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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	Jun. 30, 2026	Mar. 31, 2026	Dec. 31, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 26,580.2
Allowance for credit losses	(41.5)	$ (43.9)	$ (36.0)	$ (39.1)	$ (38.9)	$ (37.1)
Estimated fair value	24,495.7		23,886.8
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	14,999.6		14,568.8
Gross unrealized gains	38.0		137.1
Gross unrealized losses	(1,480.8)		(1,413.2)
Allowance for credit losses	(35.3)	$ (37.8)	(30.2)	$ (33.1)	$ (33.1)	$ (31.1)
Estimated fair value	13,521.5		13,262.5
United States Treasury securities and obligations of United States government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized cost	211.5		207.2
Gross unrealized gains	0.0		0.1
Gross unrealized losses	(32.6)		(30.2)
Allowance for credit losses	0.0		0.0
Estimated fair value	178.9		177.1
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost	3,269.2		3,311.4
Gross unrealized gains	19.7		25.1
Gross unrealized losses	(383.0)		(378.9)
Allowance for credit losses	(2.8)		(2.9)
Estimated fair value	2,903.1		2,954.7
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Amortized cost	133.5		130.9
Gross unrealized gains	0.6		0.9
Gross unrealized losses	(11.5)		(10.7)
Allowance for credit losses	(1.0)		(0.6)
Estimated fair value	121.6		120.5
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,989.8		1,780.8
Gross unrealized gains	6.0		14.0
Gross unrealized losses	(48.8)		(47.4)
Allowance for credit losses	(0.1)		(0.1)
Estimated fair value	1,946.9		1,747.3
Agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	711.5		838.3
Gross unrealized gains	6.7		11.4
Gross unrealized losses	(0.9)		(0.2)
Allowance for credit losses	0.0		0.0
Estimated fair value	717.3		849.5
Non-agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,474.5		1,639.1
Gross unrealized gains	30.0		37.5
Gross unrealized losses	(78.3)		(90.9)
Allowance for credit losses	0.0		0.0
Estimated fair value	1,426.2		1,585.7
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,597.7		1,142.4
Gross unrealized gains	2.7		2.8
Gross unrealized losses	(2.6)		(2.7)
Allowance for credit losses	0.0		0.0
Estimated fair value	1,597.8		1,142.5
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,192.9		2,157.5
Gross unrealized gains	2.9		6.4
Gross unrealized losses	(111.1)		(114.7)
Allowance for credit losses	(2.3)		(2.2)
Estimated fair value	2,082.4		2,047.0
Total fixed maturities, available for sale
Debt Securities, Available-for-sale [Line Items]
Amortized cost	26,580.2		25,776.4
Gross unrealized gains	106.6		235.3
Gross unrealized losses	(2,149.6)		(2,088.9)
Allowance for credit losses	(41.5)		(36.0)
Estimated fair value	$ 24,495.7		$ 23,886.8