Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets:
Derivative assets	$ 36.8	$ 26.2
Deferred proceeds (sales of receivables)	86.9	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,553.6	3,860.5
Cumulative preferred stock	200.0	145.1
Cumulative preferred stock	169.5
Cumulative preferred stock	0	60.0
Cumulative preferred stock		212.6
Derivative liabilities	27.1	40.4
IPL [Member]
Assets:
Derivative assets	31.5	17.5
Deferred proceeds (sales of receivables)	86.9	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,553.8	1,679.9
Cumulative preferred stock	200.0	145.1
Cumulative preferred stock	169.5	151.8
Derivative liabilities	6.7	16.1
WPL [Member]
Assets:
Derivative assets	5.3	8.7
Capitalization and liabilities:
Long-term debt (including current maturities)	1,541.9	1,713.3
Cumulative preferred stock	0	60.0
Cumulative preferred stock	0	60.8
Derivative liabilities	20.4	24.3
Carrying Amount [Member]
Assets:
Derivative assets	36.8	26.2
Deferred proceeds (sales of receivables)	86.9	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,153.2	3,138.1
Cumulative preferred stock	200.0
Cumulative preferred stock		205.1
Derivative liabilities	27.1	40.4
Carrying Amount [Member] | IPL [Member]
Assets:
Derivative assets	31.5	17.5
Deferred proceeds (sales of receivables)	86.9	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,374.8	1,359.5
Cumulative preferred stock	200.0	145.1
Derivative liabilities	6.7	16.1
Carrying Amount [Member] | WPL [Member]
Assets:
Derivative assets	5.3	8.7
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,332.0	1,331.5
Cumulative preferred stock	0	60.0
Derivative liabilities	20.4	24.3
Fair Value [Member]
Assets:
Derivative assets	36.8	26.2
Deferred proceeds (sales of receivables)	86.9	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	3,553.6	3,860.5
Cumulative preferred stock	169.5
Cumulative preferred stock		212.6
Derivative liabilities	27.1	40.4
Fair Value [Member] | IPL [Member]
Assets:
Derivative assets	31.5	17.5
Deferred proceeds (sales of receivables)	86.9	66.8
Capitalization and liabilities:
Long-term debt (including current maturities)	1,553.8	1,679.9
Cumulative preferred stock	169.5	151.8
Derivative liabilities	6.7	16.1
Fair Value [Member] | WPL [Member]
Assets:
Derivative assets	5.3	8.7
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,541.9	1,713.3
Cumulative preferred stock	0	60.8
Derivative liabilities	$ 20.4	$ 24.3