Fair Values of Financial Instruments - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Related allowance	$ 7,000	$ 120,000
Provision for loan losses	$ 903,000	464,000
Discount rate	10.00%
Loans classified as TDRs	$ 433,000	239,000
Commitment period to extend credit (or less)	30 days
Standby letters of credit	$ 156,000	76,000
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unpaid principal balance	0	1,103,000
Real Estate Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans classified as TDRs	68,000	58,000
Home Equity Loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans classified as TDRs	8,000
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers from Level 2 to Level 3	8,000,000
Market and Income Approach Valuation Technique
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired financing receivable, recorded investment	0	1,200,000
Related allowance		120,000
Market and Income Approach Valuation Technique | Impaired Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Provision for loan losses	$ 0	$ 95,000