Summary of Significant Accounting Policies - Recently Issued Accounting Standards (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2017 USD ($)
Research and Development
Research and development expense	$ 134	$ 143	$ 77
Recently issued accounting standards
ROU assets	390	446		$ 460
Lease liabilities	429			$ 498
Total equity	3,828	3,601	3,372		$ 3,408
Cumulative Effect, Period of Adoption, Adjustment
Recently issued accounting standards
Total equity		(3)
Accumulated Other Comprehensive Loss
Recently issued accounting standards
Total equity	(471)	(440)	(410)		(354)
Accumulated Other Comprehensive Loss | Cumulative Effect, Period of Adoption, Adjustment
Recently issued accounting standards
Total equity					(7)
Retained Earnings
Recently issued accounting standards
Total equity	$ 4,287	4,030	$ 3,770		3,750
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
Recently issued accounting standards
Total equity		$ (3)			$ 7
Designated as hedge
Derivative commodity instruments
Number of derivative agreements | item	0