SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)
Segment Reporting Information
Number of reportable segments | segment			5
Revenues	$ 868.7	$ 620.6	$ 1,790.2	$ 1,222.1
Operating income (loss)	161.7	125.2	388.1	270.6
Operating Segments | Options
Segment Reporting Information
Revenues	317.6	246.1	674.1	477.1
Operating income (loss)	93.4	80.1	236.8	167.1
Operating Segments | U.S. Equities
Segment Reporting Information
Revenues	490.5	298.7	964.2	594.5
Operating income (loss)	55.9	36.4	104.9	74.0
Operating Segments | Futures
Segment Reporting Information
Revenues	21.6	33.8	63.1	64.3
Operating income (loss)	8.3	19.4	35.2	37.7
Operating Segments | European Equities
Segment Reporting Information
Revenues	25.3	28.7	58.2	59.0
Operating income (loss)	5.4	5.7	14.9	11.5
Operating Segments | Global FX
Segment Reporting Information
Revenues	13.7	13.1	30.6	27.0
Operating income (loss)	1.1	(1.6)	4.1	(2.0)
Corporate Items and Eliminations
Segment Reporting Information
Revenues		0.2		0.2
Operating income (loss)	$ (2.4)	$ (14.8)	$ (7.8)	$ (17.7)