v2.4.0.6
Income Taxes (Schedule Of Unrecognized Tax Benefits) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Uncertain Tax Positions [Roll Forward]      
Balance, January 1 $ 23.5 [1] $ 66.7 [1] $ 101.7
Additions based on tax positions related to the current year 0.7 0.7 3.8
Additions for tax positions of prior years (a) 0 [2] 0 [2] 9.1 [2]
Reductions for tax positions of prior years (b) (23.5) [3] (43.9) [3] (31.8) [3]
Settlements with taxing authorities 0 0 (16.1)
Balance, December 31 0.7 [1] 23.5 [1] 66.7 [1]
IPL [Member]
      
Uncertain Tax Positions [Roll Forward]      
Balance, January 1 10.9 [1] 33.0 [1] 58.4
Additions based on tax positions related to the current year 0 0.1 2.5
Additions for tax positions of prior years (a) 0 [2] 0 [2] 3.8 [2]
Reductions for tax positions of prior years (b) (10.9) [3] (22.2) [3] (22.7) [3]
Settlements with taxing authorities 0 0 (9.0)
Balance, December 31 0 [1] 10.9 [1] 33.0 [1]
WPL [Member]
      
Uncertain Tax Positions [Roll Forward]      
Balance, January 1 12.6 [1] 33.7 [1] 42.1
Additions based on tax positions related to the current year 0.7 0.6 1.3
Additions for tax positions of prior years (a) 0 [2] 0 [2] 5.2 [2]
Reductions for tax positions of prior years (b) (12.6) [3] (21.7) [3] (8.0) [3]
Settlements with taxing authorities 0 0 (6.9)
Balance, December 31 0.7 [1] 12.6 [1] 33.7 [1]
Regulatory Liabilities [Member] | Alliant Energy And IPL [Member]
      
Uncertain Tax Positions [Roll Forward]      
Balance, December 31 $ 0 [1] $ 10.0 [1]  
[1] At December 31, 2012 and 2011, $0 and $10 million, respectively, of uncertain tax positions balances for both Alliant Energy and IPL included amounts recorded in regulatory liability accounts.
[2] The additions for tax positions of prior years were related to positions taken by Alliant Energy, IPL and WPL on their federal and state tax returns related to the capitalization and dispositions of property.
[3] In 2012, the reductions for tax positions of prior years were due to the finalization of Alliant Energy’s federal income tax return audits for calendar years 2005 through 2009. In 2011, the reductions for tax positions of prior years were related to guidance published by the IRS clarifying the treatment of repair expenditures for electric distribution property. In 2010, the reductions of tax positions of prior years were primarily related to deductions taken by Alliant Energy, IPL and WPL on their federal and state tax returns that were settled under audit for amounts less than the reductions of tax positions recorded.