Allowance for Credit Losses - Activity in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 120,856	$ 114,419
Loans charged-off	(1,720)	(3,379)
Charged-off loans recovered	3,024	2,001
Net recoveries (charge-offs)	1,304	(1,378)
Provision for loan losses	10,702	2,402
Ending balance	132,862	115,443
Commercial and Industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	57,232	50,820
Loans charged-off	(131)	(1,714)
Charged-off loans recovered	2,107	848
Net recoveries (charge-offs)	1,976	(866)
Provision for loan losses	7,338	1,334
Ending balance	66,546	51,288
Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	54,954	55,851
Loans charged-off	(310)	(414)
Charged-off loans recovered	369	142
Net recoveries (charge-offs)	59	(272)
Provision for loan losses	1,666	723
Ending balance	56,679	56,302
Residential Mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	3,605	3,702
Loans charged-off	(68)	(130)
Charged-off loans recovered	80	448
Net recoveries (charge-offs)	12	318
Provision for loan losses	483	(428)
Ending balance	4,100	3,592
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	5,065	4,046
Loans charged-off	(1,211)	(1,121)
Charged-off loans recovered	468	563
Net recoveries (charge-offs)	(743)	(558)
Provision for loan losses	1,215	773
Ending balance	$ 5,537	$ 4,261