v3.26.1
Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2026
Dec. 31, 2025
Mar. 31, 2026
Jun. 30, 2025
Mar. 31, 2025
Dec. 31, 2024
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Acquisition Date Fair Value $ 805.0 $ 623.2        
Impairments 9.1 9.1        
Net Carrying Value 795.9 614.1 $ 615.3 $ 534.6 $ 439.2 $ 439.2
Other intangible assets, finite-lived, accumulated amortization 57.2 38.9        
Total other intangible assets 387.8 406.1 396.4 270.8 275.9 281.1
Goodwill and intangible asset impairment 9.6 9.6        
Total goodwill and other intangible assets 1,250.5 1,068.7        
Total goodwill and other intangible assets 1,183.7 1,020.2 $ 1,011.7 $ 805.4 $ 715.1 $ 720.3
Other            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Other intangible assets, finite-lived, accumulated amortization 57.2 38.9        
Other intangible assets, impairments 0.5 0.5        
Total other intangible assets 445.5 445.5        
Total other intangible assets 387.8 406.1        
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)            
Intangible Asset, Finite-Lived, Acquired [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, Finite-Lived, Acquired [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, Finite-Lived, Acquired [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, Finite-Lived, Acquired [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, Finite-Lived, Acquired [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, Finite-Lived, Acquired [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 21.6 7.8        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value 167.2 181.0        
Total goodwill and other intangible assets $ 565.1 $ 577.7        
Operating Segments | Specialty Insurance Distribution (Distinguished) | Trade names            
Intangible Asset, Finite-Lived, Acquired [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 2.0 0.8        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value $ 22.3 $ 23.5        
Operating Segments | Specialty Insurance Distribution (Distinguished) | Agency relationships            
Intangible Asset, Finite-Lived, Acquired [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 13.5 5.0        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value $ 114.4 $ 122.9        
Operating Segments | Specialty Insurance Distribution (Distinguished) | Developed technology            
Intangible Asset, Finite-Lived, Acquired [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 6.1 2.0        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value 30.5 34.6        
Operating Segments | WTM Partners            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Acquisition Date Fair Value 238.3 57.7        
Impairments 0.0 0.0        
Net Carrying Value 238.3 57.7        
Other intangible assets, finite-lived, acquisition date fair value 37.6 37.6        
Other intangible assets, finite-lived, accumulated amortization 6.3 3.7        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value 31.3 33.9        
Total goodwill and other intangible assets $ 269.6 $ 91.5        
Operating Segments | WTM Partners | Trade names            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Other intangible assets, weighted average useful life (in years) 10 years 10 years        
Other intangible assets, finite-lived, acquisition date fair value $ 9.1 $ 9.1        
Other intangible assets, finite-lived, accumulated amortization 1.1 0.7        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value $ 8.0 $ 8.4        
Operating Segments | WTM Partners | Customer relationships            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Other intangible assets, weighted average useful life (in years) 7 years 7 years        
Other intangible assets, finite-lived, acquisition date fair value $ 28.5 $ 28.5        
Other intangible assets, finite-lived, accumulated amortization 5.2 3.0        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value 23.3 25.5        
Other Operations            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Acquisition Date Fair Value 44.4 44.4        
Impairments 9.1 9.1        
Net Carrying Value 35.3 35.3        
Other intangible assets, finite-lived, acquisition date fair value 41.2 41.2        
Other intangible assets, finite-lived, accumulated amortization 27.1 25.3        
Other intangible assets, impairments 0.5 0.5        
Other intangible assets, finite-lived, net carrying value 13.6 15.4        
Total goodwill and other intangible assets $ 48.9 $ 50.8        
Other Operations | Trade names            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Other intangible assets, weighted average useful life (in years) 13 years 13 years        
Other intangible assets, finite-lived, acquisition date fair value $ 13.3 $ 13.3        
Other intangible assets, finite-lived, accumulated amortization 7.6 7.1        
Other intangible assets, impairments 0.2 0.2        
Other intangible assets, finite-lived, net carrying value $ 5.5 $ 6.0        
Other Operations | Customer relationships            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Other intangible assets, weighted average useful life (in years) 11 years 11 years        
Other intangible assets, finite-lived, acquisition date fair value $ 24.8 $ 24.8        
Other intangible assets, finite-lived, accumulated amortization 17.5 16.2        
Other intangible assets, impairments 0.3 0.3        
Other intangible assets, finite-lived, net carrying value $ 7.0 $ 8.3        
Other Operations | Other            
Intangible Asset, Finite-Lived, Acquired [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 2.0        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value $ 1.1 $ 1.1