SEGMENT REPORTING - Summarized Financial Information by Reportable Segment (Details) $ in Millions	3 Months Ended
	Mar. 31, 2026 USD ($) segment	Mar. 31, 2025 USD ($)
Segment Reporting Information
Number of reportable segments | segment	5
Revenues	$ 1,272.8	$ 1,195.0
Cost of revenues	543.9	629.8
Revenues less cost of revenues	728.9	565.2
Depreciation and amortization	29.5	30.3
Other segment operating expenses	193.8	181.0
Operating income	505.6	353.9
Non-operating income (expenses):
Interest expense	(13.3)	(12.8)
Interest income	17.7	8.4
Earnings (loss) on investments, net	(0.7)	(3.3)
Other income (expense), net	6.2	4.0
Income before income tax provision	515.5	350.2
Income tax provision (benefit)	129.8	99.6
Net income (loss)	385.7	250.6
Options
Segment Reporting Information
Revenues	668.4	584.6
North American Equities
Segment Reporting Information
Revenues	415.7	460.1
Europe and Asia Pacific
Segment Reporting Information
Revenues	119.4	93.1
Futures
Segment Reporting Information
Revenues	38.9	35.3
Global FX
Segment Reporting Information
Revenues	30.4	21.9
Operating Segments | Options
Segment Reporting Information
Revenues	668.4	584.6
Cost of revenues	200.8	232.2
Revenues less cost of revenues	467.6	352.4
Depreciation and amortization	8.2	6.9
Other segment operating expenses	99.8	87.6
Operating income	359.6	257.9
Non-operating income (expenses):
Interest expense	0.0	0.0
Interest income	0.0	0.2
Earnings (loss) on investments, net	0.0	0.0
Other income (expense), net	0.2	0.0
Income before income tax provision	359.8	258.1
Income tax provision (benefit)	0.0	0.1
Net income (loss)	359.8	258.0
Operating Segments | North American Equities
Segment Reporting Information
Revenues	415.7	460.1
Cost of revenues	304.5	365.5
Revenues less cost of revenues	111.2	94.6
Depreciation and amortization	11.4	12.0
Other segment operating expenses	33.3	38.2
Operating income	66.5	44.4
Non-operating income (expenses):
Interest expense	0.0	0.0
Interest income	0.8	0.7
Earnings (loss) on investments, net	(0.3)	0.3
Other income (expense), net	0.1	(0.6)
Income before income tax provision	67.1	44.8
Income tax provision (benefit)	1.2	0.9
Net income (loss)	65.9	43.9
Operating Segments | Europe and Asia Pacific
Segment Reporting Information
Revenues	119.4	93.1
Cost of revenues	34.5	29.0
Revenues less cost of revenues	84.9	64.1
Depreciation and amortization	6.7	7.9
Other segment operating expenses	38.5	34.2
Operating income	39.7	22.0
Non-operating income (expenses):
Interest expense	(2.4)	(1.8)
Interest income	1.1	1.1
Earnings (loss) on investments, net	0.0	0.0
Other income (expense), net	(0.4)	(0.2)
Income before income tax provision	38.0	21.1
Income tax provision (benefit)	0.0	0.0
Net income (loss)	38.0	21.1
Operating Segments | Futures
Segment Reporting Information
Revenues	38.9	35.3
Cost of revenues	3.1	2.5
Revenues less cost of revenues	35.8	32.8
Depreciation and amortization	0.6	0.6
Other segment operating expenses	11.1	11.6
Operating income	24.1	20.6
Non-operating income (expenses):
Interest expense	0.0	0.0
Interest income	0.5	0.6
Earnings (loss) on investments, net	0.0	0.0
Other income (expense), net	0.0	0.0
Income before income tax provision	24.6	21.2
Income tax provision (benefit)	0.0	0.0
Net income (loss)	24.6	21.2
Operating Segments | Global FX
Segment Reporting Information
Revenues	30.4	21.9
Cost of revenues	1.0	0.6
Revenues less cost of revenues	29.4	21.3
Depreciation and amortization	2.6	2.9
Other segment operating expenses	9.0	8.1
Operating income	17.8	10.3
Non-operating income (expenses):
Interest expense	0.0	0.0
Interest income	0.0	0.0
Earnings (loss) on investments, net	0.0	0.0
Other income (expense), net	0.0	0.2
Income before income tax provision	17.8	10.5
Income tax provision (benefit)	0.0	(0.1)
Net income (loss)	17.8	10.6
Corporate Items and Eliminations
Segment Reporting Information
Revenues	0.0	0.0
Cost of revenues	0.0	0.0
Revenues less cost of revenues	0.0	0.0
Depreciation and amortization	0.0	0.0
Other segment operating expenses	2.1	1.3
Operating income	(2.1)	(1.3)
Non-operating income (expenses):
Interest expense	(10.9)	(11.0)
Interest income	15.3	5.8
Earnings (loss) on investments, net	(0.4)	(3.6)
Other income (expense), net	6.3	4.6
Income before income tax provision	8.2	(5.5)
Income tax provision (benefit)	128.6	98.7
Net income (loss)	$ (120.4)	$ (104.2)