Condensed Consolidated Statements of Comprehensive Income (Loss) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Collaboration revenues	$ 11,323	$ 8,582	$ 34,730	$ 21,638
Operating expenses
Research and development	30,885	23,913	91,294	73,101
General and administrative	4,312	3,306	11,650	11,239
Total operating expenses	35,197	27,219	102,944	84,340
Loss from operations	(23,874)	(18,637)	(68,214)	(62,702)
Other income and expenses
Interest income	64	8	127	51
Interest expense		(2)		(12)
Other, net	490	(93)	1,226	1,208
Net loss	(23,320)	(18,724)	(66,861)	(61,455)
Less net loss attributable to non-controlling interest	(121)	(125)	(352)	(348)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(23,199)	(18,599)	(66,509)	(61,107)
Other comprehensive income:
Unrealized gain on available-for-sale securities	59		10
Other comprehensive income	59		10
Comprehensive loss	$ (23,140)	$ (18,599)	$ (66,499)	$ (61,107)
Net loss per share available to common stockholders-basic and diluted (in dollars per share)	$ (0.25)	$ (1.81)	$ (1.05)	$ (5.92)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	93,724	11,389	65,487	11,292