Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2023	Mar. 15, 2024	Jun. 30, 2022
Document Information [Line Items]
Document Type	10-K/A
Document Annual Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Amendment Flag	true
Document Period End Date	Dec. 31, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Entity Registrant Name	MERRIMACK PHARMACEUTICALS, INC.
Entity Central Index Key	0001274792
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Title of 12(b) Security	Common stock, $0.01 par value
Trading Symbol	MACK
Security Exchange Name	NASDAQ
Entity Address, State or Province	MA
Entity Small Business	true
Entity Emerging Growth Company	false
Document Financial Statement Error Correction [Flag]	false
Entity Shell Company	false
Entity Public Float			$ 129,723,131
Entity Common Stock, Shares Outstanding		14,531,760
ICFR Auditor Attestation Flag	false
Entity File Number	001-35409
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	One Broadway
Entity Address, Address Line Two	14th Floor
Entity Address, City or Town	Cambridge
Entity Address, Postal Zip Code	02142
Entity Tax Identification Number	04-3210530
City Area Code	617
Local Phone Number	720-8606
Amendment Description	This Amendment No. 1 on Form 10-K/A (the “Amendment”) amends the Annual Report on Form 10-K (the “2023 Form 10-K”) of Merrimack Pharmaceuticals, Inc. (the “Company”) for the fiscal year ended December 31, 2023 (the “2023 Fiscal Year”), as filed with the Securities and Exchange Commission (the “SEC”) on March 7, 2024. We are filing this Amendment to amend Part III of the 2023 Form 10-K to include the information required by and not included in Part III of the 2023 Form 10-K because we do not intend to file an annual meeting definitive proxy statement within 120 days of the end of December 31, 2023 but rather intend to hold a special meeting of stockholders to approve a plan of dissolution of the Company which, if approved, will result in the Company filing a certificate of dissolution with the Delaware Secretary of State promptly following the conclusion of the special meeting. In addition, the Exhibit Index in Item 15 of Part IV of the 2023 Form 10-K is hereby amended and restated in its entirety and currently dated certifications required under Section 302 of the Sarbanes-Oxley Act of 2002 are filed as exhibits to this Amendment. Because no financial statements are contained within this Amendment, we are not filing currently dated certifications pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 nor are we filing a consent of our independent public accounting firm. Except as described above, no other changes have been made to the 2023 Form 10-K. The 2023 Form 10-K continues to speak as of the date of the 2023 Form 10-K, and we have not updated the disclosures contained therein to reflect any events which occurred at a date subsequent to the filing of the 2023 Form 10-K other than as expressly indicated in this Amendment.