Condensed Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Condensed Consolidated Statements of Comprehensive Loss
Collaboration revenues	$ 14,655	$ 11,344
Operating expenses:
Research and development	36,989	31,651
General and administrative	4,932	3,728
Total operating expenses	41,921	35,379
Loss from operations	(27,266)	(24,035)
Other income and expenses
Interest income	52	9
Interest expense	(1,220)	(1)
Other, net	111	625
Net loss	(28,323)	(23,402)
Less net loss attributable to non-controlling interest	(170)	(118)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(28,153)	(23,284)
Other comprehensive income:
Unrealized gain on available-for-sale securities	18
Other comprehensive income	18
Comprehensive loss	$ (28,135)	$ (23,284)
Net loss per share available to common stockholders-basic and diluted (in dollars per share)	$ (0.29)	$ (2.14)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted (in shares)	95,879	11,846