SEGMENT REPORTING - Summarized Financial Information By Reportable Segment (Details) $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($) segment	Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($)
Segment Reporting Information
Number of reportable segments | segment	5	6
Revenues	$ 4,714.2	$ 4,094.5	$ 3,773.5
Cost of revenues	2,285.1	2,022.1	1,855.5
Revenues less cost of revenues	2,429.1	2,072.4	1,918.0
Depreciation and amortization	122.4	133.0	158.0
Other segment operating expenses	839.6	841.0	702.1
Operating income	1,467.1	1,098.4	1,057.9
Non-operating (expenses) income:
Interest expense	(52.3)	(51.5)	(62.4)
Interest income	49.4	27.3	12.0
Earnings on investments, net	92.8	29.0	39.5
Other income (expense), net	9.6	(19.4)	0.6
Income (loss) before income tax provision (benefit)	1,566.6	1,083.8	1,047.6
Income tax provision (benefit)	466.6	318.9	286.2
Net income	1,100.0	764.9	761.4
Operating Segments | Options
Segment Reporting Information
Revenues	2,433.6	2,002.6	1,939.5
Cost of revenues	902.5	743.3	770.3
Revenues less cost of revenues	1,531.1	1,259.3	1,169.2
Depreciation and amortization	29.3	27.0	30.1
Other segment operating expenses	388.5	353.9	287.8
Operating income	1,113.3	878.4	851.3
Non-operating (expenses) income:
Interest expense	0.0	0.1	0.2
Interest income	1.1	0.5	(0.1)
Earnings on investments, net	0.0	0.0	0.0
Other income (expense), net	(1.4)	0.8	(0.1)
Income (loss) before income tax provision (benefit)	1,113.0	879.8	851.3
Income tax provision (benefit)	373.7	299.1	275.7
Net income	739.3	580.7	575.6
Operating Segments | North American Equities
Segment Reporting Information
Revenues	1,672.3	1,546.8	1,353.0
Cost of revenues	1,265.1	1,163.0	987.7
Revenues less cost of revenues	407.2	383.8	365.3
Depreciation and amortization	47.0	58.2	69.4
Other segment operating expenses	172.1	157.0	177.9
Operating income	188.1	168.6	118.0
Non-operating (expenses) income:
Interest expense	0.0	0.0	0.0
Interest income	3.3	2.4	1.4
Earnings on investments, net	0.9	0.0	0.0
Other income (expense), net	(1.2)	0.7	0.0
Income (loss) before income tax provision (benefit)	191.1	171.7	119.4
Income tax provision (benefit)	27.2	23.1	14.8
Net income	163.9	148.6	104.6
Operating Segments | Europe and Asia Pacific
Segment Reporting Information
Revenues	378.6	324.2	281.2
Cost of revenues	105.1	104.0	91.0
Revenues less cost of revenues	273.5	220.2	190.2
Depreciation and amortization	32.4	29.3	30.7
Other segment operating expenses	187.2	149.2	126.8
Operating income	53.9	41.7	32.7
Non-operating (expenses) income:
Interest expense	(8.6)	(7.8)	(7.8)
Interest income	4.3	4.1	3.0
Earnings on investments, net	0.0	0.0	0.0
Other income (expense), net	1.7	(0.1)	(0.6)
Income (loss) before income tax provision (benefit)	51.3	37.9	27.3
Income tax provision (benefit)	18.7	13.3	6.8
Net income	32.6	24.6	20.5
Operating Segments | Futures
Segment Reporting Information
Revenues	135.9	141.1	129.0
Cost of revenues	9.7	7.6	3.9
Revenues less cost of revenues	126.2	133.5	125.1
Depreciation and amortization	2.3	2.3	2.0
Other segment operating expenses	50.0	32.3	37.0
Operating income	73.9	98.9	86.1
Non-operating (expenses) income:
Interest expense	0.0	0.0	0.0
Interest income	2.4	0.0	0.0
Earnings on investments, net	0.0	0.0	0.0
Other income (expense), net	0.0	0.0	0.0
Income (loss) before income tax provision (benefit)	76.3	98.9	86.1
Income tax provision (benefit)	12.9	28.4	33.4
Net income	63.4	70.5	52.7
Operating Segments | Global FX
Segment Reporting Information
Revenues	93.8	79.9	74.9
Cost of revenues	2.7	2.3	1.4
Revenues less cost of revenues	91.1	77.6	73.5
Depreciation and amortization	11.2	13.5	18.4
Other segment operating expenses	33.9	30.9	30.4
Operating income	46.0	33.2	24.7
Non-operating (expenses) income:
Interest expense	0.0	0.0	0.0
Interest income	0.1	0.1	0.0
Earnings on investments, net	0.0	0.0	0.0
Other income (expense), net	0.2	0.0	(0.2)
Income (loss) before income tax provision (benefit)	46.3	33.3	24.5
Income tax provision (benefit)	(0.1)	0.1	0.5
Net income	46.4	33.2	24.0
Operating Segments | Digital
Segment Reporting Information
Revenues	0.0	(0.1)	(4.1)
Cost of revenues	0.0	1.9	1.2
Revenues less cost of revenues	0.0	(2.0)	(5.3)
Depreciation and amortization	0.0	2.8	7.4
Other segment operating expenses	0.0	107.6	34.0
Operating income	0.0	(112.4)	(46.7)
Non-operating (expenses) income:
Interest expense	0.0	0.0	0.0
Interest income	0.0	3.7	2.0
Earnings on investments, net	0.0	0.0	0.0
Other income (expense), net	0.0	1.5	0.0
Income (loss) before income tax provision (benefit)	0.0	(107.2)	(44.7)
Income tax provision (benefit)	0.0	(28.6)	(10.4)
Net income	0.0	(78.6)	(34.3)
Corporate Items and Eliminations
Segment Reporting Information
Revenues	0.0	0.0	0.0
Cost of revenues	0.0	0.0	0.0
Revenues less cost of revenues	0.0	0.0	0.0
Depreciation and amortization	0.2	(0.1)	0.0
Other segment operating expenses	7.9	10.1	8.2
Operating income	(8.1)	(10.0)	(8.2)
Non-operating (expenses) income:
Interest expense	(43.7)	(43.8)	(54.8)
Interest income	38.2	16.5	5.7
Earnings on investments, net	91.9	29.0	39.5
Other income (expense), net	10.3	(22.3)	1.5
Income (loss) before income tax provision (benefit)	88.6	(30.6)	(16.3)
Income tax provision (benefit)	34.2	(16.5)	(34.6)
Net income	$ 54.4	$ (14.1)	$ 18.3