Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ 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
Quarterly Financial Information Disclosure [Abstract]
Net revenues	$ 3,160	$ 2,981	$ 2,985	$ 2,901	$ 3,062	$ 2,998	$ 2,871	$ 2,765	$ 12,027	$ 11,696	$ 12,170
Pretax income	600	628	511	475	469	238	410	475	$ 2,214	$ 1,592	$ 2,142
Net Income (Loss) Available to Common Stockholders, Basic	$ 181	$ 503	$ 393	$ 403	$ 400	$ 215	$ 335	$ 364
Earnings Per Share [Abstract]
Net income (in dollars per basic share)	$ 1.20	$ 3.29	$ 2.53	$ 2.56	$ 2.49	$ 1.31	$ 1.99	$ 2.11	$ 9.60	$ 7.90	$ 8.60
Net income (in dollars per diluted share)	$ 1.18	$ 3.24	$ 2.50	$ 2.52	$ 2.46	$ 1.30	$ 1.97	$ 2.09	$ 9.44	$ 7.81	$ 8.48
Weighted average common shares outstanding
Basic	151.0	153.0	155.1	157.5	160.4	164.0	168.3	172.6	154.1	166.3	181.7
Diluted	153.8	155.4	157.5	160.1	162.4	165.8	170.1	174.4	156.7	168.2	184.2
Cash dividends declared per common share	$ 0.83	$ 0.83	$ 0.83	$ 0.75	$ 0.75	$ 0.75	$ 0.75	$ 0.67	$ 3.24	$ 2.92	$ 2.59
Common share price:
High (in dollars per share)	173.62	149.99	133.02	135.20	119.32	101.81	102.74	105.47	173.62	119.32
Low (in dollars per share)	$ 147.79	$ 128.06	$ 118.84	$ 110.56	$ 86.25	$ 84.93	$ 84.92	$ 76.00	$ 147.79	$ 86.25