Net Capital Requirement (Detail) (USD $)	Dec. 31, 2014
Net Capital Requirement
Cash balance under Proprietary Accounts for Introducing Brokers agreements	$ 1,000,000
Canadian Operations
Net Capital Requirement
Net Capital	36,800,000
Excess Net Capital	36,400,000
Ireland
Net Capital Requirement
Net Capital	63,200,000
Excess Net Capital	39,000,000
U.K.
Net Capital Requirement
Net Capital	3,900,000
Excess Net Capital	3,000,000
Australia
Net Capital Requirement
Net Capital	12,300,000
Excess Net Capital	8,200,000
Hong Kong
Net Capital Requirement
Net Capital	26,800,000
Excess Net Capital	14,100,000
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 under the Customer Protection Rule	10,000,000
ITG Inc. | U.S. Operations
Net Capital Requirement
Net Capital	99,800,000
Excess Net Capital	98,800,000
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	5,500,000
Excess Net Capital	5,300,000
ITG Derivatives
Net Capital Requirement
Minimum net capital under the basic method	1,000,000
ITG Derivatives | U.S. Operations
Net Capital Requirement
Net Capital	2,400,000
Excess Net Capital	$ 1,400,000