REVENUE RECOGNITION - Revenue by product line and Segment (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) segment item	Jun. 30, 2023 USD ($)
Segment Reporting Information
Number of types of regulatory fees the Company recognizes \| item			2
Revenues	$ 974.0	$ 907.8	$ 1,931.2	$ 1,896.0
Number of reportable segments \| segment			6
Services transferred at a point in time
Segment Reporting Information
Revenues	809.8	747.4	$ 1,604.9	1,580.5
Services transferred over time
Segment Reporting Information
Revenues	164.2	160.4	326.3	315.5
Options \| Operating Segments
Segment Reporting Information
Revenues	482.3	471.5	959.7	957.6
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	409.7	401.2	816.3	820.4
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	72.6	70.3	143.4	137.2
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	353.3	320.6	702.9	700.4
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	292.4	260.1	581.6	579.6
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	60.9	60.5	121.3	120.8
Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	81.6	68.3	162.3	140.6
Europe and Asia Pacific \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	61.8	49.5	122.7	104.2
Europe and Asia Pacific \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	19.8	18.8	39.6	36.4
Futures \| Operating Segments
Segment Reporting Information
Revenues	35.9	30.0	67.3	62.1
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	28.2	22.5	51.7	47.3
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	7.7	7.5	15.6	14.8
Global FX \| Operating Segments
Segment Reporting Information
Revenues	20.5	18.2	39.3	37.0
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	17.3	15.1	33.0	31.0
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	3.2	3.1	6.3	6.0
Digital \| Operating Segments
Segment Reporting Information
Revenues	0.4	(0.8)	(0.3)	(1.7)
Digital \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	0.4	(1.0)	(0.4)	(2.0)
Digital \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues		0.2	0.1	0.3
Transaction and clearing fees
Segment Reporting Information
Revenues	697.6	685.7	1,416.1	1,418.2
Transaction and clearing fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	385.9	384.3	775.7	780.1
Transaction and clearing fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	227.2	229.4	478.9	484.4
Transaction and clearing fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	38.9	35.6	77.7	77.7
Transaction and clearing fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	28.1	22.4	51.6	47.2
Transaction and clearing fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	17.1	15.0	32.6	30.8
Transaction and clearing fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.4	(1.0)	(0.4)	(2.0)
Access and capacity fees
Segment Reporting Information
Revenues	90.5	86.9	180.6	171.1
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	41.6	40.3	83.3	79.2
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	30.6	29.0	60.3	57.4
Access and capacity fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	10.0	9.2	20.1	18.1
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	5.5	5.5	11.2	10.8
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	2.8	2.7	5.6	5.3
Access and capacity fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues		0.2	0.1	0.3
Market data fees
Segment Reporting Information
Revenues	73.7	73.5	145.7	144.4
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	31.0	30.0	60.1	58.0
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	30.3	31.5	61.0	63.4
Market data fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	9.8	9.6	19.5	18.3
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	2.2	2.0	4.4	4.0
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.4	0.4	0.7	0.7
Regulatory fees
Segment Reporting Information
Revenues	85.9	44.4	136.1	128.9
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	22.6	15.6	37.9	37.5
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	63.2	28.7	98.1	91.3
Regulatory fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.1	0.1	0.1	0.1
Other revenue
Segment Reporting Information
Revenues	26.3	17.3	52.7	33.4
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	1.2	1.3	2.7	2.8
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	2.0	2.0	4.6	3.9
Other revenue \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	22.9	13.9	45.0	26.5
Other revenue \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.2	0.1	0.4	0.2
Cash and spot markets
Segment Reporting Information
Revenues	386.4	341.3	767.3	748.3
Cash and spot markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	283.6	279.0	588.8	590.9
Cash and spot markets \| Market data fees
Segment Reporting Information
Revenues	14.6	17.7	30.6	35.6
Cash and spot markets \| Regulatory fees
Segment Reporting Information
Revenues	63.2	28.7	98.1	91.3
Cash and spot markets \| Other revenue
Segment Reporting Information
Revenues	25.0	15.9	49.8	30.5
Data and access solutions
Segment Reporting Information
Revenues	142.1	135.3	282.3	264.7
Data and access solutions \| Access and capacity fees
Segment Reporting Information
Revenues	90.5	86.9	180.6	171.1
Data and access solutions \| Market data fees
Segment Reporting Information
Revenues	50.9	47.7	100.2	92.2
Data and access solutions \| Other revenue
Segment Reporting Information
Revenues	0.7	0.7	1.5	1.4
Derivatives markets
Segment Reporting Information
Revenues	445.5	431.2	881.6	883.0
Derivatives markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	414.0	406.7	827.3	827.3
Derivatives markets \| Market data fees
Segment Reporting Information
Revenues	8.2	8.1	14.9	16.6
Derivatives markets \| Regulatory fees
Segment Reporting Information
Revenues	22.7	15.7	38.0	37.6
Derivatives markets \| Other revenue
Segment Reporting Information
Revenues	$ 0.6	$ 0.7	$ 1.4	$ 1.5