Income Taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the change in the amount of unrecognized tax benefits
Balance – beginning of year	$ 10	$ 7	$ 0
Increase for tax positions taken in a prior year	34	0	4
Increase for tax positions taken in the current year	11	3	3
Balance – end of year	$ 55	$ 10	$ 7