Net Income Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								9 Months Ended		12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income attributable to common stockholders - basic and diluted	$ 14,400	$ 23,469	$ 10,293	$ 9,142	$ 2,857	$ 8,452	$ 7,202	$ 21,930	$ 57,304	$ 40,440	$ 57,304	$ 40,440	$ 19,224	$ 19,224
Denominator:
Weighted-average shares used in computing net income per share - basic											38,876	37,443	35,247	35,247
Effect of potentially dilutive securities:
Common stock options (in shares)									2,401	2,262				3,052
RSUs (in shares)									620	366				70
Weighted - average shares used in computing net income (loss) per share attributable to common stockholders - diluted (in shares)									41,897	40,071	41,897	40,071	38,369	38,369
Basic (usd per share)	$ 0.37	$ 0.60	$ 0.26	$ 0.24	$ 0.07	$ 0.22	$ 0.19	$ 0.60		$ 1.08	$ 1.47	$ 1.08	$ 0.55	$ 0.55
Diluted (usd per share)	$ 0.35	$ 0.56	$ 0.24	$ 0.22	$ 0.07	$ 0.21	$ 0.18	$ 0.56		$ 1.01	$ 1.37	$ 1.01	$ 0.50	$ 0.50
Stock Options
Antidilutive Securities
Common stock options (in shares)									177	742				3,241
RSUs
Antidilutive Securities
Common stock options (in shares)									22	71				24