Net Loss Per Common Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator - basic:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (23,199)	$ (18,599)	$ (66,509)	$ (61,107)
Plus: Unaccreted dividends on convertible preferred stock		(2,062)	(2,107)	(5,728)
Net loss available to common stockholders - basic	(213,199)	(20,661)	(68,616)	(66,835)
Numerator - diluted:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(23,199)	(18,599)	(66,509)	(61,107)
Plus: Unaccreted dividends on convertible preferred stock		(2,062)	(2,107)	(5,728)
Net loss available to common stockholders - diluted	$ (213,199)	$ (20,661)	$ (68,616)	$ (66,835)
Denominator:
Weighted-average common shares-basic and diluted	93,724	11,389	65,487	11,292
Net loss per share available to common stock holders-basic and diluted	$ (0.25)	$ (1.81)	$ (1.05)	$ (5.92)
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)			19,812	17,522
Convertible preferred stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)				303
Common stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)			2,891	2,937