SEGMENT REPORTING - Schedule of Underwriting Results for Operating Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Gross written premiums	$ 2,915	$ 2,437	$ 5,411	$ 4,642
Net written premiums	2,396	1,995	4,459	3,791
Premiums earned	2,131	1,954	4,200	3,783
Incurred losses and loss adjustment expenses	1,325	1,304	2,719	2,530
Commission, brokerage, taxes and fees	433	409	869	793
Other underwriting expenses	136	120	275	238
Underwriting gain (loss)	237	121	336	222
Net investment income	242	176	432	333
Net gains (losses) on investments	(22)	(378)	0	(605)
Corporate expense	(4)	(6)	(10)	(12)
Interest, fee and bond issue cost amortization expense	(33)	(24)	(65)	(48)
Other income (expense)	(10)	0	(15)	(9)
INCOME (LOSS) BEFORE TAXES	411	(110)	679	(119)
Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	1,791	1,394	3,434	2,774
Net written premiums	1,580	1,245	2,953	2,430
Premiums earned	1,417	1,295	2,784	2,504
Incurred losses and loss adjustment expenses	864	875	1,794	1,696
Commission, brokerage, taxes and fees	363	332	724	647
Other underwriting expenses	36	32	75	63
Underwriting gain (loss)	153	55	191	98
Insurance
Segment Reporting Information [Line Items]
Gross written premiums	1,125	1,043	1,977	1,868
Net written premiums	816	750	1,506	1,360
Premiums earned	714	659	1,416	1,279
Incurred losses and loss adjustment expenses	460	429	925	834
Commission, brokerage, taxes and fees	70	77	145	146
Other underwriting expenses	100	88	200	175
Underwriting gain (loss)	$ 84	$ 66	$ 145	$ 124