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Commitments and Contingencies - Additional Information (Detail)		3 Months Ended
	Mar. 31, 2020 Officer	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Contingencies And Commitments [Line Items]
Commitments related to performance bonds		$ 21,200,000	$ 15,200,000
Commitments related to performance bonds expiration month and year		2024-08
Letter of credit secured by pledge of a portion of fixed income securities		$ 18,200,000
Commitments towards private equity funds		4,900,000
Investment Commitments [Member]
Contingencies And Commitments [Line Items]
Aggregate investment committed in private equity funds		5,000,000.0
Long-term Investments [Member]
Contingencies And Commitments [Line Items]
Letter of credit secured by pledge of a portion of fixed income securities		18,100,000
Minimum [Member]
Contingencies And Commitments [Line Items]
Collateral value required to be maintained		15,000,000.0
Maximum [Member] | Restricted Cash [Member]
Contingencies And Commitments [Line Items]
Letter of credit secured by pledge of a portion of fixed income securities		$ 100,000
Current Executive Officers [Member]
Contingencies And Commitments [Line Items]
Number of officers | Officer	2
Former Executive Officers [Member]
Contingencies And Commitments [Line Items]
Number of officers | Officer	1

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- Definition

Commitments related to performance bonds.

+ References

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- Definition

Commitments related to performance bonds expiration month and year.

+ References

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- Definition

Amount of contributions made that have been applied to private equity fund commitments.

+ References

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- Definition

Contingencies and commitments.

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- Definition

Letter of credit secured by pledge of a portion of fixed income securities.

+ References

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Number of officers.

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- Definition

Amount of assets pledged to secure a debt instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

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- Definition

Cost of the investment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841

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