Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenues	$ 47,786	$ 48,921	$ 34,215
Operating expenses:
Research and development	147,139	125,858	100,630
General and administrative	21,187	15,805	14,454
Total operating expenses	168,326	141,663	115,084
Loss from operations	(120,540)	(92,742)	(80,869)
Other income and expenses
Interest income	166	184	56
Interest expense	(10,938)	(553)	(13)
Other, net	627	1,357	1,150
Net loss	(130,685)	(91,754)	(79,676)
Less net income (loss) attributable to non-controlling interest	240	(477)	(453)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(130,925)	(91,277)	(79,223)
Other comprehensive income (loss):
Unrealized loss on available-for-sale securities	14	(38)
Other comprehensive income (loss)	14	(38)
Comprehensive loss	$ (130,911)	$ (91,315)	$ (79,223)
Net loss per share available to common stockholders-basic and diluted	$ (1.32)	$ (1.28)	$ (7.67)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	98,919	72,831	11,343