Qualified Affordable Housing Project Investments - Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investments In Affordable Housing Projects [Abstract]
Investment for qualified affordable housing projects included in other assets	$ 3,792,000		$ 3,792,000		$ 3,204,000
Unfunded commitments related to the investments in qualified affordable housing projects	3,664,000		3,664,000		$ 4,510,000
Recognized amortization expense	129,000	$ 82,000	258,000	$ 165,000
Recognized tax credits and other benefits from its investment in affordable housing tax credits	207,000	138,000	426,000	276,000
Impairment losses related to its investment in qualified affordable housing projects	$ 0	$ 0	$ 0	$ 0

Amount of write-down from impairment of an investment in a qualified affordable housing project.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=114775393&loc=SL49122993-202985

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Amount of affordable housing tax credits and other tax benefits recognized.

+ References

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Amount of investment in qualified affordable housing projects.

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Amount of amortization expense of investment in qualified affordable housing projects.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount of commitment or contingent commitments to qualified affordable housing project investments.

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-Name Accounting Standards Codification
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-SubTopic 740
-Section 50
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