Contingencies (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2016	Sep. 30, 2016	Dec. 31, 1997	Dec. 31, 2015	Sep. 30, 2008	Dec. 31, 1993
Loss Contingencies [Line Items]
Investment in Trojan						87.00%
Refund to customers for Trojan Investment including interest					$ 33.0
Class action damages sought		$ 260.0
Site Contingency, Names of Other Potentially Responsible Parties	100		69
Litigation Settlement, Expense		114.0
Malpractice Loss Contingency, Letters of Credit and Surety Bonds		145.6
Loss Contingency, Damages Sought, Value		200.0
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base		514.0
Liability for Title Claims and Claims Adjustment Expense		15.0
Construction work-in-progress		194.0		$ 545.0
Minimum [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss		746.0		$ 1,500.0
Construction work-in-progress		640.0
Maximum [Member]
Loss Contingencies [Line Items]
Construction work-in-progress		$ 660.0