REVENUE RECOGNITION - Revenue by product line and Segment (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) item segment	Sep. 30, 2022 USD ($)
Segment Reporting Information
Number of types of regulatory fees the Company recognizes \| item			2
Revenues	$ 908.8	$ 993.5	$ 2,804.8	$ 2,953.8
Number of reportable segments \| segment			6
Services transferred at a point in time
Segment Reporting Information
Revenues	744.5	840.7	$ 2,325.0	2,501.8
Services transferred over time
Segment Reporting Information
Revenues	164.3	152.8	479.8	452.0
Options \| Operating Segments
Segment Reporting Information
Revenues	474.2	468.9	1,431.8	1,329.7
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	402.4	403.9	1,222.8	1,137.6
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	71.8	65.0	209.0	192.1
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	317.1	418.7	1,017.5	1,275.2
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	254.1	356.9	833.7	1,094.7
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	63.0	61.8	183.8	180.5
Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	66.8	58.9	207.4	204.9
Europe and Asia Pacific \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	48.3	42.4	152.5	154.0
Europe and Asia Pacific \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	18.5	16.5	54.9	50.9
Futures \| Operating Segments
Segment Reporting Information
Revenues	33.4	29.3	95.5	92.1
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	25.5	22.4	72.8	71.4
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	7.9	6.9	22.7	20.7
Global FX \| Operating Segments
Segment Reporting Information
Revenues	18.7	17.6	55.7	51.7
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	15.7	15.0	46.7	43.9
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	3.0	2.6	9.0	7.8
Digital \| Operating Segments
Segment Reporting Information
Revenues	(1.4)	0.1	(3.1)	0.2
Digital \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	(1.5)	0.1	(3.5)	0.2
Digital \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	0.1		0.4
Transaction and clearing fees
Segment Reporting Information
Revenues	678.6	715.5	2,096.8	2,220.4
Transaction and clearing fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	384.6	377.7	1,164.7	1,079.0
Transaction and clearing fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	221.9	264.6	706.3	899.9
Transaction and clearing fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	32.6	35.9	110.3	126.6
Transaction and clearing fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	25.4	22.3	72.6	71.1
Transaction and clearing fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	15.6	14.9	46.4	43.6
Transaction and clearing fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	(1.5)	0.1	(3.5)	0.2
Access and capacity fees
Segment Reporting Information
Revenues	87.7	81.8	258.8	241.5
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	40.4	38.6	119.6	112.7
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	29.6	27.8	87.0	81.1
Access and capacity fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	9.3	8.1	27.4	26.1
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	5.7	5.0	16.5	14.7
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	2.6	2.3	7.9	6.9
Access and capacity fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.1		0.4
Market data fees
Segment Reporting Information
Revenues	76.6	71.0	221.0	210.5
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	31.4	26.4	89.4	79.4
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	33.4	34.0	96.8	99.4
Market data fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	9.2	8.4	27.5	24.8
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	2.2	1.9	6.2	6.0
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.4	0.3	1.1	0.9
Regulatory fees
Segment Reporting Information
Revenues	47.0	114.9	175.9	244.3
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	16.6	24.4	54.1	53.0
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	30.3	90.4	121.6	191.0
Regulatory fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.1	0.1	0.2	0.3
Other revenue
Segment Reporting Information
Revenues	18.9	10.3	52.3	37.1
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	1.2	1.8	4.0	5.6
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	1.9	1.9	5.8	3.8
Other revenue \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	15.7	6.5	42.2	27.4
Other revenue \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.1	0.1	0.3	0.3
Cash and spot markets
Segment Reporting Information
Revenues	335.1	434.2	1,083.4	1,354.6
Cash and spot markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	268.6	315.5	859.5	1,070.2
Cash and spot markets \| Market data fees
Segment Reporting Information
Revenues	18.7	19.9	54.3	62.1
Cash and spot markets \| Regulatory fees
Segment Reporting Information
Revenues	30.3	90.4	121.6	191.0
Cash and spot markets \| Other revenue
Segment Reporting Information
Revenues	17.5	8.4	48.0	31.3
Data and access solutions
Segment Reporting Information
Revenues	137.0	126.1	401.7	368.9
Data and access solutions \| Access and capacity fees
Segment Reporting Information
Revenues	87.7	81.8	258.8	241.5
Data and access solutions \| Market data fees
Segment Reporting Information
Revenues	48.6	43.1	140.8	123.7
Data and access solutions \| Other revenue
Segment Reporting Information
Revenues	0.7	1.2	2.1	3.7
Derivatives markets
Segment Reporting Information
Revenues	436.7	433.2	1,319.7	1,230.3
Derivatives markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	410.0	400.0	1,237.3	1,150.2
Derivatives markets \| Market data fees
Segment Reporting Information
Revenues	9.3	8.0	25.9	24.7
Derivatives markets \| Regulatory fees
Segment Reporting Information
Revenues	16.7	24.5	54.3	53.3
Derivatives markets \| Other revenue
Segment Reporting Information
Revenues	$ 0.7	$ 0.7	$ 2.2	$ 2.1