Net Interest Income - Components of Interest Revenue and Interest Expenses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Banking and Thrift [Abstract]
Deposits with banks	$ 90	$ 41	$ 172	$ 76
U.S. Treasury and federal agencies	280	208	534	420
State and political subdivisions	44	56	96	114
Other investments	140	164	298	323
Securities purchased under resale agreements	81	69	159	115
Loans and leases	169	118	325	224
Other interest-earning assets	103	44	180	78
Total interest income	907	700	1,764	1,350
Deposits	89	14	152	57
Securities sold under repurchase agreements	6	0	7	1
Short-term borrowings	4	3	7	5
Long-term debt	97	75	194	148
Other interest-bearing liabilities	52	33	102	54
Total interest expense	248	125	462	265
Net interest income	$ 659	$ 575	$ 1,302	$ 1,085