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Goodwill and Other Intangible Assets - Schedule of Acquisition Date Fair Values, Accumulated Amortization and Net Carrying Values for Other Intangibles Assets and Goodwill, By Company (Details) - USD ($) $ in Millions | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023 | Dec. 31, 2022
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Goodwill | $ 439.2 | $ 168.8 | $ 168.8 | $ 176.5
Other intangible assets, finite-lived, accumulated amortization | 22.7 | 17.0 |  |
Total other intangible assets | 320.1 | 218.8 |  |
Total other intangible assets | 297.4 | 201.8 | 207.4 | 215.9
Total goodwill and other intangible assets | 759.3 | 387.6 |  |
Goodwill and other intangible assets | 736.6 | 370.6 | $ 376.2 | $ 392.4
Other |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Other intangible assets, finite-lived, accumulated amortization | 22.7 | 17.0 |  |
Total other intangible assets | 297.4 | 201.8 |  |
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
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 |  |
Goodwill and other intangible assets | $ 8.2 | 8.3 |  |
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 | 1.6 | 1.5 |  |
Other intangible assets, finite-lived, net carrying value | 0.6 | 0.7 |  |
Operating Segments | P&C Insurance Distribution (Bamboo) |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Goodwill | 270.4 | 0.0 |  |
Other intangible assets, finite-lived, acquisition date fair value | 101.0 | 0.0 |  |
Other intangible assets, finite-lived, accumulated amortization | 4.2 | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | 96.8 | 0.0 |  |
Goodwill and other intangible assets | $ 367.2 | 0.0 |  |
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 | 0.0 |  |
Other intangible assets, finite-lived, accumulated amortization | 0.5 | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | $ 23.0 | 0.0 |  |
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 | 0.0 |  |
Other intangible assets, finite-lived, accumulated amortization | 2.9 | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | $ 69.5 | 0.0 |  |
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 | 0.0 |  |
Other intangible assets, finite-lived, accumulated amortization | 0.4 | 0.0 |  |
Other intangible assets, finite-lived, net carrying value | $ 4.3 | 0.0 |  |
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 | 0.0 |  |
Other intangible assets, finite-lived, accumulated amortization | 0.4 | 0.0 |  |
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 | 40.9 |  |
Other intangible assets, finite-lived, accumulated amortization | 16.9 | 15.5 |  |
Other intangible assets, finite-lived, net carrying value | 24.3 | 25.4 |  |
Goodwill and other intangible assets | $ 68.7 | 69.8 |  |
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 | 4.5 | 4.1 |  |
Other intangible assets, finite-lived, net carrying value | $ 8.8 | 9.2 |  |
Other Operations | Customer relationships |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Other intangible assets, weighted average useful life (in years) | 10 years 10 months 24 days |  |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 24.8 | 24.8 |  |
Other intangible assets, finite-lived, accumulated amortization | 11.1 | 10.3 |  |
Other intangible assets, finite-lived, net carrying value | $ 13.7 | 14.5 |  |
Other Operations | Other |  |  |  |
Acquired Finite-Lived Intangible Assets [Line Items] |  |  |  |
Other intangible assets, weighted average useful life (in years) | 11 years 1 month 6 days |  |  |
Other intangible assets, finite-lived, acquisition date fair value | $ 3.1 | 2.8 |  |
Other intangible assets, finite-lived, accumulated amortization | 1.3 | 1.1 |  |
Other intangible assets, finite-lived, net carrying value | $ 1.8 | $ 1.7 |  |

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