Net Loss Per Common Share (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
Numerator - basic:
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)
Plus: Unaccreted dividends on convertible preferred stock									(2,107)	(7,789)	(11,185)
Net loss available to common stockholders - basic									(93,384)	(87,012)	(61,289)
Numerator - diluted:
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)
Plus: Unaccreted dividends on convertible preferred stock									(2,107)	(7,789)	(11,185)
Net loss available to common stockholders - diluted									$ (93,384)	$ (87,012)	$ (61,289)
Denominator:
Weighted-average common shares-basic and diluted									72,831	11,343	10,994
Net loss per share available to common stock holders-basic and diluted (in dollar per share)	$ (0.26)	$ (0.25)	$ (0.22)	$ (2.14)	$ (1.76)	$ (1.81)	$ (2.76)	$ (1.34)	$ (1.28)	$ (7.67)	$ (5.57)
Convertible preferred stock
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)										66,256	55,253
Options to purchase common stock
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)									18,066	17,617	16,214
Convertible preferred stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)										302	306
Common stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)									2,842	2,640	2,937