Condensed Consolidated Statements of Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Collaboration revenues	$ 16,440	$ 28,002	$ 67,839	$ 68,851
Operating expenses:
Research and development	37,763	43,632	116,248	107,751
General and administrative	16,956	8,095	38,460	22,240
Total operating expenses	54,719	51,727	154,708	129,991
Loss from operations	(38,279)	(23,725)	(86,869)	(61,140)
Other income and expenses:
Interest income	13	7	93	62
Interest expense	(4,476)	(4,585)	(13,524)	(13,666)
Other, net	356	265	580	662
Net loss	(42,386)	(28,038)	(99,720)	(74,082)
Less net income (loss) attributable to non-controlling interest	208	(137)	412	(487)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(42,594)	(27,901)	(100,132)	(73,595)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities	6	(40)	74	(19)
Other comprehensive income (loss)	6	(40)	74	(19)
Comprehensive loss	$ (42,588)	$ (27,941)	$ (100,058)	$ (73,614)
Net loss per share available to common stockholders-basic and diluted	$ (0.38)	$ (0.27)	$ (0.91)	$ (0.71)
Weighted-average common shares used in computing net loss per share available to common stockholders-basic and diluted	112,417	104,871	109,928	103,863