v3.26.3
Goodwill and Other Intangible Assets - Summary of Goodwill and Intangible Assets (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Sep. 30, 2026
Dec. 31, 2025
Jun. 30, 2026
Sep. 30, 2025
Jun. 30, 2025
Dec. 31, 2024
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Acquisition Date Fair Value $ 729.8 $ 623.2        
Impairments 9.1 9.1        
Net Carrying Value 720.7 614.1 $ 795.9 $ 1,092.3 $ 534.6 $ 439.2
Other intangible assets, finite-lived, accumulated amortization 69.7 38.9        
Total other intangible assets 448.7 406.1 387.8 300.6 270.8 281.1
Goodwill and intangible asset impairment 9.6 9.6        
Total goodwill and other intangible assets 1,248.7 1,068.7        
Total goodwill and other intangible assets 1,169.4 1,020.2 $ 1,183.7 $ 1,392.9 $ 805.4 $ 720.3
Other            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Other intangible assets, finite-lived, accumulated amortization 69.7 38.9        
Other intangible assets, impairments 0.5 0.5        
Total other intangible assets 518.9 445.5        
Total other intangible assets 448.7 406.1        
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 28.0 7.8        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value 160.8 181.0        
Total goodwill and other intangible assets $ 558.7 $ 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 2.6 0.8        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value $ 21.7 $ 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 17.5 5.0        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value $ 110.4 $ 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 7.9 2.0        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value 28.7 34.6        
Operating Segments | WTM Partners            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Acquisition Date Fair Value 163.1 57.7        
Impairments 0.0 0.0        
Net Carrying Value 163.1 57.7        
Other intangible assets, finite-lived, acquisition date fair value 111.0 37.6        
Other intangible assets, finite-lived, accumulated amortization 11.4 3.8        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value 99.6 33.8        
Total goodwill and other intangible assets $ 262.7 $ 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 $ 19.0 $ 9.1        
Other intangible assets, finite-lived, accumulated amortization 1.8 0.7        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value $ 17.2 $ 8.4        
Operating Segments | WTM Partners | Customer relationships            
Intangible Asset, Finite-Lived, Acquired [Line Items]            
Other intangible assets, weighted average useful life (in years) 8 years 4 months 24 days 7 years        
Other intangible assets, finite-lived, acquisition date fair value $ 92.0 $ 28.5        
Other intangible assets, finite-lived, accumulated amortization 9.6 3.1        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value 82.4 25.4        
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 28.1 25.2        
Other intangible assets, impairments 0.5 0.5        
Other intangible assets, finite-lived, net carrying value 12.6 15.5        
Total goodwill and other intangible assets $ 47.9 $ 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 7.9 7.1        
Other intangible assets, impairments 0.2 0.2        
Other intangible assets, finite-lived, net carrying value $ 5.2 $ 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 18.1 16.2        
Other intangible assets, impairments 0.3 0.3        
Other intangible assets, finite-lived, net carrying value $ 6.4 $ 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 2.1 1.9        
Other intangible assets, impairments 0.0 0.0        
Other intangible assets, finite-lived, net carrying value $ 1.0 $ 1.2