Net income per share attributable to common stockholders (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator:
Net (loss) income attributable to common stockholders, basic and diluted	$ (1,121)	$ 1,657
Denominator:
Weighted average common shares, basic	35,597,000	33,969,000
Dilutive effect of stock-based awards (in shares)		2,663,000
Weighted average common shares, dilutive	35,597,000	36,632,000
Net (loss) income per share attributable to common stockholders:
Basic (in dollars per share)	$ (0.03)	$ 0.05
Diluted (in dollars per share)	$ (0.03)	$ 0.05
Options to purchase common stock
Anti-dilutive securities
Anti-dilutive securities excluded from the computation of diluted net income per share		2,033,129