Investments - Debt Securities Unrealized Loss Positions (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Fair value
Debt securities, Fair Value, 12 Months or Less	$ 923.5	$ 1,670.9
Debt securities, Fair Value, Greater than 12 Months	218.9	364.7
Debt securities, Fair Value	1,142.4	2,035.6
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(11.0)	(23.4)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(8.4)	(16.8)
Debt securities, Gross Unrealized Losses	(19.4)	(40.2)
Asset-backed securities
Fair value
Debt securities, Fair Value, 12 Months or Less	241.0	180.4
Debt securities, Fair Value, Greater than 12 Months	6.5	23.8
Debt securities, Fair Value	247.5	204.2
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(2.1)	(1.8)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(1.4)	(2.4)
Debt securities, Gross Unrealized Losses	(3.5)	(4.2)
Residential mortgage-backed securities
Fair value
Debt securities, Fair Value, 12 Months or Less	283.3	437.4
Debt securities, Fair Value, Greater than 12 Months	125.6	223.9
Debt securities, Fair Value	408.9	661.3
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(4.4)	(7.9)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(5.9)	(12.4)
Debt securities, Gross Unrealized Losses	(10.3)	(20.3)
Commercial mortgage-backed securities
Fair value
Debt securities, Fair Value, 12 Months or Less	22.1	67.4
Debt securities, Fair Value, Greater than 12 Months	2.7	7.5
Debt securities, Fair Value	24.8	74.9
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(0.1)	(0.7)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(0.3)	(0.4)
Debt securities, Gross Unrealized Losses	(0.4)	(1.1)
Corporate debt securities
Fair value
Debt securities, Fair Value, 12 Months or Less	223.5	738.5
Debt securities, Fair Value, Greater than 12 Months	45.8	46.8
Debt securities, Fair Value	269.3	785.3
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(4.0)	(10.0)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(0.4)	(0.4)
Debt securities, Gross Unrealized Losses	(4.4)	(10.4)
U.S. government and government agency
Fair value
Debt securities, Fair Value, 12 Months or Less	146.5	247.2
Debt securities, Fair Value, Greater than 12 Months	38.3	62.7
Debt securities, Fair Value	184.8	309.9
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(0.4)	(3.0)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(0.4)	(1.2)
Debt securities, Gross Unrealized Losses	(0.8)	$ (4.2)
Non-U.S. government and government agency
Fair value
Debt securities, Fair Value, 12 Months or Less	7.1
Debt securities, Fair Value, Greater than 12 Months	0.0
Debt securities, Fair Value	7.1
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	0.0
Debt securities, Gross Unrealized Losses, Greater than 12 Months	0.0
Debt securities, Gross Unrealized Losses	$ 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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