Contingencies (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 1997	Mar. 31, 2017	Sep. 30, 2008	Dec. 31, 1993
Loss Contingencies [Line Items]
Malpractice Loss Contingency, Letters of Credit and Surety Bonds	$ 145.6
Gain Contingency, Unrecorded Amount	200.0
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base	514.0
Loss Contingency, Accrual, Current	3.0		$ 4.0
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment	637.0
Liability for Title Claims and Claims Adjustment Expense	$ 13.0
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	1,100.0
Loss Contingency, Damages Sought, Value	44.0
Maximum [Member]
Loss Contingencies [Line Items]
Loss Contingency, Damages Sought, Value	$ 117.0