Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Product revenues, net	$ 4,328								$ 4,328	$ 0	$ 0
License and collaboration revenues	17,090	$ 16,440	$ 36,558	$ 14,842	$ 33,905	$ 28,002	$ 27,815	$ 13,034	84,930	102,756	47,786
Cost of product revenues	46								46	0	0
Research and development expenses	44,740	37,763	42,806	35,679	30,744	43,632	33,795	30,324	160,988	138,495	147,139
Selling, general and administrative expenses	19,335	16,956	12,315	9,189	8,277	8,095	7,921	6,224	57,795	30,517	21,187
Net loss	(48,068)	(42,386)	(22,901)	(34,432)	(9,477)	(28,038)	(18,290)	(27,754)	(147,787)	(83,559)	(130,685)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (47,826)	$ (42,594)	$ (22,778)	$ (34,759)	$ (9,696)	$ (27,901)	$ (18,109)	$ (27,585)	$ (147,957)	$ (83,291)	$ (130,925)
Net loss per share available to common stockholders-basic and diluted	$ (0.41)	$ (0.38)	$ (0.21)	$ (0.32)	$ (0.09)	$ (0.27)	$ (0.17)	$ (0.27)	$ (1.33)	$ (0.80)	$ (1.32)