Net Loss per Share - Computation of Company's Basic and Diluted Net Loss Per Share of Common Stock (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net loss attributable to Imperva, Inc. stockholders	$ (70,279)	$ (48,882)	$ (58,963)
Shares used in computing net loss per share of common stock, basic and diluted	32,284	29,849	25,806
Net loss per share of common stock, basic and diluted	$ (2.18)	$ (1.64)	$ (2.28)