Mortgage Banking Activities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage Loan Activity [Line Items]
Mortgage loans serviced for others	$ 1,300,000,000	$ 1,200,000,000
Fair value of mortgage servicing rights	10,900,000	10,200,000	$ 9,100,000
Interest rate lock commitments for the future delivery of residential mortgage loans	40,200,000	30,200,000
Forward commitments for the future delivery of residential mortgage loans	155,000,000	124,000,000
Expense incurred for settlements	0	0	$ 554,000,000
Other reserve for settlements	0	0
FHLMC and FNMA [Member]
Mortgage Loan Activity [Line Items]
Customer escrow balances	$ 15,300,000	$ 14,300,000