Commitments and Contingencies, Variable Interest Entities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($) MW
Independent Power Producing Entities
Purchased Power Agreements [Abstract]
Generating capacity (in MW) | MW	3,770	3,537
VIE Purchase Power Agreement Expiration Date	2041
Harrington Station Power Plant [Member]
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration Date	2022
Tolk Station Power Plant [Member]
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration Date	2022
Low-Income Housing Limited Partnerships
Amount Reflected in Consolidated Balance Sheets [Abstract]
Current assets	$ 5	$ 6
Property, plant and equipment, net	42	46
Other noncurrent assets	1	1
Total assets	48	53
Current liabilities	7	9
Mortgages and other long-term debt payable	26	26
Other noncurrent liabilities	0	1
Total liabilities	$ 33	$ 36