Document and Entity Information - shares
9 Months Ended
Jun. 30, 2019
Aug. 02, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Trading Symbol ARWR  
Entity Central Index Key 0000879407  
Entity Current Reporting Status Yes  
Current Fiscal Year End Date --09-30  
Entity Filer Category Accelerated Filer  
Entity Common Stock, Shares Outstanding   95,300,992
Entity Shell Company false  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity File Number 001-38042  
Entity Tax Identification Number 460408024  
Entity Address, Address Line One 225 S. Lake Avenue  
Entity Address, Address Line Two Suite 1050  
Entity Address, City or Town Pasadena  
Entity Address, State or Province California  
Entity Address, Postal Zip Code 91101  
City Area Code 626  
Local Phone Number 304-3400