INVESTMENTS - SCHEDULE OF UNREALIZED LOSS ON INVESTMENTS (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Fair value
Less than 12 months	$ 7,636.9	$ 1,698.6
12 months or greater	1,897.5	122.2
Total	9,534.4	1,820.8
Unrealized losses
Less than 12 months	(835.2)	(15.1)
12 months or greater	(471.6)	(3.4)
Total	(1,306.8)	(18.5)
Corporate securities
Fair value
Less than 12 months	2,830.8	87.8
12 months or greater	370.4	9.2
Total	3,201.2	97.0
Unrealized losses
Less than 12 months	(329.4)	(0.4)
12 months or greater	(129.3)	(0.1)
Total	(458.7)	(0.5)
United States Treasury securities and obligations of United States government corporations and agencies
Fair value
Less than 12 months	134.4	5.7
12 months or greater	21.9	18.7
Total	156.3	24.4
Unrealized losses
Less than 12 months	(9.6)	0.0
12 months or greater	(3.4)	(0.9)
Total	(13.0)	(0.9)
States and political subdivisions
Fair value
Less than 12 months	667.0	47.3
12 months or greater	132.1	0.0
Total	799.1	47.3
Unrealized losses
Less than 12 months	(124.8)	(0.4)
12 months or greater	(58.5)	0.0
Total	(183.3)	(0.4)
Foreign governments
Fair value
Less than 12 months	35.0
12 months or greater	2.1
Total	37.1
Unrealized losses
Less than 12 months	(3.5)
12 months or greater	(0.3)
Total	(3.8)
Asset-backed securities
Fair value
Less than 12 months	914.0	210.8
12 months or greater	258.1	17.8
Total	1,172.1	228.6
Unrealized losses
Less than 12 months	(90.1)	(2.4)
12 months or greater	(53.4)	(0.7)
Total	(143.5)	(3.1)
Agency residential mortgage-backed securities
Fair value
Less than 12 months	59.7
12 months or greater	0.0
Total	59.7
Unrealized losses
Less than 12 months	(0.7)
12 months or greater	0.0
Total	(0.7)
Non-agency residential mortgage-backed securities
Fair value
Less than 12 months	861.6	380.8
12 months or greater	335.4	2.3
Total	1,197.0	383.1
Unrealized losses
Less than 12 months	(89.7)	(3.1)
12 months or greater	(102.2)	0.0
Total	(191.9)	(3.1)
Collateralized loan obligations
Fair value
Less than 12 months	553.0	271.5
12 months or greater	184.2	32.8
Total	737.2	304.3
Unrealized losses
Less than 12 months	(27.4)	(1.2)
12 months or greater	(12.2)	(0.1)
Total	(39.6)	(1.3)
Commercial mortgage-backed securities
Fair value
Less than 12 months	1,581.4	694.7
12 months or greater	593.3	41.4
Total	2,174.7	736.1
Unrealized losses
Less than 12 months	(160.0)	(7.6)
12 months or greater	(112.3)	(1.6)
Total	$ (272.3)	$ (9.2)