SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($) segment	Mar. 31, 2022 segment	Jun. 30, 2023 USD ($) segment	Jun. 30, 2022 USD ($)
Segment Reporting Information
Number of reportable segments \| segment		6	5	6
Revenues	$ 907.8	$ 985.8		$ 1,896.0	$ 1,960.3
Operating income (loss)	244.8	(237.4)		492.7	2.3
Operating Segments \| Options
Segment Reporting Information
Revenues	471.5	438.2		957.6	860.8
Operating income (loss)	203.7	178.1		404.7	340.8
Operating Segments \| North American Equities
Segment Reporting Information
Revenues	320.6	432.8		700.4	856.5
Operating income (loss)	22.8	32.6		51.2	71.2
Operating Segments \| Europe and Asia Pacific
Segment Reporting Information
Revenues	68.3	67.4		140.6	146.0
Operating income (loss)	5.5	10.7		16.8	31.6
Operating Segments \| Futures
Segment Reporting Information
Revenues	30.0	30.5		62.1	62.8
Operating income (loss)	18.4	15.3		39.3	32.3
Operating Segments \| Global FX
Segment Reporting Information
Revenues	18.2	16.8		37.0	34.1
Operating income (loss)	5.0	2.7		9.7	5.1
Operating Segments \| Digital
Segment Reporting Information
Revenues	(0.8)	0.1		(1.7)	0.1
Operating income (loss)	(11.0)	(473.6)		(22.4)	(473.6)
Corporate Items and Eliminations
Segment Reporting Information
Operating income (loss)	$ 0.4	$ (3.2)		$ (6.6)	$ (5.1)

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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