INVESTMENTS - AFS Fixed Maturity Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022	Jun. 30, 2023	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	$ 77,193,585	$ 73,600,788
Allowance for Credit Losses	(200,199)	(128,332)	$ (229,298)	$ (106,746)	$ (102,293)	$ (88,133)
Gross unrealized gains	77,543	73,312
Gross unrealized losses	(12,340,480)	(11,606,239)
Fair value	64,730,449	61,939,529
Allowance for credit losses at acquisition	(14,800)	(29,900)
U.S. government and agencies
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	565,036	438,931
Allowance for Credit Losses	0	0
Gross unrealized gains	3	304
Gross unrealized losses	(90,670)	(72,494)
Fair value	474,369	366,741
U.S. state, municipal and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	5,351,849	5,638,252
Allowance for Credit Losses	0	0
Gross unrealized gains	1,034	6,582
Gross unrealized losses	(1,337,325)	(1,233,874)
Fair value	4,015,558	4,410,960
Corporate
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	46,008,153	44,253,062
Allowance for Credit Losses	(18,440)	(1,298)	$ (22,951)	$ (791)	$ (7,842)	$ (3,238)
Gross unrealized gains	42,389	49,509
Gross unrealized losses	(8,819,906)	(7,984,341)
Fair value	37,212,196	36,316,932
Residential mortgage-backed securities, or “RMBS”
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	8,524,378	7,307,526
Allowance for Credit Losses	(133,195)	(100,554)
Gross unrealized gains	14,054	12,052
Gross unrealized losses	(812,153)	(834,212)
Fair value	7,593,084	6,384,812
Commercial mortgage-backed securities, or “CMBS”
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	7,407,932	7,269,662
Allowance for Credit Losses	(16,527)	(14,490)
Gross unrealized gains	621	36
Gross unrealized losses	(818,220)	(834,735)
Fair value	6,573,806	6,420,473
Collateralized bond obligations, or “CBOs”
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	2,964,366	3,051,850
Allowance for Credit Losses	(1,047)	(426)
Gross unrealized gains	0	0
Gross unrealized losses	(191,728)	(217,773)
Fair value	2,771,591	2,833,651
CLOs
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	3,413,888	2,726,888
Allowance for Credit Losses	(20,295)	(6,165)
Gross unrealized gains	2,941	127
Gross unrealized losses	(85,387)	(200,160)
Fair value	3,311,147	2,520,690
Asset-backed securities, or “ABSs”
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	2,957,983	2,914,617
Allowance for Credit Losses	(10,695)	(5,399)
Gross unrealized gains	16,501	4,702
Gross unrealized losses	(185,091)	(228,650)
Fair value	$ 2,778,698	$ 2,685,270