Income Taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of the change in the amount of unrecognized tax benefits
Balance – beginning of year	$ 70	$ 70	$ 55
Increase for tax positions taken in a prior year	0	0	3
Increase for tax positions taken in the current year	0	0	12
Decrease for settlements	(58)	0	0
Balance – end of year	$ 12	$ 70	$ 70