Net Income (Loss) Per Share (Calculation) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income (loss)	$ 8,317	$ (2,739)	$ 4,542	$ (3,545)
Denominator:
Weighted-average shares of common stock outstanding used in the calculation of basic net income (loss) per share	35,544	34,428	35,372	34,219
Effect of dilutive securities:
Effect of dilutive securities (in shares)	1,172		988
Weighted-average shares of common stock outstanding used in the calculation of diluted net income (loss) per share (in shares)	36,716	34,428	36,360	34,219
Basic net income (loss) per share (in dollars per share)	$ 0.23	$ (0.08)	$ 0.13	$ (0.10)
Diluted net income (loss) per share (in dollars per share)	$ 0.23	$ (0.08)	$ 0.12	$ (0.10)
Stock Options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	840		660
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities (in shares)	308		309
Employee Stock Purchase Plan
Effect of dilutive securities:
Effect of dilutive securities (in shares)	24		19