Loans - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Accounts Notes And Loans Receivable [Line Items]
Total unpaid principal balance outstanding	$ 31,204,000	$ 31,778,000	$ 31,204,000	$ 31,778,000		$ 27,499,000
Protracted litigation and reserve	546,000		546,000
Allowance for loan losses	18,234,000	17,482,000	18,234,000	17,482,000	$ 17,172,000	17,712,000	$ 16,881,000	$ 17,687,000
Amounts charged off	921,000	513,000	$ 4,615,000	2,691,000
Troubled debt restructuring period past due considered payment default			30 days
Period of cumulative homogeneous loans past due included in company analysis			90 days
Troubled Debt Restructuring [Member]
Accounts Notes And Loans Receivable [Line Items]
Recorded investment in trouble debt restructuring	28,000,000		$ 28,000,000			26,300,000
Allowance for loan losses	2,300,000		2,300,000			2,300,000
Commitment to lend additional amounts	31,000		31,000			$ 42,000
Increment in allowance for loan losses	20,000	21,000	31,000	103,000
Amounts charged off	0	0	0	0
TDRs with Subsequent Default [Member]
Accounts Notes And Loans Receivable [Line Items]
Increment in allowance for loan losses	0	0	0	0
Amounts charged off	0	$ 0	0	$ 0
Secured [Member] | Nonresidential [Member]
Accounts Notes And Loans Receivable [Line Items]
Total unpaid principal balance outstanding	$ 6,900,000		$ 6,900,000