Net Loss Per Common Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator - basic:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (28,153)	$ (23,284)
Plus: Unaccreted dividends on convertible preferred stock		(2,040)
Net loss available to common stockholders - basic	(28,153)	(25,324)
Numerator - diluted:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(28,153)	(23,284)
Plus: Unaccreted dividends on convertible preferred stock		(2,040)
Net loss available to common stockholders - diluted	$ (28,153)	$ (25,324)
Denominator:
Weighted-average common shares-basic and diluted	95,879	11,846
Net loss per share available to common stock holders-basic and diluted (in dollar per share)	$ (0.29)	$ (2.14)
Convertible preferred stock
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)		66,256
Options to purchase common stock
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)	20,664	17,515
Convertible preferred stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)		302
Common stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)	2,779	2,631