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INVESTMENTS - SCHEDULE OF AMORTIZED COST, GROSS UNREALIZED GAINS AND LOSSES, ESTIMATED FAIR VALUE, AND ALLOWANCE FOR CREDIT LOSSES (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 25,471.1
Allowance for credit losses	(38.9)	$ (37.1)	$ (39.0)	$ (42.9)
Estimated fair value	23,283.0	22,840.5
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	13,942.0	13,785.3
Gross unrealized gains	80.1	60.4
Gross unrealized losses	(1,553.8)	(1,663.3)
Allowance for credit losses	(33.1)	(31.1)	$ (37.7)	$ (41.7)
Estimated fair value	12,435.2	12,151.3
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized cost	470.0	470.0
Gross unrealized gains	0.7	18.3
Gross unrealized losses	0.0	0.0
Allowance for credit losses	0.0	0.0
Estimated fair value	470.7	488.3
United States Treasury securities and obligations of United States government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized cost	216.3	214.8
Gross unrealized gains	0.0	0.0
Gross unrealized losses	(22.9)	(28.6)
Allowance for credit losses	0.0	0.0
Estimated fair value	193.4	186.2
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost	3,297.6	3,261.9
Gross unrealized gains	23.1	12.2
Gross unrealized losses	(383.4)	(436.4)
Allowance for credit losses	(3.1)	(3.4)
Estimated fair value	2,934.2	2,834.3
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	109.0	107.3
Gross unrealized gains	0.3	0.1
Gross unrealized losses	(14.4)	(15.3)
Allowance for credit losses	(1.0)	(0.9)
Estimated fair value	93.9	91.2
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,625.7	1,574.6
Gross unrealized gains	9.3	8.3
Gross unrealized losses	(58.4)	(66.5)
Allowance for credit losses	(0.1)	(0.1)
Estimated fair value	1,576.5	1,516.3
Agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	812.1	819.8
Gross unrealized gains	8.1	5.3
Gross unrealized losses	(1.6)	(5.5)
Allowance for credit losses	0.0	0.0
Estimated fair value	818.6	819.6
Non-agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,665.0	1,636.3
Gross unrealized gains	37.0	33.5
Gross unrealized losses	(111.8)	(130.8)
Allowance for credit losses	0.0	0.0
Estimated fair value	1,590.2	1,539.0
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,009.2	1,015.2
Gross unrealized gains	2.2	5.6
Gross unrealized losses	(6.5)	(4.0)
Allowance for credit losses	0.0	0.0
Estimated fair value	1,004.9	1,016.8
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,324.2	2,379.1
Gross unrealized gains	3.6	3.7
Gross unrealized losses	(160.8)	(183.7)
Allowance for credit losses	(1.6)	(1.6)
Estimated fair value	2,165.4	2,197.5
Total fixed maturities, available for sale
Debt Securities, Available-for-sale [Line Items]
Amortized cost	25,471.1	25,264.3
Gross unrealized gains	164.4	147.4
Gross unrealized losses	(2,313.6)	(2,534.1)
Allowance for credit losses	(38.9)	(37.1)
Estimated fair value	$ 23,283.0	$ 22,840.5

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