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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
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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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