Pledged and Restricted Assets - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 40,100,000,000	$ 38,900,000,000
Fair value of accepted collateral	78,300,000	217,500,000
Average deposits maintained to meet Federal Reserve Bank reserve requirements	3,100,000,000	2,200,000,000
Us Government Agencies Debt Securities And Other Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	30,800,000,000	28,300,000,000
Obligations of States and Political Subdivisions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	684,300,000	939,800,000
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	8,600,000,000	9,600,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	11,000,000,000	9,300,000,000
Securities Available for Sale
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	833,400,000	494,700,000
Repurchase Agreements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Fair value of accepted collateral	1,200,000,000	$ 1,800,000,000
Derivative
Financial Instruments Owned and Pledged as Collateral [Line Items]
Fair value of accepted collateral	4,600,000
Amount of securities repledged or sold collateral	$ 0