Income Taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of the change in the amount of unrecognized tax benefits
Balance – beginning of year	$ 70	$ 55	$ 10
Increase for tax positions taken in a prior year	0	3	34
Increase for tax positions taken in the current year	0	12	11
Balance – end of year	$ 70	$ 70	$ 55