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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Research and development expenses	$ 10,231	$ 12,959	$ 13,678	$ 13,106	$ 12,360	$ 13,598	$ 19,751	$ 21,605	$ 49,974	$ 67,314
General and administrative expenses	4,041	3,777	3,513	4,270	4,654	3,366	14,798	5,634	15,601	28,452
Restructuring expenses									1,289	9,521
Net loss from continuing operations	(12,875)	(12,341)	(17,769)	(17,782)	(11,829)	(5,323)	(29,646)	(29,206)	(60,767)	(76,004)
(Loss) income from discontinued operations, net of tax	3,931	16,330			(1,122)	8,456	540,485	(947)	(20,261)	(546,872)
Net (loss) income	(8,944)	3,989	(17,769)	(17,782)	(12,951)	3,133	510,839	(30,153)	(40,506)	470,868
Net (loss) income attributable to Merrimack Pharmaceuticals, Inc.	$ (8,944)	$ 3,989	$ (17,769)	$ (17,782)	$ (12,951)	$ 3,102	$ 511,563	$ (29,686)	$ (40,506)	$ 472,028
Basic and dilutive net (loss) income per common share
Net loss from continuing operations	$ (0.97)	$ (0.92)	$ (1.33)	$ (1.33)	$ (0.89)	$ (0.40)	$ (2.18)	$ (2.20)	$ (4.55)	$ (5.66)
Net (loss) income from discontinued operations, net of tax	0.30	1.22			(0.08)	0.64	40.80	(0.07)	1.52	41.33
Net (loss) income per share	$ (0.67)	$ 0.30	$ (1.33)	$ (1.33)	$ (0.97)	$ 0.24	$ 38.62	$ (2.27)	$ (3.03)	$ 35.67