Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Assumptions for Stock Options Granted Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] Accounting Changes and Error Corrections [Abstract] Schedules of Error Corrections and Prior Period Adjustments Schedule of Error Corrections and Prior Period Adjustments [Table Text Block] Pro forma revenue and net income [Abstract] Pro forma revenue and net income [Abstract] Business Acquisition, Pro Forma Information, Nonrecurring Adjustments [Table] Business Acquisition, Pro Forma Information, Nonrecurring Adjustments [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Global Futures & Forex, Ltd. Global Futures & Forex Ltd. [Member] Global Futures & Forex Ltd. [Member] Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items] Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items] Total non-interest revenue Revenues, Excluding Interest and Dividends Interest revenue Interest Income, Operating Interest Expense Interest Expense Total net interest revenue Interest Income (Expense), Net Net revenue Revenue, Net Depreciation and amortization Depreciation, Depletion and Amortization, Nonproduction Purchased intangible amortization Amortization of Intangible Assets Other expense items Other Expenses Total Costs and Expenses INCOME BEFORE INCOME TAX EXPENSE Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest Income tax expense Income Tax Expense (Benefit) Net income Business Acquisition, Pro Forma Net Income (Loss) Business Combinations [Abstract] ACQUISITIONS Business Combination Disclosure [Text Block] Earnings Per Share [Abstract] EARNINGS PER COMMON SHARE Earnings Per Share [Text Block] Segment Reporting [Abstract] Number of operating segments Number of Operating Segments Statement of Stockholders' Equity [Abstract] Dividend payment, per share Common Stock, Dividends, Per Share, Cash Paid Income Tax Disclosure [Abstract] INCOME TAXES Income Tax Disclosure [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Assets and Liabilities Measured at Fair Value on Recurring Basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] 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 Carrying Value And Fair Value Of Financial Instruments Disclosure [Table Text Block] Carrying Value and Fair Value of Financial Instruments Disclosure [Table Text Block] Receivable from Brokers Schedule of Due to (from) Broker-Dealers and Clearing Organizations [Table Text Block] Fair Values of Derivative Instruments Schedule of Derivative Instruments [Table Text Block] Notional Values of Derivative Instruments Aggregate Notional Dollar Amount Of Derivatives Table [Text Block] Aggregate Notional Dollar Amount Of Derivatives Table [Text Block] Net Gains (Losses) With Respect to Derivative Instruments Which are Reflected in Trading Revenue Schedule of Derivative Instruments Included in Trading Activities [Table Text Block] Property and Equipment Including Leasehold Improvements and Capitalized Software Development Costs Property, Plant and Equipment [Table Text Block] Intangible Assets Schedule Of Intangible Assets By Major Class [Text Block] Schedule of Intangible Assets by Major Class [Table Text Block] Other Assets Schedule of Other Assets [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Global Assets Advisors, LLC Global Assets Advisors, LLC [Member] Global Assets Advisors, LLC [Member] Top Third Ag Marketing, LLC Top Third Ag Marketing, LLC [Member] Top Third Ag Marketing, LLC [Member] Global Futures & Forex Ltd Global Futures & Forex Ltd [Member] Global Futures & Forex Ltd [Member] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Customer list Customer Lists [Member] Trade name Trade Names [Member] Software Computer Software, Intangible Asset [Member] Customer relationships Customer Relationships [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Non-controlling interest Noncontrolling Interest, Increase from Business Combination Cash and cash equivalents acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Cash and cash equivalents held for customers acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash And Securities Held For Customers Acquired Business Acquisition, Purchase Price Allocation, Current Assets, the carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. This element is for unclassified presentations; for classified presentations there is a separate and distinct element. Receivable from brokers Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables Property and equipment Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Other current assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other Total tangible assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Tangible Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Tangible Assets Total liabilities assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Net assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Consideration less net assets Business Acquisition, Net Consideration of Tangible Assets Business Acquisition, Net Consideration of Tangible Assets Identifiable intangible assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill Goodwill Goodwill Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Stock Restricted Stock [Member] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Employees And Non Employee Directors Employees And Non Employee Directors [Member] Employees And Non Employee Directors [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Restricted stock, shares granted Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Stock options, shares granted Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures Restricted stock, values granted Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Aggregate Fair Value Share based Compensation Arrangement by Share based Payment Award, Equity Instruments Other than Options, Grants in Period, Aggregate Fair Value Stock options, values granted Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Aggregate Fair Value Share based Compensation Arrangement by Share based Payment Award, Options, Grants in Period, Aggregate Fair Value Provision for income taxes expense (benefit) Effective income tax rates Effective Income Tax Rate Reconciliation, Percent Reduction of provision for income taxes Unrecognized Tax Benefits, Increase Resulting from Settlements with Taxing Authorities Document Documentand Entity Information [Abstract] Document Documentand Entity Information [Abstract] Entity Registrant Name Entity Registrant Name Trading Symbol Trading Symbol Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Document Type Document Type Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding ADDITIONAL FINANCIAL INFORMATION Additional Financial Information Disclosure [Text Block] Net income Net Income (Loss) Attributable to Parent Adjustment Adjustment In Retained Earnings Adjustment in Retained Earnings Net income applicable to GAIN common shareholders Net Income Loss Available To Common Stockholders After Adjustments Net Income Loss Available To Common Stockholders After Adjustments Weighted average common shares outstanding: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Basic weighted average common shares outstanding Weighted Average Number of Shares Outstanding, Basic Effect of dilutive securities: Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract] Stock options (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements RSUs/RSAs (in shares) Incremental Common Shares Attributable to Dilutive Effect of Nonvested Shares with Forfeitable Dividends Diluted weighted average common shares outstanding Weighted Average Number of Shares Outstanding, Diluted Earnings per common share Earnings Per Share, Basic and Diluted [Abstract] Basic (usd per share) Earnings Per Share, Basic Diluted (usd per share) Earnings Per Share, Diluted Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Convertible Debt [Member] Convertible Debt [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Liability component - principal Long-term Debt, Gross Deferred bond discount Debt Instrument, Unamortized Discount Liability component - net carrying value Long-term Debt Additional paid in capital Debt Instrument, Convertible, Issuance Amount of Equity Component Debt Instrument, Convertible, Issuance Amount of Equity Component Discount attributable to equity Debt Instrument, Convertible, Unamortized Discount of Equity Component Debt Instrument, Convertible, Unamortized Discount of Equity Component Equity component Debt Instrument, Convertible, Issuance Amount of Equity Component, Net Debt Instrument, Convertible, Issuance Amount of Equity Component, Net Fair Value Disclosures [Abstract] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Accrued Expenses and Other Liabilities Accrued Expenses and Other Liabilities [Member] Accrued Expenses and Other Liabilities [Member] Other Current Assets Other Current Assets [Member] Business Combination, Contingent Consideration, Liability Business Combination, Contingent Consideration, Liability Increase in goodwill Goodwill, Acquired During Period Liabilities paid Increase (Decrease) in Accrued Liabilities Indemnification Asset Indemnification Asset Indemnification Asset Depreciation and amortization expense Domain name acquired amount Indefinite-lived Intangible Assets Acquired Related Party Transactions [Abstract] RELATED PARTY TRANSACTIONS Related Party Transactions Disclosure [Text Block] Commitments and Contingencies Disclosure [Abstract] LEGAL Legal Matters and Contingencies [Text Block] Statement of Cash Flows [Abstract] Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest CASH FLOWS FROM OPERATING ACTIVITIES: Net Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income to cash provided by operating activities Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Loss/(Gain) on foreign currency exchange rates Foreign Currency Transaction Gain (Loss), before Tax Depreciation and amortization Depreciation, Depletion and Amortization Deferred taxes Deferred Income Tax Expense (Benefit) Interest income Other Noncash Income (Expense) Amortization of deferred financing costs Amortization of Financing Costs Convertible note discount amortization Amortization of Debt Discount (Premium) Bad debt provision Provision for Loan and Lease Losses Stock compensation expense Share-based Compensation Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Cash and securities held for customers Increase Decrease In Cash And Securities Held For Customers Increase (Decrease) in Cash and Securities Held for Customers Receivables from banks and brokers Increase (Decrease) in Receivables from Brokers-Dealers and Clearing Organizations Prepaid assets Increase (Decrease) in Prepaid Expense and Other Assets Other assets Increase (Decrease) in Other Operating Assets Payables to customers, brokers, dealers, FCMs and other regulated entities Increase (Decrease) in Payables to Broker-Dealers and Clearing Organizations Accrued compensation and benefits Increase (Decrease) in Employee Related Liabilities Accrued expenses and other liabilities Increase (Decrease) in Accrued Liabilities and Other Operating Liabilities Income tax payable Increase (Decrease) in Income Taxes Payable Cash provided by/(used for) operating activities Net Cash Provided by (Used in) Operating Activities CASH FLOWS FROM INVESTING ACTIVITIES: Net Cash Provided by (Used in) Investing Activities [Abstract] Purchases of property and equipment Payments to Acquire Property, Plant, and Equipment Sale of treasury bills Proceeds from Sale and Maturity of Marketable Securities Acquisitions, net of cash acquired Payments to Acquire Businesses, Net of Cash Acquired Cash used for investing activities Net Cash Provided by (Used in) Investing Activities CASH FLOWS FROM FINANCING ACTIVITIES: Net Cash Provided by (Used in) Financing Activities [Abstract] Contractual payments for acquired assets Payments Of Contingent Consideration Payments Of Contingent Consideration Drawdown on notes payable Proceeds from Notes Payable Proceeds from exercise of stock options Proceeds from Stock Options Exercised Proceeds from employee stock purchase plan Proceeds from Stock Plans Purchase of treasury stock Payments for Repurchase of Common Stock Tax benefit from employee stock option exercises Excess Tax Benefit from Share-based Compensation, Financing Activities Dividend payment Payments of Dividends Other Proceeds from (Payments for) Other Financing Activities Cash (used for)/provided by financing activities Net Cash Provided by (Used in) Financing Activities Effect of exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash and Cash Equivalents INCREASE/(DECREASE) IN CASH AND CASH EQUIVALENTS Cash and Cash Equivalents, Period Increase (Decrease) CASH AND CASH EQUIVALENTS-Beginning of period Cash and Cash Equivalents, at Carrying Value CASH AND CASH EQUIVALENTS-End of period SUPPLEMENTARY DISCLOSURES OF CASH FLOW INFORMATION: Supplemental Cash Flow Information [Abstract] Cash (paid) / received during the year for: Cash Paid During Period For [Abstract] Cash Paid During Period For [Abstract] Interest Interest Paid Taxes Income Taxes Paid, Net Non-cash investing activities: Noncash Investing Activities [Abstract] Noncash Investing Activities [Abstract] Purchase of fixed assets in accrued expense and other liabilities Noncash or Part Noncash Acquisition, Value of Assets Acquired Common stock issued as consideration for GAA Business Combination, Consideration Transferred, Equity Interests Issued and Issuable Property, Plant and Equipment [Abstract] Schedule of Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Software Computer equipment Computer Equipment [Member] Leasehold improvements Leasehold Improvements [Member] Telephone equipment Telecommunications Equipment Group [Member] Telecommunications Equipment Group [Member] Office equipment Office Equipment [Member] Furniture and fixtures Furniture and Fixtures [Member] Website development costs Software Development [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Property and equipment Property, Plant and Equipment, Gross Less: Accumulated depreciation and amortization Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Property and equipment, net Property, Plant and Equipment, Net Statement of Financial Position [Abstract] Receivables from banks and brokers, Fair Value Receivables From Brokers Dealers And Clearing Organizations Fair Value Disclosure Receivables From Brokers Dealers And Clearing Organizations, Fair Value Disclosure Common Stock, par value Common Stock, Par or Stated Value Per Share Common Stock, shares authorized Common Stock, Shares Authorized Common Stock, shares issued Common Stock, Shares, Issued Common Stock, shares outstanding Common Stock, Shares, Outstanding Treasury stock, shares Treasury Stock, Shares Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Intangible Assets by Major Class [Table] Schedule Of Intangible Assets By Major Class [Table] Schedule of Intangible Assets by Major Class [Table] Technology Technology [Member] Technology [Member] Trademark Trademarks [Member] Non-compete agreement Noncompete Agreements [Member] Intangible Assets by Major Class [Line Items] Intangible Assets By Major Class [Line Items] Intangible Assets by Major Class [Line Items] Cost Finite-Lived Intangible Assets, Gross Cost Intangible Assets Excluding Goodwill Gross Intangible Assets Excluding Goodwill Gross Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative [Table] Derivative [Table] Derivative Instruments By Position [Axis] Derivative Instruments By Position [Axis] Derivative Instruments By Position [Axis] Derivative Instruments By Position [Domain] Derivative Instruments By Position [Domain] Derivative Instruments By Position [Domain] Notional amounts of derivative open long positions Long Position [Member] Long Position [Member] Notional amounts of derivative open short positions Short Position [Member] Short Position [Member] Derivative Instrument Risk [Axis] Derivative Instrument [Axis] Derivative Contract Type [Domain] Derivative Contract [Domain] Foreign currency exchange contracts Foreign Exchange Contract [Member] CFD contracts Other Contract [Member] Metal contracts Commodity Contract [Member] Derivative [Line Items] Derivative [Line Items] Notional amounts of derivative open long positions Derivative, Notional Amount Anti-dilutive stock options excluded from calculation of diluted earnings per share Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Receivables from Brokerage Clients [Table] Receivables From Brokerage Clients [Table] Receivables from Brokerage Clients [Table] Restricted Cash and Cash Equivalents [Axis] Restricted Cash and Cash Equivalents [Axis] Cash and Cash Equivalents [Domain] Cash and Cash Equivalents [Domain] Required collateral Cash Collateral [Member] Cash Collateral [Member] Excess From Futures Broker - Restricted [Member] Excess From Futures Broker - Restricted [Member] Excess From Futures Broker - Restricted [Member] Cash in excess of required collateral Cash In Excess Of Required Collateral [Member] Cash In Excess Of Required Collateral [Member] Open Position Open Position [Member] Open Position [Member] Investment in spot gold Investment In Spot Gold [Member] Investment In Spot Gold [Member] Receivables from Brokerage Clients [Line Items] Receivables From Brokerage Clients [Line Items] Receivables from Brokerage Clients [Line Items] Receivable from banks and brokers Receivables from Brokers-Dealers and Clearing Organizations Computation of Earnings Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Cash Payments to Acquire Businesses, Gross Payment for excess cash adjustment Payment to Acquire Business, Excess Cash Adjustment Payment to Acquire Business, Excess Cash Adjustment Loan payable Business Combination, Consideration Transferred, Liabilities Incurred Total purchase price Business Combination, Consideration Transferred Noncontrolling Interest [Abstract] Summary of redeemable noncontrolling interest treatment Redeemable Noncontrolling Interest [Table Text Block] Non-Controlling Interest Noncontrolling Interest Disclosure [Text Block] Percentage of company acquired Business Acquisition, Percentage of Voting Interests Acquired RECENT ACCOUNTING PRONOUNCEMENTS New Accounting Pronouncements and Changes in Accounting Principles [Text Block] Long-term Purchase Commitment [Table] Long-term Purchase Commitment [Table] Litigation Case [Axis] Litigation Case [Axis] Litigation Case Type [Domain] Litigation Case [Domain] Open E Cry Limited Liability Company Open E Cry Limited Liability Company [Member] Open E Cry Limited Liability Company [Member] Patent Infringement [Member] Patent Infringement [Member] Patent Infringement [Member] Long-term Purchase Commitment [Line Items] Long-term Purchase Commitment [Line Items] Number of alleged patent infringement lawsuit against the company Number Of Alleged Patents Infringed Number of alleged patents infringed Loss Contingency, Number of Patents Vacated and Remanded Loss Contingency, Number of Patents Vacated and Remanded Loss Contingency, Number of Patents Vacated and Remanded Schedule of Error Corrections and Prior Period Adjustment Restatement [Table] Schedule of Error Corrections and Prior Period Adjustment Restatement [Table] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Restatement Adjustments Restatement Adjustment [Member] Error Corrections and Prior Period Adjustments Restatement [Line Items] Error Corrections and Prior Period Adjustments Restatement [Line Items] Trading revenue Trading Revenue The caption reflecting the net gain (loss), both realized and unrealized, arising from customer trading. Referral fees Sales Commissions and Fees Trading expenses and commissions Trading Expenses And Commissions Trading expense and commissions consists primarily of compensation paid to white label partners and introducing brokers. Property and equipment, net of accumulated depreciation Schedule of Liability and Equity Components of Convertible Debt Convertible Debt [Table Text Block] Schedule of Interest Expense Schedule of Interest Expense [Table Text Block] Schedule of Interest Expense [Table Text Block] Net gains (losses) on derivative instruments Derivative, Gain (Loss) on Derivative, Net SHARE BASED COMPENSATION Disclosure of Compensation Related Costs, Share-based Payments [Text Block] RESTATEMENT Accounting Changes and Error Corrections [Text Block] Banking and Thrift [Abstract] Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Table] Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Table] Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] GFT Global Markets UK Ltd. GFT Global Markets UK Ltd [Member] GFT Global Markets UK Ltd [Member] GAIN Capital Group, LLC Gain Capital Group Limited Liability Company [Member] Gain Capital Group Limited Liability Company [Member] GAIN Capital-Forex.com U.K., Ltd. Gain Capital Forex Com United Kingdom Limited [Member] Gain Capital Forex Com United Kingdom Limited [Member] Forex.com Japan Co., Ltd. Forex Com Japan Company Limited [Member] Forex Com Japan Company Limited [Member] GAIN Capital-Forex.com Hong Kong, Ltd. Gain Capital Forex Com Hong Kong Limited [Member] Gain Capital Forex Com Hong Kong Limited [Member] GFT Global Markets Asia Pte., Ltd. GFT Global Markets Asia Pte. Ltd [Member] GFT Global Markets Asia Pte. Ltd [Member] GAIN Capital Forex.com Australia, Pty. Ltd. Gain Capital Forex Com Australia Privately Limited [Member] Gain Capital Forex Com Australia Privately Limited GAIN Capital-Forex.com Canada Ltd. Gain Capital Forex Com Canada Limited [Member] Gain Capital Forex Com Canada Limited [Member] GAIN Capital Securities, Inc. Gain Capital Securities Incorporated [Member] Gain Capital Securities Incorporated [Member] GAIN Global Markets, Inc. Gain Global Markets Incorporated [Member] Gain Global Markets Incorporated [Member] Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items] Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items] Schedule Of Compliance With Regulatory Capital Requirements For Broker Dealer [Line Items] Minimum Regulatory Capital Requirements Capital Required for Capital Adequacy Capital Levels Maintained Capital Required to be Well Capitalized Excess Net Capital Excess Capital Percent of Requirement Maintained Excess Capital To Assets Excess Capital to Assets Decrease in regulatory requirement Increase (Decrease) in Regulatory Assets and Liabilities Interest expense - stated coupon rate Interest Expense, Debt, Excluding Amortization Interest expense - amortization of deferred bond discount and costs Total interest expense - convertible note Interest Expense, Debt SEGMENT INFORMATION Segment Reporting Disclosure [Text Block] Accounting Policies [Abstract] DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION Business Description and Basis of Presentation [Text Block] Preliminary Purchase Price Schedule of Business Acquisitions, by Acquisition [Table Text Block] Schedule of Recognized Identified Assets Acquired and Liabilities Assumed Schedule of Recognized Identified Assets Acquired and Liabilities Assumed [Table Text Block] Pro Forma Information Business Acquisition, Pro Forma Information [Table Text Block] Schedule of Restructuring Costs Restructuring and Related Costs [Table Text Block] Debt Instrument, Face Amount Debt Instrument, Face Amount Debt Instrument, Interest Rate, Stated Percentage Debt Instrument, Interest Rate, Stated Percentage Proceeds from Convertible Debt Proceeds from Convertible Debt As Previously Reported Scenario, Previously Reported [Member] Cash and cash equivalents Cash and securities held for customers Restricted Cash and Cash Equivalents Total assets Assets Payables to customers, brokers, dealers, FCMs and other regulated entities Payables to Broker-Dealers and Clearing Organizations Accrued expenses and other liabilities Accrued Liabilities and Other Liabilities Income tax payable Accrued Income Taxes Total liabilities Liabilities Retained earnings Retained Earnings (Accumulated Deficit) Total GAIN Capital Holdings, Inc. Shareholders' equity Stockholders' Equity Attributable to Parent Total liabilities and shareholders' equity Liabilities and Equity Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Accumulated Retained Earnings Retained Earnings [Member] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] BEGINNING BALANCE Net income ENDING BALANCE Statement [Table] Statement [Table] Common Stock Common Stock [Member] Treasury Stock Treasury Stock [Member] Additional Paid in Capital Additional Paid-in Capital [Member] Accumulated Other Comprehensive Income/(loss) Accumulated Other Comprehensive Income (Loss) [Member] Statement [Line Items] Statement [Line Items] BEGINNING BALANCE (in shares) Shares, Outstanding Exercise of options (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Exercise of options Stock Issued During Period, Value, Stock Options Exercised Common Stock, Shares, Issued Additional Paid in Capital Additional Paid in Capital Conversion of restricted stock into common stock (in shares) Stock Issued During Period, Shares, Conversion of Convertible Securities Conversion of restricted stock into common stock Stock Issued During Period, Value, Conversion of Convertible Securities Stock compensation expense Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Tax benefit of stock options exercises Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation Dividend payment (usd per share) Dividends, Cash ENDING BALANCE (in shares) Income Statement [Abstract] REVENUE: Revenues, Net of Interest Expense [Abstract] Commission revenue Brokerage Commissions Revenue Other revenue Other Revenue, Net Total non-interest revenue Interest expense Total net interest revenue (expense) Net revenue Revenues, Net of Interest Expense EXPENSES: Costs and Expenses [Abstract] Employee compensation and benefits Labor and Related Expense Selling and marketing Marketing and Advertising Expense General and administrative General and Administrative Expense Communications and technology Communications and Information Technology Bad debt provision Provision for Doubtful Accounts Acquisition expense Business Combination, Acquisition Related Costs Restructuring Restructuring Charges Integration Business Combination, Integration Related Costs Total INCOME BEFORE INCOME TAX EXPENSE NET INCOME Net income attributable to non-controlling interest Net Income (Loss) Attributable to Noncontrolling Interest NET INCOME APPLICABLE TO GAIN CAPITAL HOLDINGS, INC. Other comprehensive income, net of tax: Other Comprehensive Income (Loss), Net of Tax [Abstract] Foreign currency translation adjustment Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax NET COMPREHENSIVE INCOME APPLICABLE TO GAIN CAPITAL HOLDINGS, INC. Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Earnings per common share: Weighted average common shares outstanding used in computing earnings per common share: Weighted Average Number Basic And Diluted Shares Outstanding [Abstract] Weighted Average Number Basic And Diluted, Shares Outstanding [Abstract] Basic (shares) Diluted (shares) Schedule of Related Party Transactions, by Related Party [Table] Schedule of Related Party Transactions, by Related Party [Table] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Management Management [Member] Related Party Transaction [Line Items] Related Party Transaction [Line Items] Payables to customers Payables to Customers Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] Business Combination, Indemnification Assets, Amount as of Acquisition Date Business Combination, Indemnification Assets, Amount as of Acquisition Date Accounts Receivable, Net Accounts Receivable, Net Vendor and security deposits Security Deposit Current tax receivable Income Taxes Receivable, Current Deferred tax assets Deferred Tax Assets, Net Investment in Kapitall, Inc. Other Investments Miscellaneous receivables Other Assets, Miscellaneous Other assets, net Other Assets Valuation Assumptions Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract] Risk-free rate Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate Expected volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Expected term (years) Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term Dividend yield Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate Redeemable Noncontrolling Interest, by Legal Entity [Table] Redeemable Noncontrolling Interest, by Legal Entity [Table] Redeemable Noncontrolling Interest [Line Items] Redeemable Noncontrolling Interest [Line Items] Redeemable noncontrolling interest retained by noncontrolling owners Redeemable Noncontrolling Interest,Ownership Percentage by Noncontrolling Owners Redeemable Noncontrolling Interest,Ownership Percentage by Noncontrolling Owners Increase (Decrease) in Temporary Equity [Roll Forward] Increase (Decrease) in Temporary Equity [Roll Forward] Redeemable noncontrolling interest, beginning balance Redeemable Noncontrolling Interest, Equity, Carrying Amount Noncontrolling interests related to acquisitions Redeemable Noncontrolling Interest, Business Combinations Redeemable Noncontrolling Interest, Business Combinations Net income attributable to noncontrolling interests Temporary Equity, Net Income Redeemable noncontrolling interest, ending balance REGULATORY REQUIREMENTS Regulatory Requirements Disclosure [Text Block] Minimum regulatory capital required to be maintained as established by regulators in the United States and other jurisdictions, and capital levels maintained and in excess of the minimum capital requirements. Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Asset Class [Axis] Asset Class [Axis] Fair Value by Asset Class [Domain] Asset Class [Domain] Derivative Assets Derivative Financial Instruments, Assets [Member] Derivative Liabilities Derivative Financial Instruments, Liabilities [Member] Derivatives, Fair Value [Line Items] Derivatives, Fair Value [Line Items] Gross amounts of assets for derivative open positions at fair value Derivative Asset, Fair Value, Gross Asset Cash Collateral Derivative, Collateral, Right to Reclaim Cash Cash Collateral Derivative, Collateral, Obligation to Return Cash Gross amounts of liability for derivative open positions at fair value Derivative Asset, Fair Value, Gross Liability Net amounts of assets/liabilities for derivative open positions at fair value Derivative Asset, Fair Value, Amount Not Offset Against Collateral Net amounts of assets/liabilities presented in the balance sheet Subsequent Events [Abstract] SUBSEQUENT EVENTS Subsequent Events [Text Block] Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Subsequent Event [Table] Subsequent Event [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent event Subsequent Event [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Common stock dividend declared per share Common Stock, Dividends, Per Share, Declared Net Cash Provided by (Used in) Operating Activities Cash and Cash Equivalents, Period Increase (Decrease) Fair Value, Assets and Liabilities Measured on Recurring Basis [Table] Fair Value Assets And Liabilities Measured On Recurring Basis [Table] Fair Value, Assets and Liabilities Measured on Recurring Basis [Table] Level 1 Fair Value, Inputs, Level 1 [Member] Level 2 Level 3 Fair Value, Inputs, Level 3 [Member] Investment Type [Axis] Investment Type [Axis] Investment Type Categorization [Domain] Investments [Domain] Money market accounts Money Market Funds [Member] Open contracts and other positions Other Financial [Member] Other Financial [Member] CIBC treasury bills Foreign Government Debt Securities [Member] Certificates of deposit Certificates of Deposit [Member] Investment in gold Commodities Investment [Member] Customer and broker open contracts and other positions Other Contracts [Member] Other Contracts [Member] Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items] Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items] Assets Assets, Fair Value Disclosure, Recurring Liabilities Liabilities, Fair Value Disclosure, Recurring Carrying Amounts and Fair Values of Financial Instruments or Liabilities [Table] Carrying Amounts And Fair Values Of Financial Instruments Or Liabilities [Table] Carrying Amounts and Fair Values of Financial Instruments or Liabilities [Table] Fair Value, by Balance Sheet Grouping, Disclosure Item Amounts [Axis] Measurement Basis [Axis] Fair Value, Disclosure Item Amounts [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement [Member] Portion at Fair Value Measurement [Member] Carrying Value Reported Value Measurement [Member] Fair Value Estimate of Fair Value Measurement [Member] Carrying Amounts and Fair Values of Financial Instruments or Liabilities [Line Items] Carrying Amounts And Fair Values Of Financial Instruments Or Liabilities [Line Items] Carrying Amounts and Fair Values of Financial Instruments or Liabilities [Line Items] Financial Assets: Assets, Fair Value Disclosure [Abstract] Financial Liabilities: Liabilities, Fair Value Disclosure [Abstract] Derivative, Collateral, Obligation to Return Cash Convertible senior notes Notes Payable Accrued Liabilities, Current Accrued Liabilities, Current Schedule of Restructuring and Related Costs [Table] Schedule of Restructuring and Related Costs [Table] Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Restructuring Reserve [Roll Forward] Restructuring Reserve [Roll Forward] Restructuring liability as of January 1, 2014 Restructuring Reserve 2014 restructuring expenses Payments made in 2014 Payments for Restructuring Liability as of March 31, 2014 ASSETS: Assets [Abstract] Short term investments, at fair value Other Short-term Investments Receivables from banks and brokers, ($10,730) and ($9,784), respectively, at fair value Prepaid assets Prepaid Expense Intangible assets, net Intangible Assets, Net (Excluding Goodwill) Other assets, net LIABILITIES AND SHAREHOLDERS' EQUITY: Liabilities and Equity [Abstract] Liabilities Liabilities [Abstract] Accrued compensation and benefits Employee-related Liabilities Commitments and contingent liabilities Commitments and Contingencies Redeemable noncontrolling interest GAIN Capital Holdings, Inc. Shareholders' equity Stockholders' Equity Attributable to Parent [Abstract] Common stock ($0.00001 par value; 60 million shares authorized; 42,460,947 shares issued and 39,964,772 shares outstanding as of March 31, 2014; 41,921,609 shares issued and 39,425,434 shares outstanding as of December 31, 2013) Common Stock, Value, Issued Accumulated other comprehensive (loss)/income Accumulated Other Comprehensive Income (Loss), Net of Tax Additional paid-in capital Additional Paid in Capital, Common Stock Treasury stock, at cost (2,496,175 shares at March 31, 2014 and 2,496,175 at December 31, 2013, respectively) Treasury Stock, Value Minimum Regulatory Capital Subsidiaries Required to Maintain Capital Requirements on Foreign Financial Institutions [Table Text Block] CONVERTIBLE SENIOR NOTES Debt Disclosure [Text Block]