Condensed Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenues	$ 27,815	$ 18,452	$ 40,849	$ 33,107
Operating expenses:
Research and development	33,795	42,465	64,119	79,454
General and administrative	7,921	5,095	14,145	10,027
Total operating expenses	41,716	47,560	78,264	89,481
Loss from operations	(13,901)	(29,108)	(37,415)	(56,374)
Other income and expenses
Interest income	20	35	55	87
Interest expense	(4,570)	(1,293)	(9,081)	(2,513)
Other, net	161	115	397	226
Net loss	(18,290)	(30,251)	(46,044)	(58,574)
Less net loss attributable to non-controlling interest	(181)	(169)	(350)	(339)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(18,109)	(30,082)	(45,694)	(58,235)
Other comprehensive income:
Unrealized gain on available-for-sale securities	5	15	21	33
Other comprehensive income	5	15	21	33
Comprehensive loss	$ (18,104)	$ (30,067)	$ (45,673)	$ (58,202)
Net loss per share available to common stockholders-basic and diluted	$ (0.17)	$ (0.31)	$ (0.44)	$ (0.61)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	103,809	96,170	103,351	96,025