INVESTMENTS - SUMMARY OF INVESTMENTS WITH UNREALIZED LOSSES BY INVESTMENT CATEGORY (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair value
Less than 12 months	$ 2,309.3	$ 1,262.8
12 months or greater	239.9	341.2
Total	2,549.2	1,604.0
Unrealized losses
Less than 12 months	(114.7)	(11.4)
12 months or greater	(10.1)	(9.7)
Total	(124.8)	(21.1)
Corporate securities
Fair value
Less than 12 months	495.4	305.5
12 months or greater	1.6	96.8
Total	497.0	402.3
Unrealized losses
Less than 12 months	(23.1)	(6.6)
12 months or greater	(0.3)	(5.7)
Total	(23.4)	(12.3)
United States Treasury securities and obligations of United States government corporations and agencies
Fair value
Less than 12 months		7.0
12 months or greater		3.5
Total		10.5
Unrealized losses
Less than 12 months		(0.1)
12 months or greater		0.0
Total		(0.1)
States and political subdivisions
Fair value
Less than 12 months	57.0	110.1
12 months or greater	0.0	0.0
Total	57.0	110.1
Unrealized losses
Less than 12 months	(4.9)	(1.5)
12 months or greater	0.0	0.0
Total	(4.9)	(1.5)
Foreign governments
Fair value
Less than 12 months		3.4
12 months or greater		0.0
Total		3.4
Unrealized losses
Less than 12 months		0.0
12 months or greater		0.0
Total		0.0
Asset-backed securities
Fair value
Less than 12 months	448.1	75.7
12 months or greater	41.9	45.5
Total	490.0	121.2
Unrealized losses
Less than 12 months	(20.9)	(0.4)
12 months or greater	(3.0)	(1.4)
Total	(23.9)	(1.8)
Agency residential mortgage-backed securities
Fair value
Less than 12 months		8.8
12 months or greater		0.0
Total		8.8
Unrealized losses
Less than 12 months		0.0
12 months or greater		0.0
Total		0.0
Non-agency residential mortgage-backed securities
Fair value
Less than 12 months	273.0	137.4
12 months or greater	39.3	67.2
Total	312.3	204.6
Unrealized losses
Less than 12 months	(9.6)	(0.7)
12 months or greater	(1.0)	(0.3)
Total	(10.6)	(1.0)
Collateralized loan obligations
Fair value
Less than 12 months	287.3	220.7
12 months or greater	155.2	115.4
Total	442.5	336.1
Unrealized losses
Less than 12 months	(9.4)	(1.1)
12 months or greater	(5.7)	(2.3)
Total	(15.1)	(3.4)
Commercial mortgage-backed securities
Fair value
Less than 12 months	748.5	394.2
12 months or greater	1.9	12.8
Total	750.4	407.0
Unrealized losses
Less than 12 months	(46.8)	(1.0)
12 months or greater	(0.1)	0.0
Total	$ (46.9)	$ (1.0)