Quarterly Financial Data (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure of quarterly financial data
Revenues	$ 179	$ 180	$ 179	$ 180	$ 180	$ 187	$ 193	$ 195	$ 718	$ 755	$ 929
Pretax income	20	70	65	57	68	45	69	70	212	252	318
Net income	$ 29	$ 60	$ 57	$ 51	$ 58	$ 39	$ 49	$ 68	$ 197	$ 214	$ 353