Income Taxes - Unrecognized tax benefit (expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, beginning of period	$ 14	$ 9	$ 12
Gross increases - tax positions in prior period	0	5	3
Gross decreases - tax positions in prior period	0	0	0
Gross decreases - Tax Reform	0	0	(6)
Balance, end of period	$ 14	$ 14	$ 9