Contingencies (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	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
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base	514.0
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment	637.0
Liability for Title Claims and Claims Adjustment Expense	$ 8.0
Site Contingency, Names of Other Potentially Responsible Parties	100	69
Litigation Settlement, Expense	$ 115.0
Loss Contingency, Estimate of Possible Loss	1,700.0
Loss Contingency, Damages Sought, Value	1,200.0
Loss Contingency, Range of Possible Loss, Portion Not Accrued	500.0
Investment in Trojan				87.00%
Class action damages sought	260.0
Refund to customers for Trojan Investment including interest			$ 33.0
Minimum [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss	1,050.0
Loss Contingency, Damages Sought, Value	44.0
Maximum [Member]
Loss Contingencies [Line Items]
Loss Contingency, Damages Sought, Value	$ 117.0