Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Nov. 01, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Entity Registrant Name Flexion Therapeutics Inc  
Entity Central Index Key 0001419600  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock Shares Outstanding   38,174,760
Entity Shell Company false  
Entity Small Business false  
Entity Emerging Growth Company false  
Title of 12(b) Security Common stock, $0.001 par value per share  
Trading Symbol FLXN  
Security Exchange Name NASDAQ  
Entity File Number 001-36287  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 26-1388364  
Entity Address, Address Line One 10 Mall Road  
Entity Address, Address Line Two Suite 301  
Entity Address, City or Town Burlington  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 01803  
City Area Code (781)  
Local Phone Number 305-7777  
Document Quarterly Report true  
Document Transition Report false