Quarterly Financial Data (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of quarterly financial data
Revenues	$ 406	$ 386	$ 351	$ 318	$ 296	$ 286	$ 253	$ 252	$ 1,461	$ 1,087	$ 718
Pretax income	108	107	93	108	72	85	52	66	416	275	212
Net income (loss)	$ 61	$ 109	$ 88	$ 129	$ (791)	$ 72	$ 57	$ 59	$ 387	$ (603)	$ 197