Allowance for Credit Losses - Activity in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 116,446	$ 108,088	$ 114,419	$ 106,178
Loans charged-off	(1,729)	(5,063)	(9,397)	(9,329)
Charged-off loans recovered	2,948	1,723	6,531	5,807
Net recoveries (charge-offs)	1,219	(3,340)	(2,866)	(3,522)
Provision for loan losses	1,301	5,949	7,413	8,041
Ending balance	118,966	110,697	118,966	110,697
Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	51,617	48,025	50,820	48,767
Loans charged-off	(265)	(3,763)	(4,889)	(5,507)
Charged-off loans recovered	2,320	902	3,480	2,418
Net recoveries (charge-offs)	2,055	(2,861)	(1,409)	(3,089)
Provision for loan losses	1,017	5,588	5,278	5,074
Ending balance	54,689	50,752	54,689	50,752
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	55,455	51,877	55,851	48,006
Loans charged-off	0	0	(553)	(519)
Charged-off loans recovered	42	44	824	1,591
Net recoveries (charge-offs)	42	44	271	1,072
Provision for loan losses	(198)	539	(823)	3,382
Ending balance	55,299	52,460	55,299	52,460
Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	4,186	3,495	3,702	4,625
Loans charged-off	(129)	(518)	(488)	(750)
Charged-off loans recovered	220	495	903	604
Net recoveries (charge-offs)	91	(23)	415	(146)
Provision for loan losses	(385)	(94)	(225)	(1,101)
Ending balance	3,892	3,378	3,892	3,378
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	5,188	4,691	4,046	4,780
Loans charged-off	(1,335)	(782)	(3,467)	(2,553)
Charged-off loans recovered	366	282	1,324	1,194
Net recoveries (charge-offs)	(969)	(500)	(2,143)	(1,359)
Provision for loan losses	867	(84)	3,183	686
Ending balance	$ 5,086	$ 4,107	$ 5,086	$ 4,107