Contingencies (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 1997	Dec. 31, 2016	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	$ 115.0
Loss Contingency, Estimate of Possible Loss	1,700.0
Malpractice Loss Contingency, Letters of Credit and Surety Bonds	145.6
Gain Contingency, Unrecorded Amount	200.0
Loss Contingency, Damages Sought, Value	1,200.0
Loss Contingency, Range of Possible Loss, Portion Not Accrued			$ 500.0
Public Utilities, Property, Plant and Equipment, Amount of Construction Work in Process Included in Rate Base	514.0
Construction in Progress, Gross	636.0
Liability for Title Claims and Claims Adjustment Expense	14.0
Construction work-in-progress	242.0		$ 213.0
Minimum [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimate of Possible Loss	1,100.0
Loss Contingency, Damages Sought, Value	44.0
Construction work-in-progress	640.0
Maximum [Member]
Loss Contingencies [Line Items]
Loss Contingency, Damages Sought, Value	$ 117.0