Fair Value Measurements (Fair Value Of Financial Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Assets:
Derivative assets	$ 35.0	$ 38.6
Capitalization and liabilities:
Derivative liabilities	47.0	37.6
IPL [Member]
Assets:
Derivative assets	30.7	28.0
Capitalization and liabilities:
Derivative liabilities	20.7	19.5
WPL [Member]
Assets:
Derivative assets	4.3	10.6
Capitalization and liabilities:
Derivative liabilities	26.3	18.1
Carrying Amount [Member]
Assets:
Derivative assets	35.0	38.6
Deferred proceeds (sales of receivables)	73.4	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	3,750.9	3,789.7
Cumulative preferred stock	200.0	200.0
Derivative liabilities	47.0	37.6
Carrying Amount [Member] | IPL [Member]
Assets:
Derivative assets	30.7	28.0
Deferred proceeds (sales of receivables)	73.4	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	1,730.2	1,768.7
Cumulative preferred stock	200.0	200.0
Derivative liabilities	20.7	19.5
Carrying Amount [Member] | WPL [Member]
Assets:
Derivative assets	4.3	10.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,574.1	1,573.9
Derivative liabilities	26.3	18.1
Fair Value [Member]
Assets:
Derivative assets	35.0	38.6
Deferred proceeds (sales of receivables)	73.4	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	4,214.4	4,418.2
Cumulative preferred stock	198.9	200.2
Derivative liabilities	47.0	37.6
Fair Value [Member] | IPL [Member]
Assets:
Derivative assets	30.7	28.0
Deferred proceeds (sales of receivables)	73.4	177.2
Capitalization and liabilities:
Long-term debt (including current maturities)	1,937.2	2,053.0
Cumulative preferred stock	198.9	200.2
Derivative liabilities	20.7	19.5
Fair Value [Member] | WPL [Member]
Assets:
Derivative assets	4.3	10.6
Capitalization and liabilities:
Long-term debt (including current maturities)	1,824.0	1,908.9
Derivative liabilities	$ 26.3	$ 18.1