Fair Value Measurement - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 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	8,300,000	$ 4,500,000	4,900,000
Specific allowance for collateral dependent loans	$ 1,000,000	678,000	548,000
Discount applied to appraisals for estimated selling costs percentage	10.00%
Mortgage servicing rights carried at fair value	$ 2,700,000	1,500,000	604,000
Mortgage servicing rights carried at fair value, valuation allowance net change during the period	474,000	219,000
Net impairment (recovery) reflected in other income	435,000	161,000
Other real estate owned carried at fair value, net	1,300,000
Other real estate owned carried at fair value		2,900,000	2,000,000
Valuation allowance related to other real estate owned	1,100,000	1,200,000	$ 1,200,000
Additions charged to expense	$ 1,000	80,000
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	$ 3,000,000	$ (35,000)
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