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) (Scenario, Forecast [Member], 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	$ 19.5
Prior service cost (credit)	0
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	19.5
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	8.0
Prior service cost (credit)	0
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	8.0
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	9.2
Prior service cost (credit)	0.3
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	9.5
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	2.4
Prior service cost (credit)	(11.9)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	(9.5)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	1.1
Prior service cost (credit)	(6.3)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	(5.2)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	1.2
Prior service cost (credit)	(3.9)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	$ (2.7)