Allowance for Loan Losses and Credit Quality Information - Schedule of Commercial Credit Exposure (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	100.00%	100.00%
Total Commercial Loans	$ 2,801,824	$ 2,656,626
Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	45,093	20,798
Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	56,348	67,065
Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	13,122	11,017
Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 5,280	$ 4,296
Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	4.00%	4.00%
Total Commercial Loans	$ 119,843	$ 103,176
Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	1.00%
Total Commercial Loans	$ 14,549	$ 15,372
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	95.00%	96.00%
Total Commercial Loans	$ 2,667,432	$ 2,538,078
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	936,765	920,072
Commercial [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	16,078	4,744
Commercial [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	35,061	42,377
Commercial [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,742	1,225
Commercial [Member] | Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,814	3,034
Commercial [Member] | Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	60,695	51,380
Commercial [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	3,159	3,269
Commercial [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	872,911	865,423
Owner Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	793,062	788,598
Owner Occupied Commercial [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	18,713	6,989
Owner Occupied Commercial [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	15,567	14,436
Owner Occupied Commercial [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,314	1,865
Owner Occupied Commercial [Member] | Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	75	609
Owner Occupied Commercial [Member] | Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	35,669	23,899
Owner Occupied Commercial [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,027	2,264
Owner Occupied Commercial [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	755,366	762,435
Commercial Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	870,152	805,459
Commercial Mortgage [Member] | Special Mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	10,302	9,065
Commercial Mortgage [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,515	9,167
Commercial Mortgage [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	7,066	7,927
Commercial Mortgage [Member] | Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	177	319
Commercial Mortgage [Member] | Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	22,060	26,478
Commercial Mortgage [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	5,884	5,976
Commercial Mortgage [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	842,208	773,005
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	201,845	142,497
Construction [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,205	1,085
Construction [Member] | Doubtful / Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	214	334
Construction [Member] | Total Special and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,419	1,419
Construction [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	3,479	3,863
Construction [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 196,947	$ 137,215