SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) segment	Sep. 30, 2023 USD ($)
Segment Reporting Information
Number of reportable segments | segment			6
Revenues	$ 1,055.7	$ 908.8	$ 2,986.9	$ 2,804.8
Operating income (loss)	307.4	271.2	799.9	763.9
Operating Segments | Options
Segment Reporting Information
Revenues	510.3	474.2	1,470.0	1,431.8
Operating income (loss)	221.2	213.9	653.6	618.6
Operating Segments | North American Equities
Segment Reporting Information
Revenues	403.6	317.1	1,106.5	1,017.5
Operating income (loss)	44.1	34.9	127.4	86.1
Operating Segments | Europe and Asia Pacific
Segment Reporting Information
Revenues	81.1	66.8	243.4	207.4
Operating income (loss)	11.3	2.6	30.7	19.4
Operating Segments | Futures
Segment Reporting Information
Revenues	39.9	33.4	107.2	95.5
Operating income (loss)	29.4	24.3	77.8	63.6
Operating Segments | Global FX
Segment Reporting Information
Revenues	20.7	18.7	60.0	55.7
Operating income (loss)	9.2	8.5	25.0	18.2
Operating Segments | Digital
Segment Reporting Information
Revenues	0.1	(1.4)	(0.2)	(3.1)
Operating income (loss)	(6.2)	(12.6)	(106.7)	(35.0)
Corporate Items and Eliminations
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Operating income (loss)	$ (1.6)	$ (0.4)	$ (7.9)	$ (7.0)