Allowance for Loan Losses and Credit Quality Information - Schedule of Commercial Credit Exposure (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	100.00%	100.00%
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 1,108,610	$ 1,061,597
Special Mention [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,379	5,620
Accrual [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	40,140	33,883
Nonaccrual [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,627	4,164
Doubtful / Nonaccrual [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 427	$ 1,164
Total Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	3.00%	3.00%
Total Special Mention and Substandard [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 46,573	$ 44,831
Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	1.00%	1.00%
Acquired Impaired Loans [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 1,711	$ 12,985
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	96.00%	96.00%
Pass [Member] | Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 1,060,326	$ 1,003,781
Owner- Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	938,330	880,643
Owner- Occupied Commercial [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	6,427	9,535
Owner- Occupied Commercial [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	23,351	22,901
Owner- Occupied Commercial [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	838	1,090
Owner- Occupied Commercial [Member] | Total Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	30,616	33,526
Owner- Occupied Commercial [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	9,757	4,688
Owner- Occupied Commercial [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	897,957	842,429
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	990,245	966,698
Commercial Mortgages [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	21,185	12,323
Commercial Mortgages [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	9,868	2,547
Commercial Mortgages [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,622	3,326
Commercial Mortgages [Member] | Total Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	32,675	18,196
Commercial Mortgages [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	10,560	10,513
Commercial Mortgages [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	947,010	937,989
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	199,039	245,773
Construction [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		8,296
Construction [Member] | Total Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans		8,296
Construction [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,094	3,544
Construction [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	194,945	233,933
Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	3,236,224	3,154,711
Total Commercial [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	31,991	27,478
Total Commercial [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	73,359	67,627
Total Commercial [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,087	8,580
Total Commercial [Member] | Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	427	1,164
Total Commercial [Member] | Total Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	109,864	104,849
Total Commercial [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	26,122	31,730
Total Commercial [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 3,100,238	$ 3,018,132