Allowance for Credit Losses - Summary of Activity in Allowance for Loan Losses (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 102,631,000	$ 106,709,000	$ 102,353,000	$ 113,617,000
Loans charged-off	(7,349,000)	(4,751,000)	(9,015,000)	(18,990,000)
Charged-off loans recovered	3,171,000	6,766,000	5,115,000	9,148,000
Net (charge-offs) recoveries	(4,178,000)	2,015,000	(3,900,000)	(9,842,000)
Provision for loan losses	4,382,000	(5,671,000)	4,382,000	(722,000)
Ending balance	102,835,000	103,053,000	102,835,000	103,053,000
Covered loans, charge-offs	0	749,000	0	749,000
Covered loans, recovered	0	462,000	0	462,000
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	44,893,000	49,932,000	43,676,000	51,551,000
Loans charged-off	(3,226,000)	(1,340,000)	(3,979,000)	(9,954,000)
Charged-off loans recovered	1,986,000	4,420,000	3,037,000	4,964,000
Net (charge-offs) recoveries	(1,240,000)	3,080,000	(942,000)	(4,990,000)
Provision for loan losses	(1,939,000)	(4,591,000)	(1,020,000)	1,860,000
Ending balance	41,714,000	48,421,000	41,714,000	48,421,000
Commercial real estate loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	41,656,000	38,382,000	42,840,000	42,343,000
Loans charged-off	(2,590,000)	(2,032,000)	(2,740,000)	(6,522,000)
Charged-off loans recovered	690,000	1,468,000	1,150,000	2,805,000
Net (charge-offs) recoveries	(1,900,000)	(564,000)	(1,590,000)	(3,717,000)
Provision for loan losses	4,429,000	(2,551,000)	2,935,000	(3,359,000)
Ending balance	44,185,000	35,267,000	44,185,000	35,267,000
Residential mortgage
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	4,092,000	6,979,000	5,093,000	7,786,000
Loans charged-off	(339,000)	(212,000)	(388,000)	(275,000)
Charged-off loans recovered	130,000	157,000	244,000	236,000
Net (charge-offs) recoveries	(209,000)	(55,000)	(144,000)	(39,000)
Provision for loan losses	1,172,000	94,000	106,000	(729,000)
Ending balance	5,055,000	7,018,000	5,055,000	7,018,000
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	4,972,000	4,107,000	5,179,000	4,359,000
Loans charged-off	(1,194,000)	(1,167,000)	(1,908,000)	(2,239,000)
Charged-off loans recovered	365,000	721,000	684,000	1,143,000
Net (charge-offs) recoveries	(829,000)	(446,000)	(1,224,000)	(1,096,000)
Provision for loan losses	1,399,000	1,707,000	1,587,000	2,105,000
Ending balance	5,542,000	5,368,000	5,542,000	5,368,000
Unallocated
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	7,018,000	7,309,000	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	(679,000)	(330,000)	774,000	(599,000)
Ending balance	$ 6,339,000	$ 6,979,000	$ 6,339,000	$ 6,979,000