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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Research and development expenses	$ 12,360	$ 13,598	$ 19,751	$ 21,605	$ 25,621	$ 28,247	$ 27,695	$ 28,002	$ 67,314	$ 109,565	$ 121,033
General and administrative expenses	4,654	3,366	14,798	5,634	11,014	6,448	8,138	6,452	28,452	32,052	24,048
Restructuring expenses					4,901	809			9,521	5,710
Loss from continuing operations	(11,829)	(5,323)	(29,646)	(29,206)	(40,091)	(26,845)	(51,633)	(37,715)	(76,004)	(156,284)	(151,619)
(Loss) income from discontinued operations, net of tax	(1,122)	8,456	540,485	(947)	6,464	(3,430)	675	(943)	(546,872)	(2,766)	(3,832)
Net (loss) income	(12,951)	3,133	510,839	(30,153)	(33,627)	(30,275)	(50,958)	(38,658)	470,868	(153,518)	(147,787)
Net (loss) income attributable to Merrimack Pharmaceuticals, Inc.	$ (12,951)	$ 3,102	$ 511,563	$ (29,686)	$ (32,449)	$ (30,068)	$ (50,750)	$ (38,473)	$ 472,028	$ (151,740)	$ (147,957)
Basic and dilutive net income (loss) per common share
Net loss from continuing operations	$ (0.89)	$ (0.40)	$ (2.18)	$ (2.20)	$ (2.93)	$ (2.06)	$ (4.07)	$ (3.23)	$ (5.66)	$ (12.33)	$ (13.63)
Net (loss) income from discontinued operations, net of tax	(0.08)	0.64	40.80	(0.07)	0.43	(0.27)	0.05	(0.08)	41.33	0.22	0.34
Net (loss) income per share	$ (0.97)	$ 0.24	$ 38.62	$ (2.27)	$ (2.50)	$ (2.33)	$ (4.02)	$ (3.31)	$ 35.67	$ (12.11)	$ (13.29)