Document and Entity Information - shares
3 Months Ended
Dec. 31, 2019
Feb. 03, 2020
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Dec. 31, 2019  
Document Fiscal Year Focus 2020  
Document Fiscal Period Focus Q1  
Trading Symbol ARWR  
Entity Central Index Key 0000879407  
Entity Current Reporting Status Yes  
Current Fiscal Year End Date --09-30  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   101,654,477
Entity Shell Company false  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity File Number 001-38042  
Entity Tax Identification Number 46-0408024  
Entity Address, Address Line One 177 E. Colorado Blvd  
Entity Address, Address Line Two Suite 700  
Entity Address, City or Town Pasadena  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 91105  
City Area Code 626  
Local Phone Number 304-3400  
Document Quarterly Report true  
Document Transition Report false  
Title of 12(b) Security Common Stock, Par Value $0.001 per share  
Security Exchange Name NASDAQ  
Entity Incorporation, State or Country Code DE  
Entity Interactive Data Current Yes