Quarterly Financial Data (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of quarterly financial data
Revenues	$ 296	$ 286	$ 253	$ 252	$ 179	$ 180	$ 179	$ 180	$ 1,087	$ 718	$ 755
Pretax income	72	85	52	66	20	70	65	57	275	212	252
Net (loss) income	$ (791)	$ 72	$ 57	$ 59	$ 29	$ 60	$ 57	$ 51	$ (603)	$ 197	$ 214