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Income Taxes - Additional Information (Detail) (USD $)
 1 Months Ended 12 Months Ended
Nov. 30, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income Tax Expenses [Line Items]          
Valuation allowance   $ 7,800,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount $ 8,400,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount $ 1,800,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount  
U.S. federal, net operating loss carryforwards   101,500,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic      
State net operating loss carryforwards   69,400,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal      
Excess tax benefits from stock options included in deferred tax assets for federal net operating losses   34,900,000impv_ExcessTaxBenefitsFromStockOptionsIncludedInFederalNetOperatingLosses      
Excess tax benefits from stock options included in deferred tax assets for state net operating losses   20,700,000impv_ExcessTaxBenefitsFromStockOptionsIncludedInStateNetOperatingLosses      
Accumulated unremitted earnings   12,900,000us-gaap_UndistributedEarningsOfForeignSubsidiaries 9,800,000us-gaap_UndistributedEarningsOfForeignSubsidiaries 7,200,000us-gaap_UndistributedEarningsOfForeignSubsidiaries  
Unrecognized tax benefits   672,000us-gaap_UnrecognizedTaxBenefits 353,000us-gaap_UnrecognizedTaxBenefits 239,000us-gaap_UnrecognizedTaxBenefits 267,000us-gaap_UnrecognizedTaxBenefits
Accrued interest on income tax expense   43,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 22,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued    
Payment to the Israeli Tax Authority 200,000impv_IncomeTaxPaidToTaxingAuthorities        
Israeli Tax Authority [Member]          
Income Tax Expenses [Line Items]          
Corporate tax rate   26.50%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ us-gaap_IncomeTaxAuthorityNameAxis
= us-gaap_ForeignCountryMember		      
Percentage of exempt profits   10.00%us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome
/ us-gaap_IncomeTaxAuthorityNameAxis
= us-gaap_ForeignCountryMember		      
Israeli Tax Authority [Member] | Previous [Member]          
Income Tax Expenses [Line Items]          
Corporate tax rate   25.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ us-gaap_IncomeTaxAuthorityNameAxis
= us-gaap_ForeignCountryMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioPreviouslyReportedMember		      
Israeli Tax Authority [Member] | Non R&D Activities [Member]          
Income Tax Expenses [Line Items]          
Corporate tax rate   26.50%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ us-gaap_IncomeStatementLocationAxis
= impv_NonResearchAndDevelopmentActivitiesMember
/ us-gaap_IncomeTaxAuthorityNameAxis
= us-gaap_ForeignCountryMember		      
Israeli Tax Authority [Member]          
Income Tax Expenses [Line Items]          
Tax years   2006 through 2010      
Period for tax exemption   2 years      
Limitation of tax benefits   12 years      
Tax exempted income       2,400,000us-gaap_IncomeTaxReconciliationTaxExemptIncome
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember		  
2011 [Member] | Israeli Tax Authority [Member]          
Income Tax Expenses [Line Items]          
Tax rate applicable for R&D activities   15.00%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment
/ us-gaap_IncomeTaxAuthorityAxis
= impv_TwoThousandElevenMember
/ us-gaap_IncomeTaxAuthorityNameAxis
= us-gaap_ForeignCountryMember		      
2012 [Member] | Israeli Tax Authority [Member]          
Income Tax Expenses [Line Items]          
Tax rate applicable for R&D activities   15.00%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment
/ us-gaap_IncomeTaxAuthorityAxis
= impv_TwoThousandTwelveMember
/ us-gaap_IncomeTaxAuthorityNameAxis
= us-gaap_ForeignCountryMember		      
2013 [Member] | Israeli Tax Authority [Member]          
Income Tax Expenses [Line Items]          
Tax rate applicable for R&D activities   12.50%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment
/ us-gaap_IncomeTaxAuthorityAxis
= impv_TwoThousandThirteenMember
/ us-gaap_IncomeTaxAuthorityNameAxis
= us-gaap_ForeignCountryMember		      
2014 [Member]          
Income Tax Expenses [Line Items]          
Tax rate applicable for R&D activities   12.50%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment
/ us-gaap_IncomeTaxAuthorityAxis
= impv_TwoThousandFourteenMember		      
2014 [Member] | Israeli Tax Authority [Member]          
Income Tax Expenses [Line Items]          
Tax rate applicable for R&D activities   16.00%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment
/ us-gaap_IncomeTaxAuthorityAxis
= impv_TwoThousandFourteenMember
/ us-gaap_IncomeTaxAuthorityNameAxis
= us-gaap_ForeignCountryMember		      
2015 and Thereafter [Member]          
Income Tax Expenses [Line Items]          
Tax rate applicable for R&D activities   12.00%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseResearchAndDevelopment
/ us-gaap_IncomeTaxAuthorityAxis
= impv_TwoThousandFifteenAndThereafterMember		      
Maximum [Member] | State [Member]          
Income Tax Expenses [Line Items]          
Expiration of net operating loss carryforwards   2034      
Maximum [Member] | Federal [Member]          
Income Tax Expenses [Line Items]          
Expiration of net operating loss carryforwards   2034      
Maximum [Member] | Israeli Tax Authority [Member]          
Income Tax Expenses [Line Items]          
Corporate tax rate   25.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Additional period for tax exemption   8 years      
Income tax liabilities       $ 600,000us-gaap_DeferredTaxLiabilitiesTaxDeferredIncome
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		  
Minimum [Member] | State [Member]          
Income Tax Expenses [Line Items]          
Expiration of net operating loss carryforwards   2018      
Minimum [Member] | Federal [Member]          
Income Tax Expenses [Line Items]          
Expiration of net operating loss carryforwards   2023      
Minimum [Member] | Israeli Tax Authority [Member]          
Income Tax Expenses [Line Items]          
Corporate tax rate   10.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_ForeignCountryMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		      
Additional period for tax exemption   5 years