Allowance for Credit Losses - Summary of Activity in Allowance for Loan Losses (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 102,835,000	$ 103,053,000	$ 102,353,000	$ 113,617,000
Loans charged-off	(2,009,000)	(2,345,000)	(11,024,000)	(21,335,000)
Charged-off loans recovered	3,725,000	1,730,000	8,840,000	10,878,000
Net (charge-offs) recoveries	1,716,000	(615,000)	(2,184,000)	(10,457,000)
Provision for loan losses	0	0	4,382,000	(722,000)
Ending balance	104,551,000	102,438,000	104,551,000	102,438,000
Covered loans, charge-offs	0	433,000	0	1,200,000
Covered loans, recovered	0	0	0	462,000
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	41,714,000	48,421,000	43,676,000	51,551,000
Loans charged-off	(1,124,000)	(1,852,000)	(5,103,000)	(11,806,000)
Charged-off loans recovered	2,550,000	1,190,000	5,587,000	6,154,000
Net (charge-offs) recoveries	1,426,000	(662,000)	484,000	(5,652,000)
Provision for loan losses	4,397,000	(1,388,000)	3,377,000	472,000
Ending balance	47,537,000	46,371,000	47,537,000	46,371,000
Commercial real estate loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	44,185,000	35,267,000	42,840,000	42,343,000
Loans charged-off	(40,000)	(181,000)	(2,780,000)	(6,703,000)
Charged-off loans recovered	536,000	26,000	1,686,000	2,831,000
Net (charge-offs) recoveries	496,000	(155,000)	(1,094,000)	(3,872,000)
Provision for loan losses	(2,403,000)	1,954,000	532,000	(1,405,000)
Ending balance	42,278,000	37,066,000	42,278,000	37,066,000
Residential mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	5,055,000	7,018,000	5,093,000	7,786,000
Loans charged-off	(111,000)	(240,000)	(499,000)	(515,000)
Charged-off loans recovered	151,000	8,000	395,000	244,000
Net (charge-offs) recoveries	40,000	(232,000)	(104,000)	(271,000)
Provision for loan losses	(546,000)	(610,000)	(440,000)	(1,339,000)
Ending balance	4,549,000	6,176,000	4,549,000	6,176,000
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	5,542,000	5,368,000	5,179,000	4,359,000
Loans charged-off	(734,000)	(72,000)	(2,642,000)	(2,311,000)
Charged-off loans recovered	488,000	506,000	1,172,000	1,649,000
Net (charge-offs) recoveries	(246,000)	434,000	(1,470,000)	(662,000)
Provision for loan losses	(829,000)	(22,000)	758,000	2,083,000
Ending balance	4,467,000	5,780,000	4,467,000	5,780,000
Unallocated
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	6,339,000	6,979,000	5,565,000	7,578,000
Loans charged-off	0	0	0	0
Charged-off loans recovered	0	0	0	0
Net (charge-offs) recoveries	0	0	0	0
Provision for loan losses	(619,000)	66,000	155,000	(533,000)
Ending balance	$ 5,720,000	$ 7,045,000	$ 5,720,000	$ 7,045,000