Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Other, accum. depr.	$ 5.3	$ 6.4
Non-regulated Generation, accum. depr.	26.4	22.4
Alliant Energy Corporate Services, Inc. and other, less accum. depr.	185.8	173.6
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	240,000,000	240,000,000
Common stock, shares outstanding	111,018,821	110,893,901
Shares in deferred compensation trust, shares	262,735	246,301
Shares in deferred compensation trust, weighted average cost per share	$ 31.68	$ 30.75
IPL [Member]
Other, accum. depr.	4.0	4.2
Common stock, par value	$ 2.5	$ 2.5
Common stock, shares authorized	24,000,000	24,000,000
Common stock, shares outstanding	13,370,788	13,370,788
WPL [Member]
Leased Sheboygan Falls Energy Facility, accumulated amortization	40.6	34.4
Other, accum. depr.	$ 1.3	$ 2.2
Common stock, par value	$ 5	$ 5
Common stock, shares authorized	18,000,000	18,000,000
Common stock, shares outstanding	13,236,601	13,236,601