Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statements of Comprehensive Loss
Collaboration revenues	$ 48,921	$ 34,215	$ 20,305
Operating expenses
Research and development	125,858	100,630	58,278
General and administrative	15,805	14,454	11,381
Contingent consideration			(178)
Total operating expenses	141,663	115,084	69,481
Loss from operations	(92,742)	(80,869)	(49,176)
Other income and expenses
Interest income	184	56	74
Interest expense	(553)	(13)	(3,726)
Other, net	1,357	1,150	2,669
Net loss	(91,754)	(79,676)	(50,159)
Less net loss attributable to non-controlling interest	(477)	(453)	(55)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(91,277)	(79,223)	(50,104)
Other comprehensive loss:
Unrealized loss on available-for-sale securities	(38)
Other comprehensive loss	(38)
Comprehensive loss	$ (91,315)	$ (79,223)	$ (50,104)
Net loss per share available to common stockholders-basic and diluted (in dollars per share)	$ (1.28)	$ (7.67)	$ (5.57)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted (in shares)	72,831	11,343	10,994