Benefit Plans (Estimated Amoritization From Regulatory Assets, Regulatory Liabilities And Accumulated Other Comprehensive Loss) (Details) (Scenario, Forecast [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2015
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	$ 35.4
Prior service cost (credit)	(0.3)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	35.1
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	15.3
Prior service cost (credit)	(0.1)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	15.2
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	16.8
Prior service cost (credit)	0.2
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	17.0
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	4.9
Prior service cost (credit)	(11.3)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	(6.4)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	2.3
Prior service cost (credit)	(6.1)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	(3.8)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	2.3
Prior service cost (credit)	(3.5)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	$ (1.2)