Quarterly Financial Data (unaudited) (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
Quarterly Financial Data [Abstract]
Total revenue	$ 2,692	$ 2,499	$ 2,292	$ 2,075	$ 1,878	$ 1,707	$ 1,515	$ 1,354	$ 9,558	$ 6,454	$ 4,854
Income before income taxes	194	228	266	225	167	231	225	214	913	837	815
Net income	$ 254	$ 147	$ 189	$ 164	$ 131	$ 179	$ 186	$ 150	$ 754	$ 646	$ 537