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, 2016 USD ($)
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	$ 37.4
Prior service cost (credit)	(0.3)
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	16.5
Prior service cost (credit)	(0.2)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	16.3
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	17.6
Prior service cost (credit)	0.2
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	17.8
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	4.8
Prior service cost (credit)	(4.1)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	0.7
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	2.6
Prior service cost (credit)	(2.6)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	0.0
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	1.8
Prior service cost (credit)	(0.9)
Estimated amortization from regulatory assets, regulatory liabilities and AOCL	$ 0.9