v2.4.0.6
Benefit Plans (Schedule Of Qualified And Non-Qualified Pension And Other Post Retirement Benefits Costs) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Pension Plans, Defined Benefit [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Aggregate additional payments to certain plan participants $ 0.1 [1] $ 10.2 [1] $ 0 [1]
Pension Plans, Defined Benefit [Member] | IPL [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Allocated qualified and non-qualified costs 4.9 [2] 5.8 [2] 2.9 [2]
Pension Plans, Defined Benefit [Member] | WPL [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Allocated qualified and non-qualified costs 3.6 [2] 4.2 [2] 1.9 [2]
Other Postretirement Benefit Plans, Defined Benefit [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Aggregate additional payments to certain plan participants 0 [1],[3] 0 [1],[3] 0 [1],[3]
Other Postretirement Benefit Plans, Defined Benefit [Member] | IPL [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Allocated qualified and non-qualified costs 0.1 0.3 2.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | WPL [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Allocated qualified and non-qualified costs 0.1 0.2 1.3
Aggregate additional payments to certain plan participants 0 [3] 0 [3] 0 [3]
Corporate Service Allocation [Member] | IPL [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Aggregate additional payments to certain plan participants   3.5  
Corporate Service Allocation [Member] | WPL [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Aggregate additional payments to certain plan participants   $ 2.7  
[1] Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Cash Balance Plan in 2011. The agreement with the IRS required Alliant Energy to amend the Cash Balance Plan, which was completed in 2011 resulting in aggregate additional benefits of $10.2 million paid by Alliant Energy to certain former participants in the Cash Balance Plan in 2011. Alliant Energy recognized $10.2 million of additional benefits costs in 2011 related to these benefits. IPL recognized $6.3 million ($2.8 million directly assigned and $3.5 million allocated by Corporate Services) and WPL recognized $3.4 million ($0.7 million directly assigned and $2.7 million allocated by Corporate Services) of additional benefits costs in 2011 related to these benefits. Refer to Note 13(c) for additional information regarding the Cash Balance Plan.
[2] Includes settlement losses related to payments made to retired executives of Alliant Energy in 2012. In 2011, additional qualified pension benefits costs resulting from the 2011 amendment to the Cash Balance Plan allocated to IPL and WPL by Corporate Services were $3.5 million and $2.7 million, respectively.
[3] In 2011, Alliant Energy, IPL and WPL amended their defined benefit postretirement health care plans resulting in a revision to the method and level of coverage provided for participants more than 65 years of age. This amendment was determined to be a significant event, which required Alliant Energy, IPL and WPL to remeasure their defined benefit postretirement health care plans in 2011. The amendment resulted in a decrease in Alliant Energy’s, IPL’s and WPL’s postretirement benefit obligations of $55 million, $30 million and $16 million, respectively, in 2011 with the impact of the remeasurement on net periodic benefit costs being recognized prospectively from the remeasurement date. The impact of the remeasurement decreased Alliant Energy’s, IPL’s and WPL’s net periodic benefit costs by $11.3 million, $7.2 million, and $3.8 million in 2011, respectively. The discount rate used for the remeasurement was 5.20%. All other assumptions used for the remeasurement were consistent with the measurement assumptions used at December 31, 2010.