Allowance for Loan Losses and Credit Quality Information - Schedule of Loans Identified as Troubled Debt Restructurings During Periods Indicated (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Modifications [Line Items]
Total loans identified during the period	$ 1,667	$ 1,075	$ 8,855	$ 3,381
Commercial
Financing Receivable, Modifications [Line Items]
Total loans identified during the period	0	0	781	1,125
Commercial mortgages
Financing Receivable, Modifications [Line Items]
Total loans identified during the period	0	0	0	0
Construction
Financing Receivable, Modifications [Line Items]
Total loans identified during the period	0	0	1,836	0
Residential
Financing Receivable, Modifications [Line Items]
Total loans identified during the period	1,058	797	1,300	1,523
Consumer
Financing Receivable, Modifications [Line Items]
Total loans identified during the period	609	278	1,867	733
Owner-occupied commercial
Financing Receivable, Modifications [Line Items]
Total loans identified during the period	$ 0	$ 0	$ 3,071	$ 0