Net Interest Income - Components of Interest Revenue and Interest Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Banking and Thrift [Abstract]
Deposits with banks	$ 180	$ 126	$ 208
U.S. Treasury and federal agencies	854	821	735
State and political subdivisions	226	224	227
Other investments	658	756	934
Securities purchased under resale agreements	264	146	62
Loans and leases	504	378	311
Other interest-earning assets	222	61	11
Total interest income	2,908	2,512	2,488
Deposits	163	85	97
Securities sold under repurchase agreements	2	1	0
Short-term borrowings	10	7	7
Long-term debt	308	260	250
Other interest-bearing liabilities	121	75	46
Total interest expense	604	428	400
Net interest income	$ 2,304	$ 2,084	$ 2,088