Net income (loss) per share attributable to common stockholders (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator:
Net income (loss) attributable to common stockholders, basic and diluted	$ 354	$ 2,777	$ (1,166)	$ 5,895
Denominator:
Weighted average common shares, basic	35,593,000	35,080,000	35,620,000	34,618,000
Dilutive effect of stock options (in shares)	1,347,000	2,257,000		2,706,000
Dilutive effect of RSUs (in shares)	189,000
Weighted average common shares, dilutive	37,129,000	37,337,000	35,620,000	37,324,000
Net income (loss) per share attributable to common stockholders:
Basic (in dollars per share)	$ 0.01	$ 0.08	$ (0.03)	$ 0.17
Diluted (in dollars per share)	$ 0.01	$ 0.07	$ (0.03)	$ 0.16
Anti-dilutive securities excluded from the computation of diluted net income per share	3,086,287	2,478,156		2,442,898