Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2012

Summary of Significant Accounting Policies

Schedule of non-controlling interest in subsidiary

(in thousands)	Non-Controlling Interest
Balance at December 31, 2010	$	1,027
Net loss attributable to Silver Creek	(348		)
Balance at September 30, 2011	$	679

	Non-Controlling Interest
Balance at December 31, 2011	$	574
Net loss attributable to Silver Creek	(352		)
Balance at September 30, 2012	$	222

Schedule of available-for-sale securities

	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
	(in thousands)
September 30, 2012:
U.S. government agencies securities	$	1,000	$	—	$	—		$	1,000
Corporate debt securities	57,749		12		(2		)	57,759
Total	$	58,749	$	12	$	(2	)	$	58,759

Schedule of marketable securities in an unrealized loss position

	Aggregate Fair Value	Unrealized Losses
	(in thousands)
September 30, 2012:
Corporate debt securities	21,979	(2	)

Schedule of comprehensive loss from operations

	Three months ended September 30,						Nine months ended September 30,
(in thousands)	2011			2012			2011			2012
Net loss attributable to Merrimack Pharmaceuticals, Inc.	$	(18,599	)	$	(23,199	)	$	(61,107	)	$	(66,509	)
Unrealized gain on available-for-sale securities	—			59			—			10
Comprehensive loss	$	(18,599	)	$	(23,140	)	$	(61,107	)	$	(66,499	)