GOODWILL, INTANGIBLE ASSETS, NET, AND DIGITAL ASSETS HELD - Intangible assets by category (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Intangible Assets
Intangible assets, net	$ 1,376.9	$ 1,561.5	$ 1,662.8
Customer relationships | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	14 years	15 years
Market data customer relationships | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	7 years	8 years
Technology | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	7 years	7 years
Trademarks and tradenames | Weighted Average Amortization Period (in years)
Intangible Assets
Weighted Average Amortization Period (in years)	5 years	6 years
Options
Intangible Assets
Accumulated amortization	$ (110.3)	$ (102.6)
Intangible assets, net	126.4	134.1	146.1
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 tradenames
Intangible Assets
Finite-lived intangible assets, gross	12.9	12.9
Options | Licensing Agreements
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	(520.3)	(471.7)
Intangible assets, net	879.6	935.3	992.8
North American Equities | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	409.7	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	55.5	56.9
North American Equities | Trademarks and tradenames
Intangible Assets
Finite-lived intangible assets, gross	8.1	8.2
North American Equities | Licensing Agreements
Intangible Assets
Indefinite-lived intangible assets, gross	603.4	606.0
North American Equities | Digital assets held
Intangible Assets
Indefinite-lived intangible assets, gross	1.2	0.0
Europe and Asia Pacific
Intangible Assets
Accumulated amortization	(184.4)	(171.4)
Intangible assets, net	326.7	352.5	359.9
Europe and Asia Pacific | Customer relationships
Intangible Assets
Finite-lived intangible assets, gross	209.2	216.1
Europe and Asia Pacific | Market data customer relationships
Intangible Assets
Finite-lived intangible assets, gross	60.8	61.6
Europe and Asia Pacific | Technology
Intangible Assets
Finite-lived intangible assets, gross	33.6	34.2
Europe and Asia Pacific | Trademarks and tradenames
Intangible Assets
Finite-lived intangible assets, gross	2.3	2.4
Europe and Asia Pacific | Licensing Agreements
Intangible Assets
Indefinite-lived intangible assets, gross	205.2	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	(183.9)	(171.9)
Intangible assets, net	44.2	56.2	72.8
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 | Licensing Agreements
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	$ 91.2
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 tradenames
Intangible Assets
Finite-lived intangible assets, gross	0.0	0.0
Digital | Licensing Agreements
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