GOODWILL, INTANGIBLE ASSETS, NET, AND DIGITAL ASSETS HELD - Intangible assets by category (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Intangible Assets
Intangible assets, net	$ 1,424.7	$ 1,561.5
Customer relationships | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	15 years	15 years
Market data customer relationships | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	8 years	8 years
Technology | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	6 years	7 years
Trademarks and trade names | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	5 years	6 years
Options
Intangible Assets
Accumulated amortization	$ (108.6)	$ (102.6)
Intangible assets, net	128.1	134.1
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	28.1	28.1
Options | Trademarks and trade names
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	(509.9)	(471.7)
Intangible assets, net	896.0	935.3
North American Equities | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	412.9	413.9
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	56.5	56.9
North American Equities | Trademarks and trade names
Intangible Assets
Finite-lived intangible assets, gross	8.2	8.2
North American Equities | Trading registrations and licenses
Intangible Assets
Indefinite-lived intangible assets, gross	605.3	606.0
North American Equities | Digital assets held
Intangible Assets
Indefinite-lived intangible assets, gross	1.0	0.0
Europe and Asia Pacific
Intangible Assets
Accumulated amortization	(191.7)	(171.4)
Intangible assets, net	353.5	352.5
Europe and Asia Pacific | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	224.4	216.1
Europe and Asia Pacific | Market data customer relationships
Intangible Assets
Finite-lived intangible assets, gross	64.6	61.6
Europe and Asia Pacific | Technology
Intangible Assets
Finite-lived intangible assets, gross	35.3	34.2
Europe and Asia Pacific | Trademarks and trade names
Intangible Assets
Finite-lived intangible assets, gross	2.5	2.4
Europe and Asia Pacific | Trading registrations and licenses
Intangible Assets
Indefinite-lived intangible assets, gross	218.4	209.6
Europe and Asia Pacific | Digital assets held
Intangible Assets
Indefinite-lived intangible assets, gross	0.0	0.0
Global FX
Intangible Assets
Accumulated amortization	(181.0)	(171.9)
Intangible assets, net	47.1	56.2
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 trade names
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
Digital
Intangible Assets
Accumulated amortization	0.0	(11.7)
Intangible assets, net	0.0	83.4
Digital | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	0.0	0.0
Digital | Market data customer relationships
Intangible Assets
Finite-lived intangible assets, gross	0.0	0.0
Digital | Technology
Intangible Assets
Finite-lived intangible assets, gross	0.0	70.0
Digital | Trademarks and trade names
Intangible Assets
Finite-lived intangible assets, gross	0.0	0.0
Digital | Trading registrations and licenses
Intangible Assets
Indefinite-lived intangible assets, gross	0.0	25.0
Digital | Digital assets held
Intangible Assets
Indefinite-lived intangible assets, gross	$ 0.0	$ 0.1