Allowance for Loan Losses and Credit Quality Information - Schedule of Residential and Consumer Credit Exposure (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 4,577,628	$ 4,469,216	$ 3,803,713
Total	100.00%	100.00%
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 259,255	$ 267,028	251,065
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	464,898	450,029	$ 359,656
Nonperforming | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	14,280	13,547
Nonperforming | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	9,030	7,863
Performing | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	244,109	252,621
Performing | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 455,605	$ 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	$ 866	$ 860
Acquired impaired | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	263	369
Total Residential and Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 724,153	$ 717,057
Total	100.00%	100.00%
Total Residential and Consumer | Nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 23,310	$ 21,410
Total	3.00%	3.00%
Total Residential and Consumer | Performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 699,714	$ 694,418
Total	97.00%	97.00%
Total Residential and Consumer | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 1,129	$ 1,229
Total	0.00%	0.00%