Allowance for Loan Losses and Credit Quality Information - Schedule of Residential and Consumer Credit Exposure (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 4,805,094	$ 4,469,216	$ 3,774,639
Total	100.00%	100.00%
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 233,487	$ 267,028	259,679
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	558,493	450,029	$ 360,249
Nonperforming | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	13,778	13,547
Nonperforming | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	7,588	7,863
Performing | Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	218,925	252,621
Performing | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 550,658	$ 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	$ 784	$ 860
Acquired impaired | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	247	369
Total Residential and Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 791,980	$ 717,057
Total	100.00%	100.00%
Total Residential and Consumer Loan | Nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 21,366	$ 21,410
Total	3.00%	3.00%
Total Residential and Consumer Loan | Performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 769,583	$ 694,418
Total	97.00%	97.00%
Total Residential and Consumer Loan | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 1,031	$ 1,229
Total	0.00%