Investments - Category and Duration of Gross Unrealized Losses on Fixed Maturity Securities and Equity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	$ 2,888.3	$ 1,133.9
Fixed maturity securities, less than 12 months, unrealized losses	(39.5)	(18.1)
Fixed maturity securities, 12 months or more, fair value	1,171.3	1,396.3
Fixed maturity securities, 12 months or more, unrealized losses	(202.5)	(212.5)
Fixed maturity securities, total, fair value	4,059.6	2,530.2
Fixed maturity securities, total, unrealized losses	(242.0)	(230.6)
U.S. government and government agencies and authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	36.1	12.5
Fixed maturity securities, less than 12 months, unrealized losses	(0.4)	0.0
Fixed maturity securities, 12 months or more, fair value	8.1	9.8
Fixed maturity securities, 12 months or more, unrealized losses	(0.9)	(0.8)
Fixed maturity securities, total, fair value	44.2	22.3
Fixed maturity securities, total, unrealized losses	(1.3)	(0.8)
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	16.0	4.1
Fixed maturity securities, less than 12 months, unrealized losses	(0.5)	(0.2)
Fixed maturity securities, 12 months or more, fair value	48.0	53.0
Fixed maturity securities, 12 months or more, unrealized losses	(4.8)	(4.9)
Fixed maturity securities, total, fair value	64.0	57.1
Fixed maturity securities, total, unrealized losses	(5.3)	(5.1)
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	193.2	93.8
Fixed maturity securities, less than 12 months, unrealized losses	(2.6)	(1.6)
Fixed maturity securities, 12 months or more, fair value	160.2	163.8
Fixed maturity securities, 12 months or more, unrealized losses	(10.4)	(9.7)
Fixed maturity securities, total, fair value	353.4	257.6
Fixed maturity securities, total, unrealized losses	(13.0)	(11.3)
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	370.0	364.0
Fixed maturity securities, less than 12 months, unrealized losses	(1.8)	(2.9)
Fixed maturity securities, 12 months or more, fair value	78.3	72.8
Fixed maturity securities, 12 months or more, unrealized losses	(6.8)	(6.2)
Fixed maturity securities, total, fair value	448.3	436.8
Fixed maturity securities, total, unrealized losses	(8.6)	(9.1)
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	133.8	35.7
Fixed maturity securities, less than 12 months, unrealized losses	(2.1)	(0.8)
Fixed maturity securities, 12 months or more, fair value	107.6	131.5
Fixed maturity securities, 12 months or more, unrealized losses	(16.5)	(18.5)
Fixed maturity securities, total, fair value	241.4	167.2
Fixed maturity securities, total, unrealized losses	(18.6)	(19.3)
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	495.8	72.9
Fixed maturity securities, less than 12 months, unrealized losses	(5.3)	(1.1)
Fixed maturity securities, 12 months or more, fair value	149.6	182.1
Fixed maturity securities, 12 months or more, unrealized losses	(33.4)	(34.3)
Fixed maturity securities, total, fair value	645.4	255.0
Fixed maturity securities, total, unrealized losses	(38.7)	(35.4)
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	1,131.0	374.6
Fixed maturity securities, less than 12 months, unrealized losses	(20.8)	(8.7)
Fixed maturity securities, 12 months or more, fair value	454.5	562.8
Fixed maturity securities, 12 months or more, unrealized losses	(98.8)	(104.6)
Fixed maturity securities, total, fair value	1,585.5	937.4
Fixed maturity securities, total, unrealized losses	(119.6)	(113.3)
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	512.4	176.3
Fixed maturity securities, less than 12 months, unrealized losses	(6.0)	(2.8)
Fixed maturity securities, 12 months or more, fair value	165.0	220.5
Fixed maturity securities, 12 months or more, unrealized losses	(30.9)	(33.5)
Fixed maturity securities, total, fair value	677.4	396.8
Fixed maturity securities, total, unrealized losses	$ (36.9)	$ (36.3)

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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