SEGMENT REPORTING - Summarized financial information by reportable segment (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Segment Reporting Information
Number of reportable segments \| segment			5
Revenues	$ 816.8	$ 792.7	$ 2,628.4	$ 2,582.9
Operating income (loss)	190.7	139.3	585.3	527.4
Operating Segments \| Options
Segment Reporting Information
Revenues	374.3	312.8	1,106.7	986.9
Operating income (loss)	136.5	94.4	390.4	331.2
Operating Segments \| North American Equities
Segment Reporting Information
Revenues	336.3	403.6	1,215.8	1,367.8
Operating income (loss)	31.5	33.1	114.0	138.0
Operating Segments \| Futures
Segment Reporting Information
Revenues	29.8	24.0	89.7	87.1
Operating income (loss)	16.3	10.1	49.1	45.3
Operating Segments \| Europe and Asia Pacific
Segment Reporting Information
Revenues	61.9	39.1	172.8	97.3
Operating income (loss)	12.6	9.3	40.4	24.2
Operating Segments \| Global FX
Segment Reporting Information
Revenues	14.5	13.2	43.1	43.8
Operating income (loss)	0.3	0.6	1.9	4.7
Corporate Items and Eliminations
Segment Reporting Information
Revenues			0.3
Operating income (loss)	$ (6.5)	$ (8.2)	$ (10.5)	$ (16.0)

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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