GOODWILL, INTANGIBLE ASSETS, NET, AND DIGITAL ASSETS HELD - Intangible Assets by Category (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Intangible Assets
Intangible assets, net	$ 1,356.2	$ 1,376.9
Customer relationships | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	13 years	14 years
Market data customer relationships | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	7 years	7 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	5 years
Options
Intangible Assets
Accumulated amortization	$ (113.5)	$ (110.3)
Intangible assets, net	123.2	126.4
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	(542.3)	(520.3)
Intangible assets, net	862.7	879.6
North American Equities | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	412.4	409.7
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	56.3	55.5
North American Equities | Trademarks and trade names
Intangible Assets
Finite-lived intangible assets, gross	8.2	8.1
North American Equities | Trading registrations and licenses
Intangible Assets
Indefinite-lived intangible assets, gross	605.0	603.4
North American Equities | Digital assets held
Intangible Assets
Indefinite-lived intangible assets, gross	1.1	1.2
Europe and Asia Pacific
Intangible Assets
Accumulated amortization	(205.3)	(184.4)
Intangible assets, net	331.0	326.7
Europe and Asia Pacific | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	207.1	209.2
Europe and Asia Pacific | Market data customer relationships
Intangible Assets
Finite-lived intangible assets, gross	66.2	60.8
Europe and Asia Pacific | Technology
Intangible Assets
Finite-lived intangible assets, gross	36.1	33.6
Europe and Asia Pacific | Trademarks and trade names
Intangible Assets
Finite-lived intangible assets, gross	2.6	2.3
Europe and Asia Pacific | Trading registrations and licenses
Intangible Assets
Indefinite-lived intangible assets, gross	224.3	205.2
Europe and Asia Pacific | Digital assets held
Intangible Assets
Indefinite-lived intangible assets, gross	0.0	0.0
Global FX
Intangible Assets
Accumulated amortization	(188.8)	(183.9)
Intangible assets, net	39.3	44.2
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