Income Taxes - Components of income tax expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of income tax expense (benefit)
Current	$ 1,018	$ 654	$ 1,033
Deferred	(216)	223	78
Total income tax expense	$ 802	$ 877	$ 1,111