Allowance for Loan Losses and Credit Quality Information - Schedule of Residential and Consumer Credit Exposure (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 4,696,231	$ 4,469,216	$ 4,394,110
Total	100.00%	100.00%
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 240,717	$ 267,028	268,711
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	520,276	450,029	$ 438,158
Nonperforming | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	14,060	13,547
Nonperforming | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	7,409	7,863
Performing | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	225,869	252,621
Performing | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 512,624	$ 441,797
Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1.00%	1.00%
Acquired impaired | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 788	$ 860
Acquired impaired | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	243	369
Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 760,993	$ 717,057
Total	100.00%	100.00%
Total Residential and Consumer | Nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 21,469	$ 21,410
Total	3.00%	3.00%
Total Residential and Consumer | Performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 738,493	$ 694,418
Total	97.00%	97.00%
Total Residential and Consumer | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 1,031	$ 1,229
Total	0.00%	0.00%