Allowance for Loan Losses - Changes in Allowance for Loss Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable Allowance For Credit Losses [Line Items]
Balance, beginning of period	$ 2,179	$ 2,583	$ 2,374	$ 2,458
Provision for (recovery of) loan losses	(80)	(24)	(508)	92
Charge-offs	(56)	(214)	(126)	(302)
Recoveries	28	20	417	117
Net (charge-offs) / Recoveries	(28)	(194)	291	(185)
Reclassification of reserve for off balance sheet commitments			(86)
Balance at end of period	2,071	2,365	2,071	2,365
Commercial [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance, beginning of period	1,243	1,460	1,334	1,401
Provision for (recovery of) loan losses	(61)	(17)	(472)	49
Charge-offs	(5)	(156)	(10)	(187)
Recoveries	6	10	363	34
Net (charge-offs) / Recoveries	1	(146)	353	(153)
Reclassification of reserve for off balance sheet commitments			(32)
Balance at end of period	1,183	1,297	1,183	1,297
Non-Commercial [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance, beginning of period	936	1,123	1,040	1,057
Provision for (recovery of) loan losses	(19)	(7)	(36)	43
Charge-offs	(51)	(58)	(116)	(115)
Recoveries	22	10	54	83
Net (charge-offs) / Recoveries	(29)	(48)	(62)	(32)
Reclassification of reserve for off balance sheet commitments			(54)
Balance at end of period	$ 888	$ 1,068	$ 888	$ 1,068