Net 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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator:
Net income (loss) attributable to common stockholders, basic (in dollars)	$ 1,400	$ 2,777	$ 1,461	$ 1,657	$ 1,855	$ 1,662	$ 1,293	$ (148)	$ 7,295	$ 4,662	$ 10,554
Accretion of convertible preferred stock (in dollars)											5,020
Net income attributable to common stockholders, diluted									$ 7,295	$ 4,662	$ 15,574
Denominator:
Weighted average common shares, basic									34,774	24,014	5,834
Effect of dilutive convertible preferred stock (in shares)											22,846
Effect of dilutive stock options (in shares)									2,543	3,457	3,204
Effect of dilutive common stock warrants (in shares)										10	15
Weighted average common shares, dilutive									37,317	27,481	31,899
Net income per share attributable to common stockholders:
Basic (in dollars per share)	$ 0.04	$ 0.08	$ 0.04	$ 0.05	$ 0.06	$ 0.05	$ 0.06	$ (0.03)	$ 0.21	$ 0.19	$ 1.81
Diluted (in dollars per share)	$ 0.04	$ 0.07	$ 0.04	$ 0.05	$ 0.05	$ 0.05	$ 0.05	$ (0.03)	$ 0.20	$ 0.17	$ 0.49
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted net income per share									2,518	9,897	26
Convertible preferred stocks
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted net income per share										7,699
Options to purchase common stock
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted net income per share									2,518	2,172
Warrants to purchase preferred stock
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted net income per share										26	26