Minimum Regulatory Capital Subsidiaries Required to Maintain (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 42.89
Capital Levels Maintained	100.70
Excess Net Capital	57.81
Percent of Requirement Maintained	235.00%
GAIN Capital Group, LLC
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	24.26
Capital Levels Maintained	42.82
Excess Net Capital	18.56
Percent of Requirement Maintained	177.00%
GAIN Capital Securities, Inc.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	0.05
Capital Levels Maintained	0.12
Excess Net Capital	0.07
Percent of Requirement Maintained	240.00%
GAIN Capital-Forex.com U.K., Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	10.83
Capital Levels Maintained	32.50
Excess Net Capital	21.67
Percent of Requirement Maintained	300.00%
Forex.com Japan Co., Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	4.02
Capital Levels Maintained	12.28
Excess Net Capital	8.26
Percent of Requirement Maintained	305.00%
GAIN Capital Forex.com Australia, Pty. Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	0.33
Capital Levels Maintained	2.45
Excess Net Capital	2.12
Percent of Requirement Maintained	742.00%
GAIN Capital-Forex.com Hong Kong, Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	1.93
Capital Levels Maintained	3.56
Excess Net Capital	1.63
Percent of Requirement Maintained	184.00%
GAIN Global Markets, Inc.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	0.10
Capital Levels Maintained	0.38
Excess Net Capital	0.28
Percent of Requirement Maintained	380.00%
Open E Cry Limited Liability Company
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	1.12
Capital Levels Maintained	4.74
Excess Net Capital	3.62
Percent of Requirement Maintained	423.00%
GAIN Capital-Forex.com Canada Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	0.25
Capital Levels Maintained	1.85
Excess Net Capital	$ 1.60
Percent of Requirement Maintained	740.00%