Accounting Policies - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change	$ (6,789)	$ (6,269)	$ (4,972)			$ (5,331)
Operating lease liability	159	180		$ 198
Operating lease right-of-use asset	139	158		174
Deferred rent and lease incentives	$ 20	$ 22		$ 24
Weighted average common shares adjustment related to stock-based compensation	500,000	1,100,000	1,600,000
Anti-dilutive potential common shares related to stock-based compensation plans	0	0	0
Maturities of short term investments	3 months
AOCI attributable to parent
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change	$ (1,273)	$ (863)	$ (48)			(813)
Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change	$ (4,149)	(4,009)	$ (3,588)			(3,248)
Cumulative effect, period of adoption, adjustment
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change						(4)
Cumulative effect, period of adoption, adjustment | AOCI attributable to parent
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change		0			$ 221	221
Cumulative effect, period of adoption, adjustment | Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change		(7)				(225)
Accounting Standards Update 2016-01 | Cumulative effect, period of adoption, adjustment | AOCI attributable to parent
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change						221
Accounting Standards Update 2016-13 | Cumulative effect, period of adoption, adjustment | Retained Earnings
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change		$ (7)
Equity securities
Significant Accounting Policies [Line Items]
Available-for-sale securities, Equity securities						1,600
Limited partnerships and similar investments | Accounting Standards Update 2016-01 | Cumulative effect, period of adoption, adjustment
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change						$ (4)