Leases (Details) $ in Millions		12 Months Ended
	Jan. 01, 2016 USD ($)	Dec. 31, 2018 USD ($) facility	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
ROU assets, operating leases		$ 133	$ 141
Lease liabilities, operating leases		141	150
ROU assets, finance leases		68	75
Lease liabilities, finance leases		$ 63	$ 72
Weighted average incremental borrowing rate at the lease inception, operating leases		4.31%	3.65%
Weighted average incremental borrowing rate at the lease inception, finance leases		2.72%	2.72%
Weighted-average remaining lease term, operating leases		19 years	19 years
Weighted-average remaining lease term, finance leases		10 years	10 years
Expected lease payments over the remaining terms of the leases		$ 330
Number of natural gas and/or oil electric generation facilities under sales-type leases | facility		3
Net investment in sales-type leases		$ 69	$ 47
Losses at commencement of sales-type leases		(20)
Expected lease payments over the remaining terms of the power sales agreements		$ 200
Accounting Standards Update 2016-02 [Member] | Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net increase to retained earnings	$ 32
Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease extension term, operating lease		1 year
Lease extension term, finance lease		1 year
Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease extension term, operating lease		20 years
Lease extension term, finance lease		20 years