Document and Entity Information
| Document and Entity Information | Mar. 10, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | MERRIMACK PHARMACEUTICALS INC |
| Amendment Flag | false |
| Entity Central Index Key | 0001274792 |
| Document Type | 8-K |
| Document Period End Date | Mar. 10, 2021 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-35409 |
| Entity Tax Identification Number | 04-3210530 |
| Entity Address, Address Line One | One Broadway |
| Entity Address, Address Line Two | 14th Floor |
| Entity Address, City or Town | Cambridge |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02142 |
| City Area Code | (617) |
| Local Phone Number | 441-1000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, $0.01 par value |
| Trading Symbol | MACK |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |