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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Product revenues, net	$ 15,752	$ 14,493	$ 12,851	$ 9,968	$ 4,328				$ 53,064	$ 4,328
License and collaboration revenues	44,057	12,417	19,332	11,313	17,090	$ 16,440	$ 36,558	$ 14,842	87,119	84,930	$ 102,756
Other revenues	1,431	1,161	1,498						4,090
Cost of revenues	3,319	1,010	1,872	711	46				6,912	46
Research and development expenses	54,961	32,078	40,996	32,882	44,740	37,763	42,806	35,679	160,917	160,988	138,495
Selling, general and administrative expenses	24,206	18,048	20,680	17,795	19,335	16,956	12,315	9,189	80,729	57,795	30,517
Restructuring expenses	5,047	809							5,856
Net loss	(33,627)	(30,275)	(50,958)	(38,658)	(48,068)	(42,386)	(22,901)	(34,432)	(153,518)	(147,787)	(83,559)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (32,449)	$ (30,068)	$ (50,750)	$ (38,473)	$ (47,826)	$ (42,594)	$ (22,778)	$ (34,759)	$ (151,740)	$ (147,957)	$ (83,291)
Net loss per share available to common stockholders—basic and diluted	$ (0.25)	$ (0.23)	$ (0.40)	$ (0.33)	$ (0.41)	$ (0.38)	$ (0.21)	$ (0.32)	$ (1.21)	$ (1.33)	$ (0.80)