Loans Receivable and Related Allowance for Loan Losses - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan	Dec. 31, 2015 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchase of residential mortgage loans		$ 6,900,000
Deferred costs	$ 1,500,000	1,300,000
Allowance for loan losses	6,127,000	5,545,000	$ 5,205,000
Loans classified as TDRs	433,000	239,000
Allowance loan losses allocated to TDRs	$ 7,000	19,000
Loans classified as TDRs which subsequently defaulted | loan	0
Home equity loans and lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 646,000	633,000	586,000
Loans classified as TDRs		$ 10,000
Number of loans modified | loan		1
Residential first mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	2,090,000	$ 1,846,000	$ 1,429,000
Loans classified as TDRs	$ 323,000
Number of loans modified | loan	1	1
Loans receivable in payment default		$ 10,000
Northern Hancock
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 0	0
United American Savings Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 0	$ 0