INVESTMENTS - Restricted assets (Details) - USD ($) $ in Thousands		12 Months Ended
Jan. 01, 2019	Dec. 31, 2020	Dec. 31, 2019
Restricted Investments [Line Items]
Collateral in Trust for inter-company agreements		$ 1,153,157	$ 1,580,689
Collateral for secured letter of credit facility		434,845	473,187
Funds at Lloyd's		1,155,832	1,314,345
Collateral in Trust for third-party agreements		1,903,274	1,856,327
Securities on deposit or in trust with regulatory authorities		265,959	76,229
Total restricted investments		4,913,067	5,300,777
Syndicate 2007 \| Debt And Equity Securities
Restricted Investments [Line Items]
Collateral in Trust for third-party agreements		45,000	365,000
Syndicate 2007 \| Cash
Restricted Investments [Line Items]
Pledged cash for third party insurance and reinsurance agreements		225,000	16,000
Syndicate 1686 \| Debt And Equity Securities
Restricted Investments [Line Items]
Collateral in Trust for third-party agreements		297,000	169,000
Syndicate 1686 \| Cash
Restricted Investments [Line Items]
Pledged cash for third party insurance and reinsurance agreements		$ 178,000	$ 181,000
Axis Corporate Capital UK Limited
Restricted Investments [Line Items]
Capital support, percentage	70.00%	70.00%
Axis Corporate Capital UK Limited II
Restricted Investments [Line Items]
Capital support, percentage	30.00%	30.00%

Pledged Cash For Third Party Insurance And Reinsurance Agreements

No definition available.

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- Definition

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for inter-company insurance and reinsurance agreements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

+ References

No definition available.

+ Details

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- Definition

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for insurance and reinsurance agreements with third parties, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

+ References

No definition available.

+ Details

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- Definition

Pledged Financial Instruments, Underwriting Support

+ References

No definition available.

+ Details

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- Definition

Restricted Investments [Line Items]

+ References

No definition available.

+ Details

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- Definition

The amount of capital support provided to Lloyd's Syndicate 1686.

+ References

No definition available.

+ Details

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- Definition

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=75763970&loc=SL75763979-209797

+ Details

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- Definition

Amount of restricted 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 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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- Definition

The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for letter of credit arrangements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

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