Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change					$ 4
Operating lease liability	$ 190		$ 198
Operating lease right-of-use asset	167		174
Deferred rent and lease incentives	$ 23		$ 24
Weighted average common shares adjustment related to stock-based compensation (shares)	1,300,000	1,800,000
Anti-dilutive potential common shares related to stock-based compensation plans (shares)	0	0
Maturities of short term investments	3 months
Equity securities
Significant Accounting Policies [Line Items]
Available-for-sale securities, Equity securities					$ 1,600
Equity securities | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
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)