Investments - Debt Securities Unrealized Loss Positions (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Fair value
12 Months or Less	$ 2,601.8	$ 739.6
Greater than 12 Months	256.7	316.8
Total	2,858.5	1,056.4
Gross unrealized losses
12 Months or Less	(32.9)	(5.1)
Greater than 12 Months	(13.7)	(8.1)
Total	(46.6)	(13.2)
Asset-backed securities
Fair value
12 Months or Less	312.7	184.5
Greater than 12 Months	36.6	20.6
Total	349.3	205.1
Gross unrealized losses
12 Months or Less	(5.7)	(1.5)
Greater than 12 Months	(7.9)	(1.8)
Total	(13.6)	(3.3)
Residential mortgage-backed securities
Fair value
12 Months or Less	369.2	84.5
Greater than 12 Months	140.5	190.5
Total	509.7	275.0
Gross unrealized losses
12 Months or Less	(4.7)	(0.5)
Greater than 12 Months	(4.8)	(4.2)
Total	(9.5)	(4.7)
Commercial mortgage-backed securities
Fair value
12 Months or Less	129.9	46.2
Greater than 12 Months	10.1	9.0
Total	140.0	55.2
Gross unrealized losses
12 Months or Less	(1.3)	(1.2)
Greater than 12 Months	(0.3)	(1.3)
Total	(1.6)	(2.5)
Corporate debt securities
Fair value
12 Months or Less	1,148.7	309.4
Greater than 12 Months	48.4	54.5
Total	1,197.1	363.9
Gross unrealized losses
12 Months or Less	(13.3)	(1.8)
Greater than 12 Months	(0.4)	(0.6)
Total	(13.7)	(2.4)
U.S. government and government agency
Fair value
12 Months or Less	630.3	107.9
Greater than 12 Months	21.1	42.2
Total	651.4	150.1
Gross unrealized losses
12 Months or Less	(7.8)	(0.1)
Greater than 12 Months	(0.3)	(0.2)
Total	(8.1)	(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	(0.1)	0.0
Greater than 12 Months	0.0	0.0
Total	$ (0.1)	$ 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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