REVENUE RECOGNITION - Revenue by product line and Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information
Revenues	$ 3,494.8	$ 3,427.1	$ 2,496.1
Services transferred at a point in time
Segment Reporting Information
Revenues	2,962.0	2,958.4	2,060.7
Services transferred over time
Segment Reporting Information
Revenues	532.8	468.7	435.4
Corporate Items and Eliminations
Segment Reporting Information
Revenues	0.3		0.2
Corporate Items and Eliminations \| Services transferred at a point in time
Segment Reporting Information
Revenues	0.3		0.2
Options \| Operating Segments
Segment Reporting Information
Revenues	1,505.0	1,330.1	983.1
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	1,296.7	1,148.9	823.3
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	208.3	181.2	159.8
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	1,570.5	1,789.5	1,213.1
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	1,337.9	1,568.3	996.1
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	232.6	221.2	217.0
Futures \| Operating Segments
Segment Reporting Information
Revenues	120.6	109.2	135.9
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	95.3	85.6	113.8
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	25.3	23.6	22.1
Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	240.3	140.5	110.8
Europe and Asia Pacific \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	183.5	106.5	81.7
Europe and Asia Pacific \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	56.8	34.0	29.1
Global FX \| Operating Segments
Segment Reporting Information
Revenues	58.1	57.8	53.0
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	48.3	49.1	45.6
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	9.8	8.7	7.4
Transaction fees.
Segment Reporting Information
Revenues	2,693.1	2,418.0	1,716.2
Transaction fees. \| Options \| Operating Segments
Segment Reporting Information
Revenues	1,231.2	1,046.3	742.9
Transaction fees. \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	1,173.1	1,147.2	744.6
Transaction fees. \| Futures \| Operating Segments
Segment Reporting Information
Revenues	95.2	84.5	110.2
Transaction fees. \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	145.3	90.9	73.1
Transaction fees. \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	48.3	49.1	45.4
Access and capacity fees
Segment Reporting Information
Revenues	280.7	236.7	221.9
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	124.0	107.0	104.1
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	98.0	84.2	78.9
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	18.7	17.0	15.6
Access and capacity fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	31.2	20.6	16.5
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	8.8	7.9	6.8
Market data fees
Segment Reporting Information
Revenues	252.1	232.0	213.5
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	84.3	74.2	55.7
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	134.6	137.0	138.1
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	6.6	6.6	6.5
Market data fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	25.6	13.4	12.6
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	1.0	0.8	0.6
Regulatory fees
Segment Reporting Information
Revenues	208.3	500.2	311.7
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	46.6	83.4	64.0
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	161.6	416.8	247.0
Regulatory fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.1		0.7
Other revenue
Segment Reporting Information
Revenues	60.6	40.2	32.8
Other revenue \| Corporate Items and Eliminations
Segment Reporting Information
Revenues	0.3		0.2
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	18.9	19.2	16.4
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	3.2	4.3	4.5
Other revenue \| Futures \| Operating Segments
Segment Reporting Information
Revenues		1.1	2.9
Other revenue \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	$ 38.2	$ 15.6	8.6
Other revenue \| Global FX \| Operating Segments
Segment Reporting Information
Revenues			$ 0.2