Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Within one year, Amortized Cost	$ 27,160	$ 61,650
After one year through five years, Amortized Cost	144,142	175,743
After five years through ten years, Amortized Cost	47,175	47,058
After ten years, Amortized Cost	177,722	182,486
Available-for-sale, Amortized Cost	425,698	479,772
Within one year, Fair Value	27,194	61,435
After one year through five years, Fair Value	146,469	174,213
After five years through ten years, Fair Value	49,419	46,725
After ten years, Fair Value	188,597	183,842
Available-for-sale, Fair Value	441,146	478,730
U.S. Agency-Based Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Without single maturity date, Amortized Cost	29,499	12,835
Available-for-sale, Amortized Cost	29,499	12,835
Without single maturity date, Fair value	29,467	12,515
Available-for-sale, Fair Value	$ 29,467	$ 12,515

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27198-111563

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

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-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

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