GOODWILL, INTANGIBLE ASSETS, NET, AND DIGITAL ASSETS HELD - Intangible assets by category (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Intangible Assets
Intangible assets, net	$ 1,297.2	$ 1,376.9	$ 1,561.5
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)	6 years	7 years
Technology | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	6 years	7 years
Trademarks and tradenames | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	4 years	5 years
Options
Intangible Assets
Accumulated amortization	$ (116.6)	$ (110.3)
Intangible assets, net	119.9	126.4	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	27.9	28.1
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	(560.8)	(520.3)
Intangible assets, net	824.6	879.6	935.3
North American Equities | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	412.1	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	55.5	55.5
North American Equities | Trademarks and tradenames
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	586.9	603.4
North American Equities | Digital assets held
Intangible Assets
Indefinite-lived intangible assets, gross	0.7	1.2
Europe and Asia Pacific
Intangible Assets
Accumulated amortization	(209.3)	(184.4)
Intangible assets, net	318.1	326.7	352.5
Europe and Asia Pacific | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	204.7	209.2
Europe and Asia Pacific | Market data customer relationships
Intangible Assets
Finite-lived intangible assets, gross	65.1	60.8
Europe and Asia Pacific | Technology
Intangible Assets
Finite-lived intangible assets, gross	35.7	33.6
Europe and Asia Pacific | Trademarks and tradenames
Intangible Assets
Finite-lived intangible assets, gross	2.5	2.3
Europe and Asia Pacific | Trading registrations and licenses
Intangible Assets
Indefinite-lived intangible assets, gross	219.4	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	(193.5)	(183.9)
Intangible assets, net	34.6	44.2	$ 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 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