Acquisitions - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended	11 Months Ended	0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 07, 2014	Dec. 31, 2014	Jan. 30, 2014
Business Acquisition [Line Items]
Residual value of intangible assets	$ 0				$ 0
Business acquisition purchase price, cash paid	12,083,000	0	0
Compensation expense, service period	2 years 10 months 24 days
Recognized compensation expense	37,273,000	22,494,000	5,968,000
Maximum [Member]
Business Acquisition [Line Items]
Estimated useful life	10 years
Skyfence Networks Ltd. [Member]
Business Acquisition [Line Items]
Date of acquisition	Feb. 07, 2014
Business acquisition purchase price, cash paid				8,558,000
Holdback payment commitment period				24 months
Holdback liability				7,157,000
Business acquisition purchase price, shares issued				884,422
Compensation expense, service period				4 years
Closing price of stock				$ 59.08
Unvested equity awards to be recorded as operating expense				1,100,000
Vested equity awards				354,000
Related acquisition costs				900,000
Recognized compensation expense					7,300,000
Total purchase price				36,924,000
Business acquisition purchase price, common stock value				20,855,000
Skyfence Networks Ltd. [Member] | Subject to a Holdback [Member]
Business Acquisition [Line Items]
Business acquisition purchase price, shares issued				29,871
Skyfence Networks Ltd. [Member] | Restricted Shares of Common Stock Subject to Repurchase [Member]
Business Acquisition [Line Items]
Business acquisition purchase price, shares issued				532,262
Skyfence Networks Ltd. [Member] | Skyfence Options [Member]
Business Acquisition [Line Items]
Options converted to purchase shares				164,000
Skyfence Networks Ltd. [Member] | Converted Imperva Options [Member]
Business Acquisition [Line Items]
Options converted to purchase shares				24,248
Skyfence Networks Ltd. [Member] | Maximum [Member]
Business Acquisition [Line Items]
Common stock subject to holdback, period				24 months
Skyfence Networks Ltd. [Member] | Purchased Technology [Member]
Business Acquisition [Line Items]
Estimated useful life				7 years
Tomium Software, LLC. [Member]
Business Acquisition [Line Items]
Date of acquisition	Jan. 30, 2014
Business acquisition purchase price, cash paid						4,600,000
Business acquisition purchase price, shares issued						60,556
Closing price of stock						$ 55.45
Related acquisition costs						300,000
Estimated useful life						10 years
Total purchase price						8,263,000
Business acquisition purchase price, assumption of liability to pay cash						300,000
Business acquisition purchase price, common stock value						$ 3,400,000