Benefit Plans (Estimated Amoritization From Regulatory Assets, Regulatory Liabilities And Accumulated Other Comprehensive Loss) (Details) - Scenario, Forecast [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	$ 37.5
Prior service cost (credit)	(0.4)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	37.1
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	16.1
Prior service cost (credit)	(0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	15.9
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	18.5
Prior service cost (credit)	0.1
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	18.6
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	3.8
Prior service cost (credit)	(0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	3.6
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	2.0
Prior service cost (credit)	0.0
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	2.0
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	1.6
Prior service cost (credit)	(0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	$ 1.4