v3.26.1
ACQUISITIONS AND STRATEGIC REALIGNMENT - Pre-tax Expenses Related to Statements of Income by Segment (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Business Combination [Line Items]        
Total strategic realignment costs $ 23.7 $ 0.2 $ 28.8 $ 0.5
Location, Statement of Income, Balance [Axis]: us-gaap:CommunicationsAndInformationTechnology        
Business Combination [Line Items]        
Total strategic realignment costs 0.1   1.9  
Location, Statement of Income, Balance [Axis]: us-gaap:LaborAndRelatedExpense        
Business Combination [Line Items]        
Total strategic realignment costs 21.7   23.3  
Location, Statement of Income, Balance [Axis]: us-gaap:OtherCostAndExpenseOperating        
Business Combination [Line Items]        
Total strategic realignment costs (0.6)   (0.4)  
Location, Statement of Income, Balance [Axis]: us-gaap:ProfessionalFees        
Business Combination [Line Items]        
Total strategic realignment costs 2.5   4.0  
Operating Segments | Options        
Business Combination [Line Items]        
Total strategic realignment costs 15.7 0.0 15.6 0.0
Operating Segments | Options | Location, Statement of Income, Balance [Axis]: us-gaap:CommunicationsAndInformationTechnology        
Business Combination [Line Items]        
Total strategic realignment costs 0.0   0.0  
Operating Segments | Options | Location, Statement of Income, Balance [Axis]: us-gaap:LaborAndRelatedExpense        
Business Combination [Line Items]        
Total strategic realignment costs 14.0   13.9  
Operating Segments | Options | Location, Statement of Income, Balance [Axis]: us-gaap:OtherCostAndExpenseOperating        
Business Combination [Line Items]        
Total strategic realignment costs 0.0   0.0  
Operating Segments | Options | Location, Statement of Income, Balance [Axis]: us-gaap:ProfessionalFees        
Business Combination [Line Items]        
Total strategic realignment costs 1.7   1.7  
Operating Segments | North American Equities        
Business Combination [Line Items]        
Total strategic realignment costs 3.8 0.0 4.1 0.0
Operating Segments | North American Equities | Location, Statement of Income, Balance [Axis]: us-gaap:CommunicationsAndInformationTechnology        
Business Combination [Line Items]        
Total strategic realignment costs 0.0   0.0  
Operating Segments | North American Equities | Location, Statement of Income, Balance [Axis]: us-gaap:LaborAndRelatedExpense        
Business Combination [Line Items]        
Total strategic realignment costs 3.4   3.7  
Operating Segments | North American Equities | Location, Statement of Income, Balance [Axis]: us-gaap:OtherCostAndExpenseOperating        
Business Combination [Line Items]        
Total strategic realignment costs 0.0   0.0  
Operating Segments | North American Equities | Location, Statement of Income, Balance [Axis]: us-gaap:ProfessionalFees        
Business Combination [Line Items]        
Total strategic realignment costs 0.4   0.4  
Operating Segments | Europe and Asia Pacific        
Business Combination [Line Items]        
Total strategic realignment costs 1.6 0.0 5.1 0.0
Operating Segments | Europe and Asia Pacific | Location, Statement of Income, Balance [Axis]: us-gaap:CommunicationsAndInformationTechnology        
Business Combination [Line Items]        
Total strategic realignment costs 0.1   1.9  
Operating Segments | Europe and Asia Pacific | Location, Statement of Income, Balance [Axis]: us-gaap:LaborAndRelatedExpense        
Business Combination [Line Items]        
Total strategic realignment costs 2.0   3.3  
Operating Segments | Europe and Asia Pacific | Location, Statement of Income, Balance [Axis]: us-gaap:OtherCostAndExpenseOperating        
Business Combination [Line Items]        
Total strategic realignment costs (0.7)   (0.6)  
Operating Segments | Europe and Asia Pacific | Location, Statement of Income, Balance [Axis]: us-gaap:ProfessionalFees        
Business Combination [Line Items]        
Total strategic realignment costs 0.2   0.5  
Operating Segments | Futures        
Business Combination [Line Items]        
Total strategic realignment costs 1.5 0.2 1.6 0.5
Operating Segments | Futures | Location, Statement of Income, Balance [Axis]: us-gaap:CommunicationsAndInformationTechnology        
Business Combination [Line Items]        
Total strategic realignment costs 0.0   0.0  
Operating Segments | Futures | Location, Statement of Income, Balance [Axis]: us-gaap:LaborAndRelatedExpense        
Business Combination [Line Items]        
Total strategic realignment costs 1.4   1.4  
Operating Segments | Futures | Location, Statement of Income, Balance [Axis]: us-gaap:OtherCostAndExpenseOperating        
Business Combination [Line Items]        
Total strategic realignment costs 0.0   0.1  
Operating Segments | Futures | Location, Statement of Income, Balance [Axis]: us-gaap:ProfessionalFees        
Business Combination [Line Items]        
Total strategic realignment costs 0.1   0.1  
Operating Segments | Global FX        
Business Combination [Line Items]        
Total strategic realignment costs 0.7 0.0 0.7 0.0
Operating Segments | Global FX | Location, Statement of Income, Balance [Axis]: us-gaap:CommunicationsAndInformationTechnology        
Business Combination [Line Items]        
Total strategic realignment costs 0.0   0.0  
Operating Segments | Global FX | Location, Statement of Income, Balance [Axis]: us-gaap:LaborAndRelatedExpense        
Business Combination [Line Items]        
Total strategic realignment costs 0.6   0.6  
Operating Segments | Global FX | Location, Statement of Income, Balance [Axis]: us-gaap:OtherCostAndExpenseOperating        
Business Combination [Line Items]        
Total strategic realignment costs 0.0   0.0  
Operating Segments | Global FX | Location, Statement of Income, Balance [Axis]: us-gaap:ProfessionalFees        
Business Combination [Line Items]        
Total strategic realignment costs 0.1   0.1  
Corporate Items and Eliminations        
Business Combination [Line Items]        
Total strategic realignment costs 0.4 $ 0.0 1.7 $ 0.0
Corporate Items and Eliminations | Location, Statement of Income, Balance [Axis]: us-gaap:CommunicationsAndInformationTechnology        
Business Combination [Line Items]        
Total strategic realignment costs 0.0   0.0  
Corporate Items and Eliminations | Location, Statement of Income, Balance [Axis]: us-gaap:LaborAndRelatedExpense        
Business Combination [Line Items]        
Total strategic realignment costs 0.3   0.4  
Corporate Items and Eliminations | Location, Statement of Income, Balance [Axis]: us-gaap:OtherCostAndExpenseOperating        
Business Combination [Line Items]        
Total strategic realignment costs 0.1   0.1  
Corporate Items and Eliminations | Location, Statement of Income, Balance [Axis]: us-gaap:ProfessionalFees        
Business Combination [Line Items]        
Total strategic realignment costs $ 0.0   $ 1.2