Cash and Cash Equivalents - Additional Information (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents	$ 37,006,000	$ 18,473,000	$ 56,265,000	$ 103,591,000
Federal Reserve Bank of Boston
Cash And Cash Equivalents [Line Items]
Reserve balance of cash and due from banks	12,000,000	12,700,000
New Hampshire
Cash And Cash Equivalents [Line Items]
Pledged amount to federal banking department	$ 500,000	$ 500,000