Fair Value Measurement - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase in fair value			$ 250,000
Derivative instrument maximum maturity period			30 years
Transfers between level 1 and level 2			$ 0		$ 0
Transfers between level 2 and level 3			0		0
Fair value of the collateral dependent loans, net carrying amount	$ 7,700,000	$ 4,500,000	7,700,000	$ 4,500,000	4,900,000
Specific allowance for collateral dependent loans	989,000	609,000	$ 989,000	609,000	548,000
Discount applied to appraisals for estimated selling costs percentage			10.00%
Mortgage servicing rights carried at fair value	3,300,000	1,100,000	$ 3,300,000	1,100,000	604,000
Mortgage servicing rights carried at fair value, valuation allowance net change during the period			741,000	150,000
Net impairment (recovery) reflected in other income	(25,000)	138,000	702,000	92,000
Other real estate owned carried at fair value, net	1,000,000		1,000,000
Other real estate owned carried at fair value		2,200,000		2,200,000	2,000,000
Valuation allowance related to other real estate owned	1,000,000	1,200,000	1,000,000	1,200,000	$ 1,200,000
Additions charged to expense	1,000	39,000	$ (25,000)	162,000
Maximum maturity period of short term borrowings			90 days
Collateral Dependent Loans [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Increase (decrease) in provision for loan losses	134,000	$ (35,000)	$ 3,700,000	$ (104,000)
Construction [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value option, loans held as assets, aggregate amount in nonaccrual status	$ 0		$ 0