Allowance for Loan Losses - Changes in Allowance for Loss Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financing Receivable Allowance For Credit Losses [Line Items]
Balance, beginning of period	$ 2,547	$ 2,677	$ 2,458	$ 2,707
Provision (recovery) charged to operations	38	(114)	116	(173)
Charge-offs	(71)	(42)	(88)	(100)
Recoveries	69	30	97	117
Net (charge-offs) / Recoveries	(2)	(12)	9	17
Balance at end of period	2,583	2,551	2,583	2,551
Commercial [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance, beginning of period	1,461	1,439	1,401	1,404
Provision (recovery) charged to operations	21	(107)	66	(135)
Charge-offs	(31)	(16)	(31)	(16)
Recoveries	9	7	24	70
Net (charge-offs) / Recoveries	(22)	(9)	(7)	54
Balance at end of period	1,460	1,323	1,460	1,323
Non-Commercial [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance, beginning of period	1,086	1,238	1,057	1,303
Provision (recovery) charged to operations	17	(7)	50	(38)
Charge-offs	(40)	(26)	(57)	(84)
Recoveries	60	23	73	47
Net (charge-offs) / Recoveries	20	(3)	16	(37)
Balance at end of period	$ 1,123	$ 1,228	$ 1,123	$ 1,228