v2.4.0.6
Commitments And Contingencies (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 9 Months Ended 9 Months Ended 12 Months Ended 3 Months Ended 9 Months Ended 9 Months Ended
Dec. 31, 2010
Sep. 30, 2012
Sep. 30, 2012
IPL [Member]
site
Sep. 30, 2012
WPL [Member]
site
Sep. 30, 2012
RMT [Member]
Vendor
Sep. 30, 2012
Manufactured Gas Plant Sites [Member]
Alliant Energy [Member]
Sep. 30, 2012
Manufactured Gas Plant Sites [Member]
IPL [Member]
Sep. 30, 2012
Manufactured Gas Plant Sites [Member]
WPL [Member]
Sep. 30, 2012
Defined Benefit Pension Plans [Member]
Dec. 31, 2011
Defined Benefit Pension Plans [Member]
Sep. 30, 2012
Defined Benefit Pension Plans [Member]
Alliant Energy [Member]
Sep. 30, 2011
Defined Benefit Pension Plans [Member]
Alliant Energy [Member]
Sep. 30, 2012
Defined Benefit Pension Plans [Member]
Alliant Energy [Member]
Sep. 30, 2011
Defined Benefit Pension Plans [Member]
Alliant Energy [Member]
Sep. 30, 2012
Minimum [Member]
Sep. 30, 2012
Maximum [Member]
Sep. 30, 2012
Performance guarantees outstanding, 2012 [Member]
Sep. 30, 2012
Performance guarantees outstanding, 2013 [Member]
Sep. 30, 2012
Performance guarantees outstanding, 2014 [Member]
Sep. 30, 2012
Performance guarantees outstanding, 2015 [Member]
Sep. 30, 2012
Scenario, Forecast [Member]
Sep. 30, 2012
Scenario, Forecast [Member]
Alliant Energy [Member]
Sep. 30, 2012
Scenario, Forecast [Member]
IPL [Member]
Sep. 30, 2012
Scenario, Forecast [Member]
WPL [Member]
Sep. 30, 2012
Scenario, Forecast [Member]
Capital Purchase Obligation [Member]
Alliant Energy and WPL [Member]
Sep. 30, 2012
EPA new source performance standards for GHG [Member]
lb
MW
Long-term Purchase Commitment [Line Items]                                                    
Minimum future commitments                                           $ 1,131 $ 605 $ 355 $ 34  
Interest crediting rate 8.20%                                                  
Aggregate additional payments to certain plan participants                   10.0 0 [1] 1.3 [1] 0 [1] 10.2 [1]                        
Estimated damages from court opinion and order                 18.7                                  
Loss contingency, damages sought, value                 9.6                                  
Damages to be paid out of common fund                 4.3                                  
Number of vendors who filed lawsuits (in vendors)         3                                          
Loss contingency accrual, carrying value, payments         16                                          
Performance guarantees outstanding   598                               262 289 47            
Surety bonds outstanding   79                             70 9                
Present value abandonment obligation   29                                                
Number of sites with ownership interest (in sites)     40 14                                            
Number of sites with no further action (in sites)     13 9                                            
Sites with no further action for soil remediation (in sites)     3                                              
Minimum remaining estimated cost incurred for investigation, remediation and monitoring                                         19   17 2    
Maximum remaining estimated cost incurred for investigation, remediation and monitoring                                         44   40 4    
Current and non-current environmental liabilities           $ 30 $ 27 $ 3                                    
Annual standard fine particle NAAQS, current (in micrograms/cubic meter)   15                                                
Annual standard fine particle NAAQS (in micrograms/cubic meter)                             12 13                    
Fossil fueled energy generating units capacity (in megawatts)                                                   25
Emissions rate limitation (in pounds of carbon dioxide per Mwh)                                                   1,000
[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.