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		2039
Capacity
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity		$ 168	$ 191	$ 231
Estimated Future Payments Under PPAs [Abstract]
2018		133
2019		87
2020		68
2021		73
2022		77
Thereafter		205
Total		643
Energy
Estimated Future Payments Under PPAs [Abstract]
2018	[1]	93
2019	[1]	99
2020	[1]	105
2021	[1]	140
2022	[1]	155
Thereafter	[1]	368
Total	[1]	$ 960

[1] Excludes contingent energy payments for renewable energy PPAs.