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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2022
Mar. 07, 2023
Jun. 30, 2022
Cover [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2022    
Document Fiscal Year Focus 2022    
Document Fiscal Period Focus FY    
Trading Symbol MACK    
Entity Registrant Name MERRIMACK PHARMACEUTICALS INC    
Entity Central Index Key 0001274792    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Entity Filer Category Non-accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding   14,255,104  
Entity Public Float     $ 57,237,893
Entity Interactive Data Current Yes    
Title of 12(b) Security Common Stock, $0.01 par value    
Security Exchange Name NASDAQ    
Entity File Number 001-35409    
Entity Incorporation, State or Country Code DE    
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 720-8606    
Document Annual Report true    
Document Transition Report false    
Auditor Name Marcum llp    
Auditor Location Boston, MA    
Auditor Firm ID 688    
Documents Incorporated by Reference [Text Block]

The registrant intends to file a definitive proxy statement pursuant to Regulation 14A in connection with its 2022 Annual Meeting of Stockholders. Portions of such proxy statement are incorporated by reference into Part III of this Annual Report on Form 10-K.