Carty Generating Station (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Public Utilities, Allowance for Funds Used During Construction, Additions	$ 501.0	$ 424.0
Phase-in Plan, Amount of Costs Deferred for Rate-making Purposes		514.0
Allowance for Funds Used During Construction, Capitalized Interest	50.0	41.0
Malpractice Loss Contingency, Letters of Credit and Surety Bonds	145.6
Construction work-in-progress	647.0	$ 545.0
Minimum [Member]
Property, Plant and Equipment [Line Items]
Construction work-in-progress	635.0
Maximum [Member]
Property, Plant and Equipment [Line Items]
Construction work-in-progress	$ 670.0