v2.4.0.6
Income Taxes (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2012
wind_projects
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2012
2009 through 2011 [Member]
Dec. 31, 2012
After 2020 [Member]
Mar. 31, 2012
State Apportionment [Member]
Dec. 31, 2010
Audit Completion Income Tax Benefits [Member]
Dec. 31, 2012
Bonus Depreciation 2012 Return [Member]
Scenario, Forecast [Member]
Dec. 31, 2011
Wisconsin Tax Legislation [Member]
Dec. 31, 2010
Federal Healthcare Legislation [Member]
Dec. 31, 2012
IPL [Member]
Dec. 31, 2011
IPL [Member]
Dec. 31, 2010
IPL [Member]
Dec. 31, 2012
IPL [Member]
After 2021 [Member]
Mar. 31, 2012
IPL [Member]
State Apportionment [Member]
Dec. 31, 2010
IPL [Member]
Audit Completion Income Tax Benefits [Member]
Dec. 31, 2012
IPL [Member]
Bonus Depreciation 2012 Return [Member]
Scenario, Forecast [Member]
Dec. 31, 2010
IPL [Member]
Federal Healthcare Legislation [Member]
Dec. 31, 2012
IPL [Member]
Whispering Willow - East Wind Project [Member]
MW
Dec. 31, 2012
WPL [Member]
Dec. 31, 2011
WPL [Member]
Dec. 31, 2010
WPL [Member]
Dec. 31, 2012
WPL [Member]
After 2023 [Member]
Mar. 31, 2012
WPL [Member]
State Apportionment [Member]
Dec. 31, 2012
WPL [Member]
Bonus Depreciation 2012 Return [Member]
Scenario, Forecast [Member]
Dec. 31, 2010
WPL [Member]
Federal Healthcare Legislation [Member]
Dec. 31, 2012
WPL [Member]
Bent Tree - Phase I Wind Project [Member]
MW
Dec. 31, 2012
WPL [Member]
Cedar Ridge Wind Project [Member]
MW
Dec. 31, 2012
Alliant Energy And IPL [Member]
Dec. 31, 2007
Alliant Energy And IPL [Member]
Dec. 31, 2012
Alliant Energy And IPL [Member]
Internal Revenue Service (IRS) [Member]
Dec. 31, 2012
Alliant Energy And IPL [Member]
Whispering Willow - East Wind Project [Member]
State Income Taxes [Member]
Dec. 31, 2011
Alliant Energy And IPL [Member]
Whispering Willow - East Wind Project [Member]
State Income Taxes [Member]
Dec. 31, 2010
Alliant Energy And IPL [Member]
Whispering Willow - East Wind Project [Member]
State Income Taxes [Member]
Dec. 31, 2012
Resources [Member]
Bonus Depreciation 2012 Return [Member]
Scenario, Forecast [Member]
Income Tax [Line Items]                                                                      
Deferred income tax expense           $ 15       $ 7         $ 8     $ 4           $ 7   $ 3                  
Wind projects generating production tax credits (in wind projects) 3                                                                    
Wattage capacity of projects generating production tax credits (in megawatts)                                     200               200 68              
Increase in income tax benefits related to change in accounting method                                                             13        
Net income tax benefits related to tax depreciation                                                               3 6 12  
Income tax benefit from changes in federal income tax returns             7                 5                                      
Income tax rates for continuing operations 20.80% 16.90% 32.30%       1.50%       (15.20%) (2.70%) 22.80%     2.70%       36.30% 33.40% 39.20%                          
Operating loss carryforwards, utilization period                 20 years                                                    
Operating losses utilized                 368                                                    
State net operating loss carryforwards, deferred tax asset, valuation allowances                 19                                                    
Deductions in income tax return               284                 67               117                   100
Taxable gain on sale of electric transmission assets                                                           $ 527          
Gain on sale of assets, income tax obligation deferral period                                                         8 years            
Gain on sale of assets, percent recognized during each year of deferral period 12.50%                                                                    
Net operating loss carryforwards, percentage expiring         99.00%                 99.00%                 99.00%                        
Statute of limitations, expiration period from extended due date of federal tax return       3 years