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Stock Plans - Additional Information (Detail)			1 Months Ended							3 Months Ended		9 Months Ended
Stock Plans - Additional Information (Detail)	Apr. 26, 2017 shares	Jan. 01, 2017 shares	Aug. 31, 2017 shares	May 31, 2016 shares	Oct. 31, 2015 shares	Aug. 31, 2014 shares	Nov. 30, 2013 shares	Jan. 31, 2012	Sep. 30, 2011 Period shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($) shares	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based restricted stock units granted												1,361,494
Number of stock option outstanding, granted										0	0	0
Intrinsic value of options exercised \| $												$ 5,800,000
Compensation cost related to unvested stock-based awards granted, but not yet recognized \| $										$ 95,500,000		$ 95,500,000
Nonvested awards, compensation cost not yet recognized, period for recognition												2 years 8 months 13 days
Capitalized stock-based compensation expense \| $										$ 0	$ 0	$ 0	$ 0
Restricted Stock Units for Common Stock [Member] \| Incapsula [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issuable, as-converted to common stock-basis							198,825
Restricted Stock Units And Outstanding Options [Member] \| Incapsula [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issuable, as-converted to common stock-basis												247,184
2003 Stock Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserve of common stock for future issuance										955,568		955,568
2003 Stock Plan [Member] \| Stock options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum percentage of fair value of common stock on grant date												85.00%
2003 Stock Plan [Member] \| Stock options [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period												10 years
2003 Stock Plan [Member] \| Stock options [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period												4 years
2003 Stock Plan [Member] \| Stock options [Member] \| Greater 10% Ownership [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum percentage of fair value of common stock on grant date												110.00%
Minimum percentage of combined voting power												10.00%
2011 Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserve of common stock for future issuance									1,000,000
Increase percentage of common stock outstanding												4.00%
Increase in reserve share	850,000			1,300,000
2011 Stock Option and Incentive Plan [Member] \| Stock options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum percentage of fair value of common stock on grant date												100.00%
Expiration period												10 years
2011 Stock Option and Incentive Plan [Member] \| Restricted Stock Units for Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period												4 years
Description of vesting terms												The Company also grants RSUs. RSUs granted to new hires and merit grants to existing employees generally vest over a four-year period with 25% vesting at the end of one year and the remainder vesting quarterly thereafter.
2011 Stock Option and Incentive Plan [Member] \| Restricted Stock Units for Common Stock [Member] \| Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage at the end of period												25.00%
2011 Stock Option and Incentive Plan [Member] \| Performance-based RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period												3 years
Vesting performance period												1 year
Vesting service period												2 years
Description of vesting terms												The PRSUs provide for a target number of shares that generally vest over three years including a one year performance period and two years of service-based vesting. The PRSUs vest contingent upon the Company’s achievement of an annual contract value of subscription bookings target for fiscal 2017 and total operating expenses for 2017 that are at 110% or less of the internal operating plan.
Vest contingent upon the company’s achievement of total operating expenses at the internal operating plan, percentage												110.00%
Award expiration date												Feb. 15, 2019
2011 Stock Option and Incentive Plan [Member] \| Performance-based RSUs [Member] \| Executives [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based restricted stock units granted												46,619
2011 Stock Option and Incentive Plan [Member] \| Performance-based RSUs [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Earned percentage on target award amount, depending on level of achievement												200.00%
2011 Stock Option and Incentive Plan [Member] \| Performance-based RSUs [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Earned percentage on target award amount, depending on level of achievement												0.00%
2011 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserve of common stock for future issuance									500,000
Increase percentage of common stock outstanding								1.00%
Increase in reserve share		330,889
Program effective date									Nov. 08, 2011
Percentage in payroll of deduction base salary to acquire shares of common stock									15.00%
Offering period									24 months
Percentage of fair value of common stock on offering period									85.00%
Number of years, shares reserved for issuance increase								8 years
2011 Employee Stock Purchase Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserve of common stock for future issuance									20,000,000
Number of purchase periods for offering \| Period									5
Inducement Stock Option Plan And Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Description of vesting terms												In accordance with the terms of the Inducement Option Plans, the Company issued options to purchase an aggregate of 290,000 shares of the Company’s common stock at an exercise price equal to the fair market value of a share of the Company’s common stock on the dates of grant of the options. The options, which have a ten-year term, vest at the rate of 25% of the shares on each of the first anniversary of the vesting commencement date with an additional 6.25% of the shares subject to the option vesting each quarter thereafter so long as the participant has not been terminated.
Number of stock option outstanding, granted						290,000
Inducement Stock Option Plan And Agreement [Member] \| Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage at the end of period						25.00%
Inducement Stock Option Plan And Agreement [Member] \| Quarterly Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage at the end of period						6.25%
Inducement Restricted Stock Unit Plan and Agreement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Description of vesting terms												In accordance with the terms of the Inducement RSU Plans, the Company issued RSUs representing an aggregate of 290,000 shares of the Company’s common stock. The RSUs, which expire following settlement, vest at the rate of 25% of the shares on each of the first anniversary of the vesting commencement date with an additional 6.25% of the shares subject to the RSU vesting each quarter thereafter so long as the participant has not been terminated.
Shares available for grant						290,000
Inducement Restricted Stock Unit Plan and Agreement [Member] \| Share-based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage at the end of period						25.00%
Inducement Restricted Stock Unit Plan and Agreement [Member] \| Quarterly Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage at the end of period						6.25%
2015 Equity Inducement Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserve of common stock for future issuance					100,000
2015 Equity Inducement Plan [Member] \| Stock options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum percentage of fair value of common stock on grant date					100.00%
Expiration period					10 years
Increase in reserve share			100,000
Incapsula 2010 Share Incentive Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserve of common stock for future issuance							10,263,211
Incapsula 2010 Share Incentive Plan [Member] \| Restricted Stock Units for Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance-based restricted stock units granted							7,095,461