Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development expenses	$ 49,974	$ 67,314
General and administrative expenses	15,601	28,452
Total operating expenses	65,575	95,766
Loss from continuing operations	(65,575)	(95,766)
Other income and expenses:
Interest income	1,299	895
Interest expense	(956)	(34,650)
Gain on deconsolidation of Silver Creek Pharmaceuticals, Inc.		10,848
Gain on sale of asset		1,703
Other (expense) income, net	(3,230)	(1,433)
Total other income and expenses	(2,887)	(22,637)
Net loss from continuing operations before income tax benefit	(68,462)	(118,403)
Income tax benefit	7,695	42,399
Net loss from continuing operations	(60,767)	(76,004)
Discontinued operations:
Income from discontinued operations, net of tax	20,261	546,872
Net (loss) income	(40,506)	470,868
Net loss attributable to non-controlling interest		(1,160)
Net (loss) income attributable to Merrimack Pharmaceuticals, Inc.	(40,506)	472,028
Other comprehensive loss:
Unrealized loss on marketable securities	(9)
Other comprehensive loss	(9)
Comprehensive (loss) income	(40,515)	472,028
Amounts attributable to Merrimack Pharmaceuticals, Inc.:
Net loss from continuing operations	(60,767)	(74,844)
Income from discontinued operations, net of tax	20,261	546,872
Net (loss) income attributable to Merrimack Pharmaceuticals, Inc.	$ (40,506)	$ 472,028
Basic and dilutive net (loss) income per common share
Net loss from continuing operations	$ (4.55)	$ (5.66)
Net income from discontinued operations, net of tax	1.52	41.33
Net (loss) income per share	$ (3.03)	$ 35.67
Weighted-average common shares used in per share calculations—basic and diluted	13,343	13,232