REVENUE RECOGNITION - Revenue by Product Line and Segment (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2025 USD ($) fee segment	Mar. 31, 2024 USD ($)	Dec. 31, 2024 segment
Segment Reporting Information
Number of reportable segments \| segment	5		6
Number of types of regulatory fees recognized \| fee	2
Revenues	$ 1,195.0	$ 957.2
Services transferred at a point in time
Segment Reporting Information
Revenues	1,019.4	795.1
Services transferred over time
Segment Reporting Information
Revenues	175.6	162.1
Options \| Operating Segments
Segment Reporting Information
Revenues	584.6	477.4
Options \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	507.0	406.6
Options \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	77.6	70.8
North American Equities \| Operating Segments
Segment Reporting Information
Revenues	460.1	349.6
North American Equities \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	394.6	289.2
North American Equities \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	65.5	60.4
Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	93.1	80.7
Europe and Asia Pacific \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	71.9	60.9
Europe and Asia Pacific \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	21.2	19.8
Futures \| Operating Segments
Segment Reporting Information
Revenues	35.3	31.4
Futures \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	27.2	23.5
Futures \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	8.1	7.9
Global FX \| Operating Segments
Segment Reporting Information
Revenues	21.9	18.8
Global FX \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	18.7	15.7
Global FX \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	3.2	3.1
Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	(0.7)
Digital \| Operating Segments \| Services transferred at a point in time
Segment Reporting Information
Revenues	0.0	(0.8)
Digital \| Operating Segments \| Services transferred over time
Segment Reporting Information
Revenues	0.0	0.1
Transaction and clearing fees
Segment Reporting Information
Revenues	832.6	718.5
Transaction and clearing fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	464.5	389.8
Transaction and clearing fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	271.7	251.7
Transaction and clearing fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	50.8	38.8
Transaction and clearing fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	27.1	23.5
Transaction and clearing fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	18.5	15.5
Transaction and clearing fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	(0.8)
Access and capacity fees
Segment Reporting Information
Revenues	97.8	90.1
Access and capacity fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	44.0	41.7
Access and capacity fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	34.5	29.7
Access and capacity fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	10.9	10.1
Access and capacity fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	5.6	5.7
Access and capacity fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	2.8	2.8
Access and capacity fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.1
Market data fees
Segment Reporting Information
Revenues	77.8	72.0
Market data fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	33.6	29.1
Market data fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	31.0	30.7
Market data fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	10.3	9.7
Market data fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	2.5	2.2
Market data fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.4	0.3
Market data fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0
Regulatory fees
Segment Reporting Information
Revenues	161.8	50.2
Regulatory fees \| Options \| Operating Segments
Segment Reporting Information
Revenues	41.1	15.3
Regulatory fees \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	120.7	34.9
Regulatory fees \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0
Regulatory fees \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0
Regulatory fees \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0
Regulatory fees \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0
Other revenue
Segment Reporting Information
Revenues	25.0	26.4
Other revenue \| Options \| Operating Segments
Segment Reporting Information
Revenues	1.4	1.5
Other revenue \| North American Equities \| Operating Segments
Segment Reporting Information
Revenues	2.2	2.6
Other revenue \| Europe and Asia Pacific \| Operating Segments
Segment Reporting Information
Revenues	21.1	22.1
Other revenue \| Futures \| Operating Segments
Segment Reporting Information
Revenues	0.1	0.0
Other revenue \| Global FX \| Operating Segments
Segment Reporting Information
Revenues	0.2	0.2
Other revenue \| Digital \| Operating Segments
Segment Reporting Information
Revenues	0.0	0.0
Cash and spot markets
Segment Reporting Information
Revenues	500.9	380.9
Cash and spot markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	341.0	305.2
Cash and spot markets \| Access and capacity fees
Segment Reporting Information
Revenues	0.0	0.0
Cash and spot markets \| Market data fees
Segment Reporting Information
Revenues	15.7	16.0
Cash and spot markets \| Regulatory fees
Segment Reporting Information
Revenues	120.7	34.9
Cash and spot markets \| Other revenue
Segment Reporting Information
Revenues	23.5	24.8
Data Vantage
Segment Reporting Information
Revenues	152.5	140.2
Data Vantage \| Transaction and clearing fees
Segment Reporting Information
Revenues	0.0	0.0
Data Vantage \| Access and capacity fees
Segment Reporting Information
Revenues	97.8	90.1
Data Vantage \| Market data fees
Segment Reporting Information
Revenues	54.0	49.3
Data Vantage \| Regulatory fees
Segment Reporting Information
Revenues	0.0	0.0
Data Vantage \| Other revenue
Segment Reporting Information
Revenues	0.7	0.8
Derivatives markets
Segment Reporting Information
Revenues	541.6	436.1
Derivatives markets \| Transaction and clearing fees
Segment Reporting Information
Revenues	491.6	413.3
Derivatives markets \| Access and capacity fees
Segment Reporting Information
Revenues	0.0	0.0
Derivatives markets \| Market data fees
Segment Reporting Information
Revenues	8.1	6.7
Derivatives markets \| Regulatory fees
Segment Reporting Information
Revenues	41.1	15.3
Derivatives markets \| Other revenue
Segment Reporting Information
Revenues	$ 0.8	$ 0.8