Condensed Consolidated Statements Of Operations And Comprehensive Income (USD $) In Thousands, except Share data	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Revenue:
Trading revenue	$ 54,084		$ 54,459		$ 93,912		$ 96,518
Other revenue	1,696		730		2,623		1,176
Total non-interest revenue	55,780		55,189		96,535		97,694
Interest revenue	151		103		253		166
Interest expense	(341)		(585)		(816)		(1,184)
Total net interest expense	(190)		(482)		(563)		(1,018)
Net revenue	55,590		54,707		95,972		96,676
Expenses:
Employee compensation and benefits	12,200		11,379		23,344		22,597
Selling and marketing	8,954		8,772		19,166		18,478
Trading expenses and commissions	8,431		7,129		15,617		12,270
Bank fees	1,137		1,142		2,123		2,184
Depreciation and amortization	968		860		1,900		1,649
Purchased intangible amortization	1,959				3,656
Communications and data processing	752		719		1,412		1,470
Occupancy and equipment	1,120		957		2,314		1,919
Bad debt provision	114		113		603		355
Professional fees	1,134		529		1,963		967
Product development, software and maintenance	1,100		855		1,950		1,564
Change in fair value of convertible, redeemable preferred stock embedded derivative			(820)				(60,283)
Other	1,719		1,888		3,686		2,673
Total	39,588		33,523		77,734		5,843
Income before income tax expense	16,002		21,184		18,238		90,833
Income tax expense	5,989		7,389		6,822		11,478
Net income	10,013		13,795		11,416		79,355
Net loss applicable to noncontrolling interest							(402)
Net income applicable to gain capital holdings, inc.	10,013		13,795		11,416		79,757
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustment	340		(257)		863		(848)
Net comprehensive income	10,353		13,538		12,279		78,909
Net income applicable to GAIN Capital Holdings, Inc. common shareholders	$ 10,013		$ 13,538		$ 11,416		$ 79,757
Earnings per common share:
Basic	$ 0.29	[1]	$ 4.62	[1]	$ 0.33	[1]	$ 26.79	[1]
Diluted	$ 0.26	[1]	$ 0.37	[1]	$ 0.29	[1]	$ 2.17	[1]
Weighted average common shares outstanding used in computing earnings per common share:
Basic	34,544,643	[1]	2,985,136	[1]	34,156,494	[1]	2,977,209	[1]
Diluted	39,086,435	[1]	36,945,784	[1]	38,978,908	[1]	36,704,088	[1]

[1] (1) In connection with the completion of the Company's initial public offering in December 2010 (the "IPO"), the Company's board of directors approved a 2.29-for-1 stock split of the Company's common stock to be effective immediately prior to the completion of the IPO. The 2.29-for-1 stock split, after giving effect to the receipt by the Company of 407,692 shares of common stock from all of the Company's pre-IPO common stockholders (on a pro-rata basis) in satisfaction of previously outstanding obligations owed by such stockholders to the Company, resulted in an effective stock split of 2.26-for-1. Accordingly, all references to share and per share data have been retroactively restated for the three months and six months ended June 30, 2010 to reflect the effective 2.26-for-1 stock split.