GAIN Capital 10-Q 9 May 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Apr. 30, 2018
Document Documentand Entity Information [Abstract]
Entity Registrant Name	GAIN Capital Holdings, Inc.
Trading Symbol	GCAP
Entity Central Index Key	1,444,363
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		44,753,644

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
ASSETS:
Cash and cash equivalents	$ 239,704	$ 209,688
Cash and securities held for customers	963,221	978,828
Receivables from brokers	72,017	78,503
Property and equipment, net	40,173	40,742
Intangible assets, net	59,152	61,969
Goodwill	33,443	33,036
Other assets	46,898	45,881
Total assets	1,454,608	1,448,647
Liabilities
Payables to customers	963,221	978,828
Payables to brokers	0	2,789
Accrued compensation and benefits	8,782	10,104
Accrued expenses and other liabilities	37,294	33,947
Income tax payable	3,630	599
Convertible senior notes	133,734	132,221
Total liabilities	1,146,661	1,158,488
Commitments and contingent liabilities
Redeemable non-controlling interests	1,953	4,411
Shareholders’ equity
Common stock ($0.00001 par value; 120 million shares authorized, 53,944,154 shares issued and 44,901,625 shares outstanding as of March 31, 2018; 120 million shares authorized, 53,612,340 shares issued and 45,152,299 shares outstanding as of December 31, 2017)	0	0
Additional paid-in capital	237,560	235,659
Retained earnings	135,762	122,686
Accumulated other comprehensive loss	(6,196)	(15,670)
Treasury stock, at cost (9,042,529 shares at March 31, 2018 and 8,460,041 at December 31, 2017)	(61,132)	(56,927)
Total shareholders’ equity	305,994	285,748
Total liabilities and shareholders’ equity	$ 1,454,608	$ 1,448,647

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Common stock, par value (in dollars per share)	$ 0.00001	$ 0.00001
Common stock, shares authorized (in shares)	120,000,000	120,000,000
Common stock, shares issued (in shares)	53,944,154	52,848,811
Common stock, shares outstanding (in shares)	44,901,625	48,220,243
Treasury stock, shares (in shares)	9,042,529	4,628,568

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Income/(Loss) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
REVENUE:
Retail revenue	$ 84,120	$ 38,931
Institutional revenue	8,455	8,367
Futures revenue	10,645	10,580
Other revenue	1,895	1,011
Total non-interest revenue	105,115	58,889
Interest revenue	2,122	841
Interest expense	374	163
Total net interest revenue	1,748	678
Net revenue	106,863	59,567
EXPENSES:
Employee compensation and benefits	27,751	24,215
Selling and marketing	6,027	9,256
Referral fees	11,948	16,441
Trading expenses	8,468	8,067
General and administrative	12,985	9,737
Depreciation and amortization	5,669	4,019
Purchased intangible amortization	4,204	3,602
Communications and technology	5,524	5,152
Bad debt provision	1,118	72
Impairment of investment	130	0
Total operating expense	83,564	80,561
OPERATING PROFIT/(LOSS)	23,299	(20,994)
Interest expense on long term borrowings	3,340	2,665
INCOME/(LOSS) BEFORE INCOME TAX EXPENSE/(BENEFIT)	19,959	(23,659)
Income tax expense/(benefit)	3,698	(4,893)
Equity in net loss of affiliate	0	(20)
NET INCOME/(LOSS)	16,261	(18,786)
Net income attributable to non-controlling interests	175	85
NET INCOME/(LOSS) APPLICABLE TO GAIN CAPITAL HOLDINGS, INC.	16,086	(18,871)
Other comprehensive income/(loss):
Foreign currency translation adjustment	9,474	4,539
NET COMPREHENSIVE INCOME/(LOSS) APPLICABLE TO GAIN CAPITAL HOLDINGS, INC.	$ 25,560	$ (14,332)
Earnings/(loss) per common share:
Basic (usd per share)	$ 0.35	$ (0.39)
Diluted (usd per share)	$ 0.35	$ (0.39)
Weighted average common shares outstanding used in computing earnings/(loss) per common share:
Basic (shares)	45,017,716	47,894,546
Diluted (shares)	45,523,766	47,894,546

Condensed Consolidated Stateme5

Condensed Consolidated Statement of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
BEGINNING BALANCE (in shares) at Dec. 31, 2016		48,220,243
BEGINNING BALANCE at Dec. 31, 2016	$ 294,182	$ 0	$ (30,767)	$ 218,392	$ 143,399	$ (36,842)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) applicable to Gain Capital Holdings, Inc.	(18,871)				(18,871)
Exercise of options (in shares)		182,751
Exercise of options	0			0
Purchase of treasury stock (in shares)		(612,423)
Purchase of treasury stock	(4,662)		(4,662)
Shares withheld for net settlements of share-based awards (in shares)		(22,598)
Shares withheld for net settlements of share-based awards	(182)		(182)
Share-based compensation	1,113			1,113
Adjustment to fair value of redeemable non-controlling interests	68				68
Dividends declared	(2,874)				(2,874)
Foreign currency translation adjustment	4,539					4,539
ENDING BALANCE (in shares) at Mar. 31, 2017		47,767,973
ENDING BALANCE at Mar. 31, 2017	273,313	$ 0	(35,611)	219,505	121,722	(32,303)
BEGINNING BALANCE (in shares) at Dec. 31, 2016		48,220,243
BEGINNING BALANCE at Dec. 31, 2016	294,182	$ 0	(30,767)	218,392	143,399	(36,842)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to fair value of redeemable non-controlling interests	(367)
ENDING BALANCE (in shares) at Dec. 31, 2017		45,152,299
ENDING BALANCE at Dec. 31, 2017	285,748	$ 0	(56,927)	235,659	122,686	(15,670)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) applicable to Gain Capital Holdings, Inc.	16,086				16,086
Exercise of options (in shares)		56,500
Exercise of options	443			443
Conversion of restricted stock into common stock (in shares)		275,314
Conversion of restricted stock into common stock	0
Issuance of common stock for the employee stock purchase plan (in shares)		0
Issuance of common stock for the employee stock purchase plan	0			0
Purchase of treasury stock (in shares)		(580,064)
Purchase of treasury stock	(4,187)		(4,187)
Shares withheld for net settlements of share-based awards (in shares)		(2,424)
Shares withheld for net settlements of share-based awards	(18)		(18)
Share-based compensation	1,458			1,458
Recognition of debt discount on convertible notes	0			0
Tax effect of debt discount on convertible notes	0			0
Repurchase of convertible notes	0			0
Adjustment to fair value of redeemable non-controlling interests	(367)				(367)
Dividends declared	(2,643)				(2,643)
Foreign currency translation adjustment	9,474					9,474
ENDING BALANCE (in shares) at Mar. 31, 2018		44,901,625
ENDING BALANCE at Mar. 31, 2018	$ 305,994	$ 0	$ (61,132)	$ 237,560	$ 135,762	$ (6,196)

Condensed Consolidated Stateme6

Condensed Consolidated Statement of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net (loss)/income	$ 16,261	$ (18,786)
Adjustments to reconcile net (loss)/income to cash (used in)/provided by operating activities
(Gain)/loss on foreign currency exchange rates	(1,325)	(117)
Depreciation and amortization	9,872	7,621
Deferred tax expense/(benefit)	(1,607)	0
Amortization of deferred financing costs	163	110
Bad debt provision	1,118	72
Convertible senior notes discount amortization	1,350	1,130
Share-based compensation	1,458	1,113
Equity in net loss of affiliate	0	20
Changes in operating assets and liabilities:
Receivables from brokers	7,304	(14,365)
Other assets	1,441	2,969
Payable(s) to customer(s)	(36,506)	86,437
Payables to brokers	(2,789)	0
Accrued compensation and benefits	(1,538)	(8,720)
Accrued expenses and other liabilities	(523)	(3,334)
Income tax payable	3,127	(2,376)
Net cash (used in)/provided by operating activities	(2,194)	51,774
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(3,938)	(5,639)
Acquisition of FXCM assets	0	(5,125)
Net cash used in investing activities	(3,938)	(10,764)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from exercise of stock options	443	0
Purchase of treasury stock	(4,205)	(4,844)
Dividend payments	(2,643)	(2,874)
Distributions to non-controlling interest holders	(90)	(811)
Net cash used in financing activities	(6,495)	(8,529)
Effect of exchange rate changes on cash and cash equivalents	27,036	13,102
NET INCREASE/ (DECREASE) IN CASH AND CASH EQUIVALENTS AND CASH AND SECURITIES HELD FOR CUSTOMERS	14,409	45,583
CASH AND CASH EQUIVALENTS AND CASH AND SECURITIES HELD FOR CUSTOMERS—Beginning of period	1,188,516	1,180,228	$ 1,180,228
CASH AND CASH EQUIVALENTS AND CASH AND SECURITIES HELD FOR CUSTOMERS—End of period	1,202,925	1,225,811	1,188,516	$ 1,180,228
Cash and cash equivalents					$ 239,704	$ 209,688	$ 183,667
Cash and securities held for customers					963,221		1,042,144
CASH AND CASH EQUIVALENTS AND CASH AND SECURITIES HELD FOR CUSTOMERS—End of period	1,188,516	1,180,228	1,180,228	1,180,228	$ 1,202,925	$ 1,188,516	$ 1,225,811
Cash (paid)/received during the year for:
Interest			(2,585)	(163)
Income taxes			(166)	(403)
Non-cash financing activities:
Adjustment to redemption value of non-controlling interests	$ (367)	$ 68	$ (367)	$ 68

DESCRIPTION OF BUSINESS AND BAS

DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION	DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION Description of Business GAIN Capital Holdings, Inc. (together with its subsidiaries, the “Company”), is a Delaware corporation formed and incorporated on March 24, 2006 . GAIN Holdings, LLC is a wholly-owned subsidiary of GAIN Capital Holdings, Inc., and owns all outstanding membership units of GAIN Capital Group, LLC (“Group, LLC”), the Company’s primary regulated entity in the United States. GAIN Capital Holdings Ltd. is the holding company of the Company’s primary regulated entity in the United Kingdom. The Company is a global provider of trading services and solutions, specializing in over-the-counter, or OTC, and exchange-traded markets. The Company operates its business in three segments. Through its retail segment, the Company provides its retail customers around the world with access to a diverse range of global financial markets, including spot forex, precious metals, as well as spread bets and contracts for difference, or CFDs, on currencies, commodities, indices, individual equities, cryptocurrencies, bonds and interest rate products, as well as OTC options. The Company’s institutional segment provides agency execution services and electronic access to spot and forward foreign exchange and precious metals markets via the electronic communications network, or ECN, GTX. The Company’s futures segment offers execution and risk management services for exchange-traded products on major U.S. and European exchanges, including Bitcoin. For more information about the Company’s segments, please see Note 18 . Group, LLC is a retail foreign exchange dealer (“RFED”) and a Futures Commission Merchant (“FCM”) registered with the Commodity Futures Trading Commission (the “CFTC”). As such, it is subject to the regulations of the CFTC, an agency of the U.S. government, and the rules of the National Futures Association (“NFA”), an industry self-regulatory organization. GAIN Capital UK Limited ("GCUK") is registered in the United Kingdom (“U.K.”) and regulated by the Financial Conduct Authority (“FCA”) as a full scope €730k IFPRU Investment Firm. Basis of Presentation and Principles of Consolidation The accompanying unaudited Condensed Consolidated Financial Statements reflect all adjustments that, in the opinion of management, are necessary for a fair presentation of the financial statements for the interim periods. The financial statements are presented in accordance with accounting principles generally accepted in the United States of America. The unaudited Condensed Consolidated Financial Statements have been prepared in accordance with the Securities and Exchange Commission's ("SEC") regulations for interim financial statements, and, in accordance with SEC rules, omit or condense certain information and footnote disclosures. Results for the interim periods are not necessarily indicative of results to be expected for any other interim period or for the full year. These financial statements should be read in conjunction with the audited Consolidated Financial Statements and related notes included in the Company’s Annual Report on Form 10-K as of and for the year ended December 31, 2017 . The unaudited Condensed Consolidated Financial Statements include the accounts of the Company and its wholly-owned subsidiaries, after the elimination of inter-company transactions and balances.

RECENT ACCOUNTING PRONOUNCEMENT

RECENT ACCOUNTING PRONOUNCEMENTS NOT YET ADOPTED	3 Months Ended
	Mar. 31, 2018
Accounting Changes and Error Corrections [Abstract]
RECENT ACCOUNTING PRONOUNCEMENTS NOT YET ADOPTED	ACCOUNTING PRONOUNCEMENTS Recently Adopted In January 2017, the Financial Accounting Standard Board (the "FASB") issued ASU No. 2017-04, “Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment,” which eliminates the requirement to calculate the implied fair value of goodwill to measure a goodwill impairment charge (Step 2) from the goodwill impairment test. Instead, an impairment charge will equal the amount by which a reporting unit’s carrying amount exceeds its fair value, not to exceed the amount of goodwill allocated to the reporting unit. The guidance will be effective prospectively for the Company for its annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019, with early adoption permitted after January 1, 2017 . The Company has adopted this ASU as of January 1, 2018, and will apply Step 2 to the extent it is required. In November 2016, the FASB issued ASU No. 2016-18, “Restricted Cash,” which requires that companies present cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents (restricted cash) when reconciling beginning-of-period and end-of-period totals on the Statement of Cash Flows. The Company has retrospectively adopted this ASU as of January 1, 2018 and as a result Net cash provided by operating activities of continuing operations on the Statement of Cash Flows increased by $86.4 million for the three months ended March 31, 2017. The Company defines restricted cash to include cash and cash equivalents held for customers and represents cash and other highly liquid assets held to fund customer liabilities in connection with trading positions. Included in this balance are funds deposited by customers and funds accruing to customers as a result of trades or contracts. The Company records a corresponding liability in connection with this amount in Payables to customers. In addition, the Company holds certain customer funds in segregated or secured broker accounts. Legally segregated balances are not available for general use, in accordance with certain jurisdictional regulatory requirements. In May 2014, the FASB issued ASU No. 2014-09, “Revenue from Contracts with Customers”, which is part of ASC Topic 606. It defines how companies report revenues from contracts with customers and also requires certain enhanced disclosures. The standard’s provisions and related amendments are effective for annual reporting periods beginning after December 15, 2017. On January 1, 2018, the Company adopted this guidance, which did not have a material impact on the Company’s financial statements. A substantial portion of revenue falls under ASC Topic 825, Financial Instruments, which is excluded from the scope of the new guidance. The Company adopted ASU No. 2014-09 using the modified retrospective approach. See Note 3 Revenue Recognition for additional disclosure. Not Yet Adopted In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842),” which amended the guidance on accounting for leases. The FASB issued this update to increase transparency and comparability among organizations. This update requires recognizing lease assets and lease liabilities on the balance sheet and disclosing key information about leasing arrangements. The guidance is effective for fiscal years beginning after December 15, 2018 , including interim periods within that fiscal year. The Company is currently assessing the impact on its Financial Statements of adopting this guidance. In February 2018, the FASB issued ASU No. 2018-02, “Income Statement - Reporting Comprehensive Income (Topic 220),” to address certain income tax effects in Accumulated Other Comprehensive Income (AOCI) resulting from the tax reform enacted in 2017. The amended guidance provides an option to reclassify tax effects within AOCI to retained earnings in the period in which the effect of the tax reform is recorded. The amendments are effective for fiscal years beginning after December 15, 2018 , including interim periods. Early adoption is permitted. The Company is currently assessing the impact on its Financial Statements of adopting this guidance.

REVENUE RECOGNITION

REVENUE RECOGNITION	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
REVENUE RECOGNITION	REVENUE RECOGNITION In May 2014, the FASB issued ASU No. 2014-09, “Revenue from Contracts with Customers”, which is part of ASC Topic 606. It defines how companies report revenues from contracts with customers and also requires certain enhanced disclosures. The standard’s provisions and related amendments are effective for annual reporting periods beginning after December 15, 2017. On January 1, 2018, the Company adopted this guidance, which did not have a material impact on the Company’s financial statements. A substantial portion of the Company's retail revenue as well as interest revenue falls under ASC Topic 825, “Financial Instruments”, which is excluded from the scope of the new Topic 606. Additionally, the Company’s institutional and futures segment revenues were not materially impacted as the satisfaction of performance obligations under the new guidance is materially consistent with the Company’s previous revenue recognition policies. Accordingly, the adoption of the new standard did not result in a transition adjustment to opening retained earnings, and as a result, revenues for contracts with customers would not have been adjusted in prior periods and are not presented herein on an adjusted basis. Similarly, the amended guidance did not have a material impact on the recognition of costs incurred to obtain new contracts. As a result of the adoption of the new guidance, the Company recorded a gross up of $0.5 million which impacted both institutional revenues as well as referral fee expense. The Company adopted ASU No. 2014-09 using the modified retrospective approach. Under ASC Topic 606, revenues are recognized when control of the promised goods or services is transferred to customers in an amount that reflects the consideration the Company expects to be entitled to in exchange for transferring those goods or services. The following is a description of the Company’s revenue recognition policies as it relates to revenue from contracts with customers. Institutional Revenue Institutional revenue consists of revenue from our GTX business, which provides a proprietary ECN trading platform and sales and trading services, principally to institutional customers. Revenue from our GTX business is generated primarily through commissions or commission-equivalents for trades executed on the GTX platform. The Company’s performance obligations consist of trade execution services and are satisfied at a point in time on the trade date; accordingly, commission revenues are recorded on the trade date. Futures Revenue Futures revenue comprises primarily commissions earned on futures and futures options trades. We act as an agent for the trades executed in our futures segment and are not exposed to market risk in connection with that activity. The Company earns commission revenue by acting as an agent on behalf of customers. Similar to the Company’s institutional revenue, the Company’s futures revenue performance obligations also consist of trade execution and are satisfied on the trade date; accordingly, commission revenues are recorded on the trade date. Disaggregation of Revenues The following table presents the Company’s revenue from contracts with customers disaggregated by the services described above. The table also includes a reconciliation of the disaggregated revenue with the reportable segments, for the three months ended March 31, 2018: For the 3 months ended March 31, 2018 Institutional Electronic Communications Network $ 7,056 Swap Dealer $ 1,527 Other $ 48 Total Segment Revenue $ 8,631 Futures Direct Customers (1) $ 2,473 Indirect Customers (2) $ 7,029 Other $ 1,989 Total Segment Revenue $ 11,491 (1) Direct customers are all other customers not classified as indirect (2) Indirect customers are referred to the Company by introducing brokers Contract Assets and Contract Liabilities The timing of the revenue recognition may differ from the timing of payment from customers. The Company records a receivable when revenue is recognized prior to payment, and when the Company has an unconditional right to payment. The Company records a contract liability when payment is received, prior to the time at which the satisfaction of the service obligation. The Company had receivables related to institutional revenues from contracts with customers which is included with Other Assets in Note 9 .

FAIR VALUE MEASUREMENT

FAIR VALUE MEASUREMENT	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENT	FAIR VALUE INFORMATION Fair value is defined as the price that would be received in exchange for an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. US GAAP establishes a three level hierarchy that ranks the quality and reliability of information used in developing fair value estimates for financial instruments. The hierarchy gives the highest priority to quoted prices in active markets and the lowest priority to unobservable data. In cases where two or more levels of inputs are used to determine fair value, a financial instrument’s level is determined based on the lowest level input that is considered significant to the fair value measurement in its entirety. The three levels of fair value hierarchy are summarized below: Level 1 - Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities; Level 2 - Quoted prices in markets that are not considered to be active or financial instruments for which all significant inputs are observable, either directly or indirectly; and Level 3 - Valuations that require inputs that are both unobservable to a market participant and significant to the fair value measurement. For assets and liabilities that are transferred between levels during the period, fair values are ascribed as if the assets or liabilities had been transferred as of the beginning of the period. The following table presents the Company’s assets and liabilities that were measured at fair value on a recurring basis during the reporting period and the related hierarchy levels (amounts in thousands): Fair Value Measurements on a Recurring Basis Level 1 Level 2 Level 3 Total Financial Assets/(Liabilities): Cash and cash equivalents: Money market accounts $ 219,908 $ — $ — $ 219,908 Cash and cash equivalents held for customers: US treasury bills: U.S. government and agency securities 104,658 — — 104,658 Receivable from brokers: Broker derivative contracts — 3,757 — 3,757 Other assets: Certificates of deposit 176 — — 176 Other 133 — — 133 Payables to customers: Customer derivative contracts — 133,573 — 133,573 Total $ 324,875 $ 137,330 $ — $ 462,205 Fair Value Measurements on a Recurring Basis Level 1 Level 2 Level 3 Total Financial Assets (Liabilities): Cash and cash equivalents: Money market accounts $ 219,286 $ — $ — $ 219,286 Cash and cash equivalents held for customers: US treasury bills: U.S. government and agency securities 105,190 — — 105,190 Receivable from brokers: Broker derivative contracts — 4,966 — 4,966 Other assets: Certificates of deposit 175 — — 175 Other 130 — — 130 Payables to customers: Customer derivative contracts — 129,966 — 129,966 Payables to brokers: Broker derivative contracts (3,170 ) (3,170 ) Total $ 324,781 $ 131,762 $ — $ 456,543 The Company has not changed its valuation techniques in measuring the fair value of any financial assets and liabilities during the three months ended March 31, 2018 , nor has there been any movement between levels during the period. Level 1 Financial Assets The Company has U.S. Treasury bills, money market accounts and certificates of deposit that are Level 1 financial instruments that are recorded based upon listed or quoted market rates. The U.S. Treasury bills and money market accounts are recorded in Cash and cash equivalents and Cash and cash equivalents held for customers and the certificates of deposit are recorded in Other Assets . Level 2 Financial Assets and Liabilities The Company has customer derivative contracts that are Level 2 financial instruments recorded in Payables to customers. The Company has broker derivative contracts that are Level 2 financial instruments recorded in Receivables from brokers and Payables to brokers. The fair values of these Level 2 financial instruments are based upon directly observable values for underlying instruments. Level 3 Financial Liabilities The Company did not have any Level 3 Financial Assets or Liabilities on March 31, 2018 or December 31, 2017 . Financial Instruments Not Measured at Fair Value The table below presents the carrying value, fair value and fair value hierarchy category of certain financial instruments that are not measured at fair value in the condensed consolidated balance sheets (amounts in thousands). Receivables from brokers comprise open trades, which are measured at fair value (disclosed above), and the Company’s posted funds with brokers that are required as collateral for holding trading positions, which are not measured at fair value but approximate fair value. These deposits approximate fair value because they are cash balances that the Company may withdraw at its discretion. Settlement would be expected to occur within a relatively short period of time once a withdrawal is initiated. Payables to customers comprise open trades, which are measured at fair value (disclosed above), and customer deposits that the Company holds for its role as clearing broker. These deposits are not measured at fair value, but approximate fair value, because they are cash balances that the Company or its customers can settle at either party’s discretion. Such settlement would occur within a relatively short period of time once a withdrawal is initiated. Payables to brokers comprise open trades, which are measured at fair value (disclosed above) and cash due to brokers. The cash within this balance is not measured at fair value but does approximate fair value, because it is immediately payable to the brokers. Settlement with brokers generally occurs either on the same or next business day following a margin call. The carrying value of Convertible senior notes represents the notes’ principal amounts net of unamortized discount (refer to Note 13 ). The Company assessed the notes’ fair value as determined by current Company-specific and risk free interest rates as of the balance sheet date. As of March 31, 2018 Fair Value Measurements using: Carrying Value Fair Value Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial Assets: Receivables from brokers $ 68,260 $ 68,260 $ — $ 68,260 $ — Financial Liabilities: Payables to customers $ 1,096,794 $ 1,096,794 $ — $ 1,096,794 $ — Convertible senior notes $ 133,734 $ 171,358 $ — $ 171,358 $ — As of December 31, 2017 Fair Value Measurements using: Carrying Value Fair Value Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial Assets: Receivables from brokers $ 73,537 $ 73,537 $ — $ 73,537 $ — Payables to brokers $ 381 $ 381 $ — $ 381 $ — Financial Liabilities: Payables to customers $ 1,108,794 $ 1,108,794 $ — $ 1,108,794 $ — Convertible senior notes $ 132,221 $ 205,073 $ — $ 205,073 $ —

DERIVATIVES

DERIVATIVES	3 Months Ended
	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DERIVATIVES	DERIVATIVES The Company’s contracts with its customers and its liquidity providers are deemed to be derivative instruments. The table below represents the fair values of the Company’s derivative instruments reported within Receivables from brokers, Payables to customers and Payables to brokers on the accompanying Condensed Consolidated Balance Sheets (amounts in thousands): March 31, 2018 Gross amounts of Gross amount of Net amounts of Derivative Instruments: Foreign currency exchange contracts $ 117,871 $ (22,893 ) $ 94,978 CFD contracts 99,795 (60,208 ) 39,587 Metals contracts 3,684 (919 ) 2,765 Total $ 221,350 $ (84,020 ) $ 137,330 March 31, 2018 Cash Collateral Net amounts of Net amounts of Derivative Assets/Liabilities: Receivables from brokers $ 68,260 $ 3,757 $ 72,017 Payables to customers $ (1,096,794 ) $ 133,573 $ (963,221 ) December 31, 2017 Gross amounts of Gross amount of Net amounts of Derivative Instruments: Foreign currency exchange contracts $ 121,104 $ (31,556 ) $ 89,548 CFD contracts 102,659 (62,322 ) 40,337 Metals contracts 4,084 (2,207 ) 1,877 Total $ 227,847 $ (96,085 ) $ 131,762 December 31, 2017 Cash Collateral Net amounts of Net amounts of Derivative Assets/Liabilities: Receivables from brokers $ 73,537 $ 4,966 $ 78,503 Payables to customers $ (1,108,794 ) $ 129,966 $ (978,828 ) Payables to brokers $ 381 $ (3,170 ) $ (2,789 ) The Company’s derivatives include different underlyings which vary in price. Foreign exchange contracts typically have prices less than two dollars, while certain metals contracts and CFDs can have considerably higher prices. The amounts reported within Receivables from brokers, Payables to customers, and Payables to brokers on the Consolidated Balance Sheets are derived from the number of contracts below (amounts in thousands): March 31, 2018 Total contracts in long positions Total contracts in short positions Derivative Instruments: Foreign currency exchange contracts 2,940,579 3,693,711 CFD contracts 128,478 166,802 Metals contracts 541 167 Total 3,069,598 3,860,680 December 31, 2017 Total contracts in long positions Total contracts in short positions Derivative Instruments: Foreign currency exchange contracts 2,075,789 4,148,056 CFD contracts 126,519 174,835 Metals contracts 414 217 Total 2,202,722 4,323,108 The Company did not designate any of its derivatives as hedging instruments. Net gains with respect to derivative instruments reflected in Retail revenue in the accompanying Condensed Consolidated Statements of Operations and Comprehensive Income/(Loss) for the three months ended March 31, 2018 and March 31, 2017 were as follows (amounts in thousands): Three Months Ended March 31, 2018 2017 Derivative Instruments: Foreign currency exchange contracts $ 41,251 $ 17,376 CFD contracts 37,147 26,791 Metals contracts 5,722 (5,236 ) Total $ 84,120 $ 38,931

RECEIVABLES FROM BROKERS

RECEIVABLES FROM BROKERS	3 Months Ended
	Mar. 31, 2018
Brokers and Dealers [Abstract]
RECEIVABLES FROM BROKERS	RECEIVABLES FROM BROKERS The Company has posted funds with brokers as collateral required by agreements for holding trading positions. These amounts are reflected as Receivables from brokers on the Condensed Consolidated Balance Sheets. Amounts receivable from brokers consisted of the following as of (amounts in thousands): March 31, 2018 December 31, 2017 Required collateral $ 68,260 $ 73,537 Open foreign exchange positions 3,757 4,966 Total $ 72,017 $ 78,503

PROPERTY AND EQUIPMENT

PROPERTY AND EQUIPMENT	3 Months Ended
	Mar. 31, 2018
Property, Plant and Equipment [Abstract]
PROPERTY AND EQUIPMENT	PROPERTY AND EQUIPMENT Property and equipment, including leasehold improvements and capitalized software development costs, consisted of the following as of (amounts in thousands): March 31, 2018 December 31, 2017 Software $ 61,635 $ 57,047 Computer equipment 19,025 18,390 Leasehold improvements 11,231 11,068 Telephone equipment 643 643 Office equipment 2,113 2,110 Furniture and fixtures 3,653 3,592 Web site development costs 378 386 Gross property and equipment 98,678 93,236 Less: Accumulated depreciation and amortization (58,505 ) (52,494 ) Property and equipment, net $ 40,173 $ 40,742 Depreciation and amortization expense for property and equipment was $5.7 million and $4.0 million for the three months ended March 31, 2018 and 2017 , respectively.

INTANGIBLE ASSETS

INTANGIBLE ASSETS	3 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
INTANGIBLE ASSETS	INTANGIBLE ASSETS The Company’s various intangible assets consisted of the following as of (amounts in thousands): March 31, 2018 December 31, 2017 Intangibles Gross Accumulated Net Gross Accumulated Net Customer lists $ 61,727 $ (34,786 ) $ 26,941 $ 60,420 $ (31,698 ) $ 28,722 Technology 73,106 (45,080 ) 28,026 72,204 (43,270 ) 28,934 Trademarks 7,933 (4,111 ) 3,822 7,680 (3,730 ) 3,950 Total finite lived intangibles 142,766 (83,977 ) 58,789 140,304 (78,698 ) 61,606 Trademark not subject to amortization (1) 363 — 363 363 — 363 Total intangibles $ 143,129 $ (83,977 ) $ 59,152 $ 140,667 $ (78,698 ) $ 61,969 (1) These indefinite-life trademarks relate to the Forex.com and foreignexchange.com domain names where management determined there was no legal, regulatory or technological limitation on their useful lives. The Company compares the recorded value of the indefinite-life intangible assets to their fair value on an annual basis and whenever circumstances arise that indicate that impairment may have occurred. The Company had the following identifiable intangible assets and weighted average amortization periods as of March 31, 2018 : Intangible Assets Amount (in thousands) Weighted average amortization period Customer lists $ 61,727 6.9 years Technology 73,106 9.0 years Trademarks (1) 8,296 6.7 years Total intangible assets $ 143,129 (1) Trademarks with an indefinite-life, as described above, comprise $0.4 million of the $8.3 million of trademarks. Amortization expense for the purchased intangibles was $4.2 million and $3.6 million for the three months ended March 31, 2018 and 2017 , respectively. On February 7, 2017 , the Company entered into an Asset Purchase Agreement (the “Purchase Agreement”) with Forex Capital Markets L.L.C. (“FXCM”). Pursuant to the terms of the Purchase Agreement, FXCM transferred substantially all of its U.S.-domiciled customer accounts to the Company effective as of February 24, 2017 . The Company paid $7.2 million to FXCM as consideration for the purchased accounts for the full year 2017, which was capitalized and included as an intangible asset and amortized on a straight line basis over its two year useful life. Goodwill Goodwill is evaluated for impairment on an annual basis during the fourth quarter and in interim periods when events or changes indicate the carrying value may not be recoverable. The Company operates under three reporting units: retail, institutional and futures. There were no additions or impairments to the carrying value of the Company’s goodwill during the three months ended March 31, 2018 . For the year ended December 31, 2017 , the Company performed a qualitative analysis to determine whether it was more likely than not that the fair value of its reporting units was less than their carrying value. As a result of this assessment, the Company determined that it was not necessary to perform a quantitative impairment test and concluded that goodwill assigned to each of its reporting units was not impaired at December 31, 2017 . As of March 31, 2018 and December 31, 2017 , the Company had recorded goodwill of approximately $33.4 million and $33.0 million , respectively. The increase of $0.4 million was related to foreign currency translation adjustments. The following represents the changes in the carrying amount of goodwill by segment (amounts in thousands): Retail Institutional Futures Total Carrying amount of goodwill as of December 31, 2017 $ 25,952 $ 4,650 $ 2,434 $ 33,036 Foreign currency translation adjustments 320 57 30 407 Carrying amount of goodwill as of March 31, 2018 $ 26,272 $ 4,707 $ 2,464 $ 33,443

OTHER ASSETS

OTHER ASSETS	3 Months Ended
	Mar. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
OTHER ASSETS	OTHER ASSETS Other assets consisted of the following as of (amounts in thousands): March 31, 2018 December 31, 2017 Vendor and security deposits $ 12,320 $ 11,923 Income tax receivable — 2,132 Deferred tax assets, net 12,621 10,698 GTX trade receivables 6,587 5,758 Customer debit positions 2,995 2,384 Allowance on customer debit positions (2,771 ) (1,959 ) Prepaid assets 8,911 9,523 Miscellaneous receivables 5,738 4,872 Deferred commitment fees 497 550 Total other assets $ 46,898 $ 45,881

RELATED PARTY TRANSACTIONS

RELATED PARTY TRANSACTIONS	3 Months Ended
	Mar. 31, 2018
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	RELATED PARTY TRANSACTIONS Certain officers and directors of the Company have personal funds on deposit in separate customer accounts with the Company. These accounts are recorded in Payables to customers on the Condensed Consolidated Balance Sheets. The aggregate amount of these funds was $0.2 million and $0.2 million at March 31, 2018 and December 31, 2017 , respectively. IPGL Limited, the majority selling shareholder in the acquisition of City Index, has a trading account with the Company which is recorded in Payables to customers on the Condensed Consolidated Balance Sheet. The aggregate amount of these funds was $20.4 million and $15.9 million as of March 31, 2018 and December 31, 2017 , respectively.

NON-CONTROLLING INTERESTS

NON-CONTROLLING INTERESTS	3 Months Ended
	Mar. 31, 2018
Noncontrolling Interest [Abstract]
NON-CONTROLLING INTERESTS	NON-CONTROLLING INTERESTS Non-controlling interests In March 2014, the Company acquired controlling interests in Global Asset Advisors (“GAA”) and Top Third (“TT”). The Company purchased 55% of each entity, and the respective sellers maintained a 45% interest in each entity, subject to immediately exercisable call options for the Company to purchase the remaining interests, as well as put options for the sellers to sell their remaining interests in each entity to the Company that were to become exercisable in 2017. In December 2016, the Company acquired an additional 24% of each entity and, accordingly, the respective sellers now maintain a 21% interest in each entity. In connection with the purchase of these additional interests, the Company and the respective sellers agreed that neither would exercise the call options or put options with respect to the remaining interests prior to December 31, 2017 . In December 2017, the Company and the sellers of TT extended their agreement that neither would exercise the relevant call options or put options through December 31, 2018. In February 2018, the minority owners of GAA notified the Company that they were exercising their put option with respect to their combined 21% ownership of GAA. The purchase of the ownership interests subject to the put option had not settled as of March 31, 2018. However, because of the issuance of the exercise notice, the related non-controlling interest in GAA was reclassified as a liability. The purchase of the minority ownership interests is expected to close in the second quarter of 2018. In accordance with ASC 480-10-S99-3A, Classification and Measurement of Redeemable Securities , non-controlling interests are classified outside of permanent equity as their redemption is not (i) mandatory, (ii) at fixed prices, and (iii) exclusively within the Company’s control. The non-controlling interest related to TT is not classified as a liability, because redemption is not mandatory or at a fixed price. It is not classified as equity because the redemption is not exclusively in the Company’s control. Therefore, the non-controlling interest is held in temporary equity in the Condensed Consolidated Balance Sheets. The non-controlling interests’ carrying value is determined by the Company’s purchase prices and the non-controlling interests’ share of the Company’s subsequent net income. This value is benchmarked against the redemption value of the sellers’ put options. The carrying value is adjusted to the latter, provided that it does not fall below the initial carrying values, as determined by the Company’s purchase price allocation. The Company reflects any changes caused by such an adjustment in retained earnings, rather than in current earnings. The table below reflects the non-controlling interests’ effects on the Company’s financial statements (amounts in thousands): Redeemable non-controlling interests January 1, 2018 $ 4,411 Adjustment to the redemption value of non-controlling interests 367 Net income attributable to non-controlling interests 175 Distributions to non-controlling interest holders (90 ) Reclassification to liabilities (2,910 ) March 31, 2018 $ 1,953

REVOLVING CREDIT ARRAGEMENT

REVOLVING CREDIT ARRAGEMENT	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
REVOLVING CREDIT ARRAGEMENT	REVOLVING CREDIT ARRANGEMENT On August 3, 2017 , the Company entered into a Credit Agreement, dated as of August 2, 2017 , for a three year U.S. $50.0 million senior secured first lien revolving credit facility that matures in August 2020. Upon request of the Company, the credit facility may be increased by up to $25.0 million , with a minimum increase of $5.0 million . The credit facility contains covenants that are customary for an issuer with senior debt. As of March 31, 2018 , the Company was in compliance with the covenants for its credit facility. The commitment fees of $0.5 million paid at establishment of the credit facility will be amortized over the life of the facility and was recorded to Other Assets . As of March 31, 2018 , there were no amounts outstanding under the revolving line of credit.

CONVERTIBLE SENIOR NOTES

CONVERTIBLE SENIOR NOTES	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
CONVERTIBLE SENIOR NOTES	CONVERTIBLE SENIOR NOTES Convertible Senior Notes due 2022 On August 22, 2017 , the Company issued $92.0 million aggregate principal amount of its 5.00% Convertible Senior Notes due 2022 , which amount includes the exercise in full of the over-allotment option granted to the initial purchasers of the Notes, in a private offering to qualified institutional buyers pursuant to Rule 144A under the Securities Act of 1933, as amended. The Notes bear interest at a fixed rate of 5.00% per year, payable semi-annually in arrears on February 15 and August 15 of each year, beginning on February 15 , 2018 . The Notes are convertible into cash, shares of the Company’s common stock, or a combination thereof, at the Company’s election. The Company currently intends to settle the debt in cash. The Notes will mature on August 15, 2022 , unless earlier converted, redeemed or repurchased. The Company may not redeem the Notes prior to August 15, 2020 . The net proceeds from the Note Offering were approximately $89.0 million , after deducting discounts to the initial purchasers but prior to taking into account any estimated offering expenses payable by the Company. Convertible Senior Notes due 2020 On April 1, 2015 , as part of the City Index acquisition consideration, the Company issued to the sellers $60.0 million aggregate principal amount of 4.125% Convertible Senior Notes maturing on April 1, 2020 . These Convertible Senior Notes pay interest semi-annually on April 1 and October 1 at a rate of 4.125% per year, which commenced on October 1, 2015 . The Company currently intends to settle the debt in cash. Convertible Senior Notes due 2018 On November 27 , 2013 , the Company issued $80.0 million principal amount of 4.125% Convertible Senior Notes maturing on December 1, 2018 . The Company received net proceeds of $77.9 million , after deducting the initial purchasers' discount. These Convertible Senior Notes pay interest semi-annually on June 1 and December 1 at a rate of 4.125% per year, which commenced on June 1, 2014 . During the first quarter of 2016 , the Company repurchased $1.9 million in principal amount of the convertible senior notes due in 2018, for an aggregate purchase price of $1.7 million . During the third quarter of 2017, the Company repurchased $71.8 million in principal amount of the convertible senior notes due in 2018, for an aggregate purchase price of $73.7 million with the proceeds from the issuance of Convertible Senior Notes due in 2022. As a result we recognized an extinguishment loss of $4.9 million . The Company currently intends to settle the remaining outstanding debt in cash upon maturity. Under accounting guidance, an entity must separately account for the liability and equity components of a convertible debt instrument that may be settled entirely or partially in cash upon conversion. The separate accounting must reflect the issuer’s economic interest cost. The balances of the liability and equity components as of March 31, 2018 and December 31, 2017 were as follows (amounts in thousands): March 31, 2018 December 31, 2017 Liability component - principal $ 158,350 $ 158,350 Deferred bond discount (24,140 ) (25,624 ) Deferred financing cost (476 ) (505 ) Liability component - net carrying value $ 133,734 $ 132,221 Additional paid in capital $ 39,405 $ 39,405 Discount attributable to equity (826 ) (826 ) Equity component $ 38,579 $ 38,579 Interest expense related to the Convertible Senior Notes, included in Interest Expense on long term borrowings in the Condensed Consolidated Statements of Operations and Comprehensive Income/(Loss) , was as follows (amounts in thousands): For the Three Months Ended March 31, 2018 2017 Interest expense - stated coupon rate $ 1,834 $ 1,425 Interest expense - amortization of deferred bond discount and costs 1,506 1,240 Total interest expense - convertible senior notes $ 3,340 $ 2,665

EARNINGS_(LOSS) PER COMMON SHAR

EARNINGS/(LOSS) PER COMMON SHARE	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
EARNINGS/(LOSS) PER COMMON SHARE	EARNINGS/(LOSS) PER COMMON SHARE Basic and diluted earnings (loss) per common share are computed by dividing net income (loss) by the weighted average number of common shares outstanding during the period. Diluted earnings (loss) per share includes the determinants of basic net income per share and, in addition, gives effect to the potential dilution that would occur if securities or other contracts to issue common stock were exercised, vested or converted into common stock, unless they are anti-dilutive. Diluted weighted average common shares include vested and unvested stock options, unvested restricted stock units and unvested restricted stock awards. Approximately 0.4 million and 0.3 million stock options were excluded from the calculation of diluted earnings (loss) per share for the three months ended March 31, 2018 and 2017 , respectively, as they were anti-dilutive. Diluted earnings (loss) per share excludes any shares of Company common stock potentially issuable under the Company’s convertible senior notes, which are discussed in Note 13. Based upon an assumed trading price of $13 for each share of the Company’s common stock, and if the relevant conditions under the indenture governing the 2018 , 2020 , and 2022 convertible senior notes were satisfied, there would be no additional dilutive shares for the 2018 note and an additional 1.5 million and 4.2 million dilutive shares as of March 31, 2018 , for the 2020 and 2022 notes, respectively. The following table sets forth the computation of earnings (loss) per share (amounts in thousands except share and per share data): For the Three Months Ended March 31, 2018 2017 Net income/(loss) applicable to GAIN Capital Holdings, Inc. $ 16,086 $ (18,871 ) Adjustment (1) (367 ) 68 Net income/(loss) available to GAIN common shareholders $ 15,719 $ (18,803 ) Weighted average common shares outstanding: Basic weighted average common shares outstanding 45,017,716 47,894,546 Effect of dilutive securities: Stock options 261,732 328,325 RSUs/RSAs 244,318 13,845 Diluted weighted average common shares outstanding 45,523,766 47,894,546 Earnings/(loss) per common share: Basic $ 0.35 $ (0.39 ) Diluted $ 0.35 $ (0.39 ) (1) During the three months ended March 31, 2018 and 2017 , the Company concluded that the carrying value of the Company's redeemable noncontrolling interests was less than their redemption value, requiring that an adjustment to the carrying value be recorded for purposes of calculating earnings (loss) per common share. The adjustment to increase or reduce the carrying value will, respectively, reduce or increase earnings (loss) per common share by reducing or increasing net income available to common shareholders.

COMMITMENT AND CONTINGENCIES

COMMITMENT AND CONTINGENCIES	3 Months Ended
	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
COMMITMENT AND CONTINGENCIES	COMMITMENT AND CONTINGENCIES From time to time the Company becomes involved in legal proceedings and in each case the Company assesses the likely liability and/or the amount of damages as appropriate. Where available information indicates that it is probable a liability had been incurred at the date of the Condensed Consolidated Financial Statements and the Company can reasonably estimate the amount of that loss, the Company accrues the estimated loss by a charge to income. In many proceedings, however, it is inherently difficult to determine whether any loss is probable or even reasonably possible or to estimate the amount of any loss. In addition, even where loss is possible or an exposure to loss exists in excess of the liability already accrued with respect to a previously recognized loss contingency, it is often not possible to reasonably estimate the size of the possible loss or range of loss. For certain legal proceedings, the Company can estimate possible losses, additional losses, ranges of loss or ranges of additional loss in excess of amounts accrued. For certain other legal proceedings, the Company cannot reasonably estimate such losses, if any, since the Company cannot predict if, how or when such proceedings will be resolved or what the eventual settlement, fine, penalty or other relief, if any, may be, particularly for proceedings that are in their early stages of development or where plaintiffs seek substantial or indeterminate damages. Numerous issues must be developed, including the need to discover and determine important factual matters and the need to address novel or unsettled legal questions relevant to the proceedings in question, before a loss or additional loss or range of loss can be reasonably estimated for any proceeding.

INCOME TAXES

INCOME TAXES	3 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The Company's provision for income taxes was approximately $3.7 million for the three months ended March 31, 2018 and a benefit of approximately $4.9 million for the three months ended March 31, 2017 . These amounts reflect the Company's estimate of the annual effective tax rates of 18.5% and 20.7% , adjusted for certain discrete items, for the three months ended March 31, 2018 and 2017 , respectively. Changes in the Company's effective tax rate arise primarily from changes in the geographic mix of revenues and expenses, as well as changes to statutory tax rates in the U.S. Deferred income taxes reflect the net tax effects of temporary differences between the financial reporting and tax basis of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when such differences are expected to reverse. Certain net deferred tax assets of the Company are included in Other assets on the Condensed Consolidated Balance Sheets.

REGULATORY REQUIREMENTS

REGULATORY REQUIREMENTS	3 Months Ended
	Mar. 31, 2018
Banking and Thrift [Abstract]
REGULATORY REQUIREMENTS	REGULATORY REQUIREMENTS The following table illustrates the minimum regulatory capital our subsidiaries were required to maintain as of March 31, 2018 and the actual amounts of capital that were maintained (amounts in millions): Entity Name Minimum Capital Excess Percent of GAIN Capital Group, LLC $ 34.8 $ 48.4 $ 13.6 139 % GAIN Capital Securities, Inc. 0.1 0.4 0.3 400 % GAIN Capital U.K., Ltd. 72.9 213.5 140.6 293 % GAIN Capital Japan Co., Ltd. 1.6 9.7 8.1 606 % GAIN Capital Australia, Pty. Ltd. 0.8 5.3 4.5 663 % GAIN Capital-Forex.com Hong Kong, Ltd. 1.9 4.5 2.6 237 % GAIN Global Markets, Inc. 0.4 2.3 1.9 575 % GAIN Capital-Forex.com Canada, Ltd. 0.2 1.6 1.4 800 % GAIN Capital Singapore Pte., Ltd. 0.3 6.7 6.4 2,233 % Trade Facts, Ltd. 0.6 3.7 3.1 617 % Global Asset Advisors, LLC — 1.7 1.7 100 % GAIN Capital Payments Ltd. 0.2 0.2 — 100 % GTX SEF, LLC 0.8 1.2 0.4 150 % Total $ 114.6 $ 299.2 $ 184.6 261 %

SEGMENT INFORMATION

SEGMENT INFORMATION	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION ASC 280, Disclosures about Segments of an Enterprise and Related Information , establishes standards for reporting information about operating segments. Operating segments are defined as components of an enterprise which engage in business activities from which they may earn revenues and incur expenses and about which separate financial information is available that is evaluated regularly by the chief operating decision-maker, or decision making group, in deciding how to allocate resources and in assessing performance. Reportable segments are defined as an operating segment that either (a) exceeds 10% of revenue, or (b) reported profit or loss in absolute amount exceeds 10% of profit of all operating segments that did not report a loss or (c) exceeds 10% of the combined assets of all operating segments. The Company’s operations relate to global trading services and solutions. During the three months ended March 31, 2018, the Company completed its implementation of global support groups in the areas of finance, legal, human resources, and treasury. These groups are now centrally managed and support all business functions. Therefore, all costs related to these groups previously recorded within the retail segment will now be classified in our corporate and other segment to better align the cost reporting with the support services. The change in segment reporting has no impact on the net profit or loss of the Company. To enable comparisons with prior period performance, historical segment information for the periods included in the tables below reflect this reporting change. Retail Segment Business in the retail segment is conducted primarily through the Company’s FOREX.com and City Index brands. The Company provides its retail customers around the world with access to over 12,500 global financial markets, including spot forex, precious metals, and CFDs on currencies, commodities, indices, individual equities, cryptocurrencies, bonds and interest rate products, as well as OTC options on forex. In the United Kingdom, the Company also offer spread bets, which are investment products similar to CFDs, but that offer more favorable tax treatment to residents of that country. Institutional Segment The institutional segment provides agency execution services and offers access to markets and self-directed trading in foreign exchange, commodities, equities, options and futures via an electronic communications network, or ECN, through the Company’s GTX platform. The Company also offers high touch sales and trading aided by a team of sales employees. Futures Segment The futures segment offers execution and related services for exchange-traded futures, including Bitcoin, and futures options on major U.S. and European exchanges. The Company offers futures services through its subsidiary, GAIN Capital Group, LLC, under the GAIN Capital Futures brand. In addition, in 2014, the Company expanded its futures business by acquiring majority interests in GAA and TT. Corporate and other Corporate and other provides general corporate services to the Company’s segments and also includes expense eliminations between operating segments which were $0.5 million and $0.5 million for the three months ended March 31, 2018 and 2017 , respectively. Corporate and other revenue primarily comprises foreign currency transaction gains and losses. Selected financial information by segment is presented in the following tables (amounts in thousands): Retail Three Months Ended March 31, 2018 2017 Net revenue $ 85,672 $ 40,086 Employee compensation and benefits 15,425 13,335 Selling and marketing 5,666 8,904 Referral fees 7,685 12,323 Other operating expenses 17,730 13,403 Segment profit $ 39,166 $ (7,879 ) Institutional Three Months Ended March 31, 2018 2017 Net revenue $ 8,631 $ 8,621 Employee compensation and benefits 3,694 4,041 Selling and marketing 56 21 Referral Fees 517 — Other operating expenses 3,333 3,228 Segment profit $ 1,031 $ 1,331 Futures Three Months Ended March 31, 2018 2017 Net revenue $ 11,491 $ 10,967 Employee compensation and benefits 2,511 2,551 Selling and marketing 241 265 Referral fees 3,746 4,118 Other operating expenses 3,794 3,742 Segment profit $ 1,199 $ 291 Corporate and Other Three Months Ended March 31, 2018 2017 Other revenue $ 1,069 $ (108 ) Employee compensation and benefits 6,121 4,537 Selling and marketing 64 64 Other operating expenses 3,238 2,407 Loss $ (8,354 ) $ (7,116 ) Reconciliation of operating segment profit to Income before income tax expense Three Months Ended March 31, 2018 2017 Retail segment $ 39,166 $ (7,879 ) Institutional segment 1,031 1,331 Futures segment 1,199 291 Corporate and other (8,354 ) (7,116 ) SEGMENT PROFIT 33,042 (13,373 ) Depreciation and amortization 5,669 4,019 Purchased intangible amortization 4,204 3,602 Impairment of investment (130 ) — OPERATING PROFIT/(LOSS) 23,299 (20,994 ) Interest expense on long term borrowings 3,340 2,665 INCOME/(LOSS) BEFORE INCOME TAX EXPENSE $ 19,959 $ (23,659 )

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	3 Months Ended
	Mar. 31, 2018
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS On April 26, 2018 , the Company announced the payment of a $0.06 dividend per share of Common Stock payable on June 19, 2018 to stockholders of record on June 12, 2018 . On April 24, 2018, the Company's Board of Directors approved an increase in the total amount of available cash for the purchase of the Company’s common stock of $25.0 million .

DESCRIPTION OF BUSINESS AND B26

DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION (Policies)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	The accompanying unaudited Condensed Consolidated Financial Statements reflect all adjustments that, in the opinion of management, are necessary for a fair presentation of the financial statements for the interim periods. The financial statements are presented in accordance with accounting principles generally accepted in the United States of America. The unaudited Condensed Consolidated Financial Statements have been prepared in accordance with the Securities and Exchange Commission's ("SEC") regulations for interim financial statements, and, in accordance with SEC rules, omit or condense certain information and footnote disclosures. Results for the interim periods are not necessarily indicative of results to be expected for any other interim period or for the full year. These financial statements should be read in conjunction with the audited Consolidated Financial Statements and related notes included in the Company’s Annual Report on Form 10-K as of and for the year ended December 31, 2017 . The unaudited Condensed Consolidated Financial Statements include the accounts of the Company and its wholly-owned subsidiaries, after the elimination of inter-company transactions and balances.
Recent Accounting Pronouncements Not Yet Adopted	ACCOUNTING PRONOUNCEMENTS Recently Adopted In January 2017, the Financial Accounting Standard Board (the "FASB") issued ASU No. 2017-04, “Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment,” which eliminates the requirement to calculate the implied fair value of goodwill to measure a goodwill impairment charge (Step 2) from the goodwill impairment test. Instead, an impairment charge will equal the amount by which a reporting unit’s carrying amount exceeds its fair value, not to exceed the amount of goodwill allocated to the reporting unit. The guidance will be effective prospectively for the Company for its annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019, with early adoption permitted after January 1, 2017 . The Company has adopted this ASU as of January 1, 2018, and will apply Step 2 to the extent it is required. In November 2016, the FASB issued ASU No. 2016-18, “Restricted Cash,” which requires that companies present cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents (restricted cash) when reconciling beginning-of-period and end-of-period totals on the Statement of Cash Flows. The Company has retrospectively adopted this ASU as of January 1, 2018 and as a result Net cash provided by operating activities of continuing operations on the Statement of Cash Flows increased by $86.4 million for the three months ended March 31, 2017. The Company defines restricted cash to include cash and cash equivalents held for customers and represents cash and other highly liquid assets held to fund customer liabilities in connection with trading positions. Included in this balance are funds deposited by customers and funds accruing to customers as a result of trades or contracts. The Company records a corresponding liability in connection with this amount in Payables to customers. In addition, the Company holds certain customer funds in segregated or secured broker accounts. Legally segregated balances are not available for general use, in accordance with certain jurisdictional regulatory requirements. In May 2014, the FASB issued ASU No. 2014-09, “Revenue from Contracts with Customers”, which is part of ASC Topic 606. It defines how companies report revenues from contracts with customers and also requires certain enhanced disclosures. The standard’s provisions and related amendments are effective for annual reporting periods beginning after December 15, 2017. On January 1, 2018, the Company adopted this guidance, which did not have a material impact on the Company’s financial statements. A substantial portion of revenue falls under ASC Topic 825, Financial Instruments, which is excluded from the scope of the new guidance. The Company adopted ASU No. 2014-09 using the modified retrospective approach. See Note 3 Revenue Recognition for additional disclosure. Not Yet Adopted In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842),” which amended the guidance on accounting for leases. The FASB issued this update to increase transparency and comparability among organizations. This update requires recognizing lease assets and lease liabilities on the balance sheet and disclosing key information about leasing arrangements. The guidance is effective for fiscal years beginning after December 15, 2018 , including interim periods within that fiscal year. The Company is currently assessing the impact on its Financial Statements of adopting this guidance. In February 2018, the FASB issued ASU No. 2018-02, “Income Statement - Reporting Comprehensive Income (Topic 220),” to address certain income tax effects in Accumulated Other Comprehensive Income (AOCI) resulting from the tax reform enacted in 2017. The amended guidance provides an option to reclassify tax effects within AOCI to retained earnings in the period in which the effect of the tax reform is recorded. The amendments are effective for fiscal years beginning after December 15, 2018 , including interim periods. Early adoption is permitted. The Company is currently assessing the impact on its Financial Statements of adopting this guidance.
Revenue Recognition

REVENUE RECOGNITION (Tables)

REVENUE RECOGNITION (Tables)	3 Months Ended
	Mar. 31, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	For the 3 months ended March 31, 2018 Institutional Electronic Communications Network $ 7,056 Swap Dealer $ 1,527 Other $ 48 Total Segment Revenue $ 8,631 Futures Direct Customers (1) $ 2,473 Indirect Customers (2) $ 7,029 Other $ 1,989 Total Segment Revenue $ 11,491 (1) Direct customers are all other customers not classified as indirect (2) Indirect customers are referred to the Company by introducing brokers
Schedule of New Accounting Pronouncements and Changes in Accounting Principles

FAIR VALUE MEASUREMENT (Tables)

FAIR VALUE MEASUREMENT (Tables)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Assets and Liabilities Measured at Fair Value on Recurring Basis	The following table presents the Company’s assets and liabilities that were measured at fair value on a recurring basis during the reporting period and the related hierarchy levels (amounts in thousands): Fair Value Measurements on a Recurring Basis Level 1 Level 2 Level 3 Total Financial Assets/(Liabilities): Cash and cash equivalents: Money market accounts $ 219,908 $ — $ — $ 219,908 Cash and cash equivalents held for customers: US treasury bills: U.S. government and agency securities 104,658 — — 104,658 Receivable from brokers: Broker derivative contracts — 3,757 — 3,757 Other assets: Certificates of deposit 176 — — 176 Other 133 — — 133 Payables to customers: Customer derivative contracts — 133,573 — 133,573 Total $ 324,875 $ 137,330 $ — $ 462,205 Fair Value Measurements on a Recurring Basis Level 1 Level 2 Level 3 Total Financial Assets (Liabilities): Cash and cash equivalents: Money market accounts $ 219,286 $ — $ — $ 219,286 Cash and cash equivalents held for customers: US treasury bills: U.S. government and agency securities 105,190 — — 105,190 Receivable from brokers: Broker derivative contracts — 4,966 — 4,966 Other assets: Certificates of deposit 175 — — 175 Other 130 — — 130 Payables to customers: Customer derivative contracts — 129,966 — 129,966 Payables to brokers: Broker derivative contracts (3,170 ) (3,170 ) Total $ 324,781 $ 131,762 $ — $ 456,543
Carrying Value and Fair Value and Fair Value Hierarchy Category of Financial Instruments Not Measured at Fair Value in Condensed Consolidated Statements of Financial Condition	As of March 31, 2018 Fair Value Measurements using: Carrying Value Fair Value Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial Assets: Receivables from brokers $ 68,260 $ 68,260 $ — $ 68,260 $ — Financial Liabilities: Payables to customers $ 1,096,794 $ 1,096,794 $ — $ 1,096,794 $ — Convertible senior notes $ 133,734 $ 171,358 $ — $ 171,358 $ — As of December 31, 2017 Fair Value Measurements using: Carrying Value Fair Value Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial Assets: Receivables from brokers $ 73,537 $ 73,537 $ — $ 73,537 $ — Payables to brokers $ 381 $ 381 $ — $ 381 $ — Financial Liabilities: Payables to customers $ 1,108,794 $ 1,108,794 $ — $ 1,108,794 $ — Convertible senior notes $ 132,221 $ 205,073 $ — $ 205,073 $ —

DERIVATIVES (Tables)

DERIVATIVES (Tables)	3 Months Ended
	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Fair Values of Derivative Instruments	The table below represents the fair values of the Company’s derivative instruments reported within Receivables from brokers, Payables to customers and Payables to brokers on the accompanying Condensed Consolidated Balance Sheets (amounts in thousands): March 31, 2018 Gross amounts of Gross amount of Net amounts of Derivative Instruments: Foreign currency exchange contracts $ 117,871 $ (22,893 ) $ 94,978 CFD contracts 99,795 (60,208 ) 39,587 Metals contracts 3,684 (919 ) 2,765 Total $ 221,350 $ (84,020 ) $ 137,330 March 31, 2018 Cash Collateral Net amounts of Net amounts of Derivative Assets/Liabilities: Receivables from brokers $ 68,260 $ 3,757 $ 72,017 Payables to customers $ (1,096,794 ) $ 133,573 $ (963,221 ) December 31, 2017 Gross amounts of Gross amount of Net amounts of Derivative Instruments: Foreign currency exchange contracts $ 121,104 $ (31,556 ) $ 89,548 CFD contracts 102,659 (62,322 ) 40,337 Metals contracts 4,084 (2,207 ) 1,877 Total $ 227,847 $ (96,085 ) $ 131,762 December 31, 2017 Cash Collateral Net amounts of Net amounts of Derivative Assets/Liabilities: Receivables from brokers $ 73,537 $ 4,966 $ 78,503 Payables to customers $ (1,108,794 ) $ 129,966 $ (978,828 ) Payables to brokers $ 381 $ (3,170 ) $ (2,789 )
Notional Values of Derivative Instruments	The amounts reported within Receivables from brokers, Payables to customers, and Payables to brokers on the Consolidated Balance Sheets are derived from the number of contracts below (amounts in thousands): March 31, 2018 Total contracts in long positions Total contracts in short positions Derivative Instruments: Foreign currency exchange contracts 2,940,579 3,693,711 CFD contracts 128,478 166,802 Metals contracts 541 167 Total 3,069,598 3,860,680 December 31, 2017 Total contracts in long positions Total contracts in short positions Derivative Instruments: Foreign currency exchange contracts 2,075,789 4,148,056 CFD contracts 126,519 174,835 Metals contracts 414 217 Total 2,202,722 4,323,108
Net Gains (Losses) With Respect to Derivative Instruments Which are Reflected in Trading Revenue	Net gains with respect to derivative instruments reflected in Retail revenue in the accompanying Condensed Consolidated Statements of Operations and Comprehensive Income/(Loss) for the three months ended March 31, 2018 and March 31, 2017 were as follows (amounts in thousands): Three Months Ended March 31, 2018 2017 Derivative Instruments: Foreign currency exchange contracts $ 41,251 $ 17,376 CFD contracts 37,147 26,791 Metals contracts 5,722 (5,236 ) Total $ 84,120 $ 38,931

RECEIVABLES FROM BROKERS (Table

RECEIVABLES FROM BROKERS (Tables)	3 Months Ended
	Mar. 31, 2018
Brokers and Dealers [Abstract]
Schedule of Receivable from Brokers	Amounts receivable from brokers consisted of the following as of (amounts in thousands): March 31, 2018 December 31, 2017 Required collateral $ 68,260 $ 73,537 Open foreign exchange positions 3,757 4,966 Total $ 72,017 $ 78,503

PROPERTY AND EQUIPMENT (Tables)

PROPERTY AND EQUIPMENT (Tables)	3 Months Ended
	Mar. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, including leasehold improvements and capitalized software development costs, consisted of the following as of (amounts in thousands): March 31, 2018 December 31, 2017 Software $ 61,635 $ 57,047 Computer equipment 19,025 18,390 Leasehold improvements 11,231 11,068 Telephone equipment 643 643 Office equipment 2,113 2,110 Furniture and fixtures 3,653 3,592 Web site development costs 378 386 Gross property and equipment 98,678 93,236 Less: Accumulated depreciation and amortization (58,505 ) (52,494 ) Property and equipment, net $ 40,173 $ 40,742

INTANGIBLE ASSETS (Tables)

INTANGIBLE ASSETS (Tables)	3 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Finite-Lived Intangible Assets	The Company’s various intangible assets consisted of the following as of (amounts in thousands): March 31, 2018 December 31, 2017 Intangibles Gross Accumulated Net Gross Accumulated Net Customer lists $ 61,727 $ (34,786 ) $ 26,941 $ 60,420 $ (31,698 ) $ 28,722 Technology 73,106 (45,080 ) 28,026 72,204 (43,270 ) 28,934 Trademarks 7,933 (4,111 ) 3,822 7,680 (3,730 ) 3,950 Total finite lived intangibles 142,766 (83,977 ) 58,789 140,304 (78,698 ) 61,606 Trademark not subject to amortization (1) 363 — 363 363 — 363 Total intangibles $ 143,129 $ (83,977 ) $ 59,152 $ 140,667 $ (78,698 ) $ 61,969 (1) These indefinite-life trademarks relate to the Forex.com and foreignexchange.com domain names where management determined there was no legal, regulatory or technological limitation on their useful lives. The Company compares the recorded value of the indefinite-life intangible assets to their fair value on an annual basis and whenever circumstances arise that indicate that impairment may have occurred. The Company had the following identifiable intangible assets and weighted average amortization periods as of March 31, 2018 : Intangible Assets Amount (in thousands) Weighted average amortization period Customer lists $ 61,727 6.9 years Technology 73,106 9.0 years Trademarks (1) 8,296 6.7 years Total intangible assets $ 143,129 (1) Trademarks with an indefinite-life, as described above, comprise $0.4 million of the $8.3 million of trademarks.
Schedule of Goodwill	The following represents the changes in the carrying amount of goodwill by segment (amounts in thousands): Retail Institutional Futures Total Carrying amount of goodwill as of December 31, 2017 $ 25,952 $ 4,650 $ 2,434 $ 33,036 Foreign currency translation adjustments 320 57 30 407 Carrying amount of goodwill as of March 31, 2018 $ 26,272 $ 4,707 $ 2,464 $ 33,443

OTHER ASSETS (Tables)

OTHER ASSETS (Tables)	3 Months Ended
	Mar. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Schedule of Other Assets	Other assets consisted of the following as of (amounts in thousands): March 31, 2018 December 31, 2017 Vendor and security deposits $ 12,320 $ 11,923 Income tax receivable — 2,132 Deferred tax assets, net 12,621 10,698 GTX trade receivables 6,587 5,758 Customer debit positions 2,995 2,384 Allowance on customer debit positions (2,771 ) (1,959 ) Prepaid assets 8,911 9,523 Miscellaneous receivables 5,738 4,872 Deferred commitment fees 497 550 Total other assets $ 46,898 $ 45,881

NON-CONTROLLING INTERESTS (Tabl

NON-CONTROLLING INTERESTS (Tables)	3 Months Ended
	Mar. 31, 2018
Noncontrolling Interest [Abstract]
Summary of redeemable noncontrolling interest treatment	The table below reflects the non-controlling interests’ effects on the Company’s financial statements (amounts in thousands): Redeemable non-controlling interests January 1, 2018 $ 4,411 Adjustment to the redemption value of non-controlling interests 367 Net income attributable to non-controlling interests 175 Distributions to non-controlling interest holders (90 ) Reclassification to liabilities (2,910 ) March 31, 2018 $ 1,953

CONVERTIBLE SENIOR NOTES (Table

CONVERTIBLE SENIOR NOTES (Tables)	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Schedule of Liability and Equity Components of Convertible Debt	The balances of the liability and equity components as of March 31, 2018 and December 31, 2017 were as follows (amounts in thousands): March 31, 2018 December 31, 2017 Liability component - principal $ 158,350 $ 158,350 Deferred bond discount (24,140 ) (25,624 ) Deferred financing cost (476 ) (505 ) Liability component - net carrying value $ 133,734 $ 132,221 Additional paid in capital $ 39,405 $ 39,405 Discount attributable to equity (826 ) (826 ) Equity component $ 38,579 $ 38,579
Schedule of Interest Expense	Interest expense related to the Convertible Senior Notes, included in Interest Expense on long term borrowings in the Condensed Consolidated Statements of Operations and Comprehensive Income/(Loss) , was as follows (amounts in thousands): For the Three Months Ended March 31, 2018 2017 Interest expense - stated coupon rate $ 1,834 $ 1,425 Interest expense - amortization of deferred bond discount and costs 1,506 1,240 Total interest expense - convertible senior notes $ 3,340 $ 2,665

EARNINGS_(LOSS) PER COMMON SH36

EARNINGS/(LOSS) PER COMMON SHARE (Tables)	3 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Computation of Earnings Per Share	The following table sets forth the computation of earnings (loss) per share (amounts in thousands except share and per share data): For the Three Months Ended March 31, 2018 2017 Net income/(loss) applicable to GAIN Capital Holdings, Inc. $ 16,086 $ (18,871 ) Adjustment (1) (367 ) 68 Net income/(loss) available to GAIN common shareholders $ 15,719 $ (18,803 ) Weighted average common shares outstanding: Basic weighted average common shares outstanding 45,017,716 47,894,546 Effect of dilutive securities: Stock options 261,732 328,325 RSUs/RSAs 244,318 13,845 Diluted weighted average common shares outstanding 45,523,766 47,894,546 Earnings/(loss) per common share: Basic $ 0.35 $ (0.39 ) Diluted $ 0.35 $ (0.39 ) (1) During the three months ended March 31, 2018 and 2017 , the Company concluded that the carrying value of the Company's redeemable noncontrolling interests was less than their redemption value, requiring that an adjustment to the carrying value be recorded for purposes of calculating earnings (loss) per common share. The adjustment to increase or reduce the carrying value will, respectively, reduce or increase earnings (loss) per common share by reducing or increasing net income available to common shareholders.

REGULATORY REQUIREMENTS (Tables

REGULATORY REQUIREMENTS (Tables)	3 Months Ended
	Mar. 31, 2018
Banking and Thrift [Abstract]
Regulatory Capital Subsidiaries Required to Maintain	The following table illustrates the minimum regulatory capital our subsidiaries were required to maintain as of March 31, 2018 and the actual amounts of capital that were maintained (amounts in millions): Entity Name Minimum Capital Excess Percent of GAIN Capital Group, LLC $ 34.8 $ 48.4 $ 13.6 139 % GAIN Capital Securities, Inc. 0.1 0.4 0.3 400 % GAIN Capital U.K., Ltd. 72.9 213.5 140.6 293 % GAIN Capital Japan Co., Ltd. 1.6 9.7 8.1 606 % GAIN Capital Australia, Pty. Ltd. 0.8 5.3 4.5 663 % GAIN Capital-Forex.com Hong Kong, Ltd. 1.9 4.5 2.6 237 % GAIN Global Markets, Inc. 0.4 2.3 1.9 575 % GAIN Capital-Forex.com Canada, Ltd. 0.2 1.6 1.4 800 % GAIN Capital Singapore Pte., Ltd. 0.3 6.7 6.4 2,233 % Trade Facts, Ltd. 0.6 3.7 3.1 617 % Global Asset Advisors, LLC — 1.7 1.7 100 % GAIN Capital Payments Ltd. 0.2 0.2 — 100 % GTX SEF, LLC 0.8 1.2 0.4 150 % Total $ 114.6 $ 299.2 $ 184.6 261 %

SEGMENT INFORMATION (Tables)

SEGMENT INFORMATION (Tables)	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Selected Financial Information by Segment	Selected financial information by segment is presented in the following tables (amounts in thousands): Retail Three Months Ended March 31, 2018 2017 Net revenue $ 85,672 $ 40,086 Employee compensation and benefits 15,425 13,335 Selling and marketing 5,666 8,904 Referral fees 7,685 12,323 Other operating expenses 17,730 13,403 Segment profit $ 39,166 $ (7,879 ) Institutional Three Months Ended March 31, 2018 2017 Net revenue $ 8,631 $ 8,621 Employee compensation and benefits 3,694 4,041 Selling and marketing 56 21 Referral Fees 517 — Other operating expenses 3,333 3,228 Segment profit $ 1,031 $ 1,331 Futures Three Months Ended March 31, 2018 2017 Net revenue $ 11,491 $ 10,967 Employee compensation and benefits 2,511 2,551 Selling and marketing 241 265 Referral fees 3,746 4,118 Other operating expenses 3,794 3,742 Segment profit $ 1,199 $ 291 Corporate and Other Three Months Ended March 31, 2018 2017 Other revenue $ 1,069 $ (108 ) Employee compensation and benefits 6,121 4,537 Selling and marketing 64 64 Other operating expenses 3,238 2,407 Loss $ (8,354 ) $ (7,116 )
Reconciliation of Operating Segment Profit to Income Before Income Tax Expense	Reconciliation of operating segment profit to Income before income tax expense Three Months Ended March 31, 2018 2017 Retail segment $ 39,166 $ (7,879 ) Institutional segment 1,031 1,331 Futures segment 1,199 291 Corporate and other (8,354 ) (7,116 ) SEGMENT PROFIT 33,042 (13,373 ) Depreciation and amortization 5,669 4,019 Purchased intangible amortization 4,204 3,602 Impairment of investment (130 ) — OPERATING PROFIT/(LOSS) 23,299 (20,994 ) Interest expense on long term borrowings 3,340 2,665 INCOME/(LOSS) BEFORE INCOME TAX EXPENSE $ 19,959 $ (23,659 )

DESCRIPTION OF BUSINESS AND B39

DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION (Details)	3 Months Ended
	Mar. 31, 2018segment
Accounting Policies [Abstract]
Number of operating segments	3

REVENUE RECOGNITION - Disaggreg

REVENUE RECOGNITION - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018USD ($)
Institutional
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 8,631
Institutional | ECN
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,056
Institutional | Swap Dealer
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,527
Institutional | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	48
Futures
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	11,491
Direct Customers | Futures
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,473
Indirect Customers | Futures
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7,029
Other | Futures
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 1,989

REVENUE RECOGNITION - Impact of

REVENUE RECOGNITION - Impact of Changes in Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Institutional revenue	$ 8,455	$ 8,367
Net revenue	106,863	59,567
Referral fees	11,948	16,441
Net Income	16,261	$ (18,786)
Adjustments | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Institutional revenue	(516)
Net revenue	(516)
Referral fees	(516)
Net Income	0
Balances without adoption of Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Institutional revenue	7,939
Net revenue	106,347
Referral fees	11,432
Net Income	$ 16,261

FAIR VALUE MEASUREMENT - Assets

FAIR VALUE MEASUREMENT - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 462,205	$ 456,543
Money market accounts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	219,908	219,286
US treasury bills: U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	104,658	105,190
Broker derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	3,757	4,966
Liabilities		(3,170)
Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	176	175
Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	133	130
Customer derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	(133,573)	(129,966)
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	324,875	324,781
Level 1 | Money market accounts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	219,908	219,286
Level 1 | US treasury bills: U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	104,658	105,190
Level 1 | Broker derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 1 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	176	175
Level 1 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	133	130
Level 1 | Customer derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	137,330	131,762
Level 2 | Money market accounts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 2 | US treasury bills: U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 2 | Broker derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	3,757	4,966
Liabilities		(3,170)
Level 2 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 2 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 2 | Customer derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	(133,573)	(129,966)
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 3 | Money market accounts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 3 | US treasury bills: U.S. government and agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 3 | Broker derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 3 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 3 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0	0
Level 3 | Customer derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	$ 0	$ 0

FAIR VALUE MEASUREMENT - Carryi

FAIR VALUE MEASUREMENT - Carrying Value and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial Assets:
Receivables from brokers	$ 72,017	$ 78,503
Payables to brokers	0	2,789
Financial Liabilities:
Convertible senior notes	133,734	132,221
Derivative Assets | Level 1
Financial Assets:
Receivables from brokers	0	0
Payables to brokers		0
Derivative Assets | Level 2
Financial Assets:
Receivables from brokers	68,260	73,537
Payables to brokers		381
Derivative Assets | Level 3
Financial Assets:
Receivables from brokers	0	0
Payables to brokers		0
Derivative Assets | Carrying Value
Financial Assets:
Receivables from brokers	68,260	73,537
Payables to brokers		381
Derivative Assets | Fair Value
Financial Assets:
Receivables from brokers	68,260	73,537
Payables to brokers		381
Derivative Liabilities | Level 1
Financial Liabilities:
Payables to customers	0	0
Convertible senior notes	0	0
Derivative Liabilities | Level 2
Financial Liabilities:
Payables to customers	1,096,794	1,108,794
Convertible senior notes	171,358	205,073
Derivative Liabilities | Level 3
Financial Liabilities:
Payables to customers	0	0
Convertible senior notes	0	0
Derivative Liabilities | Carrying Value
Financial Liabilities:
Payables to customers	1,096,794	1,108,794
Convertible senior notes	133,734	132,221
Derivative Liabilities | Fair Value
Financial Liabilities:
Payables to customers	1,096,794	1,108,794
Convertible senior notes	$ 171,358	$ 205,073

DERIVATIVES - Fair Value of Der

DERIVATIVES - Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross amounts of assets for derivative open positions at fair value	$ 221,350	$ 227,847
Gross amount of (liabilities) for derivative open positions at fair value	(84,020)	(96,085)
Net amounts of assets/(liabilities) for derivative open positions at fair value	137,330	131,762
Receivables from brokers	72,017	78,503
Payables to customers	(963,221)	(978,828)
Payables to brokers	0	(2,789)
Receivables from brokers
Derivatives, Fair Value [Line Items]
Receivables from brokers	72,017	78,503
Receivables from brokers | Derivative Assets
Derivatives, Fair Value [Line Items]
Cash Collateral	68,260	73,537
Net amounts of assets/(liabilities) for derivative open positions at fair value	3,757	4,966
Payables to customers
Derivatives, Fair Value [Line Items]
Payables to customers	(963,221)	(978,828)
Payables to customers | Derivative Liabilities
Derivatives, Fair Value [Line Items]
Cash Collateral	(1,096,794)	(1,108,794)
Net amounts of assets/(liabilities) for derivative open positions at fair value	133,573	129,966
Foreign currency exchange contracts
Derivatives, Fair Value [Line Items]
Gross amounts of assets for derivative open positions at fair value	117,871	121,104
Gross amount of (liabilities) for derivative open positions at fair value	(22,893)	(31,556)
Net amounts of assets/(liabilities) for derivative open positions at fair value	94,978	89,548
CFD contracts
Derivatives, Fair Value [Line Items]
Gross amounts of assets for derivative open positions at fair value	99,795	102,659
Gross amount of (liabilities) for derivative open positions at fair value	(60,208)	(62,322)
Net amounts of assets/(liabilities) for derivative open positions at fair value	39,587	40,337
Metals contracts
Derivatives, Fair Value [Line Items]
Gross amounts of assets for derivative open positions at fair value	3,684	4,084
Gross amount of (liabilities) for derivative open positions at fair value	(919)	(2,207)
Net amounts of assets/(liabilities) for derivative open positions at fair value	$ 2,765	1,877
Broker derivative contracts
Derivatives, Fair Value [Line Items]
Payables to brokers		(2,789)
Broker derivative contracts | Derivative Liabilities
Derivatives, Fair Value [Line Items]
Cash Collateral		(381)
Net amounts of assets/(liabilities) for derivative open positions at fair value		$ (3,170)

DERIVATIVES - Notional Values o

DERIVATIVES - Notional Values of Derivative Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Total contracts in long positions
Derivative [Line Items]
Notional amounts of derivative	$ 3,069,598	$ 2,202,722
Total contracts in long positions | Foreign currency exchange contracts
Derivative [Line Items]
Notional amounts of derivative	2,940,579	2,075,789
Total contracts in long positions | CFD contracts
Derivative [Line Items]
Notional amounts of derivative	128,478	126,519
Total contracts in long positions | Metal contracts
Derivative [Line Items]
Notional amounts of derivative	541	414
Total contracts in short positions
Derivative [Line Items]
Notional amounts of derivative	3,860,680	4,323,108
Total contracts in short positions | Foreign currency exchange contracts
Derivative [Line Items]
Notional amounts of derivative	3,693,711	4,148,056
Total contracts in short positions | CFD contracts
Derivative [Line Items]
Notional amounts of derivative	166,802	174,835
Total contracts in short positions | Metal contracts
Derivative [Line Items]
Notional amounts of derivative	$ 167	$ 217

DERIVATIVES - Net Gains (Losses

DERIVATIVES - Net Gains (Losses) on Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative [Line Items]
Net gains (losses) on derivative instruments, total	$ 84,120	$ 38,931
Foreign currency exchange contracts
Derivative [Line Items]
Net gains (losses) on derivative instruments	41,251	17,376
CFD contracts
Derivative [Line Items]
Net gains (losses) on derivative instruments	37,147	26,791
Metal contracts
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 5,722	$ (5,236)

RECEIVABLES FROM BROKERS - Sche

RECEIVABLES FROM BROKERS - Schedule of Receivables from Brokers (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Receivables from Brokerage Clients [Line Items]
Receivables from brokers	$ 72,017	$ 78,503
Required collateral
Receivables from Brokerage Clients [Line Items]
Receivables from brokers	68,260	73,537
Open foreign exchange positions
Receivables from Brokerage Clients [Line Items]
Receivables from brokers	$ 3,757	$ 4,966

PROPERTY AND EQUIPMENT - Schedu

PROPERTY AND EQUIPMENT - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Gross property and equipment	$ 98,678	$ 93,236
Less: Accumulated depreciation and amortization	(58,505)	(52,494)
Property and equipment, net	40,173	40,742
Software
Property, Plant and Equipment [Line Items]
Gross property and equipment	61,635	57,047
Computer equipment
Property, Plant and Equipment [Line Items]
Gross property and equipment	19,025	18,390
Leasehold improvements
Property, Plant and Equipment [Line Items]
Gross property and equipment	11,231	11,068
Telephone equipment
Property, Plant and Equipment [Line Items]
Gross property and equipment	643	643
Office equipment
Property, Plant and Equipment [Line Items]
Gross property and equipment	2,113	2,110
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Gross property and equipment	3,653	3,592
Website development costs
Property, Plant and Equipment [Line Items]
Gross property and equipment	$ 378	$ 386

PROPERTY AND EQUIPMENT - Additi

PROPERTY AND EQUIPMENT - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 5,669	$ 4,019

INTANGIBLE ASSETS - Intangible

INTANGIBLE ASSETS - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Intangible Assets by Major Class [Line Items]
Finite-lived intangible assets, gross	$ 142,766		$ 140,304
Accumulated Amortization	(83,977)		(78,698)
Finite-lived intangible assets, net	58,789		61,606
Total intangibles, gross	143,129		140,667
Intangible assets, net	59,152		61,969
Amortization of Intangible Assets	4,204	$ 3,602
Payments to acquire intangible assets	0	$ 5,125
Forex Capital Markets L.L.C.
Intangible Assets by Major Class [Line Items]
Payments to acquire intangible assets			7,200
Trademarks
Intangible Assets by Major Class [Line Items]
Indefinite-lived intangible assets	363		363
Customer lists
Intangible Assets by Major Class [Line Items]
Finite-lived intangible assets, gross	61,727		60,420
Accumulated Amortization	(34,786)		(31,698)
Finite-lived intangible assets, net	26,941		$ 28,722
Total intangibles, gross	$ 61,727
Weighted average amortization period	6 years 11 months
Customer lists | Forex Capital Markets L.L.C.
Intangible Assets by Major Class [Line Items]
Weighted average amortization period			2 years
Technology
Intangible Assets by Major Class [Line Items]
Finite-lived intangible assets, gross	$ 73,106		$ 72,204
Accumulated Amortization	(45,080)		(43,270)
Finite-lived intangible assets, net	28,026		28,934
Total intangibles, gross	$ 73,106
Weighted average amortization period	9 years
Trademarks
Intangible Assets by Major Class [Line Items]
Finite-lived intangible assets, gross	$ 7,933		7,680
Accumulated Amortization	(4,111)		(3,730)
Finite-lived intangible assets, net	3,822		$ 3,950
Total intangibles, gross	$ 8,296
Weighted average amortization period	6 years 8 months

INTANGIBLE ASSETS - Goodwill (D

INTANGIBLE ASSETS - Goodwill (Details)	3 Months Ended
	Mar. 31, 2018USD ($)reporting_unit
Goodwill [Line Items]
Number of reporting units | reporting_unit	3
Goodwill additions or impairments	$ 0
Goodwill	33,036,000
Goodwill [Roll Forward]
Goodwill. beginning balance	33,036,000
Foreign currency translation adjustments	407,000
Goodwill. ending balance	33,443,000
Operating Segments | Retail
Goodwill [Line Items]
Goodwill	25,952,000
Goodwill [Roll Forward]
Goodwill. beginning balance	25,952,000
Foreign currency translation adjustments	320,000
Goodwill. ending balance	26,272,000
Operating Segments | Institutional
Goodwill [Line Items]
Goodwill	4,650,000
Goodwill [Roll Forward]
Goodwill. beginning balance	4,650,000
Foreign currency translation adjustments	57,000
Goodwill. ending balance	4,707,000
Operating Segments | Futures
Goodwill [Line Items]
Goodwill	2,434,000
Goodwill [Roll Forward]
Goodwill. beginning balance	2,434,000
Foreign currency translation adjustments	30,000
Goodwill. ending balance	$ 2,464,000

OTHER ASSETS - Schedule of Othe

OTHER ASSETS - Schedule of Other Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Vendor and security deposits	$ 12,320	$ 11,923
Income tax receivable	0	2,132
Deferred tax assets, net	12,621	10,698
GTX trade receivables	6,587	5,758
Customer debit positions	2,995	2,384
Allowance on customer debit positions	(2,771)	(1,959)
Prepaid assets	8,911	9,523
Miscellaneous receivables	5,738	4,872
Deferred commitment fees	497	550
Total other assets	$ 46,898	$ 45,881

RELATED PARTY TRANSACTIONS - Ad

RELATED PARTY TRANSACTIONS - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Payables to customers	$ 963,221	$ 978,828
Management
Related Party Transaction [Line Items]
Payables to customers	200	200
IPGL Limited
Related Party Transaction [Line Items]
Payables to customers	$ 20,400	$ 15,900

NON-CONTROLLING INTERESTS (Deta

NON-CONTROLLING INTERESTS (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Feb. 28, 2018	Mar. 31, 2014
Increase (Decrease) in Temporary Equity [Roll Forward]
Redeemable noncontrolling interest, beginning balance	$ 4,411
Adjustment to the redemption value of non-controlling interests	367	$ (68)	$ 367	$ (68)
Net income attributable to non-controlling interests	175	$ 85
Distributions to non-controlling interest holders	(90)
Reclassification to Liabilities	(2,910)
Redeemable noncontrolling interest, ending balance	$ 1,953		$ 4,411
Global Asset Advisors, LLC
Redeemable Noncontrolling Interest [Line Items]
Percentage of company acquired (as percent)				24.00%		55.00%
Redeemable noncontrolling interest retained by noncontrolling owners (as percent)						45.00%
Noncontrolling interest by seller (as percent)				21.00%	21.00%
Top Third Ag Marketing, LLC
Redeemable Noncontrolling Interest [Line Items]
Percentage of company acquired (as percent)				24.00%		55.00%
Redeemable noncontrolling interest retained by noncontrolling owners (as percent)						45.00%
Noncontrolling interest by seller (as percent)				21.00%		45.00%

REVOLVING CREDIT ARRAGEMENT (De

REVOLVING CREDIT ARRAGEMENT (Details) - Revolving Credit Facility - USD ($)	Aug. 03, 2017	Mar. 31, 2018
Debt Instrument [Line Items]
Maximum amount under facility	$ 50,000,000
Commitment fee amount	500,000
Line of credit outstanding		$ 0
Maximum
Debt Instrument [Line Items]
Increase in borrowing capacity available	25,000,000
Minimum
Debt Instrument [Line Items]
Increase in borrowing capacity available	$ 5,000,000
Line of Credit
Debt Instrument [Line Items]
Term of credit facility	3 years

CONVERTIBLE SENIOR NOTES - Addi

CONVERTIBLE SENIOR NOTES - Additional Information (Details) - Convertible Debt - USD ($)	Aug. 22, 2017	Nov. 27, 2013	Sep. 30, 2017	Mar. 31, 2016	Apr. 01, 2015
Convertible Notes Due 2022
Debt Instrument [Line Items]
Principal amount	$ 92,000,000
Stated interest rate (as percent)	5.00%
Proceeds from note offering	$ 89,000,000
Convertible Notes Due 2020
Debt Instrument [Line Items]
Principal amount					$ 60,000,000
Stated interest rate (as percent)					4.125%
Convertible Notes Due 2018
Debt Instrument [Line Items]
Principal amount		$ 80,000,000	$ 71,800,000	$ 1,900,000
Stated interest rate (as percent)		4.125%
Proceeds from note offering		$ 77,900,000
Repurchase of convertible notes			73,700,000	$ 1,700,000
Loss on extinguishment of debt			$ 4,900,000

CONVERTIBLE SENIOR NOTES - Sche

CONVERTIBLE SENIOR NOTES - Schedule of Liability and Equity Components (Details) - Convertible Debt - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Liability component - principal	$ 158,350	$ 158,350
Deferred bond discount	(24,140)	(25,624)
Deferred financing cost	(476)	(505)
Liability component - net carrying value	133,734	132,221
Additional paid in capital	39,405	39,405
Discount attributable to equity	(826)	(826)
Equity component	$ 38,579	$ 38,579

CONVERTIBLE SENIOR NOTES - Sc58

CONVERTIBLE SENIOR NOTES - Schedule of Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Debt Instrument [Line Items]
Interest expense - amortization of deferred bond discount and costs	$ 163	$ 110
Convertible Debt
Debt Instrument [Line Items]
Interest expense - stated coupon rate	1,834	1,425
Interest expense - amortization of deferred bond discount and costs	1,506	1,240
Total interest expense - convertible senior notes	$ 3,340	$ 2,665

EARNINGS_(LOSS) PER COMMON SH59

EARNINGS/(LOSS) PER COMMON SHARE - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive stock options excluded from calculation of diluted earnings per share	0.4	0.3
Common stock threshold for additional dilutive shares (in dollars per share)	$ 13
Convertible Notes Due 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Dilutive securities, effect on basic earnings per share, dilutive convertible securities	$ 0
Convertible Notes Due 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Dilutive securities, effect on basic earnings per share, dilutive convertible securities	1.5
Convertible Notes Due 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Dilutive securities, effect on basic earnings per share, dilutive convertible securities	$ 4.2

EARNINGS_(LOSS) PER COMMON SH60

EARNINGS/(LOSS) PER COMMON SHARE - Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income/(loss) applicable to GAIN Capital Holdings, Inc.	$ 16,086	$ (18,871)
Adjustment to fair value of redeemable non-controlling interests	(367)	68	$ (367)	$ 68
Net income/(loss) available to GAIN common shareholders	$ 15,719	$ (18,803)
Weighted average common shares outstanding:
Basic weighted average common shares outstanding	45,017,716	47,894,546
Effect of dilutive securities:
Stock options (in shares)	261,732	328,325
RSUs/RSAs (in shares)	244,318	13,845
Diluted weighted average common shares outstanding	45,523,766	47,894,546
Earnings/(loss) per common share:
Basic (usd per share)	$ 0.35	$ (0.39)
Diluted (usd per share)	$ 0.35	$ (0.39)

INCOME TAXES - Additional Infor

INCOME TAXES - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Provision (benefit) for income taxes	$ 3,698	$ (4,893)
Effective income tax rates (as percent)	18.50%	20.70%

REGULATORY REQUIREMENTS - Regul

REGULATORY REQUIREMENTS - Regulatory Capital Subsidiaries Required to Maintain (Detail) $ in Millions	Mar. 31, 2018USD ($)
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 114.6
Capital Levels Maintained	299.2
Excess Net Capital	$ 184.6
Percent of Requirement Maintained	261.00%
GAIN Capital Group, LLC
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 34.8
Capital Levels Maintained	48.4
Excess Net Capital	$ 13.6
Percent of Requirement Maintained	139.00%
GAIN Capital Securities, Inc.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.1
Capital Levels Maintained	0.4
Excess Net Capital	$ 0.3
Percent of Requirement Maintained	400.00%
GAIN Capital U.K., Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.6
Capital Levels Maintained	3.7
Excess Net Capital	$ 3.1
Percent of Requirement Maintained	617.00%
GAIN Capital Japan Co., Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 1.9
Capital Levels Maintained	4.5
Excess Net Capital	$ 2.6
Percent of Requirement Maintained	237.00%
GAIN Capital Australia, Pty. Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.8
Capital Levels Maintained	5.3
Excess Net Capital	$ 4.5
Percent of Requirement Maintained	663.00%
GAIN Capital-Forex.com Hong Kong, Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.2
Capital Levels Maintained	1.6
Excess Net Capital	$ 1.4
Percent of Requirement Maintained	800.00%
GAIN Global Markets, Inc.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 1.6
Capital Levels Maintained	9.7
Excess Net Capital	$ 8.1
Percent of Requirement Maintained	606.00%
GAIN Capital-Forex.com Canada, Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.4
Capital Levels Maintained	2.3
Excess Net Capital	$ 1.9
Percent of Requirement Maintained	575.00%
GAIN Capital Singapore Pte., Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 72.9
Capital Levels Maintained	213.5
Excess Net Capital	$ 140.6
Percent of Requirement Maintained	293.00%
Trade Facts, Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.3
Capital Levels Maintained	6.7
Excess Net Capital	$ 6.4
Percent of Requirement Maintained	2233.00%
Global Asset Advisors, LLC
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0
Capital Levels Maintained	1.7
Excess Net Capital	$ 1.7
Percent of Requirement Maintained	100.00%
GTX SEF, LLC
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.2
Capital Levels Maintained	0.2
Excess Net Capital	$ 0
Percent of Requirement Maintained	100.00%
GTX SEF, LLC
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.8
Capital Levels Maintained	1.2
Excess Net Capital	$ 0.4
Percent of Requirement Maintained	150.00%

SEGMENT INFORMATION - Additiona

SEGMENT INFORMATION - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)segmentmarket	Mar. 31, 2017USD ($)
Segment Reporting [Abstract]
Number of operating segments | segment	3
Number of accessible global financial markets | market	12,500
Intersegment Eliminations
Segment Reporting Information [Line Items]
Net revenue | $	$ (0.5)	$ (0.5)

SEGMENT INFORMATION - Selected

SEGMENT INFORMATION - Selected Financial Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Employee compensation and benefits	$ 27,751	$ 24,215
Selling and marketing	6,027	9,256
Referral fees	11,948	16,441
Segment profit (loss)	33,042	(13,373)
Operating Segments | Retail
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net revenue	85,672	40,086
Employee compensation and benefits	15,425	13,335
Selling and marketing	5,666	8,904
Referral fees	7,685	12,323
Other operating expenses	17,730	13,403
Segment profit (loss)	39,166	(7,879)
Operating Segments | Institutional
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net revenue	8,631	8,621
Employee compensation and benefits	3,694	4,041
Selling and marketing	56	21
Referral fees	517	0
Other operating expenses	3,333	3,228
Segment profit (loss)	1,031	1,331
Operating Segments | Futures
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net revenue	11,491	10,967
Employee compensation and benefits	2,511	2,551
Selling and marketing	241	265
Referral fees	3,746	4,118
Other operating expenses	3,794	3,742
Segment profit (loss)	1,199	291
Corporate and Other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net revenue	1,069	(108)
Employee compensation and benefits	6,121	4,537
Selling and marketing	64	64
Other operating expenses	3,238	2,407
Segment profit (loss)	$ (8,354)	$ (7,116)

SEGMENT INFORMATION - Reconcili

SEGMENT INFORMATION - Reconciliation of Operating Segment Profit to Income Before Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
SEGMENT PROFIT	$ 33,042	$ (13,373)
Depreciation and amortization	5,669	4,019
Purchased intangible amortization	4,204	3,602
Impairment of investment	(130)	0
OPERATING PROFIT/(LOSS)	23,299	(20,994)
Interest expense on long term borrowings	3,340	2,665
INCOME/(LOSS) BEFORE INCOME TAX EXPENSE/(BENEFIT)	19,959	(23,659)
Operating Segments | Retail
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
SEGMENT PROFIT	39,166	(7,879)
Operating Segments | Institutional
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
SEGMENT PROFIT	1,031	1,331
Operating Segments | Futures
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
SEGMENT PROFIT	1,199	291
Corporate and Other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
SEGMENT PROFIT	$ (8,354)	$ (7,116)

SUBSEQUENT EVENTS - Additional

SUBSEQUENT EVENTS - Additional Information (Detail) - Subsequent event - USD ($) $ / shares in Units, $ in Millions	Apr. 24, 2018	Apr. 30, 2018
Subsequent Event [Line Items]
Common stock dividend declared (usd per share)		$ 0.06
Stock repurchase program, increase in authorized amount	$ 25