v3.25.2
Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($)
$ in Millions
 6 Months Ended
Jun. 30, 2025
Mar. 31, 2025
Dec. 31, 2024
Jun. 30, 2024
Mar. 31, 2024
Dec. 31, 2023
Intangible Asset, Acquired, Finite-Lived [Line Items]            
Goodwill $ 534.6 $ 439.2 $ 439.2 $ 439.2 $ 439.2 $ 168.8
Other intangible assets, finite-lived, accumulated amortization 49.3   39.0      
Total other intangible assets 320.1   320.1      
Total other intangible assets 270.8 275.9 281.1 291.8 297.4 201.8
Total goodwill and other intangible assets 854.7   759.3      
Total goodwill and other intangible assets 805.4 $ 715.1 720.3 $ 731.0 $ 736.6 $ 370.6
Other            
Intangible Asset, Acquired, Finite-Lived [Line Items]            
Other intangible assets, finite-lived, accumulated amortization 49.3   39.0      
Total other intangible assets 270.8   281.1      
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)            
Intangible Asset, Acquired, Finite-Lived [Line Items]            
Total goodwill and other intangible assets 292.5   292.5      
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark            
Intangible Asset, Acquired, Finite-Lived [Line Items]            
Goodwill 116.8   116.8      
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Underwriting Capacity            
Intangible Asset, Acquired, Finite-Lived [Line Items]            
Other intangible assets, indefinite-lived 175.7   175.7      
Operating Segments | Asset Management (Kudu)            
Intangible Asset, Acquired, Finite-Lived [Line Items]            
Goodwill 7.6   7.6      
Total goodwill and other intangible assets $ 7.8   8.0      
Operating Segments | Asset Management (Kudu) | Trade names            
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 2.0   1.8      
Other intangible assets, finite-lived, net carrying value 0.2   0.4      
Operating Segments | P&C Insurance Distribution (Bamboo)            
Intangible Asset, Acquired, Finite-Lived [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 24.4   16.4      
Other intangible assets, finite-lived, net carrying value 76.6   84.6      
Total goodwill and other intangible assets $ 347.0   355.0      
Operating Segments | P&C Insurance Distribution (Bamboo) | Trade names            
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 $ 23.5   23.5      
Other intangible assets, finite-lived, accumulated amortization 3.5   2.3      
Other intangible assets, finite-lived, net carrying value $ 20.0   21.2      
Operating Segments | P&C Insurance Distribution (Bamboo) | Agency relationships            
Intangible Asset, Acquired, Finite-Lived [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 18.1   12.1      
Other intangible assets, finite-lived, net carrying value $ 54.3   60.3      
Operating Segments | P&C Insurance Distribution (Bamboo) | Developed technology            
Intangible Asset, Acquired, Finite-Lived [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 2.4   1.6      
Other intangible assets, finite-lived, net carrying value $ 2.3   3.1      
Operating Segments | P&C Insurance Distribution (Bamboo) | Other            
Intangible Asset, Acquired, Finite-Lived [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            
Intangible Asset, Acquired, Finite-Lived [Line Items]            
Goodwill 139.8   44.4      
Other intangible assets, finite-lived, acquisition date fair value 41.2   41.2      
Other intangible assets, finite-lived, accumulated amortization 22.9   20.8      
Other intangible assets, finite-lived, net carrying value 18.3   20.4      
Total goodwill and other intangible assets $ 158.1   64.8      
Other Operations | Trade names            
Intangible Asset, Acquired, Finite-Lived [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 6.3   5.6      
Other intangible assets, finite-lived, net carrying value $ 7.0   7.7      
Other Operations | Customer relationships            
Intangible Asset, Acquired, Finite-Lived [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 14.9   13.6      
Other intangible assets, finite-lived, net carrying value $ 9.9   11.2      
Other Operations | Other            
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 1.7   1.6      
Other intangible assets, finite-lived, net carrying value $ 1.4   $ 1.5