Net Interest Income - Components of Interest Revenue and Interest Expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Banking and Thrift [Abstract]
Deposits with banks	$ 82	$ 34
U.S. Treasury and federal agencies	255	212
State and political subdivisions	52	59
Other investments	159	160
Securities purchased under resale agreements	77	46
Loans and leases	155	105
Other interest-earning assets	77	34
Total interest income	857	650
Deposits	48	44
Short-term borrowings	3	2
Long-term debt	97	73
Other interest-bearing liabilities	51	21
Total interest expense	199	140
Net interest income	$ 658	$ 510