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Summary of Significant Accounting Policies (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
item
Dec. 31, 2013
Dec. 31, 2012
Principles of Consolidation      
Interests in variable interest entities (as a percent) 0.00%us-gaap_VariableInterestEntityOwnershipPercentage    
Revenue recognition      
Additional fee on unspecified future software upgrades $ 0us-gaap_TechnologyServicesRevenue    
Number of revenue recognition criteria 4itg_NumberOfCriteriaToBeMetForRevenueRecognition    
Cash and Cash Equivalents      
Maximum original maturity period of cash equivalents 3 months    
Securities Borrowed and Loaned      
Number of parties from whom matched book securities are purchased 1itg_NumberOfPartiesFromWhomMatchedBookBusinessSecuritiesArePurchased    
Client Commission Arrangements      
Client commissions 119,400,000itg_ClientCommissionsAllocatedToResearch 123,000,000itg_ClientCommissionsAllocatedToResearch 112,100,000itg_ClientCommissionsAllocatedToResearch
Prepaid research, gross 4,300,000itg_PrepaidResearchReceivableGross 4,500,000itg_PrepaidResearchReceivableGross 5,100,000itg_PrepaidResearchReceivableGross
Allowance for prepaid research   (200,000)itg_AllowanceForPrepaidResearchReceivable (400,000)itg_AllowanceForPrepaidResearchReceivable
Prepaid research, net of allowance 4,300,000itg_PrepaidResearchReceivableNet 4,300,000itg_PrepaidResearchReceivableNet 4,700,000itg_PrepaidResearchReceivableNet
Accrued research payables 56,736,000itg_AccruedResearchPayables 52,015,000itg_AccruedResearchPayables 38,600,000itg_AccruedResearchPayables
Research and Development      
Research and development costs $ 35,800,000us-gaap_ResearchAndDevelopmentExpense $ 41,700,000us-gaap_ResearchAndDevelopmentExpense $ 45,400,000us-gaap_ResearchAndDevelopmentExpense
Maximum      
Principles of Consolidation      
Ownership percentage for equity method investments 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Ownership percentage for consolidation of affiliate 50.00%itg_OwnershipPercentageByParentForConsolidation
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Revenue recognition      
Term of agreement 3 years    
Minimum      
Principles of Consolidation      
Ownership percentage for equity method investments 20.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Revenue recognition      
Term of agreement 1 year    
Furniture, fixtures and equipment | Maximum      
Premises and equipment      
Estimated useful life 7 years    
Furniture, fixtures and equipment | Minimum      
Premises and equipment      
Estimated useful life 3 years    
Proprietary software      
Premises and equipment      
Estimated useful life 3 years