Net Loss Per Common Share - Schedule of Computation of Basic and Diluted Net Loss Per Share Available to Common Stockholders (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (33,059)	$ (39,631)	$ (30,082)	$ (28,153)	$ (24,768)	$ (23,199)	$ (20,026)	$ (23,284)	$ (130,925)	$ (91,277)	$ (79,223)
Plus: Unaccreted dividends on convertible preferred stock										(2,107)	(7,789)
Net loss available to common stockholders-basic and diluted									$ (130,925)	$ (93,384)	$ (87,012)
Denominator:
Weighted-average common shares-basic and diluted									98,919	72,831	11,343
Net loss per share available to common stockholders-basic and diluted	$ (0.33)	$ (0.39)	$ (0.31)	$ (0.29)	$ (0.26)	$ (0.25)	$ (0.22)	$ (2.14)	$ (1.32)	$ (1.28)	$ (7.67)