REVENUE RECOGNITION - Revenue by Product Line and Segment (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) fee segment	Sep. 30, 2024 USD ($)	Dec. 31, 2024 segment
Segment Reporting Information
Number of reportable segments \| segment			5		6
Number of types of regulatory fees recognized \| fee			2
Revenues	$ 1,141.7	$ 1,055.7	$ 3,510.2	$ 2,986.9
Services transferred at a point in time
Segment Reporting Information
Revenues	954.1	888.0	2,963.4	2,492.9
Services transferred over time
Segment Reporting Information
Revenues	187.6	167.7	546.8	494.0
Options \| Operating Segments
Segment Reporting Information
Revenues	616.0	510.3	1,776.4	1,470.0
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	531.8	437.5	1,533.7	1,253.8
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	84.2	72.8	242.7	216.2
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	377.8	403.6	1,279.7	1,106.5
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	308.4	340.2	1,076.4	921.8
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	69.4	63.4	203.3	184.7
Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	92.8	81.1	284.9	243.4
Europe and Asia Pacific \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	70.2	60.8	218.1	183.5
Europe and Asia Pacific \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	22.6	20.3	66.8	59.9
Futures \| Operating Segments
Segment Reporting Information
Revenues	32.0	39.9	99.8	107.2
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	23.8	31.8	75.4	83.5
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	8.2	8.1	24.4	23.7
Global FX \| Operating Segments
Segment Reporting Information
Revenues	23.1	20.7	69.4	60.0
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	19.9	17.6	59.8	50.6
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	3.2	3.1	9.6	9.4
Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.1	0.0	(0.2)
Digital \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	0.0	0.1	0.0	(0.3)
Digital \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.1
Transaction and clearing fees
Segment Reporting Information
Revenues	919.9	725.9	2,620.2	2,142.0
Transaction and clearing fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	519.0	405.3	1,451.8	1,181.0
Transaction and clearing fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	305.1	231.0	877.0	709.9
Transaction and clearing fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	52.4	40.5	157.2	118.2
Transaction and clearing fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	23.8	31.7	75.2	83.3
Transaction and clearing fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	19.6	17.4	59.0	50.0
Transaction and clearing fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0	0.0	(0.4)
Access and capacity fees
Segment Reporting Information
Revenues	103.9	94.2	302.9	274.8
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	47.0	42.3	136.3	125.6
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	36.6	33.2	106.7	93.5
Access and capacity fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	11.8	10.3	34.6	30.4
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	5.7	5.7	17.0	16.9
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	2.8	2.7	8.3	8.3
Access and capacity fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.1
Market data fees
Segment Reporting Information
Revenues	83.7	73.5	243.9	219.2
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	37.2	30.5	106.4	90.6
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	32.8	30.2	96.6	91.2
Market data fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	10.8	10.0	32.2	29.5
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	2.5	2.4	7.4	6.8
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.4	0.4	1.3	1.1
Market data fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Regulatory fees
Segment Reporting Information
Revenues	11.8	139.0	270.4	275.1
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	10.8	31.0	77.3	68.9
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	1.0	107.9	193.0	206.0
Regulatory fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Regulatory fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.1	0.1	0.2
Regulatory fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Regulatory fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Other revenue
Segment Reporting Information
Revenues	22.4	23.1	72.8	75.8
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	2.0	1.2	4.6	3.9
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	2.3	1.3	6.4	5.9
Other revenue \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	17.8	20.3	60.9	65.3
Other revenue \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0	0.1	0.0
Other revenue \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.3	0.2	0.8	0.6
Other revenue \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.1	0.0	0.1
Cash and spot markets
Segment Reporting Information
Revenues	415.0	434.1	1,403.5	1,201.4
Cash and spot markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	377.0	288.9	1,093.1	877.7
Cash and spot markets \| Access and capacity fees
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Cash and spot markets \| Market data fees
Segment Reporting Information
Revenues	16.8	15.5	49.5	46.1
Cash and spot markets \| Regulatory fees
Segment Reporting Information
Revenues	1.0	107.9	193.0	206.0
Cash and spot markets \| Other revenue
Segment Reporting Information
Revenues	20.2	21.8	67.9	71.6
Data Vantage
Segment Reporting Information
Revenues	162.1	145.6	472.9	427.9
Data Vantage \| Transaction and clearing fees
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Data Vantage \| Access and capacity fees
Segment Reporting Information
Revenues	103.9	94.2	302.9	274.8
Data Vantage \| Market data fees
Segment Reporting Information
Revenues	57.5	50.7	167.9	150.9
Data Vantage \| Regulatory fees
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Data Vantage \| Other revenue
Segment Reporting Information
Revenues	0.7	0.7	2.1	2.2
Derivatives markets
Segment Reporting Information
Revenues	564.6	476.0	1,633.8	1,357.6
Derivatives markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	542.9	437.0	1,527.1	1,264.3
Derivatives markets \| Access and capacity fees
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Derivatives markets \| Market data fees
Segment Reporting Information
Revenues	9.4	7.3	26.5	22.2
Derivatives markets \| Regulatory fees
Segment Reporting Information
Revenues	10.8	31.1	77.4	69.1
Derivatives markets \| Other revenue
Segment Reporting Information
Revenues	$ 1.5	$ 0.6	$ 2.8	$ 2.0