Net Loss per Share - Computation of Company's Basic and Diluted Net Loss Per Share of Common Stock (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net loss attributable to Imperva, Inc. stockholders	$ (58,963)	$ (25,178)	$ (7,387)
Shares used in computing net loss per share of common stock, basic and diluted	25,805,632	24,300,198	22,916,426
Net loss per share of common stock, basic and diluted	$ (2.28)	$ (1.04)	$ (0.32)