Contingencies (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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
Legal Fees	4.0	$ 3.0	7.0	$ 7.0
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
Litigation Settlement, Expense			$ 115.0
Loss Contingency, Estimate of Possible Loss	1,700.0		1,700.0
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
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		$ 1,100.0