SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)
Segment Reporting Information
Number of reportable segments | segment			5
Revenues	$ 620.6	$ 667.5	$ 1,222.1	$ 1,445.2
Operating income (loss)	125.2	129.1	270.6	296.8
Operating Segments | Options
Segment Reporting Information
Revenues	246.1	244.1	477.1	538.2
Operating income (loss)	80.1	78.1	167.1	191.7
Operating Segments | U.S. Equities
Segment Reporting Information
Revenues	298.7	343.2	594.5	733.7
Operating income (loss)	36.4	37.5	74.0	71.3
Operating Segments | Futures
Segment Reporting Information
Revenues	33.8	32.6	64.3	76.5
Operating income (loss)	19.4	16.3	37.7	42.5
Operating Segments | European Equities
Segment Reporting Information
Revenues	28.7	32.9	59.0	67.3
Operating income (loss)	5.7	6.5	11.5	12.7
Operating Segments | Global FX
Segment Reporting Information
Revenues	13.1	14.5	27.0	29.1
Operating income (loss)	(1.6)	(1.7)	(2.0)	(4.7)
Corporate Items and Eliminations
Segment Reporting Information
Revenues	0.2	0.2	0.2	0.4
Operating income (loss)	$ (14.8)	$ (7.6)	$ (17.7)	$ (16.7)