Allowance for Credit Losses - Summary of Activity in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for loan losses:
Beginning balance	$ 114,419	$ 106,178
Loans charged-off	(11,074)	(10,969)
Charged-off loans recovered	8,980	7,337
Net charge-offs	(2,094)	(3,632)
Provision for loan losses		11,873
Ending balance	120,856	114,419	$ 106,178
Provision for loan losses	8,531	11,873	7,846
Commercial and Industrial
Allowance for loan losses:
Beginning balance	50,820	48,767
Loans charged-off	(5,421)	(5,990)
Charged-off loans recovered	4,736	2,852
Net charge-offs	(685)	(3,138)
Provision for loan losses		5,191
Ending balance	57,232	50,820	48,767
Provision for loan losses	7,097
Commercial Real Estate
Allowance for loan losses:
Beginning balance	55,851	48,006
Loans charged-off	(559)	(650)
Charged-off loans recovered	1,425	2,057
Net charge-offs	866	1,407
Provision for loan losses		6,438
Ending balance	54,954	55,851	48,006
Provision for loan losses	(1,763)
Residential Mortgage
Allowance for loan losses:
Beginning balance	3,702	4,625
Loans charged-off	(530)	(866)
Charged-off loans recovered	1,016	774
Net charge-offs	486	(92)
Provision for loan losses		(831)
Ending balance	3,605	3,702	4,625
Provision for loan losses	(583)
Consumer
Allowance for loan losses:
Beginning balance	4,046	4,780
Loans charged-off	(4,564)	(3,463)
Charged-off loans recovered	1,803	1,654
Net charge-offs	(2,761)	(1,809)
Provision for loan losses		1,075
Ending balance	5,065	$ 4,046	$ 4,780
Provision for loan losses	$ 3,780