Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 759,409	$ 639,822	$ 668,440
Included in realized gains / losses	1,579	396	698
Included in other comprehensive income	14,386	32,600	(21,108)
Acquisitions	67,820	0	27,453
Sales	0	0	0
Amortization	4,563	4,609	4,753
Other	(48,784)	(25,042)	(44,947)
Transfers into Level 3	0	107,024	4,533
Transfers out of Level 3	0	0	0
Ending Balance	798,973	759,409	639,822
Asset- backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	13,177	12,982	14,049
Included in realized gains / losses	0	0	0
Included in other comprehensive income	(173)	708	(591)
Acquisitions	0	0	0
Sales	0	0	0
Amortization	0	0	0
Other	(134)	(513)	(476)
Transfers into Level 3	0	0	0
Transfers out of Level 3	0	0	0
Ending Balance	12,870	13,177	12,982
Collateralized debt obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	74,104	73,369	71,581
Included in realized gains / losses	0	0	0
Included in other comprehensive income	(2,523)	1,514	3,170
Acquisitions	0	0	0
Sales	0	0	0
Amortization	4,551	4,596	4,737
Other	(4,534)	(5,375)	(6,119)
Transfers into Level 3	0	0	0
Transfers out of Level 3	0	0	0
Ending Balance	71,598	74,104	73,369
Corporates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	672,128	553,471	582,810
Included in realized gains / losses	1,579	396	698
Included in other comprehensive income	17,082	30,378	(23,687)
Acquisitions	67,820	0	27,453
Sales	0	0	0
Amortization	12	13	16
Other	(44,116)	(19,154)	(38,352)
Transfers into Level 3	0	107,024	4,533
Transfers out of Level 3	0	0	0
Ending Balance	$ 714,505	$ 672,128	$ 553,471

Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, amortization.

No definition available.

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- Definition

Other adjustments, including capitalized interest and foreign exchange.

+ References

No definition available.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
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-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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