Net Interest Income - Components of Interest Revenue and Interest Expenses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Banking and Thrift [Abstract]
Deposits with banks	$ 45	$ 29	$ 121	$ 101
U.S. Treasury and federal agencies	207	200	627	620
State and political subdivisions	56	55	171	162
Other investments	173	208	495	581
Securities purchased under resale agreements	74	40	189	112
Loans and leases	139	97	362	281
Other interest-earning assets	67	18	146	39
Total interest income	761	647	2,111	1,896
Deposits	39	20	96	73
Securities sold under repurchase agreements	1	0	2	1
Short-term borrowings	3	2	7	4
Long-term debt	78	68	227	191
Other interest-bearing liabilities	37	20	91	57
Total interest expense	158	110	423	326
Net interest income	$ 603	$ 537	$ 1,688	$ 1,570