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Investments - Summary of Carrying Amounts and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]
Within one year, Carrying Amount	$ 63,982	$ 49,967
After one year through five years, Carrying Amount	182,355	198,025
After five years through ten years, Carrying Amount	94,035	110,460
After ten years, Carrying Amount	223,896	229,782
Held-to-maturity, Carrying Amount, Totals	572,508	599,421
Within one year, Fair Value	64,630	50,348
After one year through five years, Fair Value	190,897	202,109
After five years through ten years, Fair Value	99,644	113,877
After ten years, Fair Value	243,277	243,279
Held-to-maturity, Fair Value, Totals	607,339	621,343
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	8,070	10,967
Held-to-maturity, Carrying Amount, Totals	8,070
Without single maturity, Fair Value	8,714	11,511
Held-to-maturity, Fair Value, Totals	8,714	11,511
Asset-Backed Securities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Without single maturity, Carrying Amount	170	220
Held-to-maturity, Carrying Amount, Totals	170
Without single maturity, Fair Value	177	219
Held-to-maturity, Fair Value, Totals	$ 177	$ 219

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Held to maturity securities carrying amount.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), without single maturity date and not allocated over maturity grouping.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5B
-URI http://asc.fasb.org/extlink&oid=121645371&loc=SL120269825-111563

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Amortized cost of investment in debt security measured at amortized cost (held-to-maturity), without single maturity date and not allocated over maturity grouping.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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-Topic 320
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563

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-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
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-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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