Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Stockholders' equity	$ 12,867	$ 10,063
Lease assets	864	858
Lease liabilities	865	854
Total equity	12,931	10,126	$ 9,848	$ 11,622
ASU 2016-02 Leases | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Total equity			14
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle
Total equity	43,761	42,135	40,636	39,115
Retained Earnings | ASU 2018-02 Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Total equity			853
Retained Earnings | ASU 2016-02 Leases | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Total equity			14
Total Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle
Total equity	$ (7,661)	$ (8,139)	(6,866)	$ (7,026)
Total Accumulated Other Comprehensive Income (Loss) | ASU 2018-02 Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Total equity			$ (853)