Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Capitalization and liabilities:
Cumulative preferred stock of subsidiaries	$ 145.1	$ 145.1
Cumulative preferred stock of subsidiaries	60.0	60.0
Alliant Energy [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	3,471.9
Cumulative preferred stock of subsidiaries	218.7
IPL [Member]
Assets:
Deferred proceeds (sales of receivables)	83.2	55.3
Capitalization and liabilities:
Long-term debt (including current maturities)	1,670.3
Cumulative preferred stock of subsidiaries	145.1	145.1
Cumulative preferred stock of subsidiaries	158.1
WPL [Member]
Capitalization and liabilities:
Long-term debt (including current maturities)	1,470.3
Cumulative preferred stock of subsidiaries	60.0	60.0
Cumulative preferred stock of subsidiaries	60.6
Carrying Amount [Member] | Alliant Energy [Member]
Assets:
Derivative assets	44.4	15.7
Deferred proceeds (sales of receivables)	81.7	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	2,754.2	2,704.5
Cumulative preferred stock of subsidiaries	205.1	205.1
Derivative liabilities	65.3	78.0
Carrying Amount [Member] | IPL [Member]
Assets:
Derivative assets	31.5	10.6
Deferred proceeds (sales of receivables)	81.7	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	1,359.3	1,309.0
Cumulative preferred stock of subsidiaries	145.1	145.1
Derivative liabilities	29.3	33.6
Carrying Amount [Member] | WPL [Member]
Assets:
Derivative assets	12.9	5.1
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,082.3	1,082.2
Cumulative preferred stock of subsidiaries	60.0	60.0
Derivative liabilities	36.0	44.4
Fair Value [Member] | Alliant Energy [Member]
Assets:
Derivative assets	44.4	15.7
Deferred proceeds (sales of receivables)	81.7	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	3,471.9	3,325.3
Cumulative preferred stock of subsidiaries	218.7	222.5
Derivative liabilities	65.3	78.0
Fair Value [Member] | IPL [Member]
Assets:
Derivative assets	31.5	10.6
Deferred proceeds (sales of receivables)	81.7	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	1,670.3	1,560.4
Cumulative preferred stock of subsidiaries	158.1	164.3
Derivative liabilities	29.3	33.6
Fair Value [Member] | WPL [Member]
Assets:
Derivative assets	12.9	5.1
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,470.3	1,439.0
Cumulative preferred stock of subsidiaries	60.6	58.2
Derivative liabilities	$ 36.0	$ 44.4