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Investments - Category and Duration of Gross Unrealized Losses on Fixed Maturity Securities and Equity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	$ 2,800.3	$ 1,133.9
Fixed maturity securities, less than 12 months, unrealized losses	(52.3)	(18.1)
Fixed maturity securities, 12 months or more, fair value	1,260.1	1,396.3
Fixed maturity securities, 12 months or more, unrealized losses	(214.5)	(212.5)
Fixed maturity securities, total, fair value	4,060.4	2,530.2
Fixed maturity securities, total, unrealized losses	(266.8)	(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	19.2	12.5
Fixed maturity securities, less than 12 months, unrealized losses	(0.2)	0.0
Fixed maturity securities, 12 months or more, fair value	9.1	9.8
Fixed maturity securities, 12 months or more, unrealized losses	(0.9)	(0.8)
Fixed maturity securities, total, fair value	28.3	22.3
Fixed maturity securities, total, unrealized losses	(1.1)	(0.8)
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	13.8	4.1
Fixed maturity securities, less than 12 months, unrealized losses	(0.4)	(0.2)
Fixed maturity securities, 12 months or more, fair value	49.0	53.0
Fixed maturity securities, 12 months or more, unrealized losses	(4.8)	(4.9)
Fixed maturity securities, total, fair value	62.8	57.1
Fixed maturity securities, total, unrealized losses	(5.2)	(5.1)
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	179.8	93.8
Fixed maturity securities, less than 12 months, unrealized losses	(4.3)	(1.6)
Fixed maturity securities, 12 months or more, fair value	162.3	163.8
Fixed maturity securities, 12 months or more, unrealized losses	(11.0)	(9.7)
Fixed maturity securities, total, fair value	342.1	257.6
Fixed maturity securities, total, unrealized losses	(15.3)	(11.3)
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	371.0	364.0
Fixed maturity securities, less than 12 months, unrealized losses	(3.1)	(2.9)
Fixed maturity securities, 12 months or more, fair value	83.2	72.8
Fixed maturity securities, 12 months or more, unrealized losses	(6.4)	(6.2)
Fixed maturity securities, total, fair value	454.2	436.8
Fixed maturity securities, total, unrealized losses	(9.5)	(9.1)
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	101.8	35.7
Fixed maturity securities, less than 12 months, unrealized losses	(1.7)	(0.8)
Fixed maturity securities, 12 months or more, fair value	116.6	131.5
Fixed maturity securities, 12 months or more, unrealized losses	(17.0)	(18.5)
Fixed maturity securities, total, fair value	218.4	167.2
Fixed maturity securities, total, unrealized losses	(18.7)	(19.3)
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	357.3	72.9
Fixed maturity securities, less than 12 months, unrealized losses	(3.8)	(1.1)
Fixed maturity securities, 12 months or more, fair value	164.7	182.1
Fixed maturity securities, 12 months or more, unrealized losses	(33.7)	(34.3)
Fixed maturity securities, total, fair value	522.0	255.0
Fixed maturity securities, total, unrealized losses	(37.5)	(35.4)
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	1,200.5	374.6
Fixed maturity securities, less than 12 months, unrealized losses	(27.6)	(8.7)
Fixed maturity securities, 12 months or more, fair value	486.8	562.8
Fixed maturity securities, 12 months or more, unrealized losses	(106.0)	(104.6)
Fixed maturity securities, total, fair value	1,687.3	937.4
Fixed maturity securities, total, unrealized losses	(133.6)	(113.3)
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	556.9	176.3
Fixed maturity securities, less than 12 months, unrealized losses	(11.2)	(2.8)
Fixed maturity securities, 12 months or more, fair value	188.4	220.5
Fixed maturity securities, 12 months or more, unrealized losses	(34.7)	(33.5)
Fixed maturity securities, total, fair value	745.3	396.8
Fixed maturity securities, total, unrealized losses	$ (45.9)	$ (36.3)

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