Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change	$ 6,685	$ 5,047	$ 6,789	$ 6,269
Operating lease liability	153		159
Operating lease right-of-use asset	$ 133		$ 139
Weighted average common shares adjustment related to stock-based compensation (shares)	700,000	0
Anti-dilutive potential common shares related to stock-based compensation plans (shares)	0	800,000
Maturities of short term investments	3 months
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilities		us-gaap:OtherLiabilities
Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change	$ 4,354	$ 3,616	$ 4,149	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