Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 759,409	$ 639,822
Included in realized gains / losses	1,213	0
Included in other comprehensive income	5,786	41,759
Acquisitions	17,820	0
Sales	0	0
Amortization	3,426	3,469
Other	(41,811)	(15,465)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 745,843	$ 669,585
Percentage of total	4.00%	4.00%
Asset- backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 13,177	$ 12,982
Included in realized gains / losses	0	0
Included in other comprehensive income	(318)	1,043
Acquisitions	0	0
Sales	0	0
Amortization	0	0
Other	109	(343)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 12,968	$ 13,682
Percentage of total	0.00%	0.00%
Collateralized debt obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 74,104	$ 73,369
Included in realized gains / losses	0	0
Included in other comprehensive income	(5,220)	5,745
Acquisitions	0	0
Sales	0	0
Amortization	3,415	3,459
Other	(3,642)	(4,412)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 68,657	$ 78,161
Percentage of total	1.00%	1.00%
Corporates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 672,128	$ 553,471
Included in realized gains / losses	1,213	0
Included in other comprehensive income	11,324	34,971
Acquisitions	17,820	0
Sales	0	0
Amortization	11	10
Other	(38,278)	(10,710)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 664,218	$ 577,742
Percentage of total	3.00%	3.00%

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

Percent Of Available For Sale Securities At Fair Value By Valuation Level

+ 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

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-Publisher FASB
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-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
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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

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-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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-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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