Investments - Debt Securities Unrealized Loss Positions (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Fair value
12 Months or Less	$ 4,263.6	$ 739.6
Greater than 12 Months	434.8	316.8
Total	4,698.4	1,056.4
Gross unrealized losses
12 Months or Less	(131.2)	(5.1)
Greater than 12 Months	(33.1)	(8.1)
Total	(164.3)	(13.2)
Asset-backed securities
Fair value
12 Months or Less	368.2	184.5
Greater than 12 Months	39.5	20.6
Total	407.7	205.1
Gross unrealized losses
12 Months or Less	(8.1)	(1.5)
Greater than 12 Months	(5.4)	(1.8)
Total	(13.5)	(3.3)
Residential mortgage-backed securities
Fair value
12 Months or Less	848.2	84.5
Greater than 12 Months	172.8	190.5
Total	1,021.0	275.0
Gross unrealized losses
12 Months or Less	(29.3)	(0.5)
Greater than 12 Months	(15.3)	(4.2)
Total	(44.6)	(4.7)
Commercial mortgage-backed securities
Fair value
12 Months or Less	200.9	46.2
Greater than 12 Months	2.6	9.0
Total	203.5	55.2
Gross unrealized losses
12 Months or Less	(5.8)	(1.2)
Greater than 12 Months	(0.3)	(1.3)
Total	(6.1)	(2.5)
Corporate debt securities
Fair value
12 Months or Less	2,052.4	309.4
Greater than 12 Months	159.5	54.5
Total	2,211.9	363.9
Gross unrealized losses
12 Months or Less	(67.5)	(1.8)
Greater than 12 Months	(9.0)	(0.6)
Total	(76.5)	(2.4)
U.S. government and government agency
Fair value
12 Months or Less	782.9	107.9
Greater than 12 Months	60.4	42.2
Total	843.3	150.1
Gross unrealized losses
12 Months or Less	(20.2)	(0.1)
Greater than 12 Months	(3.1)	(0.2)
Total	(23.3)	(0.3)
Non-U.S. government and government agency
Fair value
12 Months or Less	11.0	7.1
Greater than 12 Months	0.0	0.0
Total	11.0	7.1
Gross unrealized losses
12 Months or Less	(0.3)	0.0
Greater than 12 Months	0.0	0.0
Total	$ (0.3)	$ 0.0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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