Income Taxes - Progression of the liability for uncertain tax positions, excluding interest and penalties (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Progression of the liability for uncertain tax positions, excluding interest and penalties
Balance at January 1	$ 0	$ 1	$ 1
Additions for tax positions of prior years	0	0	0
Reductions for tax positions of prior years	0	0	0
Additions for tax positions of current year	0	0	0
Settlements	0	(1)	0
Balance at December 31	$ 0	$ 0	$ 1