Commitments and Contingencies, Leases (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Operating Lease Purchase Power Agreement Expiration Date		2033
Operating Leases, Future Minimum Payments Due [Abstract]
2018		$ 57
2019		56
2020		56
2021		56
2022		56
Thereafter		604
Office Space and Other Equipment
Operating Leases [Abstract]
Total expenses under operating lease obligations		58	$ 57	$ 55
Operating Leases, Future Minimum Payments Due [Abstract]
2018		5
2019		5
2020		5
2021		5
2022		5
Thereafter		61
Purchased Power Agreements
Operating Leases [Abstract]
Payments for capacity for PPAs under operating lease obligations		51	$ 51	$ 49
Operating Leases, Future Minimum Payments Due [Abstract]
2018	[1],[2]	52
2019	[1],[2]	51
2020	[1],[2]	51
2021	[1],[2]	51
2022	[1],[2]	51
Thereafter	[1],[2]	$ 543

[1] Amounts do not include PPAs accounted for as executory contracts.

[2] PPA operating leases contractually expire through 2033.