Condensed Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Condensed Consolidated Statements of Comprehensive Loss
Collaboration revenues	$ 18,452	$ 12,063	$ 33,107	$ 23,407
Operating expenses:
Research and development	42,465	28,758	79,454	60,409
General and administrative	5,095	3,610	10,027	7,338
Total operating expenses	47,560	32,368	89,481	67,747
Loss from operations	(29,108)	(20,305)	(56,374)	(44,340)
Other income and expenses
Interest income	35	54	87	63
Interest expense	(1,293)		(2,513)
Other, net	115	112	226	736
Net loss	(30,251)	(20,139)	(58,574)	(43,541)
Less net loss attributable to non-controlling interest	(169)	(113)	(339)	(231)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(30,082)	(20,026)	(58,235)	(43,310)
Other comprehensive (loss) income:
Unrealized (loss) gain on available-for-sale securities	15	(49)	33	(49)
Other comprehensive (loss) income	15	(49)	33	(49)
Comprehensive loss	$ (30,067)	$ (20,075)	$ (58,202)	$ (43,359)
Net loss per share available to common stockholders-basic and diluted (in dollars per share)	$ (0.31)	$ (0.22)	$ (0.61)	$ (0.89)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	96,170	90,581	96,025	51,214