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Investments - Schedule of Fixed Maturity and Short-Term Investments (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Dec. 31, 2022
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost $ 2,077,885 $ 1,633,814
Gross Unrealized Gains 1,877 1,812
Gross Unrealized Losses (99,323) (90,039)
Investments, at fair value 1,980,439 1,545,587
U.S. government treasuries    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 651,446 498,841
Gross Unrealized Gains 21 99
Gross Unrealized Losses (32,381) (27,089)
Investments, at fair value 619,086 471,851
U.S. states, territories and municipalities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 4,729 4,741
Gross Unrealized Gains 0 0
Gross Unrealized Losses (375) (434)
Investments, at fair value 4,354 4,307
Non-U.S. sovereign governments and supranationals    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 44,759 14,191
Gross Unrealized Gains 413 363
Gross Unrealized Losses (1,840) (1,602)
Investments, at fair value 43,332 12,952
Corporate    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 853,433 690,900
Gross Unrealized Gains 81 363
Gross Unrealized Losses (42,177) (43,786)
Investments, at fair value 811,337 647,477
Residential mortgage-backed securities - Agency    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 148,097 111,234
Gross Unrealized Gains 0 0
Gross Unrealized Losses (20,117) (14,824)
Investments, at fair value 127,980 96,410
Residential mortgage-backed securities - Non-agency    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 4,763 5,147
Gross Unrealized Gains 2 0
Gross Unrealized Losses (821) (772)
Investments, at fair value 3,944 4,375
Commercial mortgage-backed securities - Non-agency    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 10,188 10,283
Gross Unrealized Gains 0 0
Gross Unrealized Losses (1,295) (1,064)
Investments, at fair value 8,893 9,219
Other asset-backed securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 12,847 13,347
Gross Unrealized Gains 15 1
Gross Unrealized Losses (317) (463)
Investments, at fair value 12,545 12,885
Total fixed maturities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 1,730,262 1,348,684
Gross Unrealized Gains 532 826
Gross Unrealized Losses (99,323) (90,034)
Investments, at fair value 1,631,471 1,259,476
Short-term investments    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 347,623 285,130
Gross Unrealized Gains 1,345 986
Gross Unrealized Losses 0 (5)
Investments, at fair value $ 348,968 $ 286,111