Regulatory Assets & Liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory assets	$ 416,800,000	$ 308,800,000
Regulatory liabilities	(937,200,000)	(453,700,000)
Regulatory asset amount earning a return	0
Regulatory assets currently being recovered in base rates	$ 23,000,000
Weighted average recovery period of regulatory assets currently being recovered (in years)	20 years
Deferred tax regulatory liability due to revaluation of deferred taxes at reduced federal tax rate	$ 446,000,000
Net deferred income taxes
Regulatory Assets [Line Items]
Regulatory liabilities	(459,000,000)
Removal Costs
Regulatory Assets [Line Items]
Regulatory liabilities	(477,000,000)	(452,000,000)
Future amounts recoverable from ratepayers related to:
Regulatory Assets [Line Items]
Regulatory assets	133,300,000	85,500,000
Future amounts recoverable from ratepayers related to: | Net deferred income taxes
Regulatory Assets [Line Items]
Regulatory liabilities		(17,100,000)
Future amounts recoverable from ratepayers related to: | Benefit Obligations
Regulatory Assets [Line Items]
Regulatory assets	102,800,000	102,600,000
Future amounts recoverable from ratepayers related to: | Net deferred income taxes
Regulatory Assets [Line Items]
Regulatory assets	6,200,000
Future amounts recoverable from ratepayers related to: | Asset retirement obligations & other
Regulatory Assets [Line Items]
Regulatory assets			$ 24,300,000	$ 0
Amounts deferred for future recovery related to:
Regulatory Assets [Line Items]
Regulatory assets	142,400,000	91,600,000
Amounts deferred for future recovery related to: | Cost recovery riders & other
Regulatory Assets [Line Items]
Regulatory assets	142,400,000	91,600,000
Amounts currently recovered in customer rates related to:
Regulatory Assets [Line Items]
Regulatory assets	141,100,000	131,700,000
Amounts currently recovered in customer rates related to: | Indiana authorized trackers
Regulatory Assets [Line Items]
Regulatory assets	75,900,000	64,200,000
Amounts currently recovered in customer rates related to: | Ohio authorized trackers
Regulatory Assets [Line Items]
Regulatory assets	28,400,000	22,200,000
Amounts currently recovered in customer rates related to: | Loss on reacquired debt & hedging costs
Regulatory Assets [Line Items]
Regulatory assets	22,700,000	24,100,000
Amounts currently recovered in customer rates related to: | Deferred coal costs and other
Regulatory Assets [Line Items]
Regulatory assets	$ 14,100,000	$ 21,200,000