v2.4.0.6
Benefit Plans (Estimated Amoritization From Regulatory Assets and Regulatory Liabilities On The Consolidated Balance Sheets And Accumulated Other Comprehensive Loss On Alliant Energys Consolidated Balance Sheet) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Defined Benefit Pension Plans [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Actuarial loss $ 36.2
Prior service cost (credit) 0.2
Estimated amortization from regulatory assets, regulatory liabilities and AOCL 36.4
Defined Benefit Pension Plans [Member] | IPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Actuarial loss 10.9
Prior service cost (credit) 0.4
Estimated amortization from regulatory assets, regulatory liabilities and AOCL 11.3
Defined Benefit Pension Plans [Member] | WPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Actuarial loss 13.2
Prior service cost (credit) 0.5
Estimated amortization from regulatory assets, regulatory liabilities and AOCL 13.7
Other Postretirement Benefits Plans [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Actuarial loss 4.9
Prior service cost (credit) (11.9)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL (7.0)
Other Postretirement Benefits Plans [Member] | IPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Actuarial loss 2.7
Prior service cost (credit) (6.3)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL (3.6)
Other Postretirement Benefits Plans [Member] | WPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Actuarial loss 1.9
Prior service cost (credit) (3.9)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL (2.0)
Directly Assigned Defined Benefit Pension Benefits [Member] | IPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Actuarial loss 4.2
Prior service cost (credit) (0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL 4.0
Directly Assigned Defined Benefit Pension Benefits [Member] | WPL [Member]
  
Defined Benefit Plan Disclosure [Line Items]  
Actuarial loss 3.9
Prior service cost (credit) (0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL $ 3.7