Net Capital Requirement (Details)	Dec. 31, 2016 USD ($)
Net Capital Requirement
Cash balance under Proprietary Accounts for Introducing Brokers agreements	$ 2,500,000
Canada
Net Capital Requirement
Net Capital	25,264,000
Excess Net Capital	24,892,000
Ireland
Net Capital Requirement
Net Capital	60,602,000
Excess Net Capital	36,903,000
U.K.
Net Capital Requirement
Net Capital	1,932,000
Excess Net Capital	1,159,000
Australia
Net Capital Requirement
Net Capital	12,044,000
Excess Net Capital	6,140,000
Hong Kong
Net Capital Requirement
Net Capital	24,424,000
Excess Net Capital	18,077,000
Singapore
Net Capital Requirement
Net Capital	994,000
Excess Net Capital	925,000
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 Special Reserve Bank Accounts for the benefit of customers under the Customer Protection Rule	$ 8,400,000
ITG Inc. | United States
Net Capital Requirement
Net Capital	77,469,000
Excess Net Capital	$ 76,469,000
Alter Net
Net Capital Requirement
Net capital percentage of aggregate indebtedness	6.66%
Minimum net capital under the basic method	$ 100,000
Alter Net | United States
Net Capital Requirement
Net Capital	3,435,000
Excess Net Capital	3,302,000
ITG Derivatives | United States
Net Capital Requirement
Net Capital	1,217,000
Excess Net Capital	$ 1,117,000