Selected Quarterly Financial Data - Schedule of Quarterly Financial Information (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
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenues	$ 7,823	$ 6,856	$ 18,452	$ 14,655	$ 14,191	$ 11,323	$ 12,063	$ 11,344	$ 47,786	$ 48,921	$ 34,215
Total operating expenses	36,065	42,780	47,560	41,921	38,719	35,197	32,368	35,379	168,326	141,663	115,084
Net loss	(32,348)	(39,763)	(30,251)	(28,323)	(24,893)	(23,320)	(20,139)	(23,402)	(130,685)	(91,754)	(79,676)
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)
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)