Net Loss Per Common Share (Tables)

12 Months Ended

Dec. 31, 2013

Earnings Per Share [Abstract]

Schedule of Computation of Basic and Diluted Net Loss Per Share Available to Common Stockholders

The following table presents the computation of basic and diluted net loss per share available to common stockholders:

	Years ended December 31,
(in thousands, except per share amount)	2013			2012			2011
Net Loss Per Share:
Numerator:
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(130,925	)	$	(91,277	)	$	(79,223	)
Plus: Unaccreted dividends on convertible preferred stock		—			(2,107	)		(7,789	)
Net loss available to common stockholders—basic and diluted		(130,925	)		(93,384	)		(87,012	)
Denominator:
Weighted-average common shares—basic and diluted		98,919			72,831			11,343
Net loss per share available to common stockholders—basic and diluted	$	(1.32	)	$	(1.28	)	$	(7.67	)

Schedule of Potentially Dilutive Securities Excluded from Computation of Diluted Weighted Average Shares

The potential dilutive effect of these securities is shown in the chart below:

	Years ended December 31,
(in thousands)	2013		2012		2011
Common stock warrants		2,777		2,842		2,640
Convertible preferred stock		—		—		66,256
Options to purchase common stock		20,107		18,066		17,617
Convertible preferred stock warrants		—		—		302
Conversion of the Notes		25,000		—		—