INVESTMENTS - SCHEDULE OF FIXED MATURITIES AVAILABLE FOR SALE (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 23,384.2
Allowance for credit losses	(56.0)	$ (7.6)	$ (2.2)		$ (2.1)
Estimated fair value	20,353.4	24,805.4		$ 21,293.1
Below-investment Grade
Debt Securities, Available-for-sale [Line Items]
Estimated fair value	1,318.5
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Allowance for credit losses	(54.4)	(7.4)	(1.9)		(2.1)
Corporate securities | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	13,043.6	12,384.0
Gross unrealized gains	28.9	2,229.5
Gross unrealized losses	(1,858.4)	(20.2)
Allowance for credit losses	(37.0)	(4.3)
Estimated fair value	11,177.1	14,589.0
Corporate securities | Below-investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	605.5	811.4
Gross unrealized gains	1.0	55.0
Gross unrealized losses	(53.5)	(1.5)
Allowance for credit losses	(17.4)	(3.1)
Estimated fair value	535.6	861.8
United States Treasury securities and obligations of United States government corporations and agencies | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	171.7	166.2
Gross unrealized gains	0.0	54.3
Gross unrealized losses	(13.0)	(0.9)
Allowance for credit losses	0.0	0.0
Estimated fair value	158.7	219.6
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Allowance for credit losses	(0.9)	0.0	(0.3)		0.0
States and political subdivisions | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,836.3	2,637.4
Gross unrealized gains	19.3	356.7
Gross unrealized losses	(476.0)	(1.5)
Allowance for credit losses	(0.8)	0.0
Estimated fair value	2,378.8	2,992.6
States and political subdivisions | Below-investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	10.6	11.6
Gross unrealized gains	0.0	0.0
Gross unrealized losses	(0.8)	0.0
Allowance for credit losses	(0.1)	0.0
Estimated fair value	9.7	11.6
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Allowance for credit losses	(0.4)	(0.2)	0.0		0.0
Foreign governments | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	86.3	85.4
Gross unrealized gains	0.1	13.6
Gross unrealized losses	(11.3)	(0.3)
Allowance for credit losses	(0.4)	(0.2)
Estimated fair value	74.7	98.5
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Allowance for credit losses	(0.3)	0.0	0.0		0.0
Asset-backed securities | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,312.5	983.1
Gross unrealized gains	1.0	35.2
Gross unrealized losses	(130.1)	(1.9)
Allowance for credit losses	(0.3)	0.0
Estimated fair value	1,183.1	1,016.4
Asset-backed securities | Below-investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	123.2	145.9
Gross unrealized gains	0.0	1.8
Gross unrealized losses	(19.3)	(1.2)
Allowance for credit losses	0.0	0.0
Estimated fair value	103.9	146.5
Agency residential mortgage-backed securities | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	174.3	36.7
Gross unrealized gains	1.4	3.7
Gross unrealized losses	(0.7)	0.0
Allowance for credit losses	0.0	0.0
Estimated fair value	175.0	40.4
Non-agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Allowance for credit losses	0.0	0.0	$ 0.0		$ 0.0
Non-agency residential mortgage-backed securities | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,122.6	1,141.0
Gross unrealized gains	6.0	28.4
Gross unrealized losses	(174.3)	(2.9)
Allowance for credit losses	0.0	0.0
Estimated fair value	954.3	1,166.5
Non-agency residential mortgage-backed securities | Below-investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	577.8	729.4
Gross unrealized gains	34.0	128.1
Gross unrealized losses	(17.6)	(0.2)
Allowance for credit losses	0.0	0.0
Estimated fair value	594.2	857.3
Collateralized loan obligations | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	825.2	574.2
Gross unrealized gains	0.3	2.3
Gross unrealized losses	(39.6)	(1.2)
Allowance for credit losses	0.0	0.0
Estimated fair value	785.9	575.3
Collateralized loan obligations | Below-investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost		13.1
Gross unrealized gains		0.0
Gross unrealized losses		(0.1)
Allowance for credit losses		0.0
Estimated fair value		13.0
Commercial mortgage-backed securities | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,401.3	2,064.6
Gross unrealized gains	0.1	76.3
Gross unrealized losses	(254.1)	(8.7)
Allowance for credit losses	0.0	0.0
Estimated fair value	2,147.3	2,132.2
Commercial mortgage-backed securities | Below-investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	93.3	83.6
Gross unrealized gains	0.0	1.6
Gross unrealized losses	(18.2)	(0.5)
Allowance for credit losses	0.0	0.0
Estimated fair value	75.1	84.7
Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	23,384.2	21,867.6
Gross unrealized gains	92.1	2,986.5
Gross unrealized losses	(3,066.9)	(41.1)
Allowance for credit losses	(56.0)	(7.6)
Estimated fair value	20,353.4	24,805.4
Fixed Maturities | Investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	21,973.8	20,072.6
Gross unrealized gains	57.1	2,800.0
Gross unrealized losses	(2,957.5)	(37.6)
Allowance for credit losses	(38.5)	(4.5)
Estimated fair value	19,034.9	22,830.5
Fixed Maturities | Below-investment Grade
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,410.4	1,795.0
Gross unrealized gains	35.0	186.5
Gross unrealized losses	(109.4)	(3.5)
Allowance for credit losses	(17.5)	(3.1)
Estimated fair value	$ 1,318.5	$ 1,974.9