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