Condensed Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenues	$ 13,034	$ 14,655
Operating expenses:
Research and development	30,324	36,989
General and administrative	6,224	4,932
Total operating expenses	36,548	41,921
Loss from operations	(23,514)	(27,266)
Other income and expenses
Interest income	35	52
Interest expense	(4,511)	(1,220)
Other, net	236	111
Net loss	(27,754)	(28,323)
Less net loss attributable to non-controlling interest	(169)	(170)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(27,585)	(28,153)
Other comprehensive income:
Unrealized gain on available-for-sale securities	16	18
Other comprehensive income	16	18
Comprehensive loss	$ (27,569)	$ (28,135)
Net loss per share available to common stockholders-basic and diluted	$ (0.27)	$ (0.29)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	102,888	95,879