Condensed Consolidated Statements of Comprehensive Income (Loss) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Collaboration revenues	$ 12,063	$ 6,595	$ 23,407	$ 13,056
Operating expenses
Research and development	28,758	31,187	60,409	49,188
General and administrative	3,610	4,832	7,338	7,933
Total operating expenses	32,368	36,019	67,747	57,121
Loss from operations	(20,305)	(29,424)	(44,340)	(44,065)
Other income and expenses
Interest income	54	29	63	43
Interest expense		(4)		(10)
Other, net	112	203	736	1,301
Net loss	(20,139)	(29,196)	(43,541)	(42,731)
Less net loss attributable to non-controlling interest	(113)	(145)	(231)	(223)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(20,026)	(29,051)	(43,310)	(42,508)
Other comprehensive loss:
Unrealized loss on available-for-sale securities	(49)		(49)
Other comprehensive loss	(49)		(49)
Comprehensive loss	$ (20,075)	$ (29,051)	$ (43,359)	$ (42,508)
Net loss per share available to common stockholders-basic and diluted (in dollars per share)	$ (0.22)	$ (2.76)	$ (0.89)	$ (4.14)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	90,581	11,282	51,214	11,158