Summary of Significant Accounting Policies (Details 3) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended			1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Apr. 30, 2012 Initial public offering
Public Offering
Convertible preferred stock converted into common stock (in shares)						66,255,529
Convertible Preferred Stock and Convertible Preferred Stock Warrants
Change in fair value of convertible preferred stock warrants		$ 166	$ 587	$ (742)
Preferred stock
Authorized undesignated preferred stock (in shares)	10,000,000		10,000,000		0
Preferred stock, par value (in dollars per share)	$ 0.01		$ 0.01		$ 0.01
Comprehensive Income (Loss)
Net loss attributable to Merrimack Pharmaceuticals, Inc.	(23,199)	(18,599)	(66,509)	(61,107)
Unrealized gain on available-for-sale securities	59		10
Comprehensive loss	$ (23,140)	$ (18,599)	$ (66,499)	$ (61,107)