Income Taxes - Components of income tax expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of income tax expense (benefit)
Current	$ 704	$ 1,018	$ 654
Deferred	(212)	(216)	223
Total income tax expense	$ 492	$ 802	$ 877