Condensed Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Collaboration revenues	$ 11,344	$ 6,461
Operating expenses
Research and development	31,651	18,001
General and administrative	3,728	3,101
Total operating expenses	35,379	21,102
Loss from operations	(24,035)	(14,641)
Other income and expenses
Interest income	9	14
Interest expense	(1)	(6)
Other, net	625	1,098
Net loss	(23,402)	(13,535)
Less net loss attributable to non-controlling interest	(118)	(78)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (23,284)	$ (13,457)
Net loss per share available to common stockholders-basic (in dollars per share)	$ (2.14)	$ (1.35)
Net loss per share available to common stockholders-diluted (in dollars per share)	$ (2.14)	$ (1.35)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic (in shares)	11,846	11,106
Weighted-average common shares used in computing net loss per share available to common stockholders-diluted (in shares)	11,846	11,106