Loans - Schedule of Allowances for Loan Losses by Portfolio Segment (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]
Beginning balance	$ 5,633	$ 5,454	$ 5,604	$ 5,291
Provision for loan losses	324	845	648	685
Charge-offs	(45)	(34)	(348)	(47)
Recoveries	6	24	14	360
Net charge-offs	(39)	(10)	(334)	313
Ending balance	5,918	6,289	5,918	6,289
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,884	1,704	1,813	2,207
Provision for loan losses	(31)	675	226	(156)
Charge-offs	(9)	(32)	(203)	(40)
Recoveries	5	15	13	351
Net charge-offs	(4)	(17)	(190)	311
Ending balance	1,849	2,362	1,849	2,362
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,699	1,538	1,735	1,264
Provision for loan losses	(87)	180	(61)	454
Charge-offs	0		(62)
Recoveries	0		0
Net charge-offs	0		(62)
Ending balance	1,612	1,718	1,612	1,718
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,196	1,303	1,273	1,189
Provision for loan losses	39	(47)	(1)	67
Charge-offs	0		(37)
Recoveries	1	8	1	8
Net charge-offs	1	8	(36)	8
Ending balance	1,236	1,264	1,236	1,264
Construction & Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	244	184	237	178
Provision for loan losses	35	38	42	44
Charge-offs	0		0
Recoveries	0		0
Net charge-offs	0		0
Ending balance	279	222	279	222
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	188	165	175	141
Provision for loan losses	107	18	130	47
Charge-offs	(36)	(2)	(46)	(7)
Recoveries	0	1	0	1
Net charge-offs	(36)	(1)	(46)	(6)
Ending balance	259	182	259	182
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	422	560	371	312
Provision for loan losses	261	(19)	312	229
Ending balance	$ 683	$ 541	$ 683	$ 541

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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Amount of other addition (reduction) in allowance reserve for credit loss.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount of expense related to estimated loss from loan and lease transactions.

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