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INVESTMENTS - Fixed Maturities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Available for sale
Amortized cost	$ 12,127,540		$ 12,419,905
Allowance for expected credit losses	(5,043)	$ (4,183)	(3,938)	$ (4,631)	$ (4,237)	$ (10,759)
Gross unrealized gains	155,402		48,121
Gross unrealized losses	(140,424)		(311,335)
Fair value	12,137,475		12,152,753
U.S. government and agency
Available for sale
Amortized cost	2,378,539		2,830,111
Allowance for expected credit losses	0		0
Gross unrealized gains	17,688		6,011
Gross unrealized losses	(9,052)		(33,136)
Fair value	2,387,175		2,802,986
Non-U.S. government
Available for sale
Amortized cost	767,027		753,315
Allowance for expected credit losses	(16)		0
Gross unrealized gains	27,373		2,584
Gross unrealized losses	(3,908)		(25,960)
Fair value	790,476		729,939
Corporate debt
Available for sale
Amortized cost	4,788,604		4,941,510
Allowance for expected credit losses	(4,773)		(3,690)
Gross unrealized gains	83,530		30,594
Gross unrealized losses	(55,530)		(126,224)
Fair value	4,811,831		4,842,190
Agency RMBS
Available for sale
Amortized cost	1,767,469		1,245,681
Allowance for expected credit losses	0		0
Gross unrealized gains	10,280		1,154
Gross unrealized losses	(36,769)		(61,990)
Fair value	1,740,980		1,184,845
CMBS
Available for sale
Amortized cost	840,909		852,534
Allowance for expected credit losses	0		0
Gross unrealized gains	4,906		1,244
Gross unrealized losses	(22,233)		(34,170)
Fair value	823,582		819,608
Non-agency RMBS
Available for sale
Amortized cost	190,656		132,116
Allowance for expected credit losses	(199)		(195)
Gross unrealized gains	990		597
Gross unrealized losses	(5,296)		(9,982)
Fair value	186,151		122,536
ABS
Available for sale
Amortized cost	1,329,187		1,547,350
Allowance for expected credit losses	(55)		(53)
Gross unrealized gains	10,276		5,812
Gross unrealized losses	(5,729)		(13,277)
Fair value	1,333,679		1,539,832
Municipals
Available for sale
Amortized cost	65,149		117,288
Allowance for expected credit losses	0		0
Gross unrealized gains	359		125
Gross unrealized losses	(1,907)		(6,596)
Fair value	$ 63,601		$ 110,817