Schedule I. Summary of Investments Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2020 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	$ 20,558,176
Amount at Which Shown in the Balance Sheet	20,558,176
U.S. treasuries
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	4,867,681
Fair Value	4,960,409
Amount at Which Shown in the Balance Sheet	4,960,409
Agencies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	365,387
Fair Value	368,032
Amount at Which Shown in the Balance Sheet	368,032
Non-U.S. government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	485,972
Fair Value	491,531
Amount at Which Shown in the Balance Sheet	491,531
Non-U.S. government-backed corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	333,996
Fair Value	338,014
Amount at Which Shown in the Balance Sheet	338,014
Corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	4,069,396
Fair Value	4,261,025
Amount at Which Shown in the Balance Sheet	4,261,025
Agency mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	1,095,525
Fair Value	1,113,792
Amount at Which Shown in the Balance Sheet	1,113,792
Non-agency mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	286,942
Fair Value	291,444
Amount at Which Shown in the Balance Sheet	291,444
Commercial mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	762,899
Fair Value	791,272
Amount at Which Shown in the Balance Sheet	791,272
Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	887,237
Fair Value	890,984
Amount at Which Shown in the Balance Sheet	890,984
Fixed maturity investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	13,155,035
Fair Value	13,506,503
Amount at Which Shown in the Balance Sheet	13,506,503
Short term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	4,993,735
Amount at Which Shown in the Balance Sheet	4,993,735
Equity investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	702,617
Amount at Which Shown in the Balance Sheet	702,617
Other investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	1,256,948
Amount at Which Shown in the Balance Sheet	1,256,948
Investments in other ventures, under equity method
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	98,373
Amount at Which Shown in the Balance Sheet	$ 98,373

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Amount of investment held by insurance company, excluding investment in related party.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column D))
-URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611379-123010

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Cost of investment held by insurance company, excluding investment in related party.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column B))
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column C))
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