SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2022 USD ($) segment	Mar. 31, 2022 segment	Sep. 30, 2021 USD ($)	Jun. 30, 2022 segment	Sep. 30, 2022 USD ($) segment	Sep. 30, 2021 USD ($)
Segment Reporting Information
Number of reportable segments | segment	6	5		6	6
Revenues	$ 993.5		$ 816.8		$ 2,953.8	$ 2,628.4
Operating income (loss)	236.8		190.7		239.1	585.3
Operating Segments | Options
Segment Reporting Information
Revenues	468.9		374.3		1,329.7	1,106.7
Operating income (loss)	190.8		136.5		531.6	390.4
Operating Segments | North American Equities
Segment Reporting Information
Revenues	418.7		336.3		1,275.2	1,215.8
Operating income (loss)	38.1		31.5		109.3	114.0
Operating Segments | Europe and Asia Pacific
Segment Reporting Information
Revenues	58.9		61.9		204.9	172.8
Operating income (loss)	4.9		12.6		36.5	40.4
Operating Segments | Futures
Segment Reporting Information
Revenues	29.3		29.8		92.1	89.7
Operating income (loss)	12.2		16.3		44.5	49.1
Operating Segments | Global FX
Segment Reporting Information
Revenues	17.6		14.5		51.7	43.1
Operating income (loss)	1.2		0.3		6.3	1.9
Operating Segments | Digital
Segment Reporting Information
Revenues	0.1				0.2
Operating income (loss)	(8.7)				(482.3)
Corporate Items and Eliminations
Segment Reporting Information
Revenues						0.3
Operating income (loss)	$ (1.7)		$ (6.5)		$ (6.8)	$ (10.5)