Allowance for Credit Losses - Summary of Activity in Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 106,178	$ 102,353
Loans charged-off	(2,511)	(1,666)
Charged-off loans recovered	1,019	1,944
Net (charge-offs) recoveries	(1,492)	278
Provision for loan losses	729	0
Ending balance	105,415	102,631
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	48,767	43,676
Loans charged-off	(1,251)	(753)
Charged-off loans recovered	526	1,051
Net (charge-offs) recoveries	(725)	298
Provision for loan losses	375	919
Ending balance	48,417	44,893
Commercial real estate loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	48,006	42,840
Loans charged-off	(105)	(150)
Charged-off loans recovered	89	460
Net (charge-offs) recoveries	(16)	310
Provision for loan losses	464	(1,494)
Ending balance	48,454	41,656
Residential mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	4,625	5,093
Loans charged-off	(81)	(49)
Charged-off loans recovered	15	114
Net (charge-offs) recoveries	(66)	65
Provision for loan losses	(350)	(1,066)
Ending balance	4,209	4,092
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	4,780	5,179
Loans charged-off	(1,074)	(714)
Charged-off loans recovered	389	319
Net (charge-offs) recoveries	(685)	(395)
Provision for loan losses	240	188
Ending balance	4,335	4,972
Unallocated
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	0	5,565
Loans charged-off	0	0
Charged-off loans recovered	0	0
Net (charge-offs) recoveries	0	0
Provision for loan losses	0	1,453
Ending balance	$ 0	$ 7,018