Net Capital Requirement (Details) (USD $)	Sep. 30, 2011
U.S. Operations | ITG Inc.
Net Capital Requirement
Net Capital	$ 67,300,000
Excess Net Capital	66,300,000
ITG Inc.
Net Capital Requirement
Minimum net capital under the alternative method	1,000,000
Net capital percentage of debit balances arising from customer transactions (as a percent)	2.00%
Cash balance in a Special Reserve Bank Account for the benefit of customers and brokers under the Customer Protection Rule	10,800,000
U.S. Operations | AlterNet
Net Capital Requirement
Net Capital	4,200,000
Excess Net Capital	4,000,000
AlterNet
Net Capital Requirement
Net capital percentage of aggregate indebtedness (as a percent)	6.67%
Minimum net capital under the basic method.	100,000
U.S. Operations | Blackwatch Brokerage Inc. ("Blackwatch")
Net Capital Requirement
Net Capital	2,800,000
Excess Net Capital	2,600,000
Blackwatch Brokerage Inc. ("Blackwatch")
Net Capital Requirement
Net capital percentage of aggregate indebtedness (as a percent)	6.67%
Minimum net capital under the basic method.	5,000
U.S. Operations | ITG Derivatives LLC ("ITG Derivatives")
Net Capital Requirement
Net Capital	2,500,000
Excess Net Capital	1,500,000
ITG Derivatives LLC ("ITG Derivatives")
Net Capital Requirement
Net capital percentage of aggregate indebtedness (as a percent)	6.67%
Minimum net capital under the basic method.	1,000,000
Canadian Operations
Net Capital Requirement
Net Capital	48,600,000
Excess Net Capital	48,100,000
European Operations
Net Capital Requirement
Net Capital	44,500,000
Excess Net Capital	20,900,000
Australia
Net Capital Requirement
Net Capital	8,000,000
Excess Net Capital	2,200,000
Hong Kong
Net Capital Requirement
Net Capital	31,700,000
Excess Net Capital	9,500,000
Singapore
Net Capital Requirement
Net Capital	300,000
Excess Net Capital	$ 100,000