v3.25.1
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