Allowance for Loan Losses and Credit Quality Information - Schedule of Residential and Consumer Credit Exposure (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 4,894,257	$ 4,805,094	$ 4,469,216
% of Total Loans	100.00%	100.00%
Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Total Loans	0.00%	1.00%
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 201,550	$ 233,487	267,028
Residential | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	761	784
Residential | Nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	11,017	13,778
Residential | Performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	189,772	218,925
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	680,939	558,493	$ 450,029
Consumer | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	151	247
Consumer | Nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	7,883	7,588
Consumer | Performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	672,905	550,658
Total Residential and Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 882,489	$ 791,980
% of Total Loans	100.00%	100.00%
Total Residential and Consumer Loan | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 912	$ 1,031
% of Total Loans	0.00%	0.00%
Total Residential and Consumer Loan | Nonperforming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 18,900	$ 21,366
% of Total Loans	2.00%	3.00%
Total Residential and Consumer Loan | Performing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 862,677	$ 769,583
% of Total Loans	98.00%	97.00%