Securities - Schedule of Allowance for Credit Losses on Held-to-maturity Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items]
Beginning balance	$ 326
Ending balance	290
ASU 2016-13 [Member]
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items]
Beginning balance	326
Impact of adopting ASC 326		$ 229
Current period provision	(36)	7
Ending balance	290	236
ASU 2016-13 [Member] \| Obligations of State and Political Subdivisions [Member]
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items]
Beginning balance	130
Impact of adopting ASC 326		96
Current period provision	(12)	2
Ending balance	118	98
ASU 2016-13 [Member] \| Corporate Debt Securities [Member]
Debt Securities Heldtomaturity Allowance For Credit Loss [Line Items]
Beginning balance	196
Impact of adopting ASC 326		133
Current period provision	(24)	5
Ending balance	$ 172	$ 138

Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

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Balance Type:	credit
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- Definition

Amount of expense (reversal of expense) for expected credit loss on investment in debt security measured at amortized cost (held-to-maturity), from change in methodology.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 11
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919244-210447

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- Definition

Amount of expense (reversal of expense) for expected credit loss on investment in debt security measured at amortized cost (held-to-maturity).

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+ References

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