Net Loss Per Common Share (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended		1 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jul. 31, 2013 4.50% Convertible Senior Notes	Sep. 30, 2013 4.50% Convertible Senior Notes	Sep. 30, 2013 Options to purchase common stock	Sep. 30, 2012 Options to purchase common stock	Sep. 30, 2013 Common stock warrants	Sep. 30, 2012 Common stock warrants	Sep. 30, 2013 Conversion premium on the Notes
Numerator - basic:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (39,631,000)	$ (23,199,000)	$ (97,866,000)	$ (66,509,000)
Plus: Unaccreted dividends on convertible preferred stock				(2,107,000)
Net loss available to common stockholders - basic	(39,631,000)	(23,199,000)	(97,866,000)	(68,616,000)
Numerator - diluted:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(39,631,000)	(23,199,000)	(97,866,000)	(66,509,000)
Plus: Unaccreted dividends on convertible preferred stock				(2,107,000)
Net loss available to common stockholders - diluted	(39,631,000)	(23,199,000)	(97,866,000)	(68,616,000)
Denominator:
Weighted-average common shares-basic and diluted	101,155	93,724	97,754	65,487
Net loss per share available to common stockholders-basic and diluted (in dollar per share)	$ (0.39)	$ (0.25)	$ (1.00)	$ (1.05)
Potentially dilutive securities excluded from computation of diluted weighted average shares
Aggregate principal amount					$ 125,000,000	$ 125,000,000
Interest rate (as a percent)					4.50%	4.50%
Potentially dilutive securities (in shares)							20,355	19,812	2,777	2,891	25,000