Net Capital Requirement (Details) (USD $)	Dec. 31, 2012
Canadian Operations
Net Capital Requirement
Net Capital	$ 40,300,000
Excess Net Capital	39,800,000
European Operations
Net Capital Requirement
Net Capital	49,400,000
Excess Net Capital	26,800,000
Australia
Net Capital Requirement
Net Capital	3,500,000
Excess Net Capital	1,300,000
Hong Kong
Net Capital Requirement
Net Capital	27,300,000
Excess Net Capital	15,900,000
Singapore
Net Capital Requirement
Net Capital	400,000
Excess Net Capital	200,000
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	2.00%
Cash balance in a Special Reserve Bank Account for the benefit of customers and brokers under the Customer Protection Rule	5,900,000
ITG Inc. | U.S. Operations
Net Capital Requirement
Net Capital	104,400,000
Excess Net Capital	103,400,000
Alter Net
Net Capital Requirement
Net capital percentage of aggregate indebtedness	6.66%
Minimum net capital under the basic method	100,000
Alter Net | U.S. Operations
Net Capital Requirement
Net Capital	4,600,000
Excess Net Capital	4,500,000
ITG Derivatives
Net Capital Requirement
Net capital percentage of aggregate indebtedness	6.66%
Minimum net capital under the basic method	1,000,000
ITG Derivatives | U.S. Operations
Net Capital Requirement
Net Capital	4,800,000
Excess Net Capital	$ 3,800,000