Accounting Policies - Narrative (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Net unrealized gain on equity securities included in AOCI		$ 93		$ 93			$ 840
Cumulative effect of accounting change							4
Weighted average common shares adjustment related to stock-based compensation (shares)		1,600,000	1,900,000	1,700,000	2,000,000
Anti-dilutive potential common shares related to stock-based compensation plans (shares)		0	0	0	0
Maturities of short term investments				3 months
Equity securities
Significant Accounting Policies [Line Items]
Available-for-sale securities, Equity securities							1,600
Net unrealized gain on equity securities included in AOCI	[1]						221
Equity securities, trading at fair value							62
Equity securities | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Net unrealized gain on equity securities included in AOCI							$ 221
Cumulative effect of accounting change						$ (221)
Limited partnerships and similar investments | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change						$ (4)
Equity securities still owned | Equity securities | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Net holding gains (losses) on equity securities		$ 27		$ (35)

[1] As discussed in Note A — “Accounting Policies — Investments,” effective January 1, 2018, all equity securities other than those accounted for under the equity method are carried at fair value through net earnings.