Net Income Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income	$ 5,424	$ 4,123	$ 3,316	$ 3,002	$ 25,913	$ 3,105	$ 1,657	$ (431)	$ 15,865	$ 30,244	$ 1,541
Net income attributable to participating securities									0	0	(2)
Net income attributable to common stockholders - basic									15,865	30,244	1,539
Net income attributable to common stockholders - diluted									$ 15,865	$ 30,244	$ 1,539
Denominator:
Weighted-average shares used in computing net income per share attributable to common stockholders - basic									34,050	32,979	31,914
Effect of potentially dilutive securities:
Common stock options (in shares)									4,134	4,191	4,059
Weighted - average shares used in computing net income (loss) per share attributable to common stockholders - diluted (in shares)									38,184	37,170	35,973
Basic (usd per share)	$ 0.16	$ 0.12	$ 0.10	$ 0.09	$ 0.77	$ 0.09	$ 0.05	$ (0.01)	$ 0.47	$ 0.92	$ 0.05
Diluted (usd per share)	$ 0.14	$ 0.11	$ 0.09	$ 0.08	$ 0.69	$ 0.08	$ 0.04	$ (0.01)	$ 0.42	$ 0.81	$ 0.04
Stock Options
Antidilutive Securities
Common stock options (in shares)									1,582	1,689	952