Net loss per share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		24 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013
Net loss per share
Net loss applicable to common stockholders (in dollars)	$ (5,157)	$ (2,966)	$ (21,392)
Weighted average number of common shares used in net loss per share applicable to common stockholders-basic and diluted (in shares)	13,345	1,383	4,943
Net loss per share applicable to common stockholders-basic and diluted (in dollars per share)	$ (0.39)	$ (2.14)	$ (4.33)
Series A Preferred Stock
Net loss per share
Amount excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect (in shares)		3,065
Outstanding stock options and restricted stock units
Net loss per share
Amount excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect (in shares)	1,808	661	1,808
Founders' stock
Net loss per share
Amount excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect (in shares)	1,481	2,139	1,481