Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Data (Unaudited)
Collaboration revenues	$ 14,191	$ 11,323	$ 12,063	$ 11,344	$ 12,577	$ 8,582	$ 6,595	$ 6,461	$ 48,921	$ 34,215	$ 20,305
Total operating expenses	38,719	35,197	32,368	35,379	30,744	27,219	36,019	21,102	141,663	115,084	69,481
Net loss	(24,893)	(23,320)	(20,139)	(23,402)	(18,221)	(18,724)	(29,196)	(13,535)	(91,754)	(79,676)	(50,159)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (24,768)	$ (23,199)	$ (20,026)	$ (23,284)	$ (18,116)	$ (18,599)	$ (29,051)	$ (13,457)	$ (91,277)	$ (79,223)	$ (50,104)
Net loss per share available to common stockholders-basic and diluted (in dollars per share)	$ (0.26)	$ (0.25)	$ (0.22)	$ (2.14)	$ (1.76)	$ (1.81)	$ (2.76)	$ (1.34)	$ (1.28)	$ (7.67)	$ (5.57)