Commitments and Contingencies, Leases - Future Commitments (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Operating Leases after Adoption of ASC Topic 842
2019	$ 130
2020	262
2021	267
2022	253
2023	239
Thereafter	1,095
Total minimum obligation	2,246
Interest component of obligation	(410)
Present value of minimum obligation	1,836
Less current portion	(189)
Noncurrent operating and finance lease liabilities	$ 1,647	$ 0
Weighted-average remaining lease term in years	9 years 8 months 12 days
Finance Leases after Adoption of ASC Topic 842
2019	$ 6
2020	14
2021	14
2022	12
2023	12
Thereafter	220
Total minimum obligation	278
Interest component of obligation	(195)
Present value of minimum obligation	83
Less current portion	(4)
Noncurrent operating and finance lease liabilities	$ 79
Weighted-average remaining lease term in years	37 years 2 months 12 days
Operating Leases before Adoption of ASC Topic 842
2019		239
2020		234
2021		235
2022		221
2023		208
Thereafter		1,037
Finance Leases before Adoption of ASC Topic 842
2019		14
2020		14
2021		14
2022		12
2023		12
Thereafter		220
Total minimum obligation		286
Interest component of obligation		(201)
Present value of minimum obligation		$ 85
WYCO
Lessee, Lease, Description [Line Items]
Ownership interest (as a percent)	50.00%	50.00%
PPAs
Operating Leases after Adoption of ASC Topic 842
2019	$ 118
2020	237
2021	243
2022	226
2023	218
Thereafter	959
Total minimum obligation	2,001
Interest component of obligation	(355)
Present value of minimum obligation	1,646
Operating Leases before Adoption of ASC Topic 842
2019		$ 207
2020		208
2021		210
2022		197
2023		186
Thereafter		883
Other
Operating Leases after Adoption of ASC Topic 842
2019	12
2020	25
2021	24
2022	27
2023	21
Thereafter	136
Total minimum obligation	245
Interest component of obligation	(55)
Present value of minimum obligation	$ 190
Operating Leases before Adoption of ASC Topic 842
2019		32
2020		26
2021		25
2022		24
2023		22
Thereafter		$ 154