Net Interest Income - Components of Interest Revenue and Interest Expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Banking and Thrift [Abstract]
Deposits with banks	$ 34	$ 43
U.S. Treasury and federal agencies	212	211
State and political subdivisions	59	52
Other investments	160	183
Securities purchased under resale agreements	46	36
Loans and leases	105	91
Other interest-earning assets	34	13
Total interest income	650	629
Deposits	44	38
Short-term borrowings	2	0
Long-term debt	73	61
Other interest-bearing liabilities	21	18
Total interest expense	140	117
Net interest income	$ 510	$ 512