| Document and Entity Information | Feb. 24, 2020 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001571498 |
| Document Type | 8-K |
| Document Period End Date | Feb. 24, 2020 |
| Entity Registrant Name | EPIZYME, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-35945 |
| Entity Tax Identification Number | 26-1349956 |
| Entity Address, Address Line One | 400 Technology Square |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02139 |
| City Area Code | (617) |
| Local Phone Number | 229-5872 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, $0.0001 par value |
| Trading Symbol | EPZM |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
