REVENUE RECOGNITION - Schedule of revenue by product line and Segment (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) item	Mar. 31, 2019 USD ($)
Segment Reporting Information
Number of types of regulatory fees the Company recognizes | item	2
Revenues	$ 921.5	$ 602.6
Services transferred at a point in time
Segment Reporting Information
Revenues	807.6	496.6
Services transferred over time
Segment Reporting Information
Revenues	113.9	106.0
Options
Segment Reporting Information
Revenues	356.5	232.1
Options | Operating Segments
Segment Reporting Information
Revenues	356.5	232.1
Options | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	312.1	192.4
Options | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	44.4	39.7
U.S. Equities
Segment Reporting Information
Revenues	473.7	295.8
U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	473.7	295.8
U.S. Equities | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	419.7	243.9
U.S. Equities | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	54.0	51.9
Futures
Segment Reporting Information
Revenues	41.5	30.5
Futures | Operating Segments
Segment Reporting Information
Revenues	41.5	30.5
Futures | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	35.9	25.1
Futures | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	5.6	5.4
European Equities
Segment Reporting Information
Revenues	32.9	30.3
European Equities | Operating Segments
Segment Reporting Information
Revenues	32.9	30.3
European Equities | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	24.8	23.1
European Equities | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	8.1	7.2
Global FX
Segment Reporting Information
Revenues	16.9	13.9
Global FX | Operating Segments
Segment Reporting Information
Revenues	16.9	13.9
Global FX | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	15.1	12.1
Global FX | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	1.8	1.8
Transaction fees
Segment Reporting Information
Revenues	661.5	430.4
Transaction fees | Options | Operating Segments
Segment Reporting Information
Revenues	284.2	173.8
Transaction fees | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	304.0	198.9
Transaction fees | Futures | Operating Segments
Segment Reporting Information
Revenues	35.9	24.6
Transaction fees | European Equities | Operating Segments
Segment Reporting Information
Revenues	22.3	21.1
Transaction fees | Global FX | Operating Segments
Segment Reporting Information
Revenues	15.1	12.0
Access and capacity fees
Segment Reporting Information
Revenues	57.7	54.4
Access and capacity fees | Options | Operating Segments
Segment Reporting Information
Revenues	27.2	26.0
Access and capacity fees | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	20.0	19.0
Access and capacity fees | Futures | Operating Segments
Segment Reporting Information
Revenues	4.0	3.7
Access and capacity fees | European Equities | Operating Segments
Segment Reporting Information
Revenues	4.9	4.0
Access and capacity fees | Global FX | Operating Segments
Segment Reporting Information
Revenues	1.6	1.7
Market data fees
Segment Reporting Information
Revenues	56.2	51.6
Market data fees | Options | Operating Segments
Segment Reporting Information
Revenues	17.2	13.7
Market data fees | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	34.0	32.9
Market data fees | Futures | Operating Segments
Segment Reporting Information
Revenues	1.6	1.7
Market data fees | European Equities | Operating Segments
Segment Reporting Information
Revenues	3.2	3.2
Market data fees | Global FX | Operating Segments
Segment Reporting Information
Revenues	0.2	0.1
Regulatory fees
Segment Reporting Information
Revenues	136.8	58.7
Regulatory fees | Options | Operating Segments
Segment Reporting Information
Revenues	22.3	14.5
Regulatory fees | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	114.5	43.7
Regulatory fees | Futures | Operating Segments
Segment Reporting Information
Revenues		0.5
Other revenue
Segment Reporting Information
Revenues	9.3	7.5
Other revenue | Options | Operating Segments
Segment Reporting Information
Revenues	5.6	4.1
Other revenue | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	1.2	1.3
Other revenue | European Equities | Operating Segments
Segment Reporting Information
Revenues	$ 2.5	2.0
Other revenue | Global FX | Operating Segments
Segment Reporting Information
Revenues		$ 0.1