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Organization and Significant Accounting Policies - Additional Information (Detail) (USD $)
 6 Months Ended 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Sep. 30, 2013
Schedule Of Investments [Line Items]            
Cash to fund operations $ 96,447,301us-gaap_CashAndCashEquivalentsAtCarryingValue $ 142,847,860us-gaap_CashAndCashEquivalentsAtCarryingValue $ 96,447,301us-gaap_CashAndCashEquivalentsAtCarryingValue   $ 132,510,610us-gaap_CashAndCashEquivalentsAtCarryingValue $ 19,114,444us-gaap_CashAndCashEquivalentsAtCarryingValue
Investments 31,922,240us-gaap_HeldToMaturitySecurities   31,922,240us-gaap_HeldToMaturitySecurities   44,741,378us-gaap_HeldToMaturitySecurities  
Net increase (decrease) in cash (36,063,309)us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease 123,733,416us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease        
Cash outflow related to continuing operating activities (40,569,584)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations (14,720,054)us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations        
Cash payment for acquisition 7,000,000us-gaap_PaymentsToAcquireAssetsInvestingActivities          
Capital expenditures 852,063us-gaap_PaymentsToAcquirePropertyPlantAndEquipment 607,772us-gaap_PaymentsToAcquirePropertyPlantAndEquipment        
Proceeds from sale of marketable securities 12,150,774us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities 5,010,238us-gaap_ProceedsFromSaleAndMaturityOfMarketableSecurities        
Proceeds from the exercise of warrants and stock options 313,618arwr_ProceedsFromExerciseOfWarrantsAndStockOptions 7,979,309arwr_ProceedsFromExerciseOfWarrantsAndStockOptions        
Restricted Cash 0us-gaap_RestrictedCashAndCashEquivalents   0us-gaap_RestrictedCashAndCashEquivalents   0us-gaap_RestrictedCashAndCashEquivalents  
Amount insured in FDIC per account 250,000us-gaap_CashFDICInsuredAmount   250,000us-gaap_CashFDICInsuredAmount      
Minimum            
Schedule Of Investments [Line Items]            
Property, Plant and Equipment, Useful Life 3 years          
Maximum            
Schedule Of Investments [Line Items]            
Property, Plant and Equipment, Useful Life 7 years          
Commercial bonds            
Schedule Of Investments [Line Items]            
Maturity description less than 24 months          
Investments 31,922,240us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_BondsMember		   31,922,240us-gaap_HeldToMaturitySecurities
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_BondsMember		      
Novartis            
Schedule Of Investments [Line Items]            
Cash payment for acquisition     $ 10,000,000us-gaap_PaymentsToAcquireAssetsInvestingActivities
/ arwr_AssetAcquisitionAxis
= arwr_NovartisMember		 $ 7,000,000us-gaap_PaymentsToAcquireAssetsInvestingActivities
/ arwr_AssetAcquisitionAxis
= arwr_NovartisMember