Loans - Investment in Loans by Portfolio Segment and Based on Impairment (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	$ 21,610	$ 18,970	$ 21,202	$ 19,087
Provision (recovery) for loan losses	(138)	842	269	2,317
Charge-offs	(254)	(435)	(525)	(2,305)
Recoveries	187	283	459	561
Allowance, Ending balance	21,405	19,660	21,405	19,660
Allowance, Loans individually evaluated for impairment	1,661	1,616	1,661	1,616	$ 2,059
Allowance, Loans collectively evaluated for impairment	19,689		19,689		19,088
Allowance, Loans acquired with deteriorated credit quality					55
Period-end balances, Loans individually evaluated for impairment	23,778		23,778		26,250
Period-end balances, Loans collectively evaluated for impairment	2,090,063		2,090,063		1,988,697
Period-end balances, Loans acquired with deteriorated credit quality					1,194
Total loans	2,114,932		2,114,932		2,016,141
Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Loans acquired with deteriorated credit quality	55		55
Period-end balances, Loans acquired with deteriorated credit quality	1,091		1,091
Commercial Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	13,095	10,182	12,542	10,824
Provision (recovery) for loan losses	(263)	503	133	1,025
Charge-offs	(75)	(12)	(75)	(1,323)
Recoveries	51	148	208	295
Allowance, Ending balance	12,808	10,821	12,808	10,821
Allowance, Loans individually evaluated for impairment	514	5	514	5	516
Allowance, Loans collectively evaluated for impairment	12,239		12,239		11,971
Allowance, Loans acquired with deteriorated credit quality					55
Period-end balances, Loans individually evaluated for impairment	3,119		3,119		3,356
Period-end balances, Loans collectively evaluated for impairment	896,607		896,607		818,066
Period-end balances, Loans acquired with deteriorated credit quality					1,194
Total loans	900,817		900,817		822,616
Commercial Loans [Member] \| Loans Acquired with Deteriorated Credit Quality [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Loans acquired with deteriorated credit quality	55		55
Period-end balances, Loans acquired with deteriorated credit quality	1,091		1,091
Residential Mortgage Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	5,701	5,912	5,860	5,538
Provision (recovery) for loan losses	180	295	87	865
Charge-offs	(96)	(273)	(181)	(503)
Recoveries	82	34	101	68
Allowance, Ending balance	5,867	5,968	5,867	5,968
Allowance, Loans individually evaluated for impairment	834	1,171	834	1,171	1,145
Allowance, Loans collectively evaluated for impairment	5,033		5,033		4,715
Period-end balances, Loans individually evaluated for impairment	14,455		14,455		16,140
Period-end balances, Loans collectively evaluated for impairment	914,751		914,751		903,891
Total loans	929,206		929,206		920,031
Consumer Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance, Beginning balance	2,814	2,876	2,800	2,725
Provision (recovery) for loan losses	(55)	44	49	427
Charge-offs	(83)	(150)	(269)	(479)
Recoveries	54	101	150	198
Allowance, Ending balance	2,730	2,871	2,730	2,871
Allowance, Loans individually evaluated for impairment	313	$ 440	313	$ 440	398
Allowance, Loans collectively evaluated for impairment	2,417		2,417		2,402
Period-end balances, Loans individually evaluated for impairment	6,204		6,204		6,754
Period-end balances, Loans collectively evaluated for impairment	278,705		278,705		266,740
Total loans	$ 284,909		$ 284,909		$ 273,494

Provision for (recovery) of loan losses.

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- Definition

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

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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

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

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The balance of financing receivables that were collectively evaluated for impairment.

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The balance of financing receivables that were individually evaluated for impairment.

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- Definition

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.

+ References

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

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