Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change	$ 5,601	$ 6,126	$ 5,601	$ 6,126	$ 6,685	$ 6,789	$ 5,047	$ 6,269
Operating lease liability	$ 143		$ 143			$ 159
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilities		us-gaap:OtherLiabilities			us-gaap:OtherLiabilities
Operating lease right-of-use asset	$ 124		$ 124			$ 139
Weighted average common shares adjustment related to stock-based compensation (shares)	600,000	300,000	600,000	0
Anti-dilutive potential common shares related to stock-based compensation plans (shares)	0	0	0	600,000
Maturities of short term investments			3 months
Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change	$ 4,023	$ 3,685	$ 4,023	$ 3,685	$ 4,354	$ 4,149	$ 3,616	4,009
Cumulative effect adjustment in period of adoption | Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change								7
Cumulative effect adjustment in period of adoption | Retained Earnings | Accounting Standards Update 2016-13
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change								$ 7