v2.4.0.6
Benefit Plans (Estimated Future Funding) (Details) (Scenario, Forecast [Member], USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Qualified Defined Benefit Pension Plans [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Estimated funding for calendar year 2013 $ 0
Qualified Defined Benefit Pension Plans [Member] | IPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Estimated funding for calendar year 2013 0
Qualified Defined Benefit Pension Plans [Member] | WPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Estimated funding for calendar year 2013 0
Non-qualified defined benefit pension plans [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Estimated funding for calendar year 2013 2.4 [1]
Directly assigned defined benefit pension plans [Member] | IPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Estimated funding for calendar year 2013 0.8 [2]
Directly assigned defined benefit pension plans [Member] | WPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Estimated funding for calendar year 2013 0.2 [2]
Other Postretirement Benefits Plans [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Estimated funding for calendar year 2013 3.0
Other Postretirement Benefits Plans [Member] | IPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Estimated funding for calendar year 2013 0
Other Postretirement Benefits Plans [Member] | WPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Estimated funding for calendar year 2013 $ 3.0
[1] Alliant Energy sponsors several non-qualified defined benefit pension plans that cover certain current and former key employees of IPL and WPL. Alliant Energy allocates pension costs to IPL and WPL for these plans.
[2] Amounts directly assigned to IPL and WPL for non-bargaining employees who are participants in the Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.