Fair Value Measurements (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Sep. 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]
Assets:
Money market funds, at carrying value	13.6	0
Capitalization and liabilities:
Long-term debt (including current maturities)	3,598.7
Cumulative preferred stock of subsidiaries	215.6
IPL [Member]
Assets:
Money market funds, at carrying value	1.1	0
Deferred proceeds (sales of receivables)	155.9	55.3
Capitalization and liabilities:
Long-term debt (including current maturities)	1,696.1
Cumulative preferred stock of subsidiaries	145.1	145.1
Cumulative preferred stock of subsidiaries	154.8
WPL [Member]
Assets:
Money market funds, at carrying value	12.5	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,492.4
Cumulative preferred stock of subsidiaries	60.0	60.0
Cumulative preferred stock of subsidiaries	60.8
Carrying Amount [Member] | Alliant Energy [Member]
Assets:
Money market funds, at carrying value	13.6
Derivative assets	46.5	15.7
Deferred proceeds (sales of receivables)	153.9	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	2,829.5	2,704.5
Cumulative preferred stock of subsidiaries	205.1	205.1
Derivative liabilities	57.3	78.0
Carrying Amount [Member] | IPL [Member]
Assets:
Money market funds, at carrying value	1.1
Derivative assets	28.1	10.6
Deferred proceeds (sales of receivables)	153.9	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	22.8	33.6
Carrying Amount [Member] | WPL [Member]
Assets:
Money market funds, at carrying value	12.5
Derivative assets	18.4	5.1
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,082.5	1,082.2
Cumulative preferred stock of subsidiaries	60.0	60.0
Derivative liabilities	34.5	44.4
Fair Value [Member] | Alliant Energy [Member]
Assets:
Money market funds, at carrying value	13.6
Derivative assets	46.5	15.7
Deferred proceeds (sales of receivables)	153.9	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	3,598.7	3,325.3
Cumulative preferred stock of subsidiaries	215.6	222.5
Derivative liabilities	57.3	78.0
Fair Value [Member] | IPL [Member]
Assets:
Money market funds, at carrying value	1.1
Derivative assets	28.1	10.6
Deferred proceeds (sales of receivables)	153.9	53.7
Capitalization and liabilities:
Long-term debt (including current maturities)	1,696.1	1,560.4
Cumulative preferred stock of subsidiaries	154.8	164.3
Derivative liabilities	22.8	33.6
Fair Value [Member] | WPL [Member]
Assets:
Money market funds, at carrying value	12.5
Derivative assets	18.4	5.1
Deferred proceeds (sales of receivables)	0	0
Capitalization and liabilities:
Long-term debt (including current maturities)	1,492.4	1,439.0
Cumulative preferred stock of subsidiaries	60.8	58.2
Derivative liabilities	$ 34.5	$ 44.4