SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)
Segment Reporting Information
Number of reportable segments | segment			5
Revenues	$ 675.4	$ 575.9	$ 1,897.5	$ 2,021.1
Operating income (loss)	147.4	126.1	418.0	422.9
Options
Segment Reporting Information
Revenues	266.1	228.6	743.2	766.8
Operating income (loss)	85.4	82.1	252.5	273.8
U.S. Equities
Segment Reporting Information
Revenues	330.7	271.9	925.2	1,005.6
Operating income (loss)	36.4	30.6	110.4	101.9
Futures
Segment Reporting Information
Revenues	39.6	31.2	103.9	107.7
Operating income (loss)	26.2	20.6	63.9	63.1
European Equities
Segment Reporting Information
Revenues	25.9	30.6	84.9	97.9
Operating income (loss)	4.8	5.4	16.3	18.1
Global FX
Segment Reporting Information
Revenues	13.1	13.6	40.1	42.7
Operating income (loss)	(1.3)	(1.9)	(3.3)	(6.6)
Operating Segments | Options
Segment Reporting Information
Revenues	266.1	228.6	743.2	766.8
Operating Segments | U.S. Equities
Segment Reporting Information
Revenues	330.7	271.9	925.2	1,005.6
Operating Segments | Futures
Segment Reporting Information
Revenues	39.6	31.2	103.9	107.7
Operating Segments | European Equities
Segment Reporting Information
Revenues	25.9	30.6	84.9	97.9
Operating Segments | Global FX
Segment Reporting Information
Revenues	13.1	13.6	40.1	42.7
Corporate Items and Eliminations
Segment Reporting Information
Revenues			0.2	0.4
Operating income (loss)	$ (4.1)	$ (10.7)	$ (21.8)	$ (27.4)