REVENUE RECOGNITION - Revenue by product line and Segment (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) item	Jun. 30, 2021 USD ($)
Segment Reporting Information
Number of types of regulatory fees the Company recognizes \| item			2
Revenues	$ 985.8	$ 800.8	$ 1,960.3	$ 1,811.6
Services transferred at a point in time
Segment Reporting Information
Revenues	835.2	670.9	1,661.1	1,551.5
Services transferred over time
Segment Reporting Information
Revenues	150.6	129.9	299.2	260.1
Corporate Items and Eliminations
Segment Reporting Information
Revenues				0.3
Corporate Items and Eliminations \| Services transferred at a point in time
Segment Reporting Information
Revenues				0.3
Options \| Operating Segments
Segment Reporting Information
Revenues	438.2	350.0	860.8	732.4
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	372.8	299.4	733.7	631.9
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	65.4	50.6	127.1	100.5
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	432.8	353.5	856.5	879.5
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	374.3	294.9	737.8	760.0
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	58.5	58.6	118.7	119.5
Futures \| Operating Segments
Segment Reporting Information
Revenues	30.5	28.3	62.8	59.9
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	23.2	22.3	49.0	47.9
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	7.3	6.0	13.8	12.0
Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	67.4	55.1	146.0	110.9
Europe and Asia Pacific \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	50.6	42.8	111.6	87.5
Europe and Asia Pacific \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	16.8	12.3	34.4	23.4
Global FX \| Operating Segments
Segment Reporting Information
Revenues	16.8	13.9	34.1	28.6
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	14.2	11.5	28.9	23.9
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	2.6	2.4	5.2	4.7
Digital \| Operating Segments
Segment Reporting Information
Revenues	0.1		0.1
Digital \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	0.1		0.1
Transaction and clearing fees
Segment Reporting Information
Revenues	735.3	618.2	1,504.9	1,381.4
Transaction and clearing fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	353.0	283.2	701.3	591.9
Transaction and clearing fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	304.5	268.0	635.3	649.8
Transaction and clearing fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	23.0	22.2	48.8	47.8
Transaction and clearing fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	40.6	33.3	90.7	68.0
Transaction and clearing fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	14.1	11.5	28.7	23.9
Transaction and clearing fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.1		0.1
Access and capacity fees
Segment Reporting Information
Revenues	81.8	67.1	159.7	133.5
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	38.3	29.6	74.1	59.0
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	27.3	23.8	53.3	47.2
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	5.2	4.3	9.7	8.7
Access and capacity fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	8.7	7.2	18.0	14.3
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	2.3	2.2	4.6	4.3
Market data fees
Segment Reporting Information
Revenues	68.8	62.8	139.5	126.6
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	27.1	21.0	53.0	41.5
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	31.2	34.8	65.4	72.3
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	2.1	1.7	4.1	3.3
Market data fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	8.1	5.1	16.4	9.1
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.3	0.2	0.6	0.4
Regulatory fees
Segment Reporting Information
Revenues	86.8	36.9	129.4	138.4
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	17.9	10.7	28.6	29.8
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	68.7	26.1	100.6	108.5
Regulatory fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.2	0.1	0.2	0.1
Other revenue
Segment Reporting Information
Revenues	13.1	15.8	26.8	31.7
Other revenue \| Corporate Items and Eliminations
Segment Reporting Information
Revenues				0.3
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	1.9	5.5	3.8	10.2
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	1.1	0.8	1.9	1.7
Other revenue \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	10.0	9.5	20.9	19.5
Other revenue \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.1		0.2
Cash and spot markets
Segment Reporting Information
Revenues	458.5	372.7	920.4	921.6
Cash and spot markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	359.2	312.7	754.7	741.6
Cash and spot markets \| Market data fees
Segment Reporting Information
Revenues	19.3	23.9	42.2	50.8
Cash and spot markets \| Regulatory fees
Segment Reporting Information
Revenues	68.7	26.1	100.6	108.5
Cash and spot markets \| Other revenue
Segment Reporting Information
Revenues	11.3	10.0	22.9	20.7
Data and access solutions
Segment Reporting Information
Revenues	123.9	103.0	242.8	203.6
Data and access solutions \| Access and capacity fees
Segment Reporting Information
Revenues	81.8	67.1	159.7	133.5
Data and access solutions \| Market data fees
Segment Reporting Information
Revenues	41.0	30.9	80.6	60.6
Data and access solutions \| Other revenue
Segment Reporting Information
Revenues	1.1	5.0	2.5	9.5
Derivatives markets
Segment Reporting Information
Revenues	403.4	325.1	797.1	686.4
Derivatives markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	376.1	305.5	750.2	639.8
Derivatives markets \| Market data fees
Segment Reporting Information
Revenues	8.5	8.0	16.7	15.2
Derivatives markets \| Regulatory fees
Segment Reporting Information
Revenues	18.1	10.8	28.8	29.9
Derivatives markets \| Other revenue
Segment Reporting Information
Revenues	$ 0.7	$ 0.8	$ 1.4	$ 1.5