Derivative Instruments (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 40.7	$ 12.7
Current derivative liabilities	48.4	55.9
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	29.8	9.2
Current derivative liabilities	23.2	24.5
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	10.9	3.5
Current derivative liabilities	25.2	31.4
Commodity Contracts [Member] | Alliant Energy [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	40.7	12.7
Non-current derivative assets	3.7	3.0
Current derivative liabilities	48.4	55.9
Non-current derivative liabilities	16.9	22.1
Commodity Contracts [Member] | IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	29.8	9.2
Non-current derivative assets	1.7	1.4
Current derivative liabilities	23.2	24.5
Non-current derivative liabilities	6.1	9.1
Commodity Contracts [Member] | WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	10.9	3.5
Non-current derivative assets	2.0	1.6
Current derivative liabilities	25.2	31.4
Non-current derivative liabilities	$ 10.8	$ 13.0