REVENUE RECOGNITION - Schedule of revenue by product line and Segment (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) item	Jun. 30, 2020 USD ($)
Segment Reporting Information
Number of types of regulatory fees the Company recognizes \| item			2
Revenues	$ 800.8	$ 868.7	$ 1,811.6	$ 1,790.2
Services transferred at a point in time
Segment Reporting Information
Revenues	670.9	754.3	1,551.5	1,561.9
Services transferred over time
Segment Reporting Information
Revenues	129.9	114.4	260.1	228.3
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	350.0	317.6	732.4	674.1
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	299.4	275.9	631.9	588.0
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	50.6	41.7	100.5	86.1
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	353.5	490.5	879.5	964.2
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	294.9	433.8	760.0	853.5
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	58.6	56.7	119.5	110.7
Futures \| Operating Segments
Segment Reporting Information
Revenues	28.3	21.6	59.9	63.1
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	22.3	15.8	47.9	51.7
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	6.0	5.8	12.0	11.4
Europe \| Operating Segments
Segment Reporting Information
Revenues	55.1	25.3	110.9	58.2
Europe \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	42.8	17.4	87.5	42.2
Europe \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	12.3	7.9	23.4	16.0
Global FX \| Operating Segments
Segment Reporting Information
Revenues	13.9	13.7	28.6	30.6
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	11.5	11.4	23.9	26.5
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	2.4	2.3	4.7	4.1
Transaction and clearing fees
Segment Reporting Information
Revenues	618.2	618.3	1,381.4	1,279.8
Transaction and clearing fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	283.2	250.8	591.9	535.0
Transaction and clearing fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	268.0	325.1	649.8	629.1
Transaction and clearing fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	22.2	15.8	47.8	51.7
Transaction and clearing fees \| Europe \| Operating Segments
Segment Reporting Information
Revenues	33.3	15.2	68.0	37.5
Transaction and clearing fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	11.5	11.4	23.9	26.5
Access and capacity fees
Segment Reporting Information
Revenues	67.1	55.7	133.5	113.4
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	29.6	23.9	59.0	51.1
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	23.8	20.9	47.2	40.9
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	4.3	4.1	8.7	8.1
Access and capacity fees \| Europe \| Operating Segments
Segment Reporting Information
Revenues	7.2	4.7	14.3	9.6
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	2.2	2.1	4.3	3.7
Market data fees
Segment Reporting Information
Revenues	62.8	58.7	126.6	114.9
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	21.0	17.8	41.5	35.0
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	34.8	35.8	72.3	69.8
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	1.7	1.7	3.3	3.3
Market data fees \| Europe \| Operating Segments
Segment Reporting Information
Revenues	5.1	3.2	9.1	6.4
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.2	0.2	0.4	0.4
Regulatory fees
Segment Reporting Information
Revenues	36.9	128.7	138.4	265.5
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	10.7	21.0	29.8	43.3
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	26.1	107.7	108.5	222.2
Regulatory fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.1		0.1
Other revenue
Segment Reporting Information
Revenues	15.8	7.3	31.7	16.6
Other revenue \| Corporate Items and Eliminations
Segment Reporting Information
Revenues			0.3
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	5.5	4.1	10.2	9.7
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	0.8	1.0	1.7	2.2
Other revenue \| Europe \| Operating Segments
Segment Reporting Information
Revenues	$ 9.5	$ 2.2	$ 19.5	$ 4.7