v3.10.0.1
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 0001444363  
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 2018  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Entity Common Stock, Shares Outstanding   44,642,233