Investments - Category and Duration of Gross Unrealized Losses on Fixed Maturity Securities and Equity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	$ 6,481.7	$ 1,133.9
Fixed maturity securities, less than 12 months, unrealized losses	(277.5)	(18.1)
Fixed maturity securities, 12 months or more, fair value	1,098.9	1,396.3
Fixed maturity securities, 12 months or more, unrealized losses	(252.7)	(212.5)
Fixed maturity securities, total, fair value	7,580.6	2,530.2
Fixed maturity securities, total, unrealized losses	(530.2)	(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	73.9	12.5
Fixed maturity securities, less than 12 months, unrealized losses	(2.9)	0.0
Fixed maturity securities, 12 months or more, fair value	4.7	9.8
Fixed maturity securities, 12 months or more, unrealized losses	(0.5)	(0.8)
Fixed maturity securities, total, fair value	78.6	22.3
Fixed maturity securities, total, unrealized losses	(3.4)	(0.8)
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	28.8	4.1
Fixed maturity securities, less than 12 months, unrealized losses	(1.4)	(0.2)
Fixed maturity securities, 12 months or more, fair value	27.8	53.0
Fixed maturity securities, 12 months or more, unrealized losses	(5.6)	(4.9)
Fixed maturity securities, total, fair value	56.6	57.1
Fixed maturity securities, total, unrealized losses	(7.0)	(5.1)
Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	230.4	93.8
Fixed maturity securities, less than 12 months, unrealized losses	(6.6)	(1.6)
Fixed maturity securities, 12 months or more, fair value	168.6	163.8
Fixed maturity securities, 12 months or more, unrealized losses	(11.5)	(9.7)
Fixed maturity securities, total, fair value	399.0	257.6
Fixed maturity securities, total, unrealized losses	(18.1)	(11.3)
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	321.8	364.0
Fixed maturity securities, less than 12 months, unrealized losses	(4.9)	(2.9)
Fixed maturity securities, 12 months or more, fair value	78.7	72.8
Fixed maturity securities, 12 months or more, unrealized losses	(7.3)	(6.2)
Fixed maturity securities, total, fair value	400.5	436.8
Fixed maturity securities, total, unrealized losses	(12.2)	(9.1)
Commercial mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	334.2	35.7
Fixed maturity securities, less than 12 months, unrealized losses	(11.3)	(0.8)
Fixed maturity securities, 12 months or more, fair value	111.8	131.5
Fixed maturity securities, 12 months or more, unrealized losses	(19.5)	(18.5)
Fixed maturity securities, total, fair value	446.0	167.2
Fixed maturity securities, total, unrealized losses	(30.8)	(19.3)
Residential mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	1,020.0	72.9
Fixed maturity securities, less than 12 months, unrealized losses	(44.5)	(1.1)
Fixed maturity securities, 12 months or more, fair value	135.6	182.1
Fixed maturity securities, 12 months or more, unrealized losses	(43.0)	(34.3)
Fixed maturity securities, total, fair value	1,155.6	255.0
Fixed maturity securities, total, unrealized losses	(87.5)	(35.4)
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	3,112.5	374.6
Fixed maturity securities, less than 12 months, unrealized losses	(149.5)	(8.7)
Fixed maturity securities, 12 months or more, fair value	424.2	562.8
Fixed maturity securities, 12 months or more, unrealized losses	(128.2)	(104.6)
Fixed maturity securities, total, fair value	3,536.7	937.4
Fixed maturity securities, total, unrealized losses	(277.7)	(113.3)
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturity securities, less than 12 months, fair value	1,360.1	176.3
Fixed maturity securities, less than 12 months, unrealized losses	(56.4)	(2.8)
Fixed maturity securities, 12 months or more, fair value	147.5	220.5
Fixed maturity securities, 12 months or more, unrealized losses	(37.1)	(33.5)
Fixed maturity securities, total, fair value	1,507.6	396.8
Fixed maturity securities, total, unrealized losses	$ (93.5)	$ (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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