Allowance for Loan Losses and Credit Quality Information - Schedule of Commercial Credit Exposure (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	100.00%	100.00%
Total Commercial Loans	$ 2,827,085	$ 2,656,626
Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	40,046	20,798
Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	59,842	67,065
Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	12,075	11,017
Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 1,229	$ 4,296
Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	4.00%	4.00%
Total Commercial Loans	$ 113,192	$ 103,176
Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	0.00%
Total Commercial Loans	$ 12,816	$ 15,372
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	96.00%	96.00%
Total Commercial Loans	$ 2,701,077	$ 2,538,078
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	938,060	920,072
Commercial [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	13,759	4,744
Commercial [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	36,048	42,377
Commercial [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,426	1,225
Commercial [Member] | Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	988	3,034
Commercial [Member] | Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	55,221	51,380
Commercial [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,627	3,269
Commercial [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	880,212	865,423
Owner Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	811,548	788,598
Owner Occupied Commercial [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	16,048	6,989
Owner Occupied Commercial [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	14,348	14,436
Owner Occupied Commercial [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,170	1,865
Owner Occupied Commercial [Member] | Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		609
Owner Occupied Commercial [Member] | Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	31,566	23,899
Owner Occupied Commercial [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,195	2,264
Owner Occupied Commercial [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	777,787	762,435
Commercial Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	874,316	805,459
Commercial Mortgage [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	10,239	9,065
Commercial Mortgage [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,252	9,167
Commercial Mortgage [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	6,479	7,927
Commercial Mortgage [Member] | Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	241	319
Commercial Mortgage [Member] | Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	18,211	26,478
Commercial Mortgage [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	5,400	5,976
Commercial Mortgage [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	850,705	773,005
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	203,161	142,497
Construction [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	8,194	1,085
Construction [Member] | Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		334
Construction [Member] | Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	8,194	1,419
Construction [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,594	3,863
Construction [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 192,373	$ 137,215