REVENUE RECOGNITION - Revenue by product line and Segment (Details) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($) segment	Mar. 31, 2022 segment	Jun. 30, 2023 USD ($) segment item	Jun. 30, 2022 USD ($)
Segment Reporting Information
Number of types of regulatory fees the Company recognizes \| item				2
Revenues	$ 907.8	$ 985.8		$ 1,896.0	$ 1,960.3
Number of reportable segments \| segment		6	5	6
Services transferred at a point in time
Segment Reporting Information
Revenues	747.4	$ 835.2		$ 1,580.5	1,661.1
Services transferred over time
Segment Reporting Information
Revenues	160.4	150.6		315.5	299.2
Options \| Operating Segments
Segment Reporting Information
Revenues	471.5	438.2		957.6	860.8
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	401.2	372.8		820.4	733.7
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	70.3	65.4		137.2	127.1
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	320.6	432.8		700.4	856.5
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	260.1	374.3		579.6	737.8
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	60.5	58.5		120.8	118.7
Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	68.3	67.4		140.6	146.0
Europe and Asia Pacific \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	49.5	50.6		104.2	111.6
Europe and Asia Pacific \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	18.8	16.8		36.4	34.4
Futures \| Operating Segments
Segment Reporting Information
Revenues	30.0	30.5		62.1	62.8
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	22.5	23.2		47.3	49.0
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	7.5	7.3		14.8	13.8
Global FX \| Operating Segments
Segment Reporting Information
Revenues	18.2	16.8		37.0	34.1
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	15.1	14.2		31.0	28.9
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	3.1	2.6		6.0	5.2
Digital \| Operating Segments
Segment Reporting Information
Revenues	(0.8)	0.1		(1.7)	0.1
Digital \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	(1.0)	0.1		(2.0)	0.1
Digital \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	0.2			0.3
Transaction and clearing fees
Segment Reporting Information
Revenues	685.7	735.3		1,418.2	1,504.9
Transaction and clearing fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	384.3	353.0		780.1	701.3
Transaction and clearing fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	229.4	304.5		484.4	635.3
Transaction and clearing fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	35.6	40.6		77.7	90.7
Transaction and clearing fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	22.4	23.0		47.2	48.8
Transaction and clearing fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	15.0	14.1		30.8	28.7
Transaction and clearing fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	(1.0)	0.1		(2.0)	0.1
Access and capacity fees
Segment Reporting Information
Revenues	86.9	81.8		171.1	159.7
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	40.3	38.3		79.2	74.1
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	29.0	27.3		57.4	53.3
Access and capacity fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	9.2	8.7		18.1	18.0
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	5.5	5.2		10.8	9.7
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	2.7	2.3		5.3	4.6
Access and capacity fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.2			0.3
Market data fees
Segment Reporting Information
Revenues	73.5	68.8		144.4	139.5
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	30.0	27.1		58.0	53.0
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	31.5	31.2		63.4	65.4
Market data fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	9.6	8.1		18.3	16.4
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	2.0	2.1		4.0	4.1
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.4	0.3		0.7	0.6
Regulatory fees
Segment Reporting Information
Revenues	44.4	86.8		128.9	129.4
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	15.6	17.9		37.5	28.6
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	28.7	68.7		91.3	100.6
Regulatory fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.1	0.2		0.1	0.2
Other revenue
Segment Reporting Information
Revenues	17.3	13.1		33.4	26.8
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	1.3	1.9		2.8	3.8
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	2.0	1.1		3.9	1.9
Other revenue \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	13.9	10.0		26.5	20.9
Other revenue \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.1	0.1		0.2	0.2
Cash and spot markets
Segment Reporting Information
Revenues	341.3	458.5		748.3	920.4
Cash and spot markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	279.0	359.2		590.9	754.7
Cash and spot markets \| Market data fees
Segment Reporting Information
Revenues	17.7	19.3		35.6	42.2
Cash and spot markets \| Regulatory fees
Segment Reporting Information
Revenues	28.7	68.7		91.3	100.6
Cash and spot markets \| Other revenue
Segment Reporting Information
Revenues	15.9	11.3		30.5	22.9
Data and access solutions
Segment Reporting Information
Revenues	135.3	123.9		264.7	242.8
Data and access solutions \| Access and capacity fees
Segment Reporting Information
Revenues	86.9	81.8		171.1	159.7
Data and access solutions \| Market data fees
Segment Reporting Information
Revenues	47.7	41.0		92.2	80.6
Data and access solutions \| Other revenue
Segment Reporting Information
Revenues	0.7	1.1		1.4	2.5
Derivatives markets
Segment Reporting Information
Revenues	431.2	403.4		883.0	797.1
Derivatives markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	406.7	376.1		827.3	750.2
Derivatives markets \| Market data fees
Segment Reporting Information
Revenues	8.1	8.5		16.6	16.7
Derivatives markets \| Regulatory fees
Segment Reporting Information
Revenues	15.7	18.1		37.6	28.8
Derivatives markets \| Other revenue
Segment Reporting Information
Revenues	$ 0.7	$ 0.7		$ 1.5	$ 1.4