GOODWILL, INTANGIBLE ASSETS, NET, AND DIGITAL ASSETS HELD - Intangible Assets by Category (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Intangible Assets
Intangible assets, net	$ 1,274.6	$ 1,297.2
Customer relationships | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	13 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	$ (118.1)	$ (116.6)
Intangible assets, net	118.4	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	(569.8)	(560.8)
Intangible assets, net	814.0	824.6
North American Equities | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	411.3	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	55.3	55.5
North American Equities | Trademarks and tradenames
Intangible Assets
Finite-lived intangible assets, gross	8.1	8.2
North American Equities | Trading registrations and licenses
Intangible Assets
Indefinite-lived intangible assets, gross	586.7	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	(208.9)	(209.3)
Intangible assets, net	309.8	318.1
Europe and Asia Pacific | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	202.2	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	215.0	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	(195.7)	(193.5)
Intangible assets, net	32.4	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