Allowance for Loan Losses and Credit Quality Information - Schedule of Commercial Credit Exposure (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	100.00%	100.00%
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 1,339,867	$ 1,287,731
Special mention | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	17,590	17,630
Accrual | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	41,136	45,067
Nonaccrual | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	14,660	1,693
Doubtful | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 1,860	$ 322
Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	4.00%	4.00%
Total Special Mention and Substandard | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 75,246	$ 64,712
Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	1.00%	1.00%
Acquired impaired | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 5,256	$ 6,183
Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	95.00%	95.00%
Pass | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 1,259,365	$ 1,216,836
Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,092,292	1,078,162
Owner-occupied commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	6,849	11,419
Owner-occupied commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	20,162	19,871
Owner-occupied commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,020	2,078
Owner-occupied commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	0
Owner-occupied commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	31,031	33,368
Owner-occupied commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	12,040	12,122
Owner-occupied commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,049,221	1,032,672
Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,165,556	1,163,554
Commercial mortgages | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	21,931	34,198
Commercial mortgages | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	3,216	239
Commercial mortgages | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	8,300	8,574
Commercial mortgages | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,395	1,247
Commercial mortgages | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	34,842	44,258
Commercial mortgages | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	10,450	10,386
Commercial mortgages | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,120,264	1,108,910
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	255,760	222,712
Construction | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,008	0
Construction | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,489	2,193
Construction | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	0
Construction | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	0
Construction | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	5,497	2,193
Construction | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,372	3,694
Construction | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	247,891	216,825
Total Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	3,853,475	3,752,159
Total Commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	47,378	63,247
Total Commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	69,003	67,370
Total Commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	26,980	12,345
Total Commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	3,255	1,569
Total Commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	146,616	144,531
Total Commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	30,118	32,385
Total Commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 3,676,741	$ 3,575,243