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Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Goodwill [Line Items]      
Goodwill $ 168.8 $ 176.5 $ 142.3
Indefinite-lived intangible assets 175.7    
Other intangible assets, accumulated amortization 17.0 12.2  
Other intangible assets, finite-lived, net carrying value 26.1    
Total other intangible assets 218.8 228.1  
Total other intangible assets 201.8 215.9 198.2
Total goodwill and other intangible assets 387.6 404.6  
Goodwill and other intangible assets 370.6 392.4 $ 340.5
Other Operations      
Goodwill [Line Items]      
Goodwill 44.4 52.1  
Other intangible assets, finite-lived, acquisition date fair value 40.9 50.2  
Other intangible assets, accumulated amortization 15.5 11.0  
Other intangible assets, finite-lived, net carrying value 25.4 39.2  
Goodwill and other intangible assets 69.8 91.3  
Ark/WM Outrigger | Operating Segments      
Goodwill [Line Items]      
Goodwill and other intangible assets 292.5 292.5  
Ark/WM Outrigger | Operating Segments | Ark      
Goodwill [Line Items]      
Goodwill 116.8 116.8  
Ark/WM Outrigger | Operating Segments | Ark | 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 $ 8.3 8.6  
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 17.9  
Other intangible assets, accumulated amortization 4.1 3.0  
Other intangible assets, finite-lived, net carrying value $ 9.2 14.9  
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 1.5 1.2  
Other intangible assets, finite-lived, net carrying value $ 0.7 1.0  
Customer relationships | Other Operations      
Goodwill [Line Items]      
Acquired 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 29.5  
Other intangible assets, accumulated amortization 10.3 7.5  
Other intangible assets, finite-lived, net carrying value 14.5 22.0  
Total other intangible assets      
Goodwill [Line Items]      
Other intangible assets, accumulated amortization 17.0 12.2  
Total other intangible assets $ 201.8 215.9  
Total other intangible assets | Other Operations      
Goodwill [Line Items]      
Acquired other intangible assets, weighted average useful life (in years) 12 years 1 month 6 days    
Other intangible assets, finite-lived, acquisition date fair value $ 2.8 2.8  
Other intangible assets, accumulated amortization 1.1 0.5  
Other intangible assets, finite-lived, net carrying value $ 1.7 $ 2.3