SEGMENT REPORTING - Summarized Financial Information By Reportable Segment (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Segment Reporting Information
Number of reportable segments | segment	6
Revenues	$ 4,094.5	$ 3,773.5	$ 3,958.5
Total cost of revenues	2,022.1	1,855.5	2,216.8
Revenues less cost of revenues:	2,072.4	1,918.0	1,741.7
Depreciation and amortization	133.0	158.0	166.8
Other segment operating expenses	841.0	702.1	1,085.3
Operating income (loss)	1,098.4	1,057.9	489.6
Non-operating (expenses) income:
Interest expense	(51.5)	(62.4)	(60.0)
Interest income	27.3	12.0	3.6
Earnings on investments	29.0	39.5	7.2
Other (expense) income, net	(19.4)	0.6	(7.5)
Income before income tax provision	1,083.8	1,047.6	432.9
Income tax provision	318.9	286.2	197.9
Net income	764.9	761.4	235.0
Operating Segments | Options
Segment Reporting Information
Revenues	2,002.6	1,939.5	1,823.2
Total cost of revenues	743.3	770.3	840.0
Revenues less cost of revenues:	1,259.3	1,169.2	983.2
Depreciation and amortization	27.0	30.1	26.5
Other segment operating expenses	353.9	287.8	216.2
Operating income (loss)	878.4	851.3	740.5
Non-operating (expenses) income:
Interest expense	0.1	0.2	0.0
Interest income	0.5	(0.1)	0.0
Earnings on investments	0.0	0.0	0.0
Other (expense) income, net	0.8	(0.1)	0.1
Income before income tax provision	879.8	851.3	740.6
Income tax provision	299.1	275.7	260.7
Net income	580.7	575.6	479.9
Operating Segments | North American Equities
Segment Reporting Information
Revenues	1,546.8	1,353.0	1,681.7
Total cost of revenues	1,163.0	987.7	1,302.8
Revenues less cost of revenues:	383.8	365.3	378.9
Depreciation and amortization	58.2	69.4	74.1
Other segment operating expenses	157.0	177.9	158.2
Operating income (loss)	168.6	118.0	146.6
Non-operating (expenses) income:
Interest expense	0.0	0.0	0.0
Interest income	2.4	1.4	0.4
Earnings on investments	0.0	0.0	0.0
Other (expense) income, net	0.7	0.0	(0.1)
Income before income tax provision	171.7	119.4	146.9
Income tax provision	23.1	14.8	20.5
Net income	148.6	104.6	126.4
Operating Segments | Europe and Asia Pacific
Segment Reporting Information
Revenues	324.2	281.2	264.6
Total cost of revenues	104.0	91.0	68.5
Revenues less cost of revenues:	220.2	190.2	196.1
Depreciation and amortization	29.3	30.7	37.0
Other segment operating expenses	149.2	126.8	121.0
Operating income (loss)	41.7	32.7	38.1
Non-operating (expenses) income:
Interest expense	(7.8)	(7.8)	(8.8)
Interest income	4.1	3.0	0.8
Earnings on investments	0.0	0.0	0.0
Other (expense) income, net	(0.1)	(0.6)	(0.4)
Income before income tax provision	37.9	27.3	29.7
Income tax provision	13.3	6.8	6.8
Net income	24.6	20.5	22.9
Operating Segments | Futures
Segment Reporting Information
Revenues	141.1	129.0	119.8
Total cost of revenues	7.6	3.9	3.8
Revenues less cost of revenues:	133.5	125.1	116.0
Depreciation and amortization	2.3	2.0	2.6
Other segment operating expenses	32.3	37.0	58.2
Operating income (loss)	98.9	86.1	55.2
Non-operating (expenses) income:
Interest expense	0.0	0.0	0.0
Interest income	0.0	0.0	0.0
Earnings on investments	0.0	0.0	0.0
Other (expense) income, net	0.0	0.0	0.0
Income before income tax provision	98.9	86.1	55.2
Income tax provision	28.4	33.4	42.4
Net income	70.5	52.7	12.8
Operating Segments | Global FX
Segment Reporting Information
Revenues	79.9	74.9	68.9
Total cost of revenues	2.3	1.4	1.0
Revenues less cost of revenues:	77.6	73.5	67.9
Depreciation and amortization	13.5	18.4	21.9
Other segment operating expenses	30.9	30.4	37.2
Operating income (loss)	33.2	24.7	8.8
Non-operating (expenses) income:
Interest expense	0.0	0.0	0.3
Interest income	0.1	0.0	0.1
Earnings on investments	0.0	0.0	0.0
Other (expense) income, net	0.0	(0.2)	0.0
Income before income tax provision	33.3	24.5	9.2
Income tax provision	0.1	0.5	0.1
Net income	33.2	24.0	9.1
Operating Segments | Digital
Segment Reporting Information
Revenues	(0.1)	(4.1)	0.3
Total cost of revenues	1.9	1.2	0.7
Revenues less cost of revenues:	(2.0)	(5.3)	(0.4)
Depreciation and amortization	2.8	7.4	4.7
Other segment operating expenses	107.6	34.0	486.3
Operating income (loss)	(112.4)	(46.7)	(491.4)
Non-operating (expenses) income:
Interest expense	0.0	0.0	0.0
Interest income	3.7	2.0	0.0
Earnings on investments	0.0	0.0	0.0
Other (expense) income, net	1.5	0.0	1.3
Income before income tax provision	(107.2)	(44.7)	(490.1)
Income tax provision	(28.6)	(10.4)	(119.0)
Net income	(78.6)	(34.3)	(371.1)
Corporate Items and Eliminations
Segment Reporting Information
Revenues	0.0	0.0	0.0
Total cost of revenues	0.0	0.0	0.0
Revenues less cost of revenues:	0.0	0.0	0.0
Depreciation and amortization	(0.1)	0.0	0.0
Other segment operating expenses	10.1	8.2	8.2
Operating income (loss)	(10.0)	(8.2)	(8.2)
Non-operating (expenses) income:
Interest expense	(43.8)	(54.8)	(51.5)
Interest income	16.5	5.7	2.3
Earnings on investments	29.0	39.5	7.2
Other (expense) income, net	(22.3)	1.5	(8.4)
Income before income tax provision	(30.6)	(16.3)	(58.6)
Income tax provision	(16.5)	(34.6)	(13.6)
Net income	$ (14.1)	$ 18.3	$ (45.0)