SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Segment Reporting Information
Number of reportable segments | segment	5
Revenues	$ 921.5	$ 602.6
Operating income (loss)	226.4	146.5
Options
Segment Reporting Information
Revenues	356.5	232.1
Operating income (loss)	143.4	88.1
U.S. Equities
Segment Reporting Information
Revenues	473.7	295.8
Operating income (loss)	49.0	37.6
Futures
Segment Reporting Information
Revenues	41.5	30.5
Operating income (loss)	26.9	18.3
European Equities
Segment Reporting Information
Revenues	32.9	30.3
Operating income (loss)	9.5	5.8
Global FX
Segment Reporting Information
Revenues	16.9	13.9
Operating income (loss)	3.0	(0.4)
Operating Segments | Options
Segment Reporting Information
Revenues	356.5	232.1
Operating Segments | U.S. Equities
Segment Reporting Information
Revenues	473.7	295.8
Operating Segments | Futures
Segment Reporting Information
Revenues	41.5	30.5
Operating Segments | European Equities
Segment Reporting Information
Revenues	32.9	30.3
Operating Segments | Global FX
Segment Reporting Information
Revenues	16.9	13.9
Corporate Items and Eliminations
Segment Reporting Information
Operating income (loss)	$ (5.4)	$ (2.9)