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INVESTMENTS - Fixed Maturities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Available for sale
Amortized cost	$ 12,714,210	$ 12,634,254
Allowance for expected credit losses	(4,237)	(10,759)	$ (12,645)	$ (11,733)
Gross unrealized gains	51,727	91,762
Gross unrealized losses	(492,390)	(480,515)
Fair value	12,269,310	12,234,742
U.S. government and agency
Available for sale
Amortized cost	2,649,595	3,049,445
Allowance for expected credit losses	0	0
Gross unrealized gains	1,590	13,211
Gross unrealized losses	(57,468)	(55,128)
Fair value	2,593,717	3,007,528
Non-U.S. government
Available for sale
Amortized cost	757,162	729,761
Allowance for expected credit losses	(17)	(30)
Gross unrealized gains	4,851	13,089
Gross unrealized losses	(23,798)	(18,861)
Fair value	738,198	723,959
Corporate debt
Available for sale
Amortized cost	5,095,372	4,651,654
Allowance for expected credit losses	(3,973)	(10,438)
Gross unrealized gains	37,431	49,434
Gross unrealized losses	(212,892)	(216,478)
Fair value	4,915,938	4,474,172
Agency RMBS
Available for sale
Amortized cost	1,684,869	1,706,204
Allowance for expected credit losses	0	0
Gross unrealized gains	4,273	11,495
Gross unrealized losses	(103,522)	(83,038)
Fair value	1,585,620	1,634,661
CMBS
Available for sale
Amortized cost	870,433	897,553
Allowance for expected credit losses	0	0
Gross unrealized gains	545	551
Gross unrealized losses	(50,585)	(58,408)
Fair value	820,393	839,696
Non-agency RMBS
Available for sale
Amortized cost	152,322	165,910
Allowance for expected credit losses	(198)	(194)
Gross unrealized gains	518	713
Gross unrealized losses	(12,650)	(13,033)
Fair value	139,992	153,396
ABS
Available for sale
Amortized cost	1,336,681	1,265,187
Allowance for expected credit losses	(49)	(50)
Gross unrealized gains	2,148	2,855
Gross unrealized losses	(20,875)	(25,021)
Fair value	1,317,905	1,242,971
Municipals
Available for sale
Amortized cost	167,776	168,540
Allowance for expected credit losses	0	(47)
Gross unrealized gains	371	414
Gross unrealized losses	(10,600)	(10,548)
Fair value	$ 157,547	$ 158,359