SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) segment	Jun. 30, 2020 USD ($)
Segment Reporting Information
Number of reportable segments | segment			5
Revenues	$ 800.8	$ 868.7	$ 1,811.6	$ 1,790.2
Operating income (loss)	190.0	161.7	394.6	388.1
Operating Segments | Options
Segment Reporting Information
Revenues	350.0	317.6	732.4	674.1
Operating income (loss)	125.2	93.4	253.9	236.8
Operating Segments | North American Equities
Segment Reporting Information
Revenues	353.5	490.5	879.5	964.2
Operating income (loss)	37.4	55.9	82.5	104.9
Operating Segments | Futures
Segment Reporting Information
Revenues	28.3	21.6	59.9	63.1
Operating income (loss)	15.3	8.3	32.8	35.2
Operating Segments | Europe
Segment Reporting Information
Revenues	55.1	25.3	110.9	58.2
Operating income (loss)	13.3	5.4	27.8	14.9
Operating Segments | Global FX
Segment Reporting Information
Revenues	13.9	13.7	28.6	30.6
Operating income (loss)	0.4	1.1	1.6	4.1
Corporate Items and Eliminations
Segment Reporting Information
Revenues			0.3
Operating income (loss)	$ (1.6)	$ (2.4)	$ (4.0)	$ (7.8)