Condensed Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Collaboration revenues	$ 14,842	$ 13,034
Operating expenses:
Research and development	35,679	30,324
General and administrative	9,189	6,224
Total operating expenses	44,868	36,548
Loss from operations	(30,026)	(23,514)
Other income and expenses
Interest income	47	35
Interest expense	(4,566)	(4,511)
Other, net	113	236
Net loss	(34,432)	(27,754)
Less net income (loss) attributable to non-controlling interest	327	(169)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(34,759)	(27,585)
Other comprehensive income:
Unrealized gain on available-for-sale securities	47	16
Other comprehensive income	47	16
Comprehensive loss	$ (34,712)	$ (27,569)
Net loss per share available to common stockholders-basic and diluted	$ (0.32)	$ (0.27)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	107,148	102,888