Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Other, accum depr.	$ 5.6	$ 5.3
Non-regulated Generation, accum. depr.	31.0	26.4
Alliant Energy Corporate Services, Inc. and other, accum. depr.	200.2	177.4
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	240,000,000	240,000,000
Common stock, shares outstanding (in shares)	110,987,400	111,018,821
Shares in deferred compensation trust (in shares)	216,030	262,735
Shares in deferred compensation trust, weighted average cost per share (in dollars per share)	$ 33.61	$ 31.68
IPL [Member]
Other, accum depr.	4.1	4.0
Common stock, par value	$ 2.50	$ 2.50
Common stock, shares authorized (in shares)	24,000,000	24,000,000
Common stock, shares outstanding (in shares)	13,370,788	13,370,788
WPL [Member]
Leased Sheboygan Falls Energy Facility, accum. depr.	46.7	40.6
Other, accum depr.	$ 1.5	$ 1.3
Common stock, par value	$ 5	$ 5
Common stock, shares authorized (in shares)	18,000,000	18,000,000
Common stock, shares outstanding (in shares)	13,236,601	13,236,601