Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating expenses:
Research and development expenses	$ 13,106,000	$ 21,605,000
General and administrative expenses	4,270,000	5,634,000
Total operating expenses	17,376,000	27,239,000
Loss from continuing operations	(17,376,000)	(27,239,000)
Other income and expenses:
Interest income	275,000	14,000
Interest expense		(1,979,000)
Other income (expense), net	(681,000)	(2,000)
Total other income and expenses	(406,000)	(1,967,000)
Net loss from continuing operations	(17,782,000)	(29,206,000)
Discontinued operations:
Loss from discontinued operations, net of tax	0	(947,000)
Net loss	(17,782,000)	(30,153,000)
Net loss attributable to non-controlling interest		(467,000)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(17,782,000)	(29,686,000)
Other comprehensive loss:
Unrealized loss on marketable securities	(12,000)
Other comprehensive loss	(12,000)
Comprehensive loss	(17,794,000)	(29,686,000)
Amounts attributable to Merrimack Pharmaceuticals, Inc.:
Net loss from continuing operations	(17,782,000)	(28,739,000)
Loss from discontinued operations, net of tax		(947,000)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (17,782,000)	$ (29,686,000)
Basic and dilutive net loss per common share
Net loss from continuing operations	$ (1.33)	$ (2.20)
Net loss from discontinued operations, net of tax		(0.07)
Net loss per share	$ (1.33)	$ (2.27)
Weighted-average common shares used per share calculations—basic and diluted	13,343	13,059