Commitments and Contingencies, Purchased Power Agreements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long-term Contract for Purchase of Electric Power [Line Items]
Purchase Power Agreement Expiration Date	2033
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity	$ 58	$ 57	$ 57
Estimated Future Payments Under PPAs [Abstract]
2018	58
2019	20
2020	12
2021	12
2022	13
Thereafter	18
Total	$ 133