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GOODWILL, INTANGIBLE ASSETS, NET, AND DIGITAL ASSETS HELD - Intangible Assets by Category (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Intangible Assets    
Accumulated amortization $ (106.1) $ (101.9)
Intangible assets, net $ 1,184.0 $ 1,297.2
Customer relationships | Weighted Average Amortization Period (in years)    
Intangible Assets    
Weighted Average Amortization Period (in years) 11 years 13 years
Market data customer relationships | Weighted Average Amortization Period (in years)    
Intangible Assets    
Weighted Average Amortization Period (in years) 6 years 6 years
Technology | Weighted Average Amortization Period (in years)    
Intangible Assets    
Weighted Average Amortization Period (in years) 6 years 6 years
Trademarks and tradenames | Weighted Average Amortization Period (in years)    
Intangible Assets    
Weighted Average Amortization Period (in years) 4 years 4 years
Options    
Intangible Assets    
Accumulated amortization $ (119.5) $ (116.6)
Intangible assets, net 117.0 119.9
Options | Customer relationships    
Intangible Assets    
Finite-lived intangible assets, gross 46.6 46.6
Options | Market data customer relationships    
Intangible Assets    
Finite-lived intangible assets, gross 53.6 53.6
Options | Technology    
Intangible Assets    
Finite-lived intangible assets, gross 27.9 27.9
Options | Trademarks and tradenames    
Intangible Assets    
Finite-lived intangible assets, gross 12.9 12.9
Options | Trading registrations and licenses    
Intangible Assets    
Indefinite-lived intangible assets, gross 95.5 95.5
Options | Digital assets held    
Intangible Assets    
Indefinite-lived intangible assets, gross 0.0 0.0
North American Equities    
Intangible Assets    
Accumulated amortization (542.7) (560.8)
Intangible assets, net 760.2 824.6
North American Equities | Customer relationships    
Intangible Assets    
Finite-lived intangible assets, gross 359.9 412.1
North American Equities | Market data customer relationships    
Intangible Assets    
Finite-lived intangible assets, gross 322.0 322.0
North American Equities | Technology    
Intangible Assets    
Finite-lived intangible assets, gross 40.3 55.5
North American Equities | Trademarks and tradenames    
Intangible Assets    
Finite-lived intangible assets, gross 7.6 8.2
North American Equities | Trading registrations and licenses    
Intangible Assets    
Indefinite-lived intangible assets, gross 572.7 586.9
North American Equities | Digital assets held    
Intangible Assets    
Indefinite-lived intangible assets, gross 0.4 0.7
Europe and Asia Pacific    
Intangible Assets    
Accumulated amortization (206.8) (209.3)
Intangible assets, net 276.4 318.1
Europe and Asia Pacific | Customer relationships    
Intangible Assets    
Finite-lived intangible assets, gross 170.3 204.7
Europe and Asia Pacific | Market data customer relationships    
Intangible Assets    
Finite-lived intangible assets, gross 63.9 65.1
Europe and Asia Pacific | Technology    
Intangible Assets    
Finite-lived intangible assets, gross 35.1 35.7
Europe and Asia Pacific | Trademarks and tradenames    
Intangible Assets    
Finite-lived intangible assets, gross 2.5 2.5
Europe and Asia Pacific | Trading registrations and licenses    
Intangible Assets    
Indefinite-lived intangible assets, gross 211.4 219.4
Europe and Asia Pacific | Digital assets held    
Intangible Assets    
Indefinite-lived intangible assets, gross 0.0 0.0
Global FX    
Intangible Assets    
Accumulated amortization (197.7) (193.5)
Intangible assets, net 30.4 34.6
Global FX | Customer relationships    
Intangible Assets    
Finite-lived intangible assets, gross 140.0 140.0
Global FX | Market data customer relationships    
Intangible Assets    
Finite-lived intangible assets, gross 64.4 64.4
Global FX | Technology    
Intangible Assets    
Finite-lived intangible assets, gross 22.5 22.5
Global FX | Trademarks and tradenames    
Intangible Assets    
Finite-lived intangible assets, gross 1.2 1.2
Global FX | Trading registrations and licenses    
Intangible Assets    
Indefinite-lived intangible assets, gross 0.0 0.0
Global FX | Digital assets held    
Intangible Assets    
Indefinite-lived intangible assets, gross $ 0.0 $ 0.0