Net Capital Requirement (Details) - Jun. 30, 2015 - USD ($)	Total
Canadian Operations
Net Capital Requirement
Net Capital	$ 28,600,000
Excess Net Capital	28,200,000
Cash balance in a Special Reserve Bank Account for the benefit of customers under the Customer Protection Rule	400,000
European Operations
Net Capital Requirement
Cash balance in a Special Reserve Bank Account for the benefit of customers under the Customer Protection Rule	25,900,000
Ireland
Net Capital Requirement
Net Capital	68,500,000
Excess Net Capital	18,200,000
U.K.
Net Capital Requirement
Net Capital	3,500,000
Excess Net Capital	2,700,000
Asia Pacific Operations
Net Capital Requirement
Cash balance in a Special Reserve Bank Account for the benefit of customers under the Customer Protection Rule	400,000
Australia
Net Capital Requirement
Net Capital	14,600,000
Excess Net Capital	6,000,000
Hong Kong
Net Capital Requirement
Net Capital	30,100,000
Excess Net Capital	13,400,000
Singapore
Net Capital Requirement
Net Capital	500,000
Excess Net Capital	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	2.00%
Cash balance in a Special Reserve Bank Account for the benefit of customers under the Customer Protection Rule	$ 7,300,000
Cash balance under Proprietary Accounts for Introducing Brokers agreements	1,400,000
ITG Inc. | U.S. Operations
Net Capital Requirement
Net Capital	73,100,000
Excess Net Capital	$ 72,100,000
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	3,600,000
Excess Net Capital	$ 3,300,000
ITG Derivatives
Net Capital Requirement
Net capital percentage of aggregate indebtedness	6.66%
Minimum net capital under the basic method	$ 1,000,000
Cash balance in a Special Reserve Bank Account for the benefit of customers under the Customer Protection Rule	100,000
ITG Derivatives | U.S. Operations
Net Capital Requirement
Net Capital	2,000,000
Excess Net Capital	$ 1,000,000