Fair Value Measurements (Fair Value Of Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Assets:
Derivative assets	$ 20.3	$ 38.6
Capitalization and liabilities:
Derivative liabilities	59.1	37.6
IPL [Member]
Assets:
Derivative assets	18.5	28.0
Capitalization and liabilities:
Derivative liabilities	22.0	19.5
WPL [Member]
Assets:
Derivative assets	1.8	10.6
Capitalization and liabilities:
Derivative liabilities	37.1	18.1
Carrying Amount [Member]
Assets:
Derivative assets	20.3	38.6
Deferred proceeds (sales of receivables)	195.5	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	3,858.8	3,789.7
Cumulative preferred stock	200.0	200.0
Derivative liabilities	59.1	37.6
Carrying Amount [Member] | IPL [Member]
Assets:
Derivative assets	18.5	28.0
Deferred proceeds (sales of receivables)	195.5	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	1,868.5	1,768.7
Cumulative preferred stock	200.0	200.0
Derivative liabilities	22.0	19.5
Carrying Amount [Member] | WPL [Member]
Assets:
Derivative assets	1.8	10.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,543.6	1,573.9
Derivative liabilities	37.1	18.1
Fair Value [Member]
Assets:
Derivative assets	20.3	38.6
Deferred proceeds (sales of receivables)	195.5	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	4,394.3	4,418.2
Cumulative preferred stock	201.3	200.2
Derivative liabilities	59.1	37.6
Fair Value [Member] | IPL [Member]
Assets:
Derivative assets	18.5	28.0
Deferred proceeds (sales of receivables)	195.5	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	2,115.3	2,053.0
Cumulative preferred stock	201.3	200.2
Derivative liabilities	22.0	19.5
Fair Value [Member] | WPL [Member]
Assets:
Derivative assets	1.8	10.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,825.5	1,908.9
Derivative liabilities	$ 37.1	$ 18.1