Investments - Changes in Allowance for Credit Losses by Major Security Type of Investments Classified as Held-to-Maturity (Detail) $ in Thousands	3 Months Ended
Mar. 31, 2023 USD ($)
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	$ 239
Provision for credit loss expense (benefit)	19
Balance	220
Amortized cost	480,362
AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	450,213
Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	30,149
States and Political Subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	40
Balance	40
Amortized cost	412,038
States and Political Subdivisions [Member] \| AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	409,073
States and Political Subdivisions [Member] \| Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	2,965
Corporate Bonds [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	196
Provision for credit loss expense (benefit)	18
Balance	178
Amortized cost	53,528
Corporate Bonds [Member] \| AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	26,353
Corporate Bonds [Member] \| Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	27,175
U.S. Agency-Based Mortgage-Backed Securities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	3,606
U.S. Agency-Based Mortgage-Backed Securities [Member] \| AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	3,606
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	11,125
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] \| AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	11,125
Asset-Backed Securities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Balance	3
Provision for credit loss expense (benefit)	(1)
Balance	2
Amortized cost	65
Asset-Backed Securities [Member] \| AAA/AA/A Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	56
Asset-Backed Securities [Member] \| Baa/BBB Ratings [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Amortized cost	$ 9

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 https://asc.fasb.org/extlink&oid=124255953&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 https://asc.fasb.org/extlink&oid=126970911&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 https://asc.fasb.org/extlink&oid=124255206&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 https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

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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).

+ References

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- Definition

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- Definition

Amount of investment in debt security measured at amortized cost (held-to-maturity) originated in current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919232-210447

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