Accounting Policies - Narrative (Details) - USD ($) $ in Millions		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change	$ 4		$ 4
Stranded amount reclassified from AOCI to retained earnings			$ 0
Weighted average common shares adjustment related to stock-based compensation		1,600,000	2,000,000	1,600,000
Anti-dilutive potential common shares related to stock-based compensation plans		0	0	400,000
Maturities of short term investments		3 months
Accounting Standards Update 2016-02 | Maximum
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change		0.50%
Equity securities
Significant Accounting Policies [Line Items]
Available-for-sale securities, Equity securities	1,600		$ 1,600
Equity securities, trading at fair value	62		$ 62
Equity securities | Accounting Standards Update 2016-01
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change					$ (221)
Tax Cuts and Jobs Act of 2017 | Accounting Standards Update 2018-02
Significant Accounting Policies [Line Items]
Stranded amount reclassified from AOCI to retained earnings	$ 145
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		$ (257)