Contingencies (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 1997	Sep. 30, 2008	Dec. 31, 1993
Loss Contingencies [Line Items]
Malpractice Loss Contingency, Letters of Credit and Surety Bonds	$ 145.6		$ 145.6
Gain Contingency, Unrecorded Amount	130.0		130.0
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base	514.0		514.0
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment	640.0		640.0
Liability for Title Claims and Claims Adjustment Expense	5.0		$ 5.0
Site Contingency, Names of Other Potentially Responsible Parties			100		69
Loss Contingency, Damages Sought, Value			$ 1,200.0
Loss Contingency, Range of Possible Loss, Portion Not Accrued	500.0		500.0
Investment in Trojan							87.00%
Class action damages sought	260.0		260.0
Loss Contingency, Estimate of Possible Loss	1,700.0		1,700.0
Refund to customers for Trojan Investment including interest						$ 33.0
Litigation Settlement, Expense			115.0
Proceeds from Insurance Settlement, Investing Activities			120.0	$ 0.0
Former Gain Contingency, Recognized in Current Period	10.0		10.0
Legal Fees	1.0	$ 5.0	8.0	$ 12.0
Minimum [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss	$ 1,100.0		$ 1,100.0