Condensed Consolidated Statements of Operations and Comprehensive (Loss) Income - USD ($) shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating expenses:
Research and development expenses	$ 13,678,000	$ 19,751,000	$ 26,784,000	$ 41,356,000
General and administrative expenses	3,513,000	14,798,000	7,783,000	20,432,000
Total operating expenses	17,191,000	34,549,000	34,567,000	61,788,000
Loss from continuing operations	(17,191,000)	(34,549,000)	(34,567,000)	(61,788,000)
Other income and expenses:
Interest income	282,000	382,000	557,000	396,000
Interest expense		(26,762,000)		(28,741,000)
Gain on sale of asset		1,703,000		1,703,000
Other income (expense), net	(860,000)	(659,000)	(1,541,000)	(661,000)
Total other income and expenses	(578,000)	(25,336,000)	(984,000)	(27,303,000)
Net loss from continuing operations before income tax benefit	(17,769,000)	(59,885,000)	(35,551,000)	(89,091,000)
Income tax benefit		30,239,000		30,239,000
Net loss from continuing operations	(17,769,000)	(29,646,000)	(35,551,000)	(58,852,000)
Discontinued operations:
Income from discontinued operations, net of tax	0	540,485,000	0	539,538,000
Net (loss) income	(17,769,000)	510,839,000	(35,551,000)	480,686,000
Net loss attributable to non-controlling interest		(724,000)		(1,191,000)
Net (loss) income attributable to Merrimack Pharmaceuticals, Inc.	(17,769,000)	511,563,000	(35,551,000)	481,877,000
Other comprehensive income (loss):
Unrealized gain (loss) on marketable securities	11,000		(1,000)
Other comprehensive income (loss)	11,000		(1,000)
Comprehensive (loss) income	(17,758,000)	511,563,000	(35,552,000)	481,877,000
Amounts attributable to Merrimack Pharmaceuticals, Inc.:
Net loss from continuing operations	(17,769,000)	(28,922,000)	(35,551,000)	(57,661,000)
Income from discontinued operations, net of tax		540,485,000		539,538,000
Net (loss) income attributable to Merrimack Pharmaceuticals, Inc.	$ (17,769,000)	$ 511,563,000	$ (35,551,000)	$ 481,877,000
Basic and dilutive net (loss) income per common share
Net loss from continuing operations	$ (1.33)	$ (2.18)	$ (2.66)	$ (4.38)
Net income from discontinued operations, net of tax		40.80		41.02
Net (loss) income per share	$ (1.33)	$ 38.62	$ (2.66)	$ 36.64
Weighted-average common shares used per share calculations—basic and diluted	13,343	13,246	13,343	13,153
Cash dividend paid per common share		$ 10.55		$ 10.55