Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Costs and expenses:
Research and development expenses	$ 21,605	$ 28,002
General and administrative expenses	5,634	6,452
Total costs and expenses	27,239	34,454
Loss from continuing operations	(27,239)	(34,454)
Other income and expenses:
Interest income	14	72
Interest expense	(1,979)	(3,290)
Other expense, net	(2)	(43)
Loss from continuing operations	(29,206)	(37,715)
Discontinued operations:
Loss from discontinued operations, net of tax	(947)	(943)
Net loss	(30,153)	(38,658)
Net loss attributable to non-controlling interest	(467)	(185)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(29,686)	(38,473)
Other comprehensive loss:
Unrealized loss on available-for-sale securities		(14)
Other comprehensive loss		(14)
Comprehensive loss	(29,686)	(38,487)
Amounts attributable to Merrimack Pharmaceuticals, Inc.:
Loss from continuing operations	(28,739)	(37,530)
Loss from discontinued operations	(947)	(943)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (29,686)	$ (38,473)
Basic and diluted net loss per share:
Loss from continuing operations	$ (0.22)	$ (0.32)
Loss from discontinued operations	(0.01)	(0.01)
Net loss per share	$ (0.23)	$ (0.33)
Weighted-average common shares used per share calculations—basic and diluted	130,588	116,064