Summary of Significant Accounting Policies (Details 3) (USD $)	3 Months Ended		6 Months Ended			1 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Apr. 30, 2012 Initial public offering
Public Offering
Convertible preferred stock converted into common stock (in shares)						66,255,529
Convertible Preferred Stock and Convertible Preferred Stock Warrants
Change in fair value of convertible preferred stock warrants	$ 29,000	$ 141,000	$ 587,000	$ (576,000)
Preferred stock
Authorized undesignated preferred stock (in shares)	10,000,000		10,000,000		0
Preferred stock, par value (in dollars per share)	$ 0.01		$ 0.01		$ 0.01
Comprehensive Income (Loss)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(20,026,000)	(29,051,000)	(43,310,000)	(42,508,000)
Unrealized loss on available-for-sale securities	(49,000)		(49,000)
Comprehensive loss	(20,075,000)	(29,051,000)	(43,359,000)	(42,508,000)
Other Income (Expense)
Settlement from former service provider				1,800,000
Deferred Financing Costs
Deferred financing costs					1,946,000
Deferred financing costs netted against the equity proceeds within stockholders' equity			$ 2,748,000