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Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Goodwill [Line Items] |  |  |
Goodwill | $ 439.2 | $ 168.8 | $ 176.5
Indefinite-lived intangible assets | 175.7 |  |
Other intangible assets, accumulated amortization | 39.0 | 17.0 |
Other intangible assets, finite-lived, net carrying value | 105.4 |  |
Total other intangible assets | 320.1 | 218.8 |
Total other intangible assets | 281.1 | 201.8 | 215.9
Total goodwill and other intangible assets | 759.3 | 387.6 |
Goodwill and other intangible assets | 720.3 | 370.6 | $ 392.4
Other Operations |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 44.4 | 44.4 |
Other intangible assets, finite-lived, acquisition date fair value | 41.2 | 40.9 |
Other intangible assets, accumulated amortization | 20.8 | 15.5 |
Other intangible assets, finite-lived, net carrying value | 20.4 | 25.4 |
Goodwill and other intangible assets | 64.8 | 69.8 |
Ark | Operating Segments |  |  |
Goodwill [Line Items] |  |  |
Goodwill and other intangible assets | 292.5 | 292.5 |
Ark | Operating Segments | Ark Insurance Holdings Limited - Segment |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 116.8 | 116.8 |
Ark | Operating Segments | Ark Insurance Holdings Limited - Segment | Underwriting Capacity |  |  |
Goodwill [Line Items] |  |  |
Indefinite-lived intangible assets | 175.7 | 175.7 |
Kudu | Operating Segments |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 7.6 | 7.6 |
Goodwill and other intangible assets | 8.0 | 8.3 |
Bamboo | Operating Segments |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 270.4 | 0.0 |
Other intangible assets, finite-lived, acquisition date fair value | 101.0 | 0.0 |
Other intangible assets, accumulated amortization | 16.4 | 0.0 |
Other intangible assets, finite-lived, net carrying value | 84.6 | 0.0 |
Goodwill and other intangible assets | $ 355.0 | 0.0 |
Trade names | Other Operations |  |  |
Goodwill [Line Items] |  |  |
Acquired other intangible assets, weighted average useful life (in years) | 13 years 3 months 18 days |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 13.3 | 13.3 |
Other intangible assets, accumulated amortization | 5.6 | 4.1 |
Other intangible assets, finite-lived, net carrying value | $ 7.7 | 9.2 |
Trade names | Kudu | Operating Segments |  |  |
Goodwill [Line Items] |  |  |
Acquired 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, accumulated amortization | 1.8 | 1.5 |
Other intangible assets, finite-lived, net carrying value | $ 0.4 | 0.7 |
Trade names | Bamboo | Operating Segments |  |  |
Goodwill [Line Items] |  |  |
Acquired other intangible assets, weighted average useful life (in years) | 10 years |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 23.5 | 0.0 |
Other intangible assets, accumulated amortization | 2.3 | 0.0 |
Other intangible assets, finite-lived, net carrying value | $ 21.2 | 0.0 |
Agency relationships | Bamboo | Operating Segments |  |  |
Goodwill [Line Items] |  |  |
Acquired other intangible assets, weighted average useful life (in years) | 6 years |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 72.4 | 0.0 |
Other intangible assets, accumulated amortization | 12.1 | 0.0 |
Other intangible assets, finite-lived, net carrying value | $ 60.3 | 0.0 |
Developed technology | Bamboo | Operating Segments |  |  |
Goodwill [Line Items] |  |  |
Acquired other intangible assets, weighted average useful life (in years) | 3 years |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 4.7 | 0.0 |
Other intangible assets, accumulated amortization | 1.6 | 0.0 |
Other intangible assets, finite-lived, net carrying value | $ 3.1 | 0.0 |
Customer relationships | Other Operations |  |  |
Goodwill [Line Items] |  |  |
Acquired other intangible assets, weighted average useful life (in years) | 11 years |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 24.8 | 24.8 |
Other intangible assets, accumulated amortization | 13.6 | 10.3 |
Other intangible assets, finite-lived, net carrying value | 11.2 | 14.5 |
Total other intangible assets |  |  |
Goodwill [Line Items] |  |  |
Other intangible assets, accumulated amortization | 39.0 | 17.0 |
Total other intangible assets | $ 281.1 | 201.8 |
Total other intangible assets | Other Operations |  |  |
Goodwill [Line Items] |  |  |
Acquired 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 | 2.8 |
Other intangible assets, accumulated amortization | 1.6 | 1.1 |
Other intangible assets, finite-lived, net carrying value | $ 1.5 | 1.7 |
Total other intangible assets | Bamboo | Operating Segments |  |  |
Goodwill [Line Items] |  |  |
Acquired other intangible assets, weighted average useful life (in years) | 3 months 18 days |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 0.4 | 0.0 |
Other intangible assets, accumulated amortization | 0.4 | 0.0 |
Other intangible assets, finite-lived, net carrying value | $ 0.0 | $ 0.0 |

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