Income Taxes - Income Before Income Taxes and Income Tax Provision (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income before income taxes
Domestic		$ 1,371	$ 1,308	$ 1,057
Foreign		1,149	1,218	986
Income before income tax expense (benefit)		2,520	2,526	2,043
Current tax expense:
Federal		140	266	258
State		107	92	5
Foreign		226	268	203
Current tax expense		473	626	466
Deferred tax expense (benefit):
Federal		29	(677)	71
State		9	33	76
Foreign		(11)	(10)	(27)
Deferred tax expense (benefit)	$ (764)	27	(654)	120
Total income tax expense (benefit)		$ 500	$ (28)	$ 586