Commitments and Contingencies, Variable Interest Entities (Details) - MW	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Purchased Power Agreements [Abstract]
VIE Purchase Power Agreement Expiration (year)	2033
Independent Power Producing Entities
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	827	827
Harrington Station Power Plant
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration (year)	2016
Tolk Station Power Plant
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration (year)	2017