Basis of Presentation and significant accounting policies (Details) In Thousands, unless otherwise specified	3 Months Ended		36 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014
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)	3,048	4,309	3,048
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)	2,225	2,664	2,225
Unvested 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)	823	1,645	823