Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Costs and expenses:
Research and development expenses	$ 67,314	$ 109,565	$ 121,033
General and administrative expenses	28,452	32,052	24,048
Restructuring expenses	9,521	5,710
Total costs and expenses	95,766	147,327	145,081
Loss from continuing operations	(95,766)	(147,327)	(145,081)
Other income and expenses:
Interest income	895	276	99
Interest expense	(34,650)	(22,449)	(18,769)
Gain on deconsolidation of Silver Creek Pharmaceuticals, Inc.	10,848
Gain on sale of asset	1,703
Other (expense) income, net	(1,433)	(8)	917
Net loss from continuing operations before income tax benefit	(118,403)	(169,508)	(162,834)
Income tax benefit	42,399	13,224	11,215
Net loss from continuing operations	(76,004)	(156,284)	(151,619)
Discontinued operations:
Income from discontinued operations, net of tax	546,872	2,766	3,832
Net income (loss)	470,868	(153,518)	(147,787)
Net (loss) income attributable to non-controlling interest	(1,160)	(1,778)	170
Net income (loss) attributable to Merrimack Pharmaceuticals, Inc.	472,028	(151,740)	(147,957)
Other comprehensive income:
Unrealized gain on available-for-sale securities			74
Other comprehensive income			74
Comprehensive income (loss)	472,028	(151,740)	(147,883)
Amounts attributable to Merrimack Pharmaceuticals, Inc.:
Net loss from continuing operations	(74,844)	(154,506)	(151,789)
Income from discontinued operations, net of tax	$ 546,872	$ 2,766	$ 3,832
Basic and dilutive net income (loss) per common share
Net loss from continuing operations	$ (5.66)	$ (12.33)	$ (13.63)
Net income from discontinued operations, net of tax	41.33	0.22	0.34
Net income (loss) per share	$ 35.67	$ (12.11)	$ (13.29)
Weighted-average common shares used in per share calculations—basic and diluted	13,232	12,533	11,136
Cash dividend paid per common share	$ 10.55