v3.26.1
Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2026
Dec. 31, 2025
Mar. 31, 2025
Dec. 31, 2024
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Acquisition Date Fair Value $ 624.4 $ 623.2    
Impairments 9.1 9.1    
Net Carrying Value 615.3 614.1 $ 439.2 $ 439.2
Other intangible assets, finite-lived, accumulated amortization 48.6 38.9    
Total other intangible assets 396.4 406.1 275.9 281.1
Goodwill and intangible asset impairment 9.6 9.6    
Total goodwill and other intangible assets 1,069.9 1,068.7    
Total goodwill and other intangible assets 1,011.7 1,020.2 $ 715.1 $ 720.3
Other        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Other intangible assets, finite-lived, accumulated amortization 48.6 38.9    
Other intangible assets, impairments 0.5 0.5    
Total other intangible assets 445.5 445.5    
Total other intangible assets 396.4 406.1    
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]        
Acquisition Date Fair Value 116.8 116.8    
Impairments 0.0 0.0    
Net Carrying Value 116.8 116.8    
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Underwriting Capacity        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Acquisition Date Fair Value 175.7 175.7    
Other intangible assets, indefinite-lived 175.7 175.7    
Operating Segments | Asset Management (Kudu)        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Acquisition Date Fair Value 7.6 7.6    
Impairments 0.0 0.0    
Net Carrying Value 7.6 7.6    
Total goodwill and other intangible assets $ 7.6 $ 7.7    
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 7 years    
Other intangible assets, finite-lived, acquisition date fair value $ 2.2 $ 2.2    
Other intangible assets, finite-lived, accumulated amortization 2.2 2.1    
Other intangible assets, impairments 0.0 0.0    
Other intangible assets, finite-lived, net carrying value 0.0 0.1    
Operating Segments | Specialty Insurance Distribution (Distinguished)        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Acquisition Date Fair Value 397.9 396.7    
Impairments 0.0 0.0    
Net Carrying Value 397.9 396.7    
Other intangible assets, finite-lived, acquisition date fair value 188.8 188.8    
Other intangible assets, finite-lived, accumulated amortization 15.2 7.8    
Other intangible assets, impairments 0.0 0.0    
Other intangible assets, finite-lived, net carrying value 173.6 181.0    
Total goodwill and other intangible assets $ 571.5 $ 577.7    
Operating Segments | Specialty Insurance Distribution (Distinguished) | Trade names        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Other intangible assets, weighted average useful life (in years) 10 years 10 years    
Other intangible assets, finite-lived, acquisition date fair value $ 24.3 $ 24.3    
Other intangible assets, finite-lived, accumulated amortization 1.4 0.8    
Other intangible assets, impairments 0.0 0.0    
Other intangible assets, finite-lived, net carrying value $ 22.9 $ 23.5    
Operating Segments | Specialty Insurance Distribution (Distinguished) | Agency relationships        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Other intangible assets, weighted average useful life (in years) 8 years 8 years    
Other intangible assets, finite-lived, acquisition date fair value $ 127.9 $ 127.9    
Other intangible assets, finite-lived, accumulated amortization 9.5 5.0    
Other intangible assets, impairments 0.0 0.0    
Other intangible assets, finite-lived, net carrying value $ 118.4 $ 122.9    
Operating Segments | Specialty Insurance Distribution (Distinguished) | Developed technology        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Other intangible assets, weighted average useful life (in years) 5 years 5 years    
Other intangible assets, finite-lived, acquisition date fair value $ 36.6 $ 36.6    
Other intangible assets, finite-lived, accumulated amortization 4.3 2.0    
Other intangible assets, impairments 0.0 0.0    
Other intangible assets, finite-lived, net carrying value 32.3 34.6    
Other Operations        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Acquisition Date Fair Value 102.1 102.1    
Impairments 9.1 9.1    
Net Carrying Value 93.0 93.0    
Other intangible assets, finite-lived, acquisition date fair value 78.8 78.8    
Other intangible assets, finite-lived, accumulated amortization 31.2 29.0    
Other intangible assets, impairments 0.5 0.5    
Other intangible assets, finite-lived, net carrying value 47.1 49.3    
Total goodwill and other intangible assets $ 140.1 $ 142.3    
Other Operations | Trade names        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Other intangible assets, weighted average useful life (in years) 11 years 8 months 12 days 11 years 8 months 12 days    
Other intangible assets, finite-lived, acquisition date fair value $ 22.4 $ 22.4    
Other intangible assets, finite-lived, accumulated amortization 8.1 7.7    
Other intangible assets, impairments 0.2 0.2    
Other intangible assets, finite-lived, net carrying value $ 14.1 $ 14.5    
Other Operations | Customer relationships        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Other intangible assets, weighted average useful life (in years) 8 years 9 months 18 days 8 years 9 months 18 days    
Other intangible assets, finite-lived, acquisition date fair value $ 53.3 $ 53.3    
Other intangible assets, finite-lived, accumulated amortization 21.1 19.4    
Other intangible assets, impairments 0.3 0.3    
Other intangible assets, finite-lived, net carrying value $ 31.9 $ 33.6    
Other Operations | Other        
Intangible Asset, Acquired, Finite-Lived [Line Items]        
Other intangible assets, weighted average useful life (in years) 11 years 8 months 12 days 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 2.0 1.9    
Other intangible assets, impairments 0.0 0.0    
Other intangible assets, finite-lived, net carrying value $ 1.1 $ 1.2