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Stock-Based Compensation - Additional Information (Detail) (USD $)
6 Months Ended 12 Months Ended 3 Months Ended 6 Months Ended
Mar. 31, 2015
Sep. 30, 2014
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2014
Sep. 30, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of Options Outstanding 5,399,340us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber 3,850,840us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber 5,399,340us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber     3,419,285us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Number of Options Outstanding, Granted 1,609,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 1,039,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross        
Stock options outstanding expired 43,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod 152,582us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod        
Share-based compensation, description At vesting each RSU will be exchanged for one share of the Company’s Common Stock. Restricted stock unit awards generally vest subject to the satisfaction of service requirements or the satisfaction of both service requirements and achievement of certain performance targets.          
Employee Stock Option            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Stock-based compensation expense 2,180,290us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
  $ 1,198,891us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 627,737us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
$ 1,148,875us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Fair value of the options granted 5,703,692arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
  2,114,074arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
7,085,676arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
7,617,186arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Intrinsic value of options exercised 113,728us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
  89,954us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
2,903,309us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
3,218,167us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Unrecognized pre-tax compensation expense 13,982,343us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
  13,982,343us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Weighted average period to recognize pre-tax compensation expense 3 years 1 month 6 days          
Dividend yield 0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
         
Restricted Stock Units (RSUs)            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of restricted stock units outstanding, granted 705,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
510,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
675,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
470,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
470,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Stock-based compensation expense 2,039,645us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
  1,006,188us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
570,707us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
570,707us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Weighted average period to recognize pre-tax compensation expense 1 year 9 months 18 days          
Unrecognized pre-tax compensation expense 6,147,120us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
  $ 6,147,120us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
     
2004 Equity Incentive Plan            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Shares reserve for issuance 2,546,018us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsFourEquityIncentivePlanMember
  2,546,018us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsFourEquityIncentivePlanMember
     
2004 Equity Incentive Plan | Employee Stock Option            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of Options Outstanding 2,546,018us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsFourEquityIncentivePlanMember
  2,546,018us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsFourEquityIncentivePlanMember
     
Number of Options Outstanding, Granted 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsFourEquityIncentivePlanMember
  0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsFourEquityIncentivePlanMember
     
2013 Incentive Plan            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Shares reserve for issuance 5,094,314us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
  5,094,314us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
     
2013 Incentive Plan | Employee Stock Option            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of Options Outstanding 2,306,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
  2,306,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
     
Number of Options Outstanding, Granted 1,489,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
  625,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
     
2013 Incentive Plan | Restricted Stock Units (RSUs)            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of restricted stock units outstanding, granted 1,080,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
  675,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
     
Number of restricted stock units outstanding 1,080,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
  1,080,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
     
Number of Options Outstanding, Granted 675,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
         
Outside Of Equity Compensation Plans            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Shares reserve for issuance 547,322us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= arwr_OutsideOfEquityCompensationPlansMember
  547,322us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= arwr_OutsideOfEquityCompensationPlansMember
     
Outside Of Equity Compensation Plans | Employee Stock Option            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Units granted outside of Equity Incentive plans 120,000arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardInducementGrants
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_OutsideOfEquityCompensationPlansMember
  0arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardInducementGrants
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
/ us-gaap_PlanNameAxis
= arwr_OutsideOfEquityCompensationPlansMember
     
Outside Of Equity Compensation Plans | Restricted Stock Units (RSUs)            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Number of restricted stock units outstanding, granted 70,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_OutsideOfEquityCompensationPlansMember
         
Units granted outside of Equity Incentive plans 30,000arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardInducementGrants
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_OutsideOfEquityCompensationPlansMember
  0arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardInducementGrants
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_OutsideOfEquityCompensationPlansMember
  0arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardInducementGrants
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_OutsideOfEquityCompensationPlansMember
 
2000 Stock Option Plan            
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Stock options outstanding expired 38,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsStockOptionPlanMember