Net (loss) income per share attributable to common stockholders (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator:
Net (loss) income attributable to common stockholders, basic and diluted													$ (3,793)	$ 7,295	$ 4,662
Denominator:
Weighted average common shares, basic													35,578	34,774	24,014
Effect of dilutive stock options (in shares)														2,543	3,457
Effect of dilutive common stock warrants (in shares)															10
Weighted average common stock, diluted													35,578	37,317	27,481
Net (loss) income per share attributable to common stockholders:
Basic (in dollars per share)	$ (0.07)	$ 0.01	$ (0.01)	$ (0.03)	$ 0.04	$ 0.08	$ 0.04	$ 0.05	$ 0.06	$ 0.05	$ 0.06	$ (0.03)	$ (0.11)	$ 0.21	$ 0.19
Diluted (in dollars per share)	$ (0.07)	$ 0.01	$ (0.01)	$ (0.03)	$ 0.04	$ 0.07	$ 0.04	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ (0.03)	$ (0.11)	$ 0.20	$ 0.17
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted net income per share														2,518	9,897
Convertible preferred stocks
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted net income per share															7,699
Stock options
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted net income per share														2,518	2,172
Preferred stock warrants
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted net income per share															26