Carrying Amount [Member]
Assets:
Derivative assets	23.6	26.2
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,123.4	3,138.1
Cumulative preferred stock	200.0
Cumulative preferred stock		205.1
Derivative liabilities	17.1	40.4
Carrying Amount [Member] | IPL [Member]
Assets:
Derivative assets	12.5	17.5
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,344.5	1,359.5
Cumulative preferred stock	200.0	145.1
Derivative liabilities	6.1	16.1
Carrying Amount [Member] | WPL [Member]
Assets:
Derivative assets	11.1	8.7
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,331.7	1,331.5
Cumulative preferred stock	0	60.0
Derivative liabilities	11.0	24.3
Fair Value [Member]
Assets:
Derivative assets	23.6	26.2
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,806.5	3,860.5
Cumulative preferred stock	201.8
Cumulative preferred stock		212.6
Derivative liabilities	17.1	40.4
Fair Value [Member] | IPL [Member]
Assets:
Derivative assets	12.5	17.5
Deferred proceeds (sales of receivables)	122.1	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,643.8	1,679.9
Cumulative preferred stock	201.8	151.8
Derivative liabilities	6.1	16.1
Fair Value [Member] | WPL [Member]
Assets:
Derivative assets	11.1	8.7
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,691.6	1,713.3
Cumulative preferred stock	0	60.8
Derivative liabilities	$ 11.0	$ 24.3