Fair Value Measurement - Quantitative Information about Level 3 Fair Value Measurements (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Impaired Loans [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Estimate	$ 1,345	$ 1,040
Fair Value Measurements, Valuation Technique	Appraisal of collateral	Appraisal of collateral
Impaired Loans [Member] \| Minimum [Member] \| Appraisal of Collateral [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Assumptions, Appraisal adjustments	10.00%	0.00%
Fair Value Assumptions, Liquidation expense	0.00%	0.00%
Fair Value Assumptions, Holding period	0 months	0 months
Impaired Loans [Member] \| Maximum [Member] \| Appraisal of Collateral [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Assumptions, Appraisal adjustments	38.00%	30.00%
Fair Value Assumptions, Liquidation expense	10.00%	10.00%
Fair Value Assumptions, Holding period	30 months	30 months
Impaired Loans [Member] \| Weighted Average [Member] \| Appraisal of Collateral [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Assumptions, Appraisal adjustments	31.00%	16.00%
Fair Value Assumptions, Liquidation expense	3.00%	8.00%
Fair Value Assumptions, Holding period	20 months	20 months
Other Real Estate Owned [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Estimate	$ 11	$ 16
Fair Value Measurements, Valuation Technique	Appraisal of collateral	Appraisal of collateral
Other Real Estate Owned [Member] \| Minimum [Member] \| Appraisal of Collateral [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Assumptions, Appraisal adjustments	10.00%	10.00%
Fair Value Assumptions, Liquidation expense	0.00%	0.00%
Other Real Estate Owned [Member] \| Maximum [Member] \| Appraisal of Collateral [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Assumptions, Appraisal adjustments	30.00%	30.00%
Fair Value Assumptions, Liquidation expense	10.00%	10.00%
Other Real Estate Owned [Member] \| Weighted Average [Member] \| Appraisal of Collateral [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Assumptions, Appraisal adjustments	10.00%	10.00%
Fair Value Assumptions, Liquidation expense	10.00%	10.00%

Percentage of adjustment to the observed price for a similar asset, used as an input to measure fair value.

No definition available.

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- Definition

Fair value inputs holding period.

+ References

No definition available.

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- Definition

Percentage of adjustment to the observed price to dispose of asset, used as an input to measure fair value.

+ References

No definition available.

+ Details

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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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.

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- Definition

Description of the inputs and valuation technique(s) used to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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