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Stock-Based Compensation - Additional Information (Detail) (USD $)
3 Months Ended 12 Months Ended 3 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Dec. 31, 2013
Oct. 01, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Number of Options Outstanding, Granted 984,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 1,039,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross    
Number of Options Outstanding 4,794,340us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber 3,850,840us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber   3,419,285us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Stock options outstanding expired 38,000us-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. The RSUs issued during the three months ended December 31, 2014 vest in equal annual installments over three years from the date of grant.      
Employee Stock Option        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Stock-based compensation expense 981,399us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
  $ 521,138us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Fair value of the options granted 3,596,618arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
  531,510arwr_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Intrinsic value of options exercised 23,774us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
  314,858us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
 
Unrecognized pre-tax compensation expense 13,093,446us-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 30,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
510,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
   
Stock-based compensation expense 1,033,457us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
  0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Weighted average period to recognize pre-tax compensation expense 1 year 4 months 24 days      
Unrecognized pre-tax compensation expense 4,036,658us-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 2,657,352us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsFourEquityIncentivePlanMember
     
Number of Options Outstanding, Granted 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ 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 2,561,018us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ 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 4,000,000us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
     
Number of Options Outstanding, Granted 864,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ 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 1,686,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ 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 470,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
  30,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= arwr_TwoThousandsThirteenIncentivePlanMember
 
Number of restricted stock units outstanding 470,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ 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 617,322us-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
     
Outside Of Equity Compensation Plans | Restricted Stock Units (RSUs)        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Units granted outside of Equity Incentive plans 30,000arwr_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