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Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Acquisition Date Fair Value | $ 623.2 |  |
Goodwill, impairment loss | 9.1 |  |
Goodwill | 614.1 | $ 439.2 | $ 168.8
Other intangible assets, indefinite-lived | 175.7 |  |
Other intangible assets, finite-lived, accumulated amortization | 38.9 | 39.0 |
Impairments | 0.5 |  |
Other intangible assets, finite-lived, net carrying value | 230.4 |  |
Total other intangible assets | 445.5 | 320.1 |
Total other intangible assets | 406.1 | 281.1 | 201.8
Goodwill and intangible asset impairment | 9.6 |  |
Total goodwill and other intangible assets | 1,068.7 | 759.3 |
Total goodwill and other intangible assets | $ 1,020.2 | 720.3 | $ 370.6
Goodwill, Impairment Loss, Statement of Income or Comprehensive Income [Extensible Enumeration] | General and administrative expenses |  |
Other Operations |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Acquisition Date Fair Value | $ 102.1 |  |
Goodwill, impairment loss | 9.1 |  |
Goodwill | 93.0 | 44.4 |
Other intangible assets, finite-lived, acquisition date fair value | 78.8 | 41.2 |
Other intangible assets, finite-lived, accumulated amortization | 29.0 | 20.8 |
Impairments | 0.5 |  |
Other intangible assets, finite-lived, net carrying value | 49.3 | 20.4 |
Total goodwill and other intangible assets | 142.3 | 64.8 |
Ark | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Total goodwill and other intangible assets | 292.5 | 292.5 |
Ark | Operating Segments | Ark |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Acquisition Date Fair Value | 116.8 |  |
Goodwill, impairment loss | 0.0 |  |
Goodwill | 116.8 | 116.8 |
Ark | Operating Segments | Ark | Underwriting Capacity |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, indefinite-lived | 175.7 | 175.7 |
Kudu | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Acquisition Date Fair Value | 7.6 |  |
Goodwill, impairment loss | 0.0 |  |
Goodwill | 7.6 | 7.6 |
Total goodwill and other intangible assets | 7.7 | 8.0 |
Bamboo | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Acquisition Date Fair Value | 0.0 |  |
Goodwill, impairment loss | 0.0 |  |
Goodwill | 0.0 | 270.4 |
Other intangible assets, finite-lived, acquisition date fair value | 0.0 | 101.0 |
Other intangible assets, finite-lived, accumulated amortization | 0.0 | 16.4 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | 0.0 | 84.6 |
Distinguished Partners | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Acquisition Date Fair Value | 396.7 |  |
Goodwill, impairment loss | 0.0 |  |
Goodwill | 396.7 | 0.0 |
Other intangible assets, finite-lived, acquisition date fair value | 188.8 | 0.0 |
Other intangible assets, finite-lived, accumulated amortization | 7.8 | 0.0 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | 181.0 | 0.0 |
Total goodwill and other intangible assets | $ 577.7 | 0.0 |
Trade names | Other Operations |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, weighted average useful life (in years) | 11 years 8 months 12 days |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 22.4 | 13.3 |
Other intangible assets, finite-lived, accumulated amortization | 7.7 | 5.6 |
Impairments | 0.2 |  |
Other intangible assets, finite-lived, net carrying value | $ 14.5 | 7.7 |
Trade names | Kudu | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, weighted average useful life (in years) | 7 years |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 2.2 | 2.2 |
Other intangible assets, finite-lived, accumulated amortization | 2.1 | 1.8 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | 0.1 | 0.4 |
Trade names | Bamboo | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, finite-lived, acquisition date fair value | 0.0 | 23.5 |
Other intangible assets, finite-lived, accumulated amortization | 0.0 | 2.3 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | $ 0.0 | 21.2 |
Trade names | Distinguished Partners | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, weighted average useful life (in years) | 10 years |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 24.3 | 0.0 |
Other intangible assets, finite-lived, accumulated amortization | 0.8 | 0.0 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | 23.5 | 0.0 |
Agency relationships | Bamboo | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, finite-lived, acquisition date fair value | 0.0 | 72.4 |
Other intangible assets, finite-lived, accumulated amortization | 0.0 | 12.1 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | $ 0.0 | 60.3 |
Agency relationships | Distinguished Partners | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, weighted average useful life (in years) | 8 years |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 36.6 | 0.0 |
Other intangible assets, finite-lived, accumulated amortization | 2.0 | 0.0 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | 125.9 | 0.0 |
Developed technology | Bamboo | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, finite-lived, acquisition date fair value | 0.0 | 4.7 |
Other intangible assets, finite-lived, accumulated amortization | 0.0 | 1.6 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | $ 0.0 | 3.1 |
Developed technology | Distinguished Partners | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, weighted average useful life (in years) | 5 years |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 127.9 | 0.0 |
Other intangible assets, finite-lived, accumulated amortization | 5.0 | 0.0 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | $ 31.6 | 0.0 |
Customer relationships | Other Operations |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, weighted average useful life (in years) | 8 years 9 months 18 days |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 53.3 | 24.8 |
Other intangible assets, finite-lived, accumulated amortization | 19.4 | 13.6 |
Impairments | 0.3 |  |
Other intangible assets, finite-lived, net carrying value | 33.6 | 11.2 |
Total other intangible assets |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, finite-lived, accumulated amortization | 38.9 | 39.0 |
Impairments | 0.5 |  |
Total other intangible assets | $ 406.1 | 281.1 |
Total other intangible assets | Other Operations |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, weighted average useful life (in years) | 11 years 9 months 18 days |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 3.1 | 3.1 |
Other intangible assets, finite-lived, accumulated amortization | 1.9 | 1.6 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | 1.2 | 1.5 |
Total other intangible assets | Bamboo | Operating Segments |  |  |
Intangible Asset, Acquired, Finite-Lived [Line Items] |  |  |
Other intangible assets, finite-lived, acquisition date fair value | 0.0 | 0.4 |
Other intangible assets, finite-lived, accumulated amortization | 0.0 | 0.4 |
Impairments | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | $ 0.0 | $ 0.0 |

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