REVENUE RECOGNITION - Schedule of revenue by product line and Segment (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) item	Jun. 30, 2019 USD ($)
Segment Reporting Information
Number of types of regulatory fees the Company recognizes | item			2
Revenues	$ 868.7	$ 620.6	$ 1,790.2	$ 1,222.1
Services transferred at a point in time
Segment Reporting Information
Revenues	754.3	514.3	1,561.9	1,009.8
Services transferred over time
Segment Reporting Information
Revenues	114.4	106.3	228.3	212.3
Corporate Items and Eliminations
Segment Reporting Information
Revenues		0.2		0.2
Corporate Items and Eliminations | Services transferred at a point in time
Segment Reporting Information
Revenues		0.2		0.2
Options | Operating Segments
Segment Reporting Information
Revenues	317.6	246.1	674.1	477.1
Options | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	275.9	206.5	588.0	397.8
Options | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	41.7	39.6	86.1	79.3
U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	490.5	298.7	964.2	594.5
U.S. Equities | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	433.8	246.3	853.5	490.2
U.S. Equities | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	56.7	52.4	110.7	104.3
Futures | Operating Segments
Segment Reporting Information
Revenues	21.6	33.8	63.1	64.3
Futures | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	15.8	28.4	51.7	53.5
Futures | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	5.8	5.4	11.4	10.8
European Equities | Operating Segments
Segment Reporting Information
Revenues	25.3	28.7	58.2	59.0
European Equities | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	17.4	21.6	42.2	44.7
European Equities | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	7.9	7.1	16.0	14.3
Global FX | Operating Segments
Segment Reporting Information
Revenues	13.7	13.1	30.6	27.0
Global FX | Operating Segments | Services transferred at a point in time
Segment Reporting Information
Revenues	11.4	11.3	26.5	23.4
Global FX | Operating Segments | Services transferred over time
Segment Reporting Information
Revenues	2.3	1.8	4.1	3.6
Transaction fees
Segment Reporting Information
Revenues	618.3	426.9	1,279.8	856.2
Transaction fees | Options | Operating Segments
Segment Reporting Information
Revenues	250.8	185.9	535.0	358.6
Transaction fees | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	325.1	182.0	629.1	380.9
Transaction fees | Futures | Operating Segments
Segment Reporting Information
Revenues	15.8	28.3	51.7	52.9
Transaction fees | European Equities | Operating Segments
Segment Reporting Information
Revenues	15.2	19.5	37.5	40.6
Transaction fees | Global FX | Operating Segments
Segment Reporting Information
Revenues	11.4	11.2	26.5	23.2
Access and capacity fees
Segment Reporting Information
Revenues	55.7	54.5	113.4	108.9
Access and capacity fees | Options | Operating Segments
Segment Reporting Information
Revenues	23.9	25.6	51.1	51.6
Access and capacity fees | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	20.9	19.6	40.9	38.6
Access and capacity fees | Futures | Operating Segments
Segment Reporting Information
Revenues	4.1	3.8	8.1	7.5
Access and capacity fees | European Equities | Operating Segments
Segment Reporting Information
Revenues	4.7	3.9	9.6	7.9
Access and capacity fees | Global FX | Operating Segments
Segment Reporting Information
Revenues	2.1	1.6	3.7	3.3
Market data fees
Segment Reporting Information
Revenues	58.7	51.8	114.9	103.4
Market data fees | Options | Operating Segments
Segment Reporting Information
Revenues	17.8	14.0	35.0	27.7
Market data fees | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	35.8	32.8	69.8	65.7
Market data fees | Futures | Operating Segments
Segment Reporting Information
Revenues	1.7	1.6	3.3	3.3
Market data fees | European Equities | Operating Segments
Segment Reporting Information
Revenues	3.2	3.2	6.4	6.4
Market data fees | Global FX | Operating Segments
Segment Reporting Information
Revenues	0.2	0.2	0.4	0.3
Regulatory fees
Segment Reporting Information
Revenues	128.7	79.7	265.5	138.4
Regulatory fees | Options | Operating Segments
Segment Reporting Information
Revenues	21.0	16.7	43.3	31.2
Regulatory fees | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	107.7	63.0	222.2	106.7
Regulatory fees | Futures | Operating Segments
Segment Reporting Information
Revenues				0.5
Other revenue
Segment Reporting Information
Revenues	7.3	7.7	16.6	15.2
Other revenue | Corporate Items and Eliminations
Segment Reporting Information
Revenues		0.2		0.2
Other revenue | Options | Operating Segments
Segment Reporting Information
Revenues	4.1	3.9	9.7	8.0
Other revenue | U.S. Equities | Operating Segments
Segment Reporting Information
Revenues	1.0	1.3	2.2	2.6
Other revenue | Futures | Operating Segments
Segment Reporting Information
Revenues		0.1		0.1
Other revenue | European Equities | Operating Segments
Segment Reporting Information
Revenues	$ 2.2	2.1	$ 4.7	4.1
Other revenue | Global FX | Operating Segments
Segment Reporting Information
Revenues		$ 0.1		$ 0.2