Consolidated Subsidiaries (Details) (Hermes, USD $)	0 Months Ended	12 Months Ended
	Oct. 06, 2009	Dec. 31, 2010
Consolidated subsidiaries
Shares of common stock issued	4,383,000
Common shares of Merrimack Pharmaceuticals, Inc.	$ 9,292,000
Number of additional shares that could have been required to be issued as contingent consideration		400,000
Contingent consideration	178,000
Gain recognized on settlement of contingent consideration		$ 178,000
Minimum
Consolidated subsidiaries
Number of additional shares that could have been required to be issued as contingent consideration	0
Maximum
Consolidated subsidiaries
Number of additional shares that could have been required to be issued as contingent consideration	1,100,000
Period for issuance of additional shares as contingent consideration	24 months