Loans - Allowance for Loan 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 [Roll Forward]
Allowance for loan losses, Beginning Balance	$ 20,756	$ 16,546	$ 19,693	$ 16,330
Charge-offs	(710)	(764)	(1,436)	(1,695)
Recoveries	171	353	329	484
Provision for loan losses	1,905	1,134	3,536	2,150
Allowance for loan losses, Ending Balance	22,122	17,269	22,122	17,269
Commercial, Industrial, and Agricultural
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	6,282	4,785	6,160	5,428
Charge-offs	0	(29)	(31)	(30)
Recoveries	94	119	162	131
Provision for loan losses	767	688	852	34
Allowance for loan losses, Ending Balance	7,143	5,563	7,143	5,563
Commercial mortgages
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	10,020	7,357	9,007	6,753
Charge-offs	0	0	0	0
Recoveries	0	192	0	194
Provision for loan losses	595	92	1,608	694
Allowance for loan losses, Ending Balance	10,615	7,641	10,615	7,641
Residential Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	2,052	2,022	2,033	1,653
Charge-offs	(77)	(130)	(77)	(198)
Recoveries	9	2	12	73
Provision for loan losses	(84)	(224)	(68)	142
Allowance for loan losses, Ending Balance	1,900	1,670	1,900	1,670
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	2,065	2,089	2,179	2,215
Charge-offs	(551)	(531)	(1,141)	(1,266)
Recoveries	35	12	84	14
Provision for loan losses	607	498	1,034	1,105
Allowance for loan losses, Ending Balance	2,156	2,068	2,156	2,068
Credit Cards
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	123	105	120	93
Charge-offs	(26)	(14)	(45)	(72)
Recoveries	17	4	24	15
Provision for loan losses	(13)	47	2	106
Allowance for loan losses, Ending Balance	101	142	101	142
Overdrafts
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses, Beginning Balance	214	188	194	188
Charge-offs	(56)	(60)	(142)	(129)
Recoveries	16	24	47	57
Provision for loan losses	33	33	108	69
Allowance for loan losses, Ending Balance	$ 207	$ 185	$ 207	$ 185

A valuation allowance for financing receivables that are expected to be uncollectible.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

Amount of recovery of financing receivables doubtful of collection that were previously charged off.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
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-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
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-Paragraph 11B
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-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
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-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

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