Condensed Consolidated Statements of Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Comprehensive Income [Abstract]
Collaboration revenues	$ 36,558	$ 27,815	$ 51,399	$ 40,849
Operating expenses:
Research and development	42,806	33,795	78,485	64,119
General and administrative	12,315	7,921	21,504	14,145
Total operating expenses	55,121	41,716	99,989	78,264
Loss from operations	(18,563)	(13,901)	(48,590)	(37,415)
Other income and expenses
Interest income	34	20	80	55
Interest expense	(4,482)	(4,570)	(9,048)	(9,081)
Other, net	110	161	224	397
Net loss	(22,901)	(18,290)	(57,334)	(46,044)
Less net income (loss) attributable to non-controlling interest	(123)	(181)	204	(350)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(22,778)	(18,109)	(57,538)	(45,694)
Other comprehensive income:
Unrealized gain on available-for-sale securities	21	5	68	21
Other comprehensive income	21	5	68	21
Comprehensive loss	$ (22,757)	$ (18,104)	$ (57,470)	$ (45,673)
Net loss per share available to common stockholders-basic and diluted	$ (0.21)	$ (0.17)	$ (0.53)	$ (0.44)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	109,975	103,809	108,662	103,351