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Investments - Debt Securities Unrealized Loss Positions (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Fair value
Debt securities, Fair Value, 12 Months or Less	$ 911.8	$ 1,670.9
Debt securities, Fair Value, Greater than 12 Months	335.4	364.7
Debt securities, Fair Value	1,247.2	2,035.6
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(9.3)	(23.4)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(12.2)	(16.8)
Debt securities, Gross Unrealized Losses	(21.5)	(40.2)
Asset-backed securities
Fair value
Debt securities, Fair Value, 12 Months or Less	246.2	180.4
Debt securities, Fair Value, Greater than 12 Months	37.6	23.8
Debt securities, Fair Value	283.8	204.2
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(1.1)	(1.8)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(2.2)	(2.4)
Debt securities, Gross Unrealized Losses	(3.3)	(4.2)
Residential mortgage-backed securities
Fair value
Debt securities, Fair Value, 12 Months or Less	195.7	437.4
Debt securities, Fair Value, Greater than 12 Months	223.2	223.9
Debt securities, Fair Value	418.9	661.3
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(2.1)	(7.9)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(8.8)	(12.4)
Debt securities, Gross Unrealized Losses	(10.9)	(20.3)
Commercial mortgage-backed securities
Fair value
Debt securities, Fair Value, 12 Months or Less	45.1	67.4
Debt securities, Fair Value, Greater than 12 Months	6.0	7.5
Debt securities, Fair Value	51.1	74.9
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(0.2)	(0.7)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(0.4)	(0.4)
Debt securities, Gross Unrealized Losses	(0.6)	(1.1)
Corporate debt securities
Fair value
Debt securities, Fair Value, 12 Months or Less	312.4	738.5
Debt securities, Fair Value, Greater than 12 Months	25.1	46.8
Debt securities, Fair Value	337.5	785.3
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(5.2)	(10.0)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(0.2)	(0.4)
Debt securities, Gross Unrealized Losses	(5.4)	(10.4)
U.S. government and government agency
Fair value
Debt securities, Fair Value, 12 Months or Less	112.4	247.2
Debt securities, Fair Value, Greater than 12 Months	43.5	62.7
Debt securities, Fair Value	155.9	309.9
Gross unrealized losses
Debt securities, Gross Unrealized Losses, 12 Months or Less	(0.7)	(3.0)
Debt securities, Gross Unrealized Losses, Greater than 12 Months	(0.6)	(1.2)
Debt securities, Gross Unrealized Losses	$ (1.3)	$ (4.2)

X

- Definition

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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