Condensed Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Statement of Comprehensive Income [Abstract]
Collaboration revenues	$ 28,002	$ 6,856	$ 68,851	$ 39,963
Operating expenses:
Research and development	43,632	37,630	107,751	117,084
General and administrative	8,095	5,150	22,240	15,177
Total operating expenses	51,727	42,780	129,991	132,261
Loss from operations	(23,725)	(35,924)	(61,140)	(92,298)
Other income and expenses
Interest income	7	36	62	123
Interest expense	(4,585)	(3,946)	(13,666)	(6,459)
Other, net	265	71	662	297
Net loss	(28,038)	(39,763)	(74,082)	(98,337)
Less net loss attributable to non- controlling interest	(137)	(132)	(487)	(471)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(27,901)	(39,631)	(73,595)	(97,866)
Other comprehensive (loss) income:
Unrealized (loss) gain on available-for-sale securities	(40)	(11)	(19)	22
Other comprehensive (loss) income	(40)	(11)	(19)	22
Comprehensive loss	$ (27,941)	$ (39,642)	$ (73,614)	$ (97,844)
Net loss per share available to common stockholders-basic and diluted	$ (0.27)	$ (0.39)	$ (0.71)	$ (1.00)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	104,871	101,155	103,863	97,754