Net Loss Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator - basic:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$ (30,082)	$ (20,026)	$ (58,235)	$ (43,310)
Plus: Unaccreted dividends on convertible preferred stock		(67)		(2,107)
Net loss available to common stockholders - basic	(30,082)	(20,093)	(58,235)	(45,417)
Numerator - diluted:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(30,082)	(20,026)	(58,235)	(43,310)
Plus: Unaccreted dividends on convertible preferred stock		(67)		(2,107)
Net loss available to common stockholders - diluted	$ (30,082)	$ (20,093)	$ (58,235)	$ (45,417)
Denominator:
Weighted-average common shares-basic and diluted	96,170,000	90,581,000	96,025,000	51,214,000
Net loss per share available to common stockholders-basic and diluted (in dollar per share)	$ (0.31)	$ (0.22)	$ (0.61)	$ (0.89)
Options to purchase common stock
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)			20,447,000	17,455,000
Common stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)			2,777,000	2,905,000
Convertible preferred stock
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)				66,256,000
Convertible preferred stock warrants
Potentially dilutive securities excluded from computation of diluted weighted average shares
Potentially dilutive securities (in shares)				302,000