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Securities Owned and Sold, Not Yet Purchased (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Securities owned and sold, not yet purchased    
Securities owned, at fair value $ 12,073us-gaap_FinancialInstrumentsOwnedAtFairValue $ 7,436us-gaap_FinancialInstrumentsOwnedAtFairValue
Securities sold, not yet purchased, at fair value 8,253us-gaap_FinancialInstrumentsSoldNotYetPurchasedAtFairValue 2,953us-gaap_FinancialInstrumentsSoldNotYetPurchasedAtFairValue
Corporate stocks-trading securities    
Securities owned and sold, not yet purchased    
Securities owned, at fair value 8,160us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis
= us-gaap_TradingAccountAssetsMember		 2,894us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis
= us-gaap_TradingAccountAssetsMember
Securities sold, not yet purchased, at fair value 8,253us-gaap_FinancialInstrumentsSoldNotYetPurchasedAtFairValue
/ us-gaap_SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis
= us-gaap_TradingAccountAssetsMember		 2,953us-gaap_FinancialInstrumentsSoldNotYetPurchasedAtFairValue
/ us-gaap_SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis
= us-gaap_TradingAccountAssetsMember
Mutual funds    
Securities owned and sold, not yet purchased    
Securities owned, at fair value $ 3,913us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis
= itg_MutualFundsMember		 $ 4,542us-gaap_FinancialInstrumentsOwnedAtFairValue
/ us-gaap_SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis
= itg_MutualFundsMember