Derivative Instruments (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 35.9	$ 12.7
Current derivative liabilities	37.8	55.9
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	25.9	9.2
Current derivative liabilities	17.9	24.5
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	10.0	3.5
Current derivative liabilities	19.9	31.4
Commodity Contracts [Member] | Alliant Energy [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	35.9	12.7
Non-current derivative assets	10.6	3.0
Current derivative liabilities	37.8	55.9
Non-current derivative liabilities	19.5	22.1
Commodity Contracts [Member] | IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	25.9	9.2
Non-current derivative assets	2.2	1.4
Current derivative liabilities	17.9	24.5
Non-current derivative liabilities	4.9	9.1
Commodity Contracts [Member] | WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	10.0	3.5
Non-current derivative assets	8.4	1.6
Current derivative liabilities	19.9	31.4
Non-current derivative liabilities	$ 14.6	$ 13.0