Amortized Cost and Approximate Fair Values, Together with Gross Unrealized Gains and Losses of Securities (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Amortized Cost	$ 87,683	$ 45,249
Available-for-sale Securities, Gross Unrealized Gains	10	0
Available-for-sale Securities, Gross Unrealized Losses	(1,227)	(290)
Available-for-sale securities, Fair Value	86,466	44,959
U.S. government agencies
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Amortized Cost	45,250	45,249
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	(820)	(290)
Available-for-sale securities, Fair Value	44,430	$ 44,959
State and political subdivisions
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Amortized Cost	42,433
Available-for-sale Securities, Gross Unrealized Gains	10
Available-for-sale Securities, Gross Unrealized Losses	(407)
Available-for-sale securities, Fair Value	$ 42,036

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

No definition available.

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

No definition available.

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- Definition

Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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- Definition

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.

+ References

No definition available.

+ Details

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