Variable Interest Entities - Revenues and Net Income from Entities (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
Variable Interest Entity [Line Items]
Total revenue	$ 2,692	$ 2,499	$ 2,292	$ 2,075	$ 1,878	$ 1,707	$ 1,515	$ 1,354	$ 9,558	$ 6,454	$ 4,854
Net income	$ 254	$ 147	$ 189	$ 164	$ 131	$ 179	$ 186	$ 150	754	646	537
Other VIE [Member]
Variable Interest Entity [Line Items]
Total revenue									210	191	192
Net income									$ 29	$ 29	$ 28