v2.4.0.6
Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Amortization of:      
Postreitrement health care plan, minimum age of participants covered 65 years    
Increase (decrease) in pension and other postretirement benefits obligations $ 51.3 $ 8.9 $ (20.1)
Discount rate for benefit obligations   5.20%  
Defined Benefit Pension Plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 13.5 11.4 11.9
Interest cost 51.6 52.0 52.3
Expected return on plan assets (68.8) [1] (63.8) [1] (62.1) [1]
Amortization of:      
Transition obligation 0 [2] 0 [2] 0 [2]
Prior service cost (credit) 0.3 [2] 0.7 [2] 0.9 [2]
Actuarial loss 33.3 [2] 21.1 [2] 23.8 [2]
Additional benefit costs 0.1 [3] 10.2 [3] 0 [3]
Settlement losses 5.4 [4] 1.1 [4] 1.4 [4]
Total 35.4 32.7 28.2
Discount rate for benefit obligations 4.11% 4.86% 5.56%
Other Postretirement Benefits Plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 6.9 [5] 7.0 [5] 9.3 [5]
Interest cost 10.2 [5] 12.3 [5] 14.9 [5]
Expected return on plan assets (7.5) [1],[5] (7.9) [1],[5] (7.7) [1],[5]
Amortization of:      
Transition obligation 0 [2],[5] 0 [2],[5] 0.1 [2],[5]
Prior service cost (credit) (12.0) [2],[5] (10.0) [2],[5] (2.4) [2],[5]
Actuarial loss 6.3 [2],[5] 5.3 [2],[5] 7.4 [2],[5]
Additional benefit costs 0 [3],[5] 0 [3],[5] 0 [3],[5]
Settlement losses 0 [4],[5] 0 [4],[5] 0 [4],[5]
Total 3.9 [5] 6.7 [5] 21.6 [5]
Increase (decrease) in pension and other postretirement benefits obligations   (55.0)  
Increase (decrease) of net periodic benefit costs due to impact of remeasurement   (11.3)  
Discount rate for benefit obligations 3.82% 4.60% 5.25%
Cash Balance Plan [Member]
      
Amortization of:      
Additional benefit costs   10.2  
IPL [Member]
      
Amortization of:      
Increase (decrease) in pension and other postretirement benefits obligations 21.0 (8.3) (9.0)
IPL [Member] | Defined Benefit Pension Plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 7.5 6.1  
Interest cost 17.1 16.7  
Amortization of:      
Discount rate for benefit obligations 4.20% 4.95% 5.70%
IPL [Member] | Other Postretirement Benefits Plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 3.0 [5] 2.6 [5] 3.4 [5]
Interest cost 4.4 [5] 5.5 [5] 6.8 [5]
Expected return on plan assets (5.1) [1],[5] (5.4) [1],[5] (5.3) [1],[5]
Amortization of:      
Transition obligation 0 [2],[5] 0 [2],[5] 0.1 [2],[5]
Prior service cost (credit) (6.3) [2],[5] (5.0) [2],[5] (1.1) [2],[5]
Actuarial loss 3.5 [2],[5] 2.9 [2],[5] 4.0 [2],[5]
Total (0.5) [5] 0.6 [5] 7.9 [5]
Increase (decrease) in pension and other postretirement benefits obligations   (30.0)  
Increase (decrease) of net periodic benefit costs due to impact of remeasurement   (7.2)  
Discount rate for benefit obligations 3.76% 4.60% 5.25%
IPL [Member] | Qualified Defined Benefit Pension Plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 7.5 6.1 6.2
Interest cost 17.1 16.7 16.5
Expected return on plan assets (23.0) [1] (20.0) [1] (19.5) [1]
Amortization of:      
Transition obligation 0 [2] 0 [2] 0 [2]
Prior service cost (credit) 0.4 [2] 0.5 [2] 0.6 [2]
Actuarial loss 10.2 [2] 5.7 [2] 7.2 [2]
Total 12.2 9.0 11.0
IPL [Member] | Directly assigned defined benefit pension plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Interest cost 7.0 7.3 7.8
Expected return on plan assets (9.6) [1] (9.7) [1] (9.7) [1]
Amortization of:      
Prior service cost (credit) (0.2) [2] (0.2) [2] (0.3) [2]
Actuarial loss 3.9 [2] 3.0 [2] 2.9 [2]
Additional benefit costs 0 [3] 2.8 [3] 0 [3]
Total 1.1 3.2 0.7
IPL [Member] | Cash Balance Plan [Member]
      
Amortization of:      
Additional benefit costs   6.3  
IPL [Member] | Corporate Service Allocation [Member]
      
Amortization of:      
Additional benefit costs   3.5  
WPL [Member]
      
Amortization of:      
Increase (decrease) in pension and other postretirement benefits obligations 31.7 8.8 (2.5)
WPL [Member] | Defined Benefit Pension Plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 5.2 4.5  
Interest cost 16.4 16.1  
Amortization of:      
Discount rate for benefit obligations 4.20% 4.95% 5.70%
WPL [Member] | Other Postretirement Benefits Plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 2.7 [5] 2.9 [5] 3.6 [5]
Interest cost 4.1 [5] 4.9 [5] 5.5 [5]
Expected return on plan assets (1.3) [1],[5] (1.3) [1],[5] (1.3) [1],[5]
Amortization of:      
Prior service cost (credit) (3.9) [2],[5] (3.4) [2],[5] (0.7) [2],[5]
Actuarial loss 2.3 [2],[5] 2.1 [2],[5] 2.5 [2],[5]
Additional benefit costs 0 [5] 0 [5] 0 [5]
Total 3.9 [5] 5.2 [5] 9.6 [5]
Increase (decrease) in pension and other postretirement benefits obligations   (16.0)  
Increase (decrease) of net periodic benefit costs due to impact of remeasurement   (3.8)  
Discount rate for benefit obligations 3.81% 4.60% 5.25%
WPL [Member] | Qualified Defined Benefit Pension Plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 5.2 4.5 4.9
Interest cost 16.4 16.1 15.7
Expected return on plan assets (22.3) [1] (20.0) [1] (19.1) [1]
Amortization of:      
Prior service cost (credit) 0.5 [2] 0.5 [2] 0.5 [2]
Actuarial loss 12.1 [2] 7.1 [2] 8.5 [2]
Additional benefit costs 0.1 0 0
Total 12.0 8.2 10.5
WPL [Member] | Directly assigned defined benefit pension plans [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Interest cost 5.2 5.5 5.6
Expected return on plan assets (7.3) [1] (7.3) [1] (7.3) [1]
Amortization of:      
Prior service cost (credit) (0.1) [2] (0.2) [2] (0.2) [2]
Actuarial loss 3.6 [2] 3.0 [2] 2.8 [2]
Additional benefit costs 0 [3] 0.7 [3] 0 [3]
Total 1.4 1.7 0.9
WPL [Member] | Cash Balance Plan [Member]
      
Amortization of:      
Additional benefit costs   3.4  
WPL [Member] | Corporate Service Allocation [Member]
      
Amortization of:      
Additional benefit costs   $ 2.7  
[1] The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.
[2] Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Alliant Energy Cash Balance Pension Plan (Cash Balance Plan) where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits. Unrecognized prior service costs (credits) for the postretirement benefits plans are amortized over the average future service period to full eligibility of the participants of each plan.
[3] 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.
[4] Settlement losses related to payments made to retired executives of Alliant Energy.
[5] 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.