Net Capital Requirement (Details)	Sep. 30, 2017 USD ($)
Canadian Operations
Net Capital Requirement
Net Capital	$ 25,878,000
Excess Net Capital	25,478,000
Ireland
Net Capital Requirement
Net Capital	75,352,000
Excess Net Capital	41,882,000
U.K.
Net Capital Requirement
Net Capital	2,019,000
Excess Net Capital	1,156,000
Australia
Net Capital Requirement
Net Capital	25,082,000
Excess Net Capital	8,560,000
Hong Kong
Net Capital Requirement
Net Capital	6,212,000
Excess Net Capital	5,723,000
Singapore
Net Capital Requirement
Net Capital	1,075,000
Excess Net Capital	1,001,000
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	$ 8,900,000
Cash balance under Proprietary Accounts for Introducing Brokers agreements	2,500,000
ITG Inc. | U.S. Operations
Net Capital Requirement
Net Capital	84,131,000
Excess Net Capital	$ 83,131,000
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	4,550,000
Excess Net Capital	$ 4,450,000
ITG Derivatives
Net Capital Requirement
Net capital percentage of aggregate indebtedness	6.66%
ITG Derivatives | U.S. Operations
Net Capital Requirement
Net Capital	$ 723,000
Excess Net Capital	$ 623,000