Benefit Plans (Estimated Amortization 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, 2012
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	$ 33.2
Prior service cost (credit)	0.3
Estimated amortization from "Regulatory assets", "Regulatory liabilities" , and "AOCL"	33.5
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	10.2
Prior service cost (credit)	0.4
Estimated amortization from "Regulatory assets", "Regulatory liabilities" , and "AOCL"	10.6
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	12.1
Prior service cost (credit)	0.5
Estimated amortization from "Regulatory assets", "Regulatory liabilities" , and "AOCL"	12.6
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	6.3
Prior service cost (credit)	(12.0)
Estimated amortization from "Regulatory assets", "Regulatory liabilities" , and "AOCL"	(5.7)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	3.6
Prior service cost (credit)	(6.3)
Estimated amortization from "Regulatory assets", "Regulatory liabilities" , and "AOCL"	(2.7)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	2.3
Prior service cost (credit)	(3.9)
Estimated amortization from "Regulatory assets", "Regulatory liabilities" , and "AOCL"	(1.6)
Directly Assigned Defined Benefit Pension Benefits [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	3.9
Prior service cost (credit)	(0.2)
Defined Benefit Plan Amounts That Will Be Amortized From Regulatory Assets Regulatory Liabilities In Next Fiscal Year	3.7
Directly Assigned Defined Benefit Pension Benefits [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	3.6
Prior service cost (credit)	(0.1)
Defined Benefit Plan Amounts That Will Be Amortized From Regulatory Assets Regulatory Liabilities In Next Fiscal Year	$ 3.5