GAIN Capital 10-Q 9 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Jul. 27, 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	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		44,642,233

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
ASSETS:
Cash and cash equivalents	$ 360,312	$ 209,688
Cash and cash equivalents held for customers	920,366	978,828
Receivables from brokers	52,335	78,503
Property and equipment, net	36,462	40,742
Intangible assets, net	40,117	61,969
Goodwill	28,182	33,036
Other assets	36,743	45,881
Total assets	1,474,517	1,448,647
Liabilities
Payables to customers	920,366	978,828
Payables to brokers	2,973	2,789
Accrued compensation and benefits	10,623	10,104
Accrued expenses and other liabilities	39,749	33,947
Income tax payable	10,154	599
Convertible senior notes	135,289	132,221
Total liabilities	1,119,154	1,158,488
Commitments and contingent liabilities
Redeemable non-controlling interests	2,562	4,411
Shareholders’ equity
Common stock ($0.00001 par value; 120 million shares authorized, 54,323,940 shares issued and 44,755,615 shares outstanding as of June 30, 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	239,745	235,659
Retained earnings	199,683	122,686
Accumulated other comprehensive loss	(21,353)	(15,670)
Treasury stock, at cost (9,568,325 shares at June 30, 2018 and 8,460,041 at December 31, 2017)	(65,274)	(56,927)
Total shareholders’ equity	352,801	285,748
Total liabilities and shareholders’ equity	$ 1,474,517	$ 1,448,647

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 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)	54,323,940	53,612,340
Common stock, shares outstanding (in shares)	44,755,615	45,152,299
Treasury stock, shares (in shares)	9,568,325	8,460,041

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
REVENUE:
Total non-interest revenue	$ 81,750	$ 89,466	$ 178,410	$ 139,988
Interest revenue	2,827	1,261	4,901	2,082
Interest expense	392	136	766	299
Total net interest revenue	2,435	1,125	4,135	1,783
Net revenue	84,185	90,591	182,545	141,771
EXPENSES:
Employee compensation and benefits	22,473	23,011	46,811	43,534
Selling and marketing	6,767	7,517	12,737	16,752
Referral fees	10,464	13,312	21,895	29,753
Trading expenses	5,541	4,863	11,352	10,334
General and administrative	14,228	11,835	26,692	21,064
Depreciation and amortization	5,341	4,121	10,709	7,949
Purchased intangible amortization	3,612	3,817	7,280	6,883
Communications and technology	5,495	4,497	10,886	9,526
Bad debt provision	282	(166)	1,400	(94)
Restructuring expenses	25	0	25	0
Impairment of investment	0	0	(130)	0
Total operating expense	74,228	72,807	149,657	145,701
OPERATING PROFIT/(LOSS)	9,957	17,784	32,888	(3,930)
Interest expense on long term borrowings	3,388	2,694	6,728	5,359
INCOME/(LOSS) BEFORE INCOME TAX EXPENSE/(BENEFIT)	6,569	15,090	26,160	(9,289)
Income tax (benefit)/expense	(263)	1,533	7,413	(4,401)
Equity in net loss of affiliate	0	(34)	0	(54)
Net income/(loss) from continuing operations	6,832	13,523	18,747	(4,942)
Income from discontinued operations, including gain on sale of $69,581, net of income tax of $9,806	60,642	569	64,988	248
NET INCOME/(LOSS)	67,474	14,092	83,735	(4,694)
Less income attributable to non-controlling interest	326	153	501	238
NET INCOME/(LOSS) APPLICABLE TO GAIN CAPITAL HOLDINGS, INC.	67,148	13,939	83,234	(4,932)
Other comprehensive income:
Foreign currency translation adjustment	(15,157)	7,761	(5,683)	12,300
COMPREHENSIVE INCOME APPLICABLE TO GAIN CAPITAL HOLDINGS, INC.	$ 51,991	$ 21,700	$ 77,551	$ 7,368
Earnings/(loss) per common share:
Basic earnings/(loss) from continuing operations (usd per share)	$ 0.13	$ 0.30	$ 0.39	$ (0.09)
Basic earnings from discontinued operations (usd per share)	1.36	0.01	1.45	0.01
Basic (usd per share)	1.49	0.31	1.84	(0.08)
Diluted earnings/(loss) from continuing operations (usd per share)	0.13	0.30	0.38	(0.09)
Diluted earnings from discontinued operations (usd per share)	1.34	0.01	1.43	0.01
Diluted (usd per share)	$ 1.47	$ 0.31	$ 1.81	$ (0.08)
Weighted average common shares outstanding used in computing earnings/(loss) per common share:
Basic (shares)	44,797,103	47,687,214	44,906,800	47,790,307
Diluted (shares)	45,309,002	47,894,648	45,415,774	47,790,307
Retail revenue
REVENUE:
Revenue from contract with customer	$ 72,032	$ 79,079	$ 156,152	$ 118,010
Futures revenue
REVENUE:
Revenue from contract with customer	11,129	9,565	21,774	20,145
Other (loss)/revenue
REVENUE:
Revenue from contract with customer	$ (1,411)	$ 822	$ 484	$ 1,833

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.	(4,932)				(4,932)
Exercise of options (in shares)		82,774
Exercise of options	335			335
Conversion of restricted stock into common stock (in shares)		510,793
Issuance of common stock for the employee stock purchase plan (in shares)		64,546
Issuance of common stock for the employee stock purchase plan	402			402
Purchase of treasury stock (in shares)		(1,373,685)
Purchase of treasury stock	(9,426)		(9,426)
Shares withheld for net settlements of share-based awards (in shares)		(55,190)
Shares withheld for net settlements of share-based awards	(381)		(381)
Share-based compensation	2,856			2,856
Adjustment to fair value of redeemable non-controlling interests	983				983
Dividends declared	(5,774)				(5,774)
Foreign currency translation adjustment	12,300					12,300
ENDING BALANCE (in shares) at Jun. 30, 2017		47,449,481
ENDING BALANCE at Jun. 30, 2017	290,545	$ 0	(40,574)	221,985	133,676	(24,542)
BEGINNING BALANCE (in shares) at Dec. 31, 2017		45,152,299
BEGINNING 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.	83,234				83,234
Exercise of options (in shares)		84,500
Exercise of options	587			587
Conversion of restricted stock into common stock (in shares)		585,565
Conversion of restricted stock into common stock	0
Issuance of common stock for the employee stock purchase plan (in shares)		41,535
Issuance of common stock for the employee stock purchase plan	314			314
Purchase of treasury stock (in shares)		(1,098,880)
Purchase of treasury stock	(8,271)		(8,271)
Shares withheld for net settlements of share-based awards (in shares)		(9,404)
Shares withheld for net settlements of share-based awards	(76)		(76)
Share-based compensation	3,185			3,185
Adjustment to fair value of redeemable non-controlling interests	(890)				(890)
Dividends declared	(5,347)				(5,347)
Foreign currency translation adjustment	(5,683)					(5,683)
ENDING BALANCE (in shares) at Jun. 30, 2018		44,755,615
ENDING BALANCE at Jun. 30, 2018	$ 352,801	$ 0	$ (65,274)	$ 239,745	$ 199,683	$ (21,353)

Condensed Consolidated Stateme6

Condensed Consolidated Statement of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income/(loss)	$ 67,474	$ 14,092	$ 83,735	$ (4,694)
Adjustments to reconcile net income/(loss) to cash (used in)/provided by operating activities
Loss/(gain) on foreign currency exchange rates			950	(96)
Depreciation and amortization			19,271	16,315
Deferred tax (benefit)/expense			(1,227)	622
Amortization of deferred financing costs			324	221
Bad debt provision/(recovery)	282	(166)	1,400	(94)
Convertible senior notes discount amortization			2,743	2,289
Share-based compensation			3,185	2,856
Equity in net loss of affiliate	0	34	0	54
Gain on sale of GTX			(69,581)	0
Changes in operating assets and liabilities:
Receivables from brokers			25,687	(20,037)
Other assets			3,055	518
Payables to customers			(67,826)	(20,183)
Payables to brokers			192	0
Accrued compensation and benefits			(2,056)	(4,891)
Accrued expenses and other liabilities			2,822	(6,176)
Income tax payable			9,345	(2,699)
Net cash provided by/(used in) operating activities			12,019	(35,995)
Cash provided by/(used in) operating activities - continuing operations			8,233	(24,521)
Cash used in operating activities - discontinued operations			3,786	(11,474)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment			(8,736)	(10,182)
Proceeds from sale of GTX			96,764	0
Acquisition of FXCM assets			0	(6,960)
Net cash provided by/(used in) investing activities			88,028	(17,142)
Cash used in investing activities - continuing operations			(7,891)	(16,806)
Cash provided by/(used in) investing activities - discontinued operations			95,919	(336)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from exercise of stock options			587	335
Proceeds from employee stock purchase plan			314	402
Purchase of treasury stock			(8,347)	(9,807)
Dividend payments			(5,347)	(5,774)
Distributions to non-controlling interest holders			(199)	(873)
Net cash used in financing activities			(12,992)	(15,717)
Effect of exchange rate changes on cash and cash equivalents			5,107	29,091
NET INCREASE/(DECREASE) IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH			92,162	(39,763)
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH — Beginning of period			1,188,516	1,180,228	$ 1,180,228
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH — End of period	1,280,678	1,140,465	1,280,678	1,140,465	1,188,516
Cash and cash equivalents						$ 360,312	$ 209,688	$ 193,137
Cash and cash equivalents held for customers						920,366		947,328
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH — End of period	1,280,678	1,140,465	1,188,516	1,180,228	$ 1,180,228	$ 1,280,678	$ 1,188,516	$ 1,140,465
Cash (paid)/received during the year for:
Interest			(4,346)	(3,148)
Income taxes			(3,040)	(4,386)
Non-cash financing activities:
Adjustment to redemption value of non-controlling interests	$ (523)	$ 915	$ (890)	$ 983

DESCRIPTION OF BUSINESS AND BAS

DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION	6 Months Ended
	Jun. 30, 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 two 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 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 19 . 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. Sale of GTX ECN Business On June 29, 2018, the Company completed the sale of the GTX ECN business, which previously comprised the Company's institutional segment, to Deutsche Börse Group via its FX unit, 360T, for a total purchase price of $100 million subject to a customary working capital adjustment. The Company determined that the institutional segment met the discontinued operations criteria set forth in Accounting Standards Codification (ASC) Subtopic 205-20-45, Presentation of Financial Statements , in the quarter ended June 30, 2018. As such, the institutional segment results have been classified as discontinued operations in the accompanying Condensed Consolidated Statements of Operations and Comprehensive Income. For more information relating to the discontinued operations of the Company's GTX ECN business, please see Note 3 . 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.

ACCOUNTING PRONOUNCEMENTS

ACCOUNTING PRONOUNCEMENTS	6 Months Ended
	Jun. 30, 2018
Accounting Changes and Error Corrections [Abstract]
ACCOUNTING PRONOUNCEMENTS	ACCOUNTING PRONOUNCEMENTS Recently Adopted In January 2017, the Financial Accounting Standards 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 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 going forward 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 includes cash and cash equivalents held for customers as restricted cash. As a result, Net cash used in operating activities on the Statement of Cash Flows increased by $20.2 million for the six months ended June 30, 2017. The Company defines restricted cash to include cash and cash equivalents held for customers, which 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 4 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.

DISCONTINUED OPERATIONS

DISCONTINUED OPERATIONS	6 Months Ended
	Jun. 30, 2018
Discontinued Operations and Disposal Groups [Abstract]
DISCONTINUED OPERATIONS	DISCONTINUED OPERATIONS On June 29, 2018, the Company completed the sale of its GTX ECN business, which previously comprised the Company's institutional segment, to Deutsche Börse via its FX unit, 360T, for a total purchase price of $100 million , subject to a customary working capital adjustment. The Company recorded a gain of $69.6 million on the sale of the GTX business subject to the final working capital adjustment between the parties. The Company determined that the sale of the GTX business qualifies as a discontinued operation under the criteria set forth in Accounting Standards Codification 205-20-45, Presentation of Financial Statements and the Company does not have any significant continuing involvement in these operations. The results of operations from the discontinued segment for the three and six months ended June 30, 2018 are as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 REVENUE: Institutional revenue $ 7,923 $ 7,435 $ 16,379 $ 15,802 Other revenue (2 ) — (2 ) — Total non-interest revenue 7,921 7,435 16,377 15,802 Interest revenue 55 28 103 48 Total net interest revenue 55 28 103 48 Net revenue $ 7,976 $ 7,463 $ 16,480 $ 15,850 EXPENSES: Employee compensation and benefits $ 2,466 $ 3,314 $ 5,879 $ 7,006 Trading expenses 2,782 2,315 5,439 4,911 Other expenses 1,861 1,804 3,924 3,183 Total operating expense 7,109 7,433 15,242 15,100 OPERATING PROFIT 867 30 1,238 750 Gain on sale of discontinued operations 69,581 — 69,581 — INCOME BEFORE INCOME TAX EXPENSE/(BENEFIT) 70,448 30 70,819 750 Income tax expense/(benefit) 9,806 (539 ) 5,831 502 NET INCOME FROM DISCONTINUED OPERATIONS $ 60,642 $ 569 $ 64,988 $ 248 Since the sale of the GTX ECN business closed on June 29, 2018, there are no assets held for sale as of June 30, 2018. As of December 31, 2017, total assets of $28.5 million and total liabilities of $3.8 million were related to the discontinued segment.

REVENUE RECOGNITION

REVENUE RECOGNITION	6 Months Ended
	Jun. 30, 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 Topic 606. Additionally, the Company’s futures segment revenue was 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 and $1.0 million for the three and six months ended June 30, 2018 , respectively, which impacted referral fee expense in the Company's discontinued operations. 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 exchange for an amount that reflects the consideration the Company expects to be entitled to in transferring those goods or services. The following is a description of the Company’s revenue recognition policies as they relate to revenue from contracts with customers. 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. 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 for the futures segment for the three and six months ended June 30, 2018 : Three months ended June 30, 2018 Six months ended June 30, 2018 Futures Direct Customers (1) $ 2,693 $ 5,166 Indirect Customers (2) $ 6,555 $ 13,583 Other $ 2,852 $ 4,842 Total Segment Revenue $ 12,100 $ 23,591 (1) Direct customers are all customers not classified as indirect (2) Indirect customers are referred to the Company by introducing brokers Contract Assets and Contract Liabilities The timing of 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 service obligation is satisfied.

FAIR VALUE MEASUREMENT

FAIR VALUE MEASUREMENT	6 Months Ended
	Jun. 30, 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. U.S. Generally Accepted Accounting Principles ("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 $ 252,311 $ — $ — $ 252,311 Cash and cash equivalents held for customers: US treasury bills: U.S. government and agency securities 103,891 — — 103,891 Receivable from brokers: Broker derivative contracts — (10,391 ) — (10,391 ) Other assets: Certificates of deposit 176 — — 176 Other 125 — — 125 Payables to customers: Customer derivative contracts — 124,807 — 124,807 Payables to brokers: Broker derivative contracts — 2,482 — 2,482 Total $ 356,503 $ 116,898 $ — $ 473,401 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 approaches or techniques in measuring the fair value of any financial assets and liabilities during the six months ended June 30, 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 June 30, 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 14 ). 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 June 30, 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 $ 62,726 $ 62,726 $ — $ 62,726 $ — Financial Liabilities: Payables to customers $ 1,045,173 $ 1,045,173 $ — $ 1,045,173 $ — Payable to brokers $ 5,455 $ 5,455 $ — $ 5,455 $ — Convertible senior notes $ 135,289 $ 179,847 $ — $ 179,847 $ — 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	6 Months Ended
	Jun. 30, 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): June 30, 2018 Gross amounts of Gross amount of Net amounts of Derivative Instruments: Foreign currency exchange contracts $ 105,405 $ (23,079 ) $ 82,326 CFD contracts 91,411 (60,328 ) 31,083 Metals contracts 6,363 (2,874 ) 3,489 Total $ 203,179 $ (86,281 ) $ 116,898 June 30, 2018 Cash Collateral Net amounts of Net amounts of Derivative Assets/(Liabilities): Receivables from brokers $ 62,726 $ (10,391 ) $ 52,335 Payables to customers $ (1,045,173 ) $ 124,807 $ (920,366 ) Payable to brokers $ (5,455 ) $ 2,482 $ (2,973 ) 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): June 30, 2018 Total contracts in long positions Total contracts in short positions Derivative Instruments: Foreign currency exchange contracts 3,019,520 3,598,235 CFD contracts 115,947 168,033 Metals contracts 485 120 Total 3,135,952 3,766,388 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 for the three and six months ended June 30, 2018 and June 30, 2017 were as follows (amounts in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Derivative Instruments: Foreign currency exchange contracts $ 51,579 $ 43,633 $ 92,830 $ 61,009 CFD contracts 14,681 26,422 51,828 53,213 Metals contracts 5,759 9,018 11,481 3,782 Total $ 72,019 $ 79,073 $ 156,139 $ 118,004

RECEIVABLES FROM BROKERS

RECEIVABLES FROM BROKERS	6 Months Ended
	Jun. 30, 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): June 30, 2018 December 31, 2017 Required collateral $ 62,726 $ 73,537 Open foreign exchange positions (10,391 ) 4,966 Total $ 52,335 $ 78,503

PROPERTY AND EQUIPMENT

PROPERTY AND EQUIPMENT	6 Months Ended
	Jun. 30, 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): June 30, 2018 December 31, 2017 Software $ 59,713 $ 57,047 Computer equipment 18,964 18,390 Leasehold improvements 10,963 11,068 Telephone equipment 642 643 Office equipment 2,048 2,110 Furniture and fixtures 3,544 3,592 Web site development costs 635 386 Gross property and equipment 96,509 93,236 Less: Accumulated depreciation and amortization (60,047 ) (52,494 ) Property and equipment, net $ 36,462 $ 40,742 Depreciation and amortization expense for property and equipment was $5.3 million and $4.1 million for the three months ended June 30, 2018 and 2017 , respectively, and $10.7 million and $7.9 million for the six months ended June 30, 2018 and 2017 , respectively.

INTANGIBLE ASSETS

INTANGIBLE ASSETS	6 Months Ended
	Jun. 30, 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): June 30, 2018 December 31, 2017 Intangibles Gross Accumulated Net Gross Accumulated Net Customer lists $ 59,658 $ (36,210 ) $ 23,448 $ 60,420 $ (31,698 ) $ 28,722 Technology 50,233 (37,273 ) 12,960 72,204 (43,270 ) 28,934 Trademarks 7,533 (4,187 ) 3,346 7,680 (3,730 ) 3,950 Total finite lived intangibles 117,424 (77,670 ) 39,754 140,304 (78,698 ) 61,606 Trademark not subject to amortization (1) 363 — 363 363 — 363 Total intangibles $ 117,787 $ (77,670 ) $ 40,117 $ 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 June 30, 2018 : Intangible Assets Amount (in thousands) Weighted average amortization period Customer lists $ 59,658 6.9 years Technology 50,233 8.6 years Trademarks (1) 7,896 6.7 years Total intangible assets $ 117,787 (1) Trademarks with an indefinite-life, as described above, comprise $0.4 million of the $7.9 million of trademarks. Amortization expense for the purchased intangibles was $3.6 million and $3.8 million for the three months ended June 30, 2018 and 2017 , respectively, and $7.3 million and $6.9 million for the six months ended June 30, 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 on October 31 and in interim periods when events or changes indicate the carrying value may not be recoverable. As a result of the sale of the GTX ECN business, the Company tested the institutional goodwill for impairment using data as of June 30, 2018 and concluded there was no impairment. The $4.6 million of institutional goodwill was allocated to discontinued operations. The Company operates under two reporting units: retail and futures. There were no additions or impairments to the carrying value of the Company’s goodwill during the six months ended June 30, 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 June 30, 2018 and December 31, 2017 , the Company had recorded goodwill of approximately $28.2 million and $33.0 million , respectively. The decrease of $4.8 million was primarily related to the discontinued operations of GTX. 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 (186 ) (34 ) (18 ) (238 ) Discontinued operations — (4,616 ) — (4,616 ) Carrying amount of goodwill as of June 30, 2018 $ 25,766 $ — $ 2,416 $ 28,182

OTHER ASSETS

OTHER ASSETS	6 Months Ended
	Jun. 30, 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): June 30, 2018 December 31, 2017 Vendor and security deposits $ 6,932 $ 11,923 Income tax receivable 282 2,132 Deferred tax assets, net 12,053 10,698 GTX trade receivables 616 5,758 Customer debit positions 4,067 2,384 Allowance on customer debit positions (3,700 ) (1,959 ) Prepaid assets 8,801 9,523 Miscellaneous receivables 7,248 4,872 Deferred commitment fees 444 550 Total other assets $ 36,743 $ 45,881

RELATED PARTY TRANSACTIONS

RELATED PARTY TRANSACTIONS	6 Months Ended
	Jun. 30, 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 June 30, 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 Sheets. The aggregate amount of these funds was $20.2 million and $15.9 million as of June 30, 2018 and December 31, 2017 , respectively. The net revenue generated by any individual related party was not deemed to be material in any period.

NON-CONTROLLING INTERESTS

NON-CONTROLLING INTERESTS	6 Months Ended
	Jun. 30, 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 June 30, 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 third 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 890 Net income attributable to non-controlling interests 371 Distributions to non-controlling interest holders (199 ) Reclassification to liabilities (2,911 ) June 30, 2018 $ 2,562

REVOLVING CREDIT ARRANGEMENT

REVOLVING CREDIT ARRANGEMENT	6 Months Ended
	Jun. 30, 2018
Debt Disclosure [Abstract]
REVOLVING CREDIT ARRANGEMENT	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. The commitment fees of $0.4 million is amortized over the life of the facility and is recorded to Other Assets . As of June 30, 2018 , the Company was in compliance with the covenants for its credit facility. As of June 30, 2018 , there were no amounts outstanding under the revolving line of credit.

CONVERTIBLE SENIOR NOTES

CONVERTIBLE SENIOR NOTES	6 Months Ended
	Jun. 30, 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 , 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. 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 offering were approximately $89.0 million , after deducting discounts to the initial purchasers but prior to other 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 due 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 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 2018, for an aggregate purchase price of $73.7 million with the proceeds from the issuance of Convertible Senior Notes due 2022. As a result, the Company 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 June 30, 2018 and December 31, 2017 were as follows (amounts in thousands): June 30, 2018 December 31, 2017 Liability component - principal $ 158,350 $ 158,350 Deferred bond discount (22,615 ) (25,624 ) Deferred financing cost (446 ) (505 ) Liability component - net carrying value $ 135,289 $ 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 , was as follows (amounts in thousands): For the Three Months Ended June 30, For the Six Months Ended June 30, 2018 2017 2018 2017 Interest expense - stated coupon rate $ 1,834 $ 1,425 $ 3,668 $ 2,850 Interest expense - amortization of deferred bond discount and costs 1,554 1,269 3,060 2,509 Total interest expense - convertible senior notes $ 3,388 $ 2,694 $ 6,728 $ 5,359

EARNINGS_(LOSS) PER COMMON SHAR

EARNINGS/(LOSS) PER COMMON SHARE	6 Months Ended
	Jun. 30, 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 0.3 million and 0.4 million stock options were excluded from the calculation of diluted earnings per share for the three and six months ended June 30, 2018 , respectively, as they were anti-dilutive. Approximately 0.2 million and 0.3 million stock options were excluded from the calculation of diluted earnings per share for the three and six months ended June 30, 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 14 . 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 June 30, 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 June 30, For the Six Months Ended June 30, 2018 2017 2018 2017 Net income/(loss) from continuing operations $ 6,506 $ 13,370 $ 18,246 $ (5,180 ) Adjustment (1) (523 ) 915 (890 ) 983 Net income/(loss) available to GAIN common shareholders from continuing operations $ 5,983 $ 14,285 $ 17,356 $ (4,197 ) Net income from discontinued operations 60,642 569 64,988 248 Weighted average common shares outstanding: Basic weighted average common shares outstanding 44,797,103 47,687,214 44,906,800 47,790,307 Effect of dilutive securities: Stock options 276,899 207,434 269,316 267,880 RSUs/RSAs 235,000 — 239,658 6,923 Diluted weighted average common shares outstanding 45,309,002 47,894,648 45,415,774 47,790,307 Basic earnings/(loss) from continuing operations $ 0.13 $ 0.30 $ 0.39 $ (0.09 ) Basic earnings from discontinued operations $ 1.36 $ 0.01 $ 1.45 $ 0.01 Diluted earnings/(loss) from continuing operations $ 0.13 $ 0.30 $ 0.38 $ (0.09 ) Diluted earnings from discontinued operations $ 1.34 $ 0.01 $ 1.43 $ 0.01 (1) During the three and six months ended June 30, 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	6 Months Ended
	Jun. 30, 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	6 Months Ended
	Jun. 30, 2018
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The Company's ( benefit )/provision for income taxes was approximately $(0.3) million and $7.4 million for the three and six months ended June 30, 2018 and a provision /(benefit) of approximately $1.5 million and $(4.4) million for the three and six months ended June 30, 2017 . These amounts reflect the Company's estimate of the annual effective tax rates of (4.0)% and 10.2% , adjusted for certain discrete items, for the three months ended June 30, 2018 and 2017 , respectively. The Company's effective tax rates of 28.3% and 47.4% for the six months ended June 30, 2018 and 2017 , respectively, reflect the Company's estimate of the annual effective tax rate adjusted for certain discrete items, primarily changes in uncertain tax positions. 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. 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	6 Months Ended
	Jun. 30, 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 June 30, 2018 and the actual amounts of capital that were maintained (amounts in millions): Entity Name Minimum Capital Excess Percent of GAIN Capital Group, LLC $ 35.8 $ 48.8 $ 13.0 136 % GAIN Capital Securities, Inc. 0.1 0.4 0.3 400 % GAIN Capital U.K., Ltd. 66.4 201.2 134.8 303 % GAIN Capital Japan Co., Ltd. 1.6 9.5 7.9 594 % GAIN Capital Australia, Pty. Ltd. 0.7 5.4 4.7 771 % GAIN Capital-Forex.com Hong Kong, Ltd. 1.9 3.4 1.5 179 % GAIN Global Markets, Inc. 0.4 2.0 1.6 500 % GAIN Capital-Forex.com Canada, Ltd. 0.2 1.4 1.2 700 % GAIN Capital Singapore Pte., Ltd. 0.3 7.4 7.1 2,467 % Trade Facts, Ltd. 0.6 3.5 2.9 583 % Global Asset Advisors, LLC — 1.4 1.4 100 % GAIN Capital Payments Ltd. 0.1 0.4 0.3 400 % GTX SEF, LLC 0.8 1.0 0.2 125 % Total $ 108.9 $ 285.8 $ 176.9 262 %

SEGMENT INFORMATION

SEGMENT INFORMATION	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION ASC Topic 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) the reported profit or loss in absolute amount of which 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 first quarter of 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. On June 29, 2018, the Company completed the sale of its GTX ECN business, which previously comprised the Company's institutional segment, to Deutsche Börse Group via its FX unit, 360T, for a total purchase price of $100 million subject to a customary working capital adjustment. The Company determined that the institutional segment met the discontinued operations criteria set forth in ASC Subtopic 205-20-45, Presentation of Financial Statements , in the quarter ended June 30, 2018. As such, the institutional segment results have been classified as discontinued operations in the accompanying Condensed Consolidated Statements of Operations and Comprehensive Income. For more information relating to the discontinued operations of the Company's GTX ECN business, please see Note 3 . 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. Futures Segment The futures segment offers execution and related services for exchange-traded futures, including futures on Bitcoin, and futures options on major U.S. and European exchanges. The Company offers futures services through its subsidiary, GAIN Capital Group, LLC, under several brands, including GAIN Capital Futures. 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 $0.4 million and $0.6 million for the three months ended June 30, 2018 and 2017 , respectively, and $0.9 million and $1.1 million for the six months ended June 30, 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 June 30, Six Months Ended June 30, 2018 2017 2018 2017 Net revenue $ 74,532 $ 80,605 $ 160,205 $ 120,692 Employee compensation and benefits 13,648 14,405 29,072 27,740 Selling and marketing 6,496 7,244 12,161 16,150 Referral fees 7,040 9,826 14,726 22,149 Other operating expenses 18,279 15,217 36,011 28,620 Segment profit $ 29,069 $ 33,913 $ 68,235 $ 26,033 Futures Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Net revenue $ 12,100 $ 10,013 $ 23,591 $ 20,980 Employee compensation and benefits 2,863 2,494 5,374 5,045 Selling and marketing 202 221 443 486 Referral fees 3,424 3,486 7,169 7,604 Other operating expenses 3,556 3,234 7,349 6,976 Segment profit $ 2,055 $ 578 $ 3,256 $ 869 Corporate and Other Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Other revenue/(loss) $ (2,447 ) $ (27 ) $ (1,251 ) $ 99 Employee compensation and benefits 5,962 6,112 12,365 10,749 Selling and marketing 69 52 133 116 Other operating expenses 3,711 2,578 6,970 5,234 Loss $ (12,189 ) $ (8,769 ) $ (20,719 ) $ (16,000 ) Reconciliation of operating segment profit to Income before income tax expense Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Retail segment $ 29,069 $ 33,913 $ 68,235 $ 26,033 Futures segment 2,055 578 3,256 869 Corporate and other (12,189 ) (8,769 ) (20,719 ) (16,000 ) SEGMENT PROFIT 18,935 25,722 50,772 10,902 Depreciation and amortization 5,341 4,121 10,709 7,949 Purchased intangible amortization 3,612 3,817 7,280 6,883 Restructuring expenses 25 — 25 — Impairment of investment — — (130 ) — OPERATING PROFIT/(LOSS) 9,957 17,784 32,888 (3,930 ) Interest expense on long term borrowings 3,388 2,694 6,728 5,359 INCOME/(LOSS) BEFORE INCOME TAX EXPENSE $ 6,569 $ 15,090 $ 26,160 $ (9,289 )

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	6 Months Ended
	Jun. 30, 2018
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS On July 26, 2018 , the Company announced the payment of a $0.06 dividend per share of Common Stock payable on September 18, 2018 to stockholders of record on September 11, 2018 .

DESCRIPTION OF BUSINESS AND B27

DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION (Policies)	6 Months Ended
	Jun. 30, 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.
Accounting Pronouncements Recently Adopted and Not Yet Adopted	ACCOUNTING PRONOUNCEMENTS Recently Adopted In January 2017, the Financial Accounting Standards 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 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 going forward 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 includes cash and cash equivalents held for customers as restricted cash. As a result, Net cash used in operating activities on the Statement of Cash Flows increased by $20.2 million for the six months ended June 30, 2017. The Company defines restricted cash to include cash and cash equivalents held for customers, which 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 4 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	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 Topic 606. Additionally, the Company’s futures segment revenue was 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 and $1.0 million for the three and six months ended June 30, 2018 , respectively, which impacted referral fee expense in the Company's discontinued operations. 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 exchange for an amount that reflects the consideration the Company expects to be entitled to in transferring those goods or services. The following is a description of the Company’s revenue recognition policies as they relate to revenue from contracts with customers. 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. 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.

DISCONTINUED OPERATIONS (Tables

DISCONTINUED OPERATIONS (Tables)	6 Months Ended
	Jun. 30, 2018
Discontinued Operations and Disposal Groups [Abstract]
Disposal Groups, Including Discontinued Operations	Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 REVENUE: Institutional revenue $ 7,923 $ 7,435 $ 16,379 $ 15,802 Other revenue (2 ) — (2 ) — Total non-interest revenue 7,921 7,435 16,377 15,802 Interest revenue 55 28 103 48 Total net interest revenue 55 28 103 48 Net revenue $ 7,976 $ 7,463 $ 16,480 $ 15,850 EXPENSES: Employee compensation and benefits $ 2,466 $ 3,314 $ 5,879 $ 7,006 Trading expenses 2,782 2,315 5,439 4,911 Other expenses 1,861 1,804 3,924 3,183 Total operating expense 7,109 7,433 15,242 15,100 OPERATING PROFIT 867 30 1,238 750 Gain on sale of discontinued operations 69,581 — 69,581 — INCOME BEFORE INCOME TAX EXPENSE/(BENEFIT) 70,448 30 70,819 750 Income tax expense/(benefit) 9,806 (539 ) 5,831 502 NET INCOME FROM DISCONTINUED OPERATIONS $ 60,642 $ 569 $ 64,988 $ 248 Since the sale of the GTX ECN business closed on June 29, 2018, there are no assets held for sale as of June 30, 2018. As of December 31, 2017, total assets of $28.5 million and total liabilities of $3.8 million were related to the discontinued segment.

REVENUE RECOGNITION (Tables)

REVENUE RECOGNITION (Tables)	6 Months Ended
	Jun. 30, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	Three months ended June 30, 2018 Six months ended June 30, 2018 Futures Direct Customers (1) $ 2,693 $ 5,166 Indirect Customers (2) $ 6,555 $ 13,583 Other $ 2,852 $ 4,842 Total Segment Revenue $ 12,100 $ 23,591 (1) Direct customers are all customers not classified as indirect (2) Indirect customers are referred to the Company by introducing brokers

FAIR VALUE MEASUREMENT (Tables)

FAIR VALUE MEASUREMENT (Tables)	6 Months Ended
	Jun. 30, 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 $ 252,311 $ — $ — $ 252,311 Cash and cash equivalents held for customers: US treasury bills: U.S. government and agency securities 103,891 — — 103,891 Receivable from brokers: Broker derivative contracts — (10,391 ) — (10,391 ) Other assets: Certificates of deposit 176 — — 176 Other 125 — — 125 Payables to customers: Customer derivative contracts — 124,807 — 124,807 Payables to brokers: Broker derivative contracts — 2,482 — 2,482 Total $ 356,503 $ 116,898 $ — $ 473,401 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	The carrying value of Convertible senior notes represents the notes’ principal amounts net of unamortized discount (refer to Note 14 ). 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 June 30, 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 $ 62,726 $ 62,726 $ — $ 62,726 $ — Financial Liabilities: Payables to customers $ 1,045,173 $ 1,045,173 $ — $ 1,045,173 $ — Payable to brokers $ 5,455 $ 5,455 $ — $ 5,455 $ — Convertible senior notes $ 135,289 $ 179,847 $ — $ 179,847 $ — 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)	6 Months Ended
	Jun. 30, 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): June 30, 2018 Gross amounts of Gross amount of Net amounts of Derivative Instruments: Foreign currency exchange contracts $ 105,405 $ (23,079 ) $ 82,326 CFD contracts 91,411 (60,328 ) 31,083 Metals contracts 6,363 (2,874 ) 3,489 Total $ 203,179 $ (86,281 ) $ 116,898 June 30, 2018 Cash Collateral Net amounts of Net amounts of Derivative Assets/(Liabilities): Receivables from brokers $ 62,726 $ (10,391 ) $ 52,335 Payables to customers $ (1,045,173 ) $ 124,807 $ (920,366 ) Payable to brokers $ (5,455 ) $ 2,482 $ (2,973 ) 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): June 30, 2018 Total contracts in long positions Total contracts in short positions Derivative Instruments: Foreign currency exchange contracts 3,019,520 3,598,235 CFD contracts 115,947 168,033 Metals contracts 485 120 Total 3,135,952 3,766,388 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 for the three and six months ended June 30, 2018 and June 30, 2017 were as follows (amounts in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Derivative Instruments: Foreign currency exchange contracts $ 51,579 $ 43,633 $ 92,830 $ 61,009 CFD contracts 14,681 26,422 51,828 53,213 Metals contracts 5,759 9,018 11,481 3,782 Total $ 72,019 $ 79,073 $ 156,139 $ 118,004

RECEIVABLES FROM BROKERS (Table

RECEIVABLES FROM BROKERS (Tables)	6 Months Ended
	Jun. 30, 2018
Brokers and Dealers [Abstract]
Schedule of Receivable from Brokers	Amounts receivable from brokers consisted of the following as of (amounts in thousands): June 30, 2018 December 31, 2017 Required collateral $ 62,726 $ 73,537 Open foreign exchange positions (10,391 ) 4,966 Total $ 52,335 $ 78,503

PROPERTY AND EQUIPMENT (Tables)

PROPERTY AND EQUIPMENT (Tables)	6 Months Ended
	Jun. 30, 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): June 30, 2018 December 31, 2017 Software $ 59,713 $ 57,047 Computer equipment 18,964 18,390 Leasehold improvements 10,963 11,068 Telephone equipment 642 643 Office equipment 2,048 2,110 Furniture and fixtures 3,544 3,592 Web site development costs 635 386 Gross property and equipment 96,509 93,236 Less: Accumulated depreciation and amortization (60,047 ) (52,494 ) Property and equipment, net $ 36,462 $ 40,742

INTANGIBLE ASSETS (Tables)

INTANGIBLE ASSETS (Tables)	6 Months Ended
	Jun. 30, 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): June 30, 2018 December 31, 2017 Intangibles Gross Accumulated Net Gross Accumulated Net Customer lists $ 59,658 $ (36,210 ) $ 23,448 $ 60,420 $ (31,698 ) $ 28,722 Technology 50,233 (37,273 ) 12,960 72,204 (43,270 ) 28,934 Trademarks 7,533 (4,187 ) 3,346 7,680 (3,730 ) 3,950 Total finite lived intangibles 117,424 (77,670 ) 39,754 140,304 (78,698 ) 61,606 Trademark not subject to amortization (1) 363 — 363 363 — 363 Total intangibles $ 117,787 $ (77,670 ) $ 40,117 $ 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 June 30, 2018 : Intangible Assets Amount (in thousands) Weighted average amortization period Customer lists $ 59,658 6.9 years Technology 50,233 8.6 years Trademarks (1) 7,896 6.7 years Total intangible assets $ 117,787 (1) Trademarks with an indefinite-life, as described above, comprise $0.4 million of the $7.9 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 (186 ) (34 ) (18 ) (238 ) Discontinued operations — (4,616 ) — (4,616 ) Carrying amount of goodwill as of June 30, 2018 $ 25,766 $ — $ 2,416 $ 28,182

OTHER ASSETS (Tables)

OTHER ASSETS (Tables)	6 Months Ended
	Jun. 30, 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): June 30, 2018 December 31, 2017 Vendor and security deposits $ 6,932 $ 11,923 Income tax receivable 282 2,132 Deferred tax assets, net 12,053 10,698 GTX trade receivables 616 5,758 Customer debit positions 4,067 2,384 Allowance on customer debit positions (3,700 ) (1,959 ) Prepaid assets 8,801 9,523 Miscellaneous receivables 7,248 4,872 Deferred commitment fees 444 550 Total other assets $ 36,743 $ 45,881

NON-CONTROLLING INTERESTS (Tabl

NON-CONTROLLING INTERESTS (Tables)	6 Months Ended
	Jun. 30, 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 890 Net income attributable to non-controlling interests 371 Distributions to non-controlling interest holders (199 ) Reclassification to liabilities (2,911 ) June 30, 2018 $ 2,562

CONVERTIBLE SENIOR NOTES (Table

CONVERTIBLE SENIOR NOTES (Tables)	6 Months Ended
	Jun. 30, 2018
Debt Disclosure [Abstract]
Schedule of Liability and Equity Components of Convertible Debt	The balances of the liability and equity components as of June 30, 2018 and December 31, 2017 were as follows (amounts in thousands): June 30, 2018 December 31, 2017 Liability component - principal $ 158,350 $ 158,350 Deferred bond discount (22,615 ) (25,624 ) Deferred financing cost (446 ) (505 ) Liability component - net carrying value $ 135,289 $ 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 , was as follows (amounts in thousands): For the Three Months Ended June 30, For the Six Months Ended June 30, 2018 2017 2018 2017 Interest expense - stated coupon rate $ 1,834 $ 1,425 $ 3,668 $ 2,850 Interest expense - amortization of deferred bond discount and costs 1,554 1,269 3,060 2,509 Total interest expense - convertible senior notes $ 3,388 $ 2,694 $ 6,728 $ 5,359

EARNINGS_(LOSS) PER COMMON SH38

EARNINGS/(LOSS) PER COMMON SHARE (Tables)	6 Months Ended
	Jun. 30, 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 June 30, For the Six Months Ended June 30, 2018 2017 2018 2017 Net income/(loss) from continuing operations $ 6,506 $ 13,370 $ 18,246 $ (5,180 ) Adjustment (1) (523 ) 915 (890 ) 983 Net income/(loss) available to GAIN common shareholders from continuing operations $ 5,983 $ 14,285 $ 17,356 $ (4,197 ) Net income from discontinued operations 60,642 569 64,988 248 Weighted average common shares outstanding: Basic weighted average common shares outstanding 44,797,103 47,687,214 44,906,800 47,790,307 Effect of dilutive securities: Stock options 276,899 207,434 269,316 267,880 RSUs/RSAs 235,000 — 239,658 6,923 Diluted weighted average common shares outstanding 45,309,002 47,894,648 45,415,774 47,790,307 Basic earnings/(loss) from continuing operations $ 0.13 $ 0.30 $ 0.39 $ (0.09 ) Basic earnings from discontinued operations $ 1.36 $ 0.01 $ 1.45 $ 0.01 Diluted earnings/(loss) from continuing operations $ 0.13 $ 0.30 $ 0.38 $ (0.09 ) Diluted earnings from discontinued operations $ 1.34 $ 0.01 $ 1.43 $ 0.01 (1) During the three and six months ended June 30, 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)	6 Months Ended
	Jun. 30, 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 June 30, 2018 and the actual amounts of capital that were maintained (amounts in millions): Entity Name Minimum Capital Excess Percent of GAIN Capital Group, LLC $ 35.8 $ 48.8 $ 13.0 136 % GAIN Capital Securities, Inc. 0.1 0.4 0.3 400 % GAIN Capital U.K., Ltd. 66.4 201.2 134.8 303 % GAIN Capital Japan Co., Ltd. 1.6 9.5 7.9 594 % GAIN Capital Australia, Pty. Ltd. 0.7 5.4 4.7 771 % GAIN Capital-Forex.com Hong Kong, Ltd. 1.9 3.4 1.5 179 % GAIN Global Markets, Inc. 0.4 2.0 1.6 500 % GAIN Capital-Forex.com Canada, Ltd. 0.2 1.4 1.2 700 % GAIN Capital Singapore Pte., Ltd. 0.3 7.4 7.1 2,467 % Trade Facts, Ltd. 0.6 3.5 2.9 583 % Global Asset Advisors, LLC — 1.4 1.4 100 % GAIN Capital Payments Ltd. 0.1 0.4 0.3 400 % GTX SEF, LLC 0.8 1.0 0.2 125 % Total $ 108.9 $ 285.8 $ 176.9 262 %

SEGMENT INFORMATION (Tables)

SEGMENT INFORMATION (Tables)	6 Months Ended
	Jun. 30, 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 June 30, Six Months Ended June 30, 2018 2017 2018 2017 Net revenue $ 74,532 $ 80,605 $ 160,205 $ 120,692 Employee compensation and benefits 13,648 14,405 29,072 27,740 Selling and marketing 6,496 7,244 12,161 16,150 Referral fees 7,040 9,826 14,726 22,149 Other operating expenses 18,279 15,217 36,011 28,620 Segment profit $ 29,069 $ 33,913 $ 68,235 $ 26,033 Futures Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Net revenue $ 12,100 $ 10,013 $ 23,591 $ 20,980 Employee compensation and benefits 2,863 2,494 5,374 5,045 Selling and marketing 202 221 443 486 Referral fees 3,424 3,486 7,169 7,604 Other operating expenses 3,556 3,234 7,349 6,976 Segment profit $ 2,055 $ 578 $ 3,256 $ 869 Corporate and Other Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Other revenue/(loss) $ (2,447 ) $ (27 ) $ (1,251 ) $ 99 Employee compensation and benefits 5,962 6,112 12,365 10,749 Selling and marketing 69 52 133 116 Other operating expenses 3,711 2,578 6,970 5,234 Loss $ (12,189 ) $ (8,769 ) $ (20,719 ) $ (16,000 )
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 June 30, Six Months Ended June 30, 2018 2017 2018 2017 Retail segment $ 29,069 $ 33,913 $ 68,235 $ 26,033 Futures segment 2,055 578 3,256 869 Corporate and other (12,189 ) (8,769 ) (20,719 ) (16,000 ) SEGMENT PROFIT 18,935 25,722 50,772 10,902 Depreciation and amortization 5,341 4,121 10,709 7,949 Purchased intangible amortization 3,612 3,817 7,280 6,883 Restructuring expenses 25 — 25 — Impairment of investment — — (130 ) — OPERATING PROFIT/(LOSS) 9,957 17,784 32,888 (3,930 ) Interest expense on long term borrowings 3,388 2,694 6,728 5,359 INCOME/(LOSS) BEFORE INCOME TAX EXPENSE $ 6,569 $ 15,090 $ 26,160 $ (9,289 )

DESCRIPTION OF BUSINESS AND B41

DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018segment	Jun. 29, 2018USD ($)
Accounting Policies [Abstract]
Number of operating segments | segment	2
Discontinued Operations, Disposed of by Sale | GTX ECN
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, including discontinued operation, consideration | $		$ 100

ACCOUNTING PRONOUNCEMENTS (Deta

ACCOUNTING PRONOUNCEMENTS (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by operating activities of continuing operations	$ 8,233	$ (24,521)
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by operating activities of continuing operations		$ (20,200)

DISCONTINUED OPERATIONS (Detail

DISCONTINUED OPERATIONS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 29, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
NET INCOME FROM DISCONTINUED OPERATIONS	$ 60,642	$ 569	$ 64,988	$ 248
GTX ECN | Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group, including discontinued operation, consideration					$ 100,000
Gain on sale of discontinued operations	69,581	0	69,581	0
Total non-interest revenue	7,921	7,435	16,377	15,802
Interest revenue	55	28	103	48
Total net interest revenue	55	28	103	48
Net revenue	7,976	7,463	16,480	15,850
Employee compensation and benefits	2,466	3,314	5,879	7,006
Trading expenses	2,782	2,315	5,439	4,911
Other expenses	1,861	1,804	3,924	3,183
Total operating expense	7,109	7,433	15,242	15,100
OPERATING PROFIT	867	30	1,238	750
Gain on sale of discontinued operations	69,581	0	69,581	0
INCOME BEFORE INCOME TAX EXPENSE/(BENEFIT)	70,448	30	70,819	750
Income tax expense/(benefit)	9,806	(539)	5,831	502
NET INCOME FROM DISCONTINUED OPERATIONS	60,642	569	64,988	248
Total assets of discontinued operations					28,500
Total liabilities of discontinued operations					$ 3,800
GTX ECN | Discontinued Operations, Disposed of by Sale | Institutional revenue
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total non-interest revenue	7,923	7,435	16,379	15,802
GTX ECN | Discontinued Operations, Disposed of by Sale | Other (loss)/revenue
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total non-interest revenue	$ (2)	$ 0	$ (2)	$ 0

REVENUE RECOGNITION - Narrative

REVENUE RECOGNITION - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer	$ 0.5	$ 1

REVENUE RECOGNITION - Disaggreg

REVENUE RECOGNITION - Disaggregation of Revenue (Details) - Futures - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 12,100	$ 23,591
Direct Customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2,693	5,166
Indirect Customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	6,555	13,583
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 2,852	$ 4,842

FAIR VALUE MEASUREMENT - Assets

FAIR VALUE MEASUREMENT - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 473,401	$ 456,543
Money market accounts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	252,311	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	103,891	105,190
Broker derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	(10,391)	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	125	130
Customer derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	124,807	129,966
Broker derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	2,482
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	356,503	324,781
Level 1 | Money market accounts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	252,311	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	103,891	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	125	130
Level 1 | Customer derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0	0
Level 1 | Broker derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	116,898	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	(10,391)	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	124,807	129,966
Level 2 | Broker derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	2,482
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
Level 3 | Broker derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	$ 0

FAIR VALUE MEASUREMENT - Carryi

FAIR VALUE MEASUREMENT - Carrying Value and Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financial Assets:
Receivables from brokers	$ 52,335	$ 78,503
Financial Liabilities:
Payables to brokers	2,973	2,789
Convertible senior notes	135,289	132,221
Derivative Assets | Level 1
Financial Assets:
Receivables from brokers	0	0
Financial Liabilities:
Payables to brokers		0
Derivative Assets | Level 2
Financial Assets:
Receivables from brokers	62,726	73,537
Financial Liabilities:
Payables to brokers		381
Derivative Assets | Level 3
Financial Assets:
Receivables from brokers	0	0
Financial Liabilities:
Payables to brokers		0
Derivative Assets | Carrying Value
Financial Assets:
Receivables from brokers	62,726	73,537
Financial Liabilities:
Payables to brokers		381
Derivative Assets | Fair Value
Financial Assets:
Receivables from brokers	62,726	73,537
Financial Liabilities:
Payables to brokers		381
Derivative Liabilities | Level 1
Financial Liabilities:
Payables to customers	0	0
Payables to brokers	0
Convertible senior notes	0	0
Derivative Liabilities | Level 2
Financial Liabilities:
Payables to customers	1,045,173	1,108,794
Payables to brokers	5,455
Convertible senior notes	179,847	205,073
Derivative Liabilities | Level 3
Financial Liabilities:
Payables to customers	0	0
Payables to brokers	0
Convertible senior notes	0	0
Derivative Liabilities | Carrying Value
Financial Liabilities:
Payables to customers	1,045,173	1,108,794
Payables to brokers	5,455
Convertible senior notes	135,289	132,221
Derivative Liabilities | Fair Value
Financial Liabilities:
Payables to customers	1,045,173	1,108,794
Payables to brokers	5,455
Convertible senior notes	$ 179,847	$ 205,073

DERIVATIVES - Fair Value of Der

DERIVATIVES - Fair Value of Derivative Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross amounts of assets for derivative open positions at fair value	$ 203,179	$ 227,847
Gross amount of (liabilities) for derivative open positions at fair value	(86,281)	(96,085)
Net amounts of assets/(liabilities) for derivative open positions at fair value	116,898	131,762
Receivables from brokers	52,335	78,503
Payables to customers	(920,366)	(978,828)
Payables to brokers	(2,973)	(2,789)
Receivables from brokers
Derivatives, Fair Value [Line Items]
Receivables from brokers	52,335	78,503
Receivables from brokers | Derivative Assets
Derivatives, Fair Value [Line Items]
Cash Collateral	62,726	73,537
Net amounts of assets/(liabilities) for derivative open positions at fair value	(10,391)	4,966
Payables to customers
Derivatives, Fair Value [Line Items]
Payables to customers	(920,366)	(978,828)
Payables to customers | Derivative Liabilities
Derivatives, Fair Value [Line Items]
Cash Collateral	1,045,173	1,108,794
Net amounts of assets/(liabilities) for derivative open positions at fair value	124,807	129,966
Foreign currency exchange contracts
Derivatives, Fair Value [Line Items]
Gross amounts of assets for derivative open positions at fair value	105,405	121,104
Gross amount of (liabilities) for derivative open positions at fair value	(23,079)	(31,556)
Net amounts of assets/(liabilities) for derivative open positions at fair value	82,326	89,548
CFD contracts
Derivatives, Fair Value [Line Items]
Gross amounts of assets for derivative open positions at fair value	91,411	102,659
Gross amount of (liabilities) for derivative open positions at fair value	(60,328)	(62,322)
Net amounts of assets/(liabilities) for derivative open positions at fair value	31,083	40,337
Metals contracts
Derivatives, Fair Value [Line Items]
Gross amounts of assets for derivative open positions at fair value	6,363	4,084
Gross amount of (liabilities) for derivative open positions at fair value	(2,874)	(2,207)
Net amounts of assets/(liabilities) for derivative open positions at fair value	3,489	1,877
Broker derivative contracts
Derivatives, Fair Value [Line Items]
Payables to brokers	(2,973)	(2,789)
Broker derivative contracts | Derivative Liabilities
Derivatives, Fair Value [Line Items]
Cash Collateral	(5,455)	381
Net amounts of assets/(liabilities) for derivative open positions at fair value	$ 2,482	$ (3,170)

DERIVATIVES - Notional Values o

DERIVATIVES - Notional Values of Derivative Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Total contracts in long positions
Derivative [Line Items]
Notional amounts of derivative	$ 3,135,952	$ 2,202,722
Total contracts in long positions | Foreign currency exchange contracts
Derivative [Line Items]
Notional amounts of derivative	3,019,520	2,075,789
Total contracts in long positions | CFD contracts
Derivative [Line Items]
Notional amounts of derivative	115,947	126,519
Total contracts in long positions | Metal contracts
Derivative [Line Items]
Notional amounts of derivative	485	414
Total contracts in short positions
Derivative [Line Items]
Notional amounts of derivative	3,766,388	4,323,108
Total contracts in short positions | Foreign currency exchange contracts
Derivative [Line Items]
Notional amounts of derivative	3,598,235	4,148,056
Total contracts in short positions | CFD contracts
Derivative [Line Items]
Notional amounts of derivative	168,033	174,835
Total contracts in short positions | Metal contracts
Derivative [Line Items]
Notional amounts of derivative	$ 120	$ 217

DERIVATIVES - Net Gains (Losses

DERIVATIVES - Net Gains (Losses) on Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative [Line Items]
Net gains (losses) on derivative instruments, total	$ 72,019	$ 79,073	$ 156,139	$ 118,004
Foreign currency exchange contracts
Derivative [Line Items]
Net gains (losses) on derivative instruments	51,579	43,633	92,830	61,009
CFD contracts
Derivative [Line Items]
Net gains (losses) on derivative instruments	14,681	26,422	51,828	53,213
Metal contracts
Derivative [Line Items]
Net gains (losses) on derivative instruments	$ 5,759	$ 9,018	$ 11,481	$ 3,782

RECEIVABLES FROM BROKERS - Sche

RECEIVABLES FROM BROKERS - Schedule of Receivables from Brokers (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Receivables from Brokerage Clients [Line Items]
Receivables from brokers	$ 52,335	$ 78,503
Required collateral
Receivables from Brokerage Clients [Line Items]
Receivables from brokers	62,726	73,537
Open foreign exchange positions
Receivables from Brokerage Clients [Line Items]
Receivables from brokers	$ (10,391)	$ 4,966

PROPERTY AND EQUIPMENT - Schedu

PROPERTY AND EQUIPMENT - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Gross property and equipment	$ 96,509	$ 93,236
Less: Accumulated depreciation and amortization	(60,047)	(52,494)
Property and equipment, net	36,462	40,742
Software
Property, Plant and Equipment [Line Items]
Gross property and equipment	59,713	57,047
Computer equipment
Property, Plant and Equipment [Line Items]
Gross property and equipment	18,964	18,390
Leasehold improvements
Property, Plant and Equipment [Line Items]
Gross property and equipment	10,963	11,068
Telephone equipment
Property, Plant and Equipment [Line Items]
Gross property and equipment	642	643
Office equipment
Property, Plant and Equipment [Line Items]
Gross property and equipment	2,048	2,110
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Gross property and equipment	3,544	3,592
Website development costs
Property, Plant and Equipment [Line Items]
Gross property and equipment	$ 635	$ 386

PROPERTY AND EQUIPMENT - Additi

PROPERTY AND EQUIPMENT - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 5,341	$ 4,121	$ 10,709	$ 7,949

INTANGIBLE ASSETS - Intangible

INTANGIBLE ASSETS - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Intangible Assets by Major Class [Line Items]
Finite-lived intangible assets, gross	$ 117,424		$ 117,424		$ 140,304
Accumulated Amortization	(77,670)		(77,670)		(78,698)
Finite-lived intangible assets, net	39,754		39,754		61,606
Total intangibles, gross	117,787		117,787		140,667
Intangible assets, net	40,117		40,117		61,969
Amortization of Intangible Assets	3,612	$ 3,817	7,280	$ 6,883
Payments to acquire intangible assets			0	$ 6,960
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		363
Customer lists
Intangible Assets by Major Class [Line Items]
Finite-lived intangible assets, gross	59,658		59,658		60,420
Accumulated Amortization	(36,210)		(36,210)		(31,698)
Finite-lived intangible assets, net	23,448		23,448		$ 28,722
Total intangibles, gross	59,658		$ 59,658
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	50,233		$ 50,233		$ 72,204
Accumulated Amortization	(37,273)		(37,273)		(43,270)
Finite-lived intangible assets, net	12,960		12,960		28,934
Total intangibles, gross	50,233		$ 50,233
Weighted average amortization period			8 years 7 months
Trademarks
Intangible Assets by Major Class [Line Items]
Finite-lived intangible assets, gross	7,533		$ 7,533		7,680
Accumulated Amortization	(4,187)		(4,187)		(3,730)
Finite-lived intangible assets, net	3,346		3,346		$ 3,950
Total intangibles, gross	$ 7,896		$ 7,896
Weighted average amortization period			6 years 8 months

INTANGIBLE ASSETS - Goodwill (D

INTANGIBLE ASSETS - Goodwill (Details)	6 Months Ended
	Jun. 30, 2018USD ($)reporting_unit
Goodwill [Line Items]
Number of reporting units | reporting_unit	2
Goodwill additions or impairments	$ 0
Goodwill	33,036,000
Decrease in goodwill	(4,800,000)
Goodwill [Roll Forward]
Goodwill. beginning balance	33,036,000
Foreign currency translation adjustments	(238,000)
Discontinued operations	(4,616,000)
Goodwill. ending balance	28,182,000
Operating Segments | Retail
Goodwill [Line Items]
Goodwill	25,952,000
Goodwill [Roll Forward]
Goodwill. beginning balance	25,952,000
Foreign currency translation adjustments	(186,000)
Discontinued operations	0
Goodwill. ending balance	25,766,000
Operating Segments | Institutional
Goodwill [Line Items]
Goodwill	4,650,000
Goodwill [Roll Forward]
Goodwill. beginning balance	4,650,000
Foreign currency translation adjustments	(34,000)
Discontinued operations	(4,616,000)
Goodwill. ending balance	0
Operating Segments | Futures
Goodwill [Line Items]
Goodwill	2,434,000
Goodwill [Roll Forward]
Goodwill. beginning balance	2,434,000
Foreign currency translation adjustments	(18,000)
Discontinued operations	0
Goodwill. ending balance	$ 2,416,000

OTHER ASSETS - Schedule of Othe

OTHER ASSETS - Schedule of Other Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Vendor and security deposits	$ 6,932	$ 11,923
Income tax receivable	282	2,132
Deferred tax assets, net	12,053	10,698
GTX trade receivables	616	5,758
Customer debit positions	4,067	2,384
Allowance on customer debit positions	(3,700)	(1,959)
Prepaid assets	8,801	9,523
Miscellaneous receivables	7,248	4,872
Deferred commitment fees	444	550
Total other assets	$ 36,743	$ 45,881

RELATED PARTY TRANSACTIONS - Ad

RELATED PARTY TRANSACTIONS - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Payables to customers	$ 920,366	$ 978,828
Management
Related Party Transaction [Line Items]
Payables to customers	200	200
IPGL Limited
Related Party Transaction [Line Items]
Payables to customers	$ 20,200	$ 15,900

NON-CONTROLLING INTERESTS (Deta

NON-CONTROLLING INTERESTS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Feb. 28, 2018	Dec. 31, 2016	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	$ 523	$ (915)	890	$ (983)
Net income attributable to non-controlling interests			371
Distributions to non-controlling interest holders			(199)
Reclassification to liabilities			(2,911)
Redeemable noncontrolling interest, ending balance	$ 2,562		$ 2,562
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 ARRANGEMENT (D

REVOLVING CREDIT ARRANGEMENT (Details) - Revolving Credit Facility - USD ($)	Aug. 03, 2017	Jun. 30, 2018
Debt Instrument [Line Items]
Maximum amount under facility	$ 50,000,000
Commitment fee amount	400,000
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	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Liability component - principal	$ 158,350	$ 158,350
Deferred bond discount	(22,615)	(25,624)
Deferred financing cost	(446)	(505)
Liability component - net carrying value	135,289	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 - Sc62

CONVERTIBLE SENIOR NOTES - Schedule of Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Instrument [Line Items]
Interest expense - amortization of deferred bond discount and costs			$ 324	$ 221
Convertible Debt
Debt Instrument [Line Items]
Interest expense - stated coupon rate	$ 1,834	$ 1,425	3,668	2,850
Interest expense - amortization of deferred bond discount and costs	1,554	1,269	3,060	2,509
Total interest expense - convertible senior notes	$ 3,388	$ 2,694	$ 6,728	$ 5,359

EARNINGS_(LOSS) PER COMMON SH63

EARNINGS/(LOSS) PER COMMON SHARE - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive stock options excluded from calculation of diluted earnings per share	0.3	0.2	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 SH64

EARNINGS/(LOSS) PER COMMON SHARE - Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income/(loss) from continuing operations	$ 6,506	$ 13,370	$ 18,246	$ (5,180)
Adjustment to fair value of redeemable non-controlling interests	(523)	915	(890)	983
Net income/(loss) available to GAIN common shareholders from continuing operations	5,983	14,285	17,356	(4,197)
Net income from discontinued operations	$ 60,642	$ 569	$ 64,988	$ 248
Weighted average common shares outstanding:
Basic weighted average common shares outstanding	44,797,103	47,687,214	44,906,800	47,790,307
Effect of dilutive securities:
Stock options (in shares)	276,899	207,434	269,316	267,880
RSUs/RSAs (in shares)	235,000	0	239,658	6,923
Diluted weighted average common shares outstanding	45,309,002	47,894,648	45,415,774	47,790,307
Earnings/(loss) per common share:
Basic earnings/(loss) from continuing operations (usd per share)	$ 0.13	$ 0.30	$ 0.39	$ (0.09)
Basic earnings from discontinued operations (usd per share)	1.36	0.01	1.45	0.01
Diluted earnings/(loss) from continuing operations (usd per share)	0.13	0.30	0.38	(0.09)
Diluted earnings from discontinued operations (usd per share)	$ 1.34	$ 0.01	$ 1.43	$ 0.01

INCOME TAXES - Additional Infor

INCOME TAXES - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Provision (benefit) for income taxes	$ (263)	$ 1,533	$ 7,413	$ (4,401)
Effective income tax rates (as percent)	(4.00%)	10.20%	28.30%	47.40%

REGULATORY REQUIREMENTS - Regul

REGULATORY REQUIREMENTS - Regulatory Capital Subsidiaries Required to Maintain (Detail) $ in Millions	Jun. 30, 2018USD ($)
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 108.9
Capital Levels Maintained	285.8
Excess Net Capital	$ 176.9
Percent of Requirement Maintained	262.00%
GAIN Capital Group, LLC
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 35.8
Capital Levels Maintained	48.8
Excess Net Capital	$ 13
Percent of Requirement Maintained	136.00%
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	$ 66.4
Capital Levels Maintained	201.2
Excess Net Capital	$ 134.8
Percent of Requirement Maintained	303.00%
GAIN Capital Japan Co., Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 1.6
Capital Levels Maintained	9.5
Excess Net Capital	$ 7.9
Percent of Requirement Maintained	594.00%
GAIN Capital Australia, Pty. Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.7
Capital Levels Maintained	5.4
Excess Net Capital	$ 4.7
Percent of Requirement Maintained	771.00%
GAIN Capital-Forex.com Hong Kong, Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 1.9
Capital Levels Maintained	3.4
Excess Net Capital	$ 1.5
Percent of Requirement Maintained	179.00%
GAIN Global Markets, Inc.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.4
Capital Levels Maintained	2
Excess Net Capital	$ 1.6
Percent of Requirement Maintained	500.00%
GAIN Capital-Forex.com Canada, Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.2
Capital Levels Maintained	1.4
Excess Net Capital	$ 1.2
Percent of Requirement Maintained	700.00%
GAIN Capital Singapore Pte., Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.3
Capital Levels Maintained	7.4
Excess Net Capital	$ 7.1
Percent of Requirement Maintained	2467.00%
Trade Facts, Ltd.
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0.6
Capital Levels Maintained	3.5
Excess Net Capital	$ 2.9
Percent of Requirement Maintained	583.00%
Global Asset Advisors, LLC
Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items]
Minimum Regulatory Capital Requirements	$ 0
Capital Levels Maintained	1.4
Excess Net Capital	$ 1.4
Percent of Requirement Maintained	100.00%
GAIN Capital Payments Ltd.
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%
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
Excess Net Capital	$ 0.2
Percent of Requirement Maintained	125.00%

SEGMENT INFORMATION - Additiona

SEGMENT INFORMATION - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)market	Jun. 30, 2017USD ($)	Jun. 29, 2018USD ($)
Segment Reporting Information [Line Items]
Number of accessible global financial markets | market			12,500
Expenses	$ (74,228)	$ (72,807)	$ (149,657)	$ (145,701)
Intersegment Eliminations
Segment Reporting Information [Line Items]
Expenses	$ 400	$ 600	$ 900	$ 1,100
GTX ECN | Discontinued Operations, Disposed of by Sale
Segment Reporting Information [Line Items]
Disposal group, including discontinued operation, consideration					$ 100,000

SEGMENT INFORMATION - Selected

SEGMENT INFORMATION - Selected Financial Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Employee compensation and benefits	$ 22,473	$ 23,011	$ 46,811	$ 43,534
Selling and marketing	6,767	7,517	12,737	16,752
Referral fees	10,464	13,312	21,895	29,753
Segment profit (loss)	18,935	25,722	50,772	10,902
Operating Segments | Retail
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net revenue	74,532	80,605	160,205	120,692
Employee compensation and benefits	13,648	14,405	29,072	27,740
Selling and marketing	6,496	7,244	12,161	16,150
Referral fees	7,040	9,826	14,726	22,149
Other operating expenses	18,279	15,217	36,011	28,620
Segment profit (loss)	29,069	33,913	68,235	26,033
Operating Segments | Futures
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net revenue	12,100	10,013	23,591	20,980
Employee compensation and benefits	2,863	2,494	5,374	5,045
Selling and marketing	202	221	443	486
Referral fees	3,424	3,486	7,169	7,604
Other operating expenses	3,556	3,234	7,349	6,976
Segment profit (loss)	2,055	578	3,256	869
Corporate and Other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net revenue	(2,447)	(27)	(1,251)	99
Employee compensation and benefits	5,962	6,112	12,365	10,749
Selling and marketing	69	52	133	116
Other operating expenses	3,711	2,578	6,970	5,234
Segment profit (loss)	$ (12,189)	$ (8,769)	$ (20,719)	$ (16,000)

SEGMENT INFORMATION - Reconcili

SEGMENT INFORMATION - Reconciliation of Operating Segment Profit to Income Before Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
SEGMENT PROFIT	$ 18,935	$ 25,722	$ 50,772	$ 10,902
Depreciation and amortization	5,341	4,121	10,709	7,949
Purchased intangible amortization	3,612	3,817	7,280	6,883
Restructuring expenses	25	0	25	0
Impairment of investment	0	0	(130)	0
OPERATING PROFIT/(LOSS)	9,957	17,784	32,888	(3,930)
Interest expense on long term borrowings	3,388	2,694	6,728	5,359
INCOME/(LOSS) BEFORE INCOME TAX EXPENSE/(BENEFIT)	6,569	15,090	26,160	(9,289)
Operating Segments | Retail
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
SEGMENT PROFIT	29,069	33,913	68,235	26,033
Operating Segments | Futures
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
SEGMENT PROFIT	2,055	578	3,256	869
Corporate and Other
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
SEGMENT PROFIT	$ (12,189)	$ (8,769)	$ (20,719)	$ (16,000)

SUBSEQUENT EVENTS - Additional

SUBSEQUENT EVENTS - Additional Information (Detail)	Jul. 26, 2018$ / shares
Subsequent event
Subsequent Event [Line Items]
Common stock dividend declared (usd per share)	$ 0.06