Income Taxes - Unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Reconciliation of beginning and ending amount of unrecognized tax benefits
Balance at beginning of year	$ 2	$ 2
Additions based on tax positions related to the current year	0	0
Additions for tax positions of prior years	0	0
Reductions for tax positions of prior years	0	0
Settlements	0	0
Balance at end of year	$ 2	$ 2