Carty Generating Station (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015	Mar. 31, 2016
Property, Plant and Equipment [Line Items]
Public Utilities, Property, Plant and Equipment, Other Property, Plant and Equipment	$ 587.0	$ 424.0
Phase-in Plan, Amount of Costs Deferred for Rate-making Purposes		514.0
Allowance for Funds Used During Construction, Capitalized Interest	59.0	41.0
Malpractice Loss Contingency, Letters of Credit and Surety Bonds			$ 145.6
Construction work-in-progress	746.0	$ 545.0
Liability for Title Claims and Claims Adjustment Expense	15.0
Minimum [Member]
Property, Plant and Equipment [Line Items]
Construction work-in-progress	640.0
Maximum [Member]
Property, Plant and Equipment [Line Items]
Construction work-in-progress	$ 660.0