Income Taxes - Components of Pretax Income and Provision for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States	$ 418	$ 1,529	$ 396
International	1,858	2,122	2,010
Income before income taxes	2,276	3,651	2,406
Current:
Federal	1,426	689	363
State and local	(17)	55	22
Foreign	150	178	106
Current income tax expense (benefit)	1,559	922	491
Deferred:
Federal	1,788	77	(53)
State and local	4	0	(2)
Foreign	(63)	(4,633)	23
Deferred income tax expense (benefit)	1,729	(4,556)	(32)
Income tax expense (benefit)	$ 3,288	$ (3,634)	$ 459