Net loss per share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended	12 Months Ended	21 Months Ended
	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2012
Net loss per share
Net loss applicable to common stockholders (in dollars)	$ (2,725)	$ (13,510)	$ (16,235)
Weighted average number of common shares used in net loss per share applicable to common stockholders-basic and diluted (in shares)	909	5,810	3,734
Net loss per share applicable to common stockholders-basic and diluted (in dollars per share)	$ (3.00)	$ (2.33)	$ (4.35)
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)	618	1,410	1,410
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)	2,319	1,645	1,645