Commitments and Contingencies, Variable Interest Entities (Details) - MW	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Purchased Power Agreements [Abstract]
VIE Purchase Power Agreement Expiration Date	2041
Independent Power Producing Entities
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	897	897
Harrington Station Power Plant
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration Date	2022
Tolk Station Power Plant
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration Date	2022