v2.4.0.6
Benefit Plans (Estimated Future And Actual Employer Contributions) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended
Sep. 30, 2012
Pension Plans, Defined Benefit [Member]
Alliant Energy [Member]
Sep. 30, 2011
Pension Plans, Defined Benefit [Member]
Alliant Energy [Member]
Sep. 30, 2012
Pension Plans, Defined Benefit [Member]
Alliant Energy [Member]
Sep. 30, 2011
Pension Plans, Defined Benefit [Member]
Alliant Energy [Member]
Sep. 30, 2012
Qualified Defined Benefit Pension Plans [Member]
Alliant Energy [Member]
Sep. 30, 2012
Qualified Defined Benefit Pension Plans [Member]
IPL [Member]
Sep. 30, 2012
Qualified Defined Benefit Pension Plans [Member]
WPL [Member]
Sep. 30, 2012
Non-qualified defined benefit pension plans [Member]
Alliant Energy [Member]
Sep. 30, 2012
Directly assigned defined benefit pension plans [Member]
IPL [Member]
Sep. 30, 2012
Directly assigned defined benefit pension plans [Member]
WPL [Member]
Sep. 30, 2012
Other postretirement benefits plans [Member]
Alliant Energy [Member]
Sep. 30, 2011
Other postretirement benefits plans [Member]
Alliant Energy [Member]
Sep. 30, 2012
Other postretirement benefits plans [Member]
Alliant Energy [Member]
Sep. 30, 2011
Other postretirement benefits plans [Member]
Alliant Energy [Member]
Sep. 30, 2012
Other postretirement benefits plans [Member]
IPL [Member]
Sep. 30, 2012
Other postretirement benefits plans [Member]
WPL [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Pension Plans, Defined Benefit [Member]
Alliant Energy [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Qualified Defined Benefit Pension Plans [Member]
Alliant Energy [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Qualified Defined Benefit Pension Plans [Member]
IPL [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Qualified Defined Benefit Pension Plans [Member]
WPL [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Non-qualified defined benefit pension plans [Member]
Alliant Energy [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Directly assigned defined benefit pension plans [Member]
IPL [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Directly assigned defined benefit pension plans [Member]
WPL [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Other postretirement benefits plans [Member]
Alliant Energy [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Other postretirement benefits plans [Member]
IPL [Member]
Dec. 31, 2012
Scenario, Forecast [Member]
Other postretirement benefits plans [Member]
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]                                                    
Estimated for Calendar Year 2012                                   $ 0 $ 0 $ 0 $ 16.3 [1] $ 0.8 [2] $ 0.2 [2] $ 4.4 $ 2.2 $ 2.0
Actual Through September 30, 2012         0 0 0 3.1 [1] 0.7 [2] 0.1 [2]     4.4   2.2 2.0                    
Settlement loss $ 0 $ 1.1 $ 0 $ 1.1             $ 0 $ 0 $ 0 $ 0     $ 5.0                  
[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. Estimated amounts for calendar year 2012 include amounts paid to a retired executive in the fourth quarter of 2012, which are expected to result in a settlement loss of $5 million in the fourth quarter of 2012.
[2] Amounts directly assigned to IPL and WPL for non-bargaining employees who are participants in Alliant Energy and Corporate Services sponsored qualified and non-qualified defined benefit pension plans.