REVENUE RECOGNITION - Schedule of revenue by product line and Segment (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) item	Sep. 30, 2020 USD ($)
Segment Reporting Information
Number of types of regulatory fees the Company recognizes \| item			2
Revenues	$ 816.8	$ 792.7	$ 2,628.4	$ 2,582.9
Services transferred at a point in time
Segment Reporting Information
Revenues	682.0	672.6	2,233.5	2,234.5
Services transferred over time
Segment Reporting Information
Revenues	134.8	120.1	394.9	348.4
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	374.3	312.8	1,106.7	986.9
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	322.4	265.5	954.3	853.5
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	51.9	47.3	152.4	133.4
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	336.3	403.6	1,215.8	1,367.8
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	279.3	348.1	1,039.3	1,201.6
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	57.0	55.5	176.5	166.2
Futures \| Operating Segments
Segment Reporting Information
Revenues	29.8	24.0	89.7	87.1
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	23.2	17.9	71.1	69.6
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	6.6	6.1	18.6	17.5
Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	61.9	39.1	172.8	97.3
Europe and Asia Pacific \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	45.2	30.2	132.7	72.4
Europe and Asia Pacific \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	16.7	8.9	40.1	24.9
Global FX \| Operating Segments
Segment Reporting Information
Revenues	14.5	13.2	43.1	43.8
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	11.9	10.9	35.8	37.4
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	2.6	2.3	7.3	6.4
Transaction and clearing fees
Segment Reporting Information
Revenues	632.9	545.5	2,014.3	1,825.3
Transaction and clearing fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	308.4	241.4	900.3	776.4
Transaction and clearing fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	253.2	253.0	903.0	882.1
Transaction and clearing fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	23.2	16.8	71.0	68.5
Transaction and clearing fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	36.2	23.4	104.2	60.9
Transaction and clearing fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	11.9	10.9	35.8	37.4
Access and capacity fees
Segment Reporting Information
Revenues	72.8	60.6	206.3	174.0
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	31.0	27.5	90.0	78.6
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	25.2	21.2	72.4	62.1
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	4.9	4.4	13.6	12.5
Access and capacity fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	9.4	5.4	23.7	15.0
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	2.3	2.1	6.6	5.8
Market data fees
Segment Reporting Information
Revenues	62.0	59.5	188.6	174.4
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	20.9	19.8	62.4	54.8
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	31.8	34.3	104.1	104.1
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	1.7	1.7	5.0	5.0
Market data fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	7.3	3.5	16.4	9.9
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.3	0.2	0.7	0.6
Regulatory fees
Segment Reporting Information
Revenues	34.6	113.8	173.0	379.3
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	9.3	19.7	39.1	63.0
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	25.3	94.1	133.8	316.3
Regulatory fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues			0.1
Other revenue
Segment Reporting Information
Revenues	14.5	13.3	46.2	29.9
Other revenue \| Corporate Items and Eliminations
Segment Reporting Information
Revenues			0.3
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	4.7	4.4	14.9	14.1
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	0.8	1.0	2.5	3.2
Other revenue \| Futures \| Operating Segments
Segment Reporting Information
Revenues		1.1		1.1
Other revenue \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	$ 9.0	$ 6.8	$ 28.5	$ 11.5