Condensed Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Condensed Consolidated Statements of Comprehensive Loss
Collaboration revenues	$ 6,856	$ 11,323	$ 39,963	$ 34,730
Operating expenses:
Research and development	37,630	30,885	117,084	91,294
General and administrative	5,150	4,312	15,177	11,650
Total operating expenses	42,780	35,197	132,261	102,944
Loss from operations	(35,924)	(23,874)	(92,298)	(68,214)
Other income and expenses
Interest income	36	64	123	127
Interest expense	(3,946)		(6,459)
Other, net	71	490	297	1,226
Net loss	(39,763)	(23,320)	(98,337)	(66,861)
Less net loss attributable to non-controlling interest	(132)	(121)	(471)	(352)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(39,631)	(23,199)	(97,866)	(66,509)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities	(11)	59	22	10
Other comprehensive income (loss)	(11)	59	22	10
Comprehensive loss	$ (39,642)	$ (23,140)	$ (97,844)	$ (66,499)
Net loss per share available to common stockholders-basic and diluted (in dollars per share)	$ (0.39)	$ (0.25)	$ (1.00)	$ (1.05)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	101,155	93,724	97,754	65,487