Commitments and Contingencies, Purchased Power Agreements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-term Contract for Purchase of Electric Power [Line Items]
Purchase Power Agreement Expiration Date	2034
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity	$ 131	$ 168	$ 191
Estimated Future Payments Under PPAs [Abstract]
2019	86
2020	70
2021	78
2022	77
2023	79
Thereafter	125
Total	515
Energy
Estimated Future Payments Under PPAs [Abstract]
2019	99
2020	109
2021	157
2022	173
2023	177
Thereafter	328
Total	$ 1,043
Maximum
Long-term Contract for Purchase of Electric Power [Line Items]
Purchase Power Agreement Expiration Date	2039