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
 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended
Sep. 30, 2011
Corporate Service Allocation [Member]
IPL [Member]
Sep. 30, 2011
Corporate Service Allocation [Member]
IPL [Member]
Sep. 30, 2011
Corporate Service Allocation [Member]
WPL [Member]
Sep. 30, 2011
Corporate Service Allocation [Member]
WPL [Member]
Dec. 31, 2011
Defined Benefit Pension Plans [Member]
Sep. 30, 2012
Defined Benefit Pension Plans [Member]
IPL [Member]
Sep. 30, 2011
Defined Benefit Pension Plans [Member]
IPL [Member]
Sep. 30, 2012
Defined Benefit Pension Plans [Member]
IPL [Member]
Sep. 30, 2011
Defined Benefit Pension Plans [Member]
IPL [Member]
Sep. 30, 2012
Defined Benefit Pension Plans [Member]
WPL [Member]
Sep. 30, 2011
Defined Benefit Pension Plans [Member]
WPL [Member]
Sep. 30, 2012
Defined Benefit Pension Plans [Member]
WPL [Member]
Sep. 30, 2011
Defined Benefit Pension Plans [Member]
WPL [Member]
Sep. 30, 2012
Defined Benefit Pension Plans [Member]
Alliant Energy Corporation [Member]
Sep. 30, 2011
Defined Benefit Pension Plans [Member]
Alliant Energy Corporation [Member]
Sep. 30, 2012
Defined Benefit Pension Plans [Member]
Alliant Energy Corporation [Member]
Sep. 30, 2011
Defined Benefit Pension Plans [Member]
Alliant Energy Corporation [Member]
Sep. 30, 2012
Other Postretirement Benefits Costs [Member]
IPL [Member]
Sep. 30, 2011
Other Postretirement Benefits Costs [Member]
IPL [Member]
Sep. 30, 2012
Other Postretirement Benefits Costs [Member]
IPL [Member]
Sep. 30, 2011
Other Postretirement Benefits Costs [Member]
IPL [Member]
Sep. 30, 2012
Other Postretirement Benefits Costs [Member]
WPL [Member]
Sep. 30, 2011
Other Postretirement Benefits Costs [Member]
WPL [Member]
Sep. 30, 2012
Other Postretirement Benefits Costs [Member]
WPL [Member]
Sep. 30, 2011
Other Postretirement Benefits Costs [Member]
WPL [Member]
Sep. 30, 2012
Other Postretirement Benefits Costs [Member]
Alliant Energy Corporation [Member]
Sep. 30, 2011
Other Postretirement Benefits Costs [Member]
Alliant Energy Corporation [Member]
Sep. 30, 2012
Other Postretirement Benefits Costs [Member]
Alliant Energy Corporation [Member]
Sep. 30, 2011
Other Postretirement Benefits Costs [Member]
Alliant Energy Corporation [Member]
Sep. 30, 2011
Cash Balance Plan [Member]
Sep. 30, 2011
Cash Balance Plan [Member]
Sep. 30, 2011
Cash Balance Plan [Member]
IPL [Member]
Sep. 30, 2011
Cash Balance Plan [Member]
IPL [Member]
Sep. 30, 2011
Cash Balance Plan [Member]
WPL [Member]
Sep. 30, 2011
Cash Balance Plan [Member]
WPL [Member]
Sep. 30, 2012
Directly assigned defined benefit pension plans [Member]
IPL [Member]
Sep. 30, 2011
Directly assigned defined benefit pension plans [Member]
IPL [Member]
Sep. 30, 2012
Directly assigned defined benefit pension plans [Member]
IPL [Member]
Sep. 30, 2011
Directly assigned defined benefit pension plans [Member]
IPL [Member]
Sep. 30, 2012
Directly assigned defined benefit pension plans [Member]
WPL [Member]
Sep. 30, 2011
Directly assigned defined benefit pension plans [Member]
WPL [Member]
Sep. 30, 2012
Directly assigned defined benefit pension plans [Member]
WPL [Member]
Sep. 30, 2011
Directly assigned defined benefit pension plans [Member]
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]                                                                                      
Aggregate additional payments to certain plan participants $ 0.2 $ 3.5 $ 0.3 $ 2.7 $ 10.0                 $ 0 [1] $ 1.3 [1] $ 0 [1] $ 10.2 [1]                 $ 0 [1] $ 0 [1] $ 0 [1] $ 0 [1] $ 1.3 $ 10.2 $ 0.8 $ 6.3 $ 0.4 $ 3.4 $ 0 [1] $ 0.6 [1] $ 0 [1] $ 2.8 [1] $ 0 [1] $ 0.1 [1] $ 0 [1] $ 0.7 [1]
Benefits costs associated with Corporate Services employees           0.4 [2] 1.3 [2] 1.4 [2] 5.4 [2] 0.4 [2] 0.9 [2] 1.0 [2] 3.9 [2]         0 0 0.1 0.4 0.1 (0.1) 0.1 0.2                                    
Settlement loss                           $ 0 $ 1.1 $ 0 $ 1.1                 $ 0 $ 0 $ 0 $ 0                            
[1] Alliant Energy reached an agreement with the IRS, which resulted in a favorable determination letter for the Cash Balance Plan during the first quarter of 2011. The agreement with the IRS required Alliant Energy to amend the Cash Balance Plan, which was completed in the second quarter of 2011, resulting in aggregate additional benefits of $10.2 million paid by Alliant Energy to certain former participants in the Cash Balance Plan in the second half of 2011. Alliant Energy recognized $1.3 million and $10.2 million of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. IPL recognized $0.8 million ($0.6 million directly assigned and $0.2 million allocated by Corporate Services) and $6.3 million ($2.8 million directly assigned and $3.5 million allocated by Corporate Services) of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. WPL recognized $0.4 million ($0.1 million directly assigned and $0.3 million allocated by Corporate Services) and $3.4 million ($0.7 million directly assigned and $2.7 million allocated by Corporate Services) of additional benefits costs during the three and nine months ended September 30, 2011, respectively, related to these benefits. Refer to Note 11(c) for additional information regarding the Cash Balance Plan.
[2] For the three and nine months ended September 30, 2011, additional qualified pension benefits costs resulting from the amendment to the Cash Balance Plan in the second quarter of 2011 allocated to IPL were $0.2 million and $3.5 million, and to WPL were $0.3 million and $2.7 million, respectively.