v3.8.0.1
Document And Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Mar. 12, 2018
Jun. 30, 2017
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Entity Registrant Name Gastar Exploration Inc.    
Trading Symbol GST    
Entity Central Index Key 0001431372    
Current Fiscal Year End Date --12-31    
Entity Filer Category Accelerated Filer    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 116.0
Entity Common Stock, Shares Outstanding   220,895,069