REVENUE RECOGNITION - Revenue by product line and Segment (Details) $ in Millions	3 Months Ended
Mar. 31, 2022 USD ($) item	Mar. 31, 2021 USD ($)
Segment Reporting Information
Number of types of regulatory fees the Company recognizes \| item	2
Revenues	$ 974.5	$ 1,010.8
Services transferred at a point in time
Segment Reporting Information
Revenues	825.9	880.6
Services transferred over time
Segment Reporting Information
Revenues	148.6	130.2
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	422.6	382.4
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	360.9	332.5
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	61.7	49.9
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	423.7	526.0
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	363.5	465.1
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	60.2	60.9
Futures \| Operating Segments
Segment Reporting Information
Revenues	32.3	31.6
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	25.8	25.6
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	6.5	6.0
Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	78.6	55.8
Europe and Asia Pacific \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	61.0	44.7
Europe and Asia Pacific \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	17.6	11.1
Global FX \| Operating Segments
Segment Reporting Information
Revenues	17.3	14.7
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	14.7	12.4
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	2.6	2.3
Transaction and clearing fees
Segment Reporting Information
Revenues	769.6	763.2
Transaction and clearing fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	348.3	308.7
Transaction and clearing fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	330.8	381.8
Transaction and clearing fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	25.8	25.6
Transaction and clearing fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	50.1	34.7
Transaction and clearing fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	14.6	12.4
Access and capacity fees
Segment Reporting Information
Revenues	77.9	66.4
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	35.8	29.4
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	26.0	23.4
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	4.5	4.4
Access and capacity fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	9.3	7.1
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	2.3	2.1
Market data fees
Segment Reporting Information
Revenues	70.7	63.8
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	25.9	20.5
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	34.2	37.5
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	2.0	1.6
Market data fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	8.3	4.0
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.3	0.2
Regulatory fees
Segment Reporting Information
Revenues	42.6	101.5
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	10.7	19.1
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	31.9	82.4
Other revenue
Segment Reporting Information
Revenues	13.7	15.9
Other revenue \| Corporate Items and Eliminations
Segment Reporting Information
Revenues		0.3
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	1.9	4.7
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	0.8	0.9
Other revenue \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	10.9	10.0
Other revenue \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.1
Cash and spot markets
Segment Reporting Information
Revenues	461.9	548.9
Cash and spot markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	395.5	428.9
Cash and spot markets \| Market data fees
Segment Reporting Information
Revenues	22.9	26.9
Cash and spot markets \| Regulatory fees
Segment Reporting Information
Revenues	31.9	82.4
Cash and spot markets \| Other revenue
Segment Reporting Information
Revenues	11.6	10.7
Data and access solutions
Segment Reporting Information
Revenues	118.9	100.6
Data and access solutions \| Access and capacity fees
Segment Reporting Information
Revenues	77.9	66.4
Data and access solutions \| Market data fees
Segment Reporting Information
Revenues	39.6	29.7
Data and access solutions \| Other revenue
Segment Reporting Information
Revenues	1.4	4.5
Derivatives markets
Segment Reporting Information
Revenues	393.7	361.3
Derivatives markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	374.1	334.3
Derivatives markets \| Market data fees
Segment Reporting Information
Revenues	8.2	7.2
Derivatives markets \| Regulatory fees
Segment Reporting Information
Revenues	10.7	19.1
Derivatives markets \| Other revenue
Segment Reporting Information
Revenues	$ 0.7	$ 0.7