| Document and Entity Information | Oct. 07, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | Arrowhead Pharmaceuticals, Inc. |
| Amendment Flag | false |
| Entity Central Index Key | 0000879407 |
| Document Type | 8-K |
| Document Period End Date | Oct. 7, 2020 |
| Entity Incorporation, State or Country Code | DE |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common Stock, Par Value $0.001 per share |
| Trading Symbol | ARWR |
| Security Exchange Name | NASDAQ |
