Net Interest Income - Components of Interest Revenue and Interest Expenses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Banking and Thrift [Abstract]
Deposits with banks	$ 94	$ 45	$ 267	$ 121
U.S. Treasury and federal agencies	300	207	835	627
State and political subdivisions	29	56	125	171
Other investments	136	173	434	495
Securities purchased under resale agreements	87	74	246	189
Loans and leases	173	139	498	362
Other interest-earning assets	97	67	275	146
Total interest income	916	761	2,680	2,111
Deposits	84	39	236	96
Securities sold under repurchase agreements	4	1	11	2
Other short-term borrowings	4	3	12	7
Long-term debt	100	78	293	227
Other interest-bearing liabilities	52	37	154	91
Total interest expense	244	158	706	423
Net interest income	$ 672	$ 603	$ 1,974	$ 1,688