Commitments and Contingencies, Variable Interest Entities (Details) - MW	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Independent Power Producing Entities
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	1,197	897
VIE Purchase Power Agreement Expiration	2041
Tolk Station Power Plant
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration	2022