Net Interest Income - Components of Interest Revenue and Interest Expenses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Banking and Thrift [Abstract]
Deposits with banks	$ 41	$ 30	$ 76	$ 73
U.S. Treasury and federal agencies	208	209	420	420
State and political subdivisions	56	54	114	108
Other investments	164	189	323	371
Securities purchased under resale agreements	69	35	115	71
Loans and leases	118	93	224	184
Other interest-earning assets	44	10	78	22
Total interest income	700	620	1,350	1,249
Deposits	14	16	57	54
Securities sold under repurchase agreements	0	0	1	1
Short-term borrowings	3	2	5	2
Long-term debt	75	62	148	122
Other interest-bearing liabilities	33	19	54	37
Total interest expense	125	99	265	216
Net interest income	$ 575	$ 521	$ 1,085	$ 1,033