Net Capital Requirement (Details)	Jun. 30, 2018 USD ($)
Canada
Net Capital Requirement
Net Capital	$ 23,118,000
Excess Net Capital	22,738,000
Ireland
Net Capital Requirement
Net Capital	43,807,000
Excess Net Capital	13,648,000
U.K.
Net Capital Requirement
Net Capital	1,513,000
Excess Net Capital	526,000
Australia
Net Capital Requirement
Net Capital	29,273,000
Excess Net Capital	15,746,000
Hong Kong
Net Capital Requirement
Net Capital	3,137,000
Excess Net Capital	2,747,000
Singapore
Net Capital Requirement
Net Capital	1,122,000
Excess Net Capital	1,049,000
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	$ 9,200,000
Cash balance under Proprietary Accounts for Introducing Brokers agreements	3,500,000
ITG Inc. | US
Net Capital Requirement
Net Capital	81,659,000
Excess Net Capital	$ 80,659,000
Alter Net
Net Capital Requirement
Net capital percentage of aggregate indebtedness	6.66%
Minimum net capital under the basic method	$ 100,000
Alter Net | US
Net Capital Requirement
Net Capital	5,447,000
Excess Net Capital	$ 5,347,000