SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) segment	Sep. 30, 2022 USD ($)
Segment Reporting Information
Number of reportable segments \| segment			6
Revenues	$ 908.8	$ 993.5	$ 2,804.8	$ 2,953.8
Operating income (loss)	271.2	236.8	763.9	239.1
Operating Segments \| Options
Segment Reporting Information
Revenues	474.2	468.9	1,431.8	1,329.7
Operating income (loss)	213.9	190.8	618.6	531.6
Operating Segments \| North American Equities
Segment Reporting Information
Revenues	317.1	418.7	1,017.5	1,275.2
Operating income (loss)	34.9	38.1	86.1	109.3
Operating Segments \| Europe and Asia Pacific
Segment Reporting Information
Revenues	66.8	58.9	207.4	204.9
Operating income (loss)	2.6	4.9	19.4	36.5
Operating Segments \| Futures
Segment Reporting Information
Revenues	33.4	29.3	95.5	92.1
Operating income (loss)	24.3	12.2	63.6	44.5
Operating Segments \| Global FX
Segment Reporting Information
Revenues	18.7	17.6	55.7	51.7
Operating income (loss)	8.5	1.2	18.2	6.3
Operating Segments \| Digital
Segment Reporting Information
Revenues	(1.4)	0.1	(3.1)	0.2
Operating income (loss)	(12.6)	(8.7)	(35.0)	(482.3)
Corporate Items and Eliminations
Segment Reporting Information
Operating income (loss)	$ (0.4)	$ (1.7)	$ (7.0)	$ (6.8)

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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