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Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($) $ in Millions | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2025 | Dec. 31, 2024 | Mar. 31, 2024 | Dec. 31, 2023
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Goodwill | $ 439.2 | $ 439.2 | $ 439.2 | $ 168.8
Other intangible assets, finite-lived, accumulated amortization | 44.2 | 39.0 |  |
Total other intangible assets | 320.1 | 320.1 |  |
Total other intangible assets | 275.9 | 281.1 | 297.4 | 201.8
Total goodwill and other intangible assets | 759.3 | 759.3 |  |
Total goodwill and other intangible assets | 715.1 | 720.3 | $ 736.6 | $ 370.6
Other |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Other intangible assets, finite-lived, accumulated amortization | 44.2 | 39.0 |  |
Total other intangible assets | 275.9 | 281.1 |  |
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Total goodwill and other intangible assets | 292.5 | 292.5 |  |
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Goodwill | 116.8 | 116.8 |  |
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Underwriting Capacity |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Other intangible assets, indefinite-lived | 175.7 | 175.7 |  |
Operating Segments | Asset Management (Kudu) |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Goodwill | 7.6 | 7.6 |  |
Total goodwill and other intangible assets | $ 7.9 | 8.0 |  |
Operating Segments | Asset Management (Kudu) | Trade names |  |  |  |
Acquired Finite-Lived Intangible Assets [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 | 1.9 | 1.8 |  |
Other intangible assets, finite-lived, net carrying value | 0.3 | 0.4 |  |
Operating Segments | P&C Insurance Distribution (Bamboo) |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Goodwill | 270.4 | 270.4 |  |
Other intangible assets, finite-lived, acquisition date fair value | 101.0 | 101.0 |  |
Other intangible assets, finite-lived, accumulated amortization | 20.4 | 16.4 |  |
Other intangible assets, finite-lived, net carrying value | 80.6 | 84.6 |  |
Total goodwill and other intangible assets | $ 351.0 | 355.0 |  |
Operating Segments | P&C Insurance Distribution (Bamboo) | Trade names |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Other intangible assets, weighted average useful life (in years) | 10 years |  |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 23.5 | 23.5 |  |
Other intangible assets, finite-lived, accumulated amortization | 2.9 | 2.3 |  |
Other intangible assets, finite-lived, net carrying value | $ 20.6 | 21.2 |  |
Operating Segments | P&C Insurance Distribution (Bamboo) | Agency relationships |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Other intangible assets, weighted average useful life (in years) | 6 years |  |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 72.4 | 72.4 |  |
Other intangible assets, finite-lived, accumulated amortization | 15.1 | 12.1 |  |
Other intangible assets, finite-lived, net carrying value | $ 57.3 | 60.3 |  |
Operating Segments | P&C Insurance Distribution (Bamboo) | Developed technology |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Other intangible assets, weighted average useful life (in years) | 3 years |  |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 4.7 | 4.7 |  |
Other intangible assets, finite-lived, accumulated amortization | 2.0 | 1.6 |  |
Other intangible assets, finite-lived, net carrying value | $ 2.7 | 3.1 |  |
Operating Segments | P&C Insurance Distribution (Bamboo) | Other |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Other intangible assets, weighted average useful life (in years) | 3 months 18 days |  |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 0.4 | 0.4 |  |
Other intangible assets, finite-lived, accumulated amortization | 0.4 | 0.4 |  |
Other intangible assets, finite-lived, net carrying value | 0.0 | 0.0 |  |
Other Operations |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Goodwill | 44.4 | 44.4 |  |
Other intangible assets, finite-lived, acquisition date fair value | 41.2 | 41.2 |  |
Other intangible assets, finite-lived, accumulated amortization | 21.9 | 20.8 |  |
Other intangible assets, finite-lived, net carrying value | 19.3 | 20.4 |  |
Total goodwill and other intangible assets | $ 63.7 | 64.8 |  |
Other Operations | Trade names |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
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, finite-lived, accumulated amortization | 6.0 | 5.6 |  |
Other intangible assets, finite-lived, net carrying value | $ 7.3 | 7.7 |  |
Other Operations | Customer relationships |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
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, finite-lived, accumulated amortization | 14.3 | 13.6 |  |
Other intangible assets, finite-lived, net carrying value | $ 10.5 | 11.2 |  |
Other Operations | Other |  |  |  |
Acquired Finite-Lived Intangible Assets [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 | 1.6 | 1.6 |  |
Other intangible assets, finite-lived, net carrying value | $ 1.5 | $ 1.5 |  |

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