| Document and Entity Information | Mar. 10, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | Flexion Therapeutics, Inc. |
| Document Type | 8-K |
| Amendment Flag | false |
| Entity Central Index Key | 0001419600 |
| Document Period End Date | Mar. 10, 2021 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-36287 |
| Entity Incorporation State 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.001 per share |
| Trading Symbol | FLXN |
| Security Exchange Name | NASDAQ |
