Subsequent Events	12 Months Ended
Sep. 30, 2016
Subsequent Events [Abstract]
Subsequent Events	NOTE 13. SUBSEQUENT EVENTS The Company and certain executive officers were named as defendants in related putative securities class actions filed on November 15, 2016 and December 2, 2016 in the Central District of California and respectively captioned Meller v. Arrowhead Pharmaceuticals, Inc., et al., No. 2:16-cv-08505, and Siegel v. Arrowhead Pharmaceuticals, Inc., et al., No. 2:16-cv-8954. The plaintiff brings claims under Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 regarding certain public statements in connection with the Company’s drug research programs and seek damages in an unspecified amount. The Company believes it has meritorious defenses and intends to vigorously defend itself in these matters. The Company makes provisions for liabilities when it is both probable that a liability has been incurred and the amount can be reasonably estimated. No such liability has been recorded related to these matters. The Company cannot predict the ultimate outcome of this matter and cannot accurately estimate any potential liability the Company may incur or the impact of the results of this matter on the Company. With regard to legal fees, such as attorney fees related to these matters or any other legal matters, the Company’s recognizes such costs as incurred. On November 18, 2016, the Company and Amgen received Hart-Scott-Rodino clearance with regard to the Second Collaboration and License Agreement discussed in Note 2 above. Based on the terms of this agreement, and the Common Stock Purchase Agreement, the Company issued 1,745,810 shares of Common Stock to Amgen, and received proceeds of approximately $12.5 million. Additionally, the Company received a $30 million upfront payment due under the Second Collaboration and License Agreement discussed above. On November 29, 2016, the Company announced that it would be discontinuing its clinical programs ARC-520, ARC-AAT and ARC-521, and redeploying its resources and focus toward utilizing the Company’s new proprietary subcutaneous and extra-hepatic delivery systems. Each of these clinical candidates utilized the intravenously administered DPC_iv, or EX1, delivery vehicle. The decision to discontinue development of EX1-containing programs was based primarily on two factors. First, during ongoing discussions with regulatory agencies and outside experts, it became apparent that there would be substantial delays in all clinical programs that utilize EX1, while the Company further explored the cause of deaths in a non-clinical toxicology study in non-human primates. Second, the Company has made substantial advances in RNA chemistry and targeting resulting in large potency gains for subcutaneous administered and extra-hepatic RNAi-based development programs. In preclinical studies with the subcutaneous platform, the Company has obtained depth and duration of target gene knockdown approaching that of intravenously administered EX1-containing candidates, at lower doses and with good safety margins. ARO-HBV and ARO-AAT are the Company’s subcutaneous administered preclinical candidates for chronic hepatitis B virus and liver disease associated with AATD, respectively. Because of the discontinuation of its existing clinical programs, the Company has also reduced its workforce by approximately 30%, while maintaining resourcing necessary to support current and potential future partner-based programs and the Company’s pipeline.

NOTE 13. SUBSEQUENT EVENTS

The Company and certain executive officers were named as defendants in related putative securities class actions filed on November 15, 2016 and December 2, 2016 in the Central District of California and respectively captioned Meller v. Arrowhead Pharmaceuticals, Inc., et al., No. 2:16-cv-08505, and Siegel v. Arrowhead Pharmaceuticals, Inc., et al., No. 2:16-cv-8954. The plaintiff brings claims under Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 regarding certain public statements in connection with the Company’s drug research programs and seek damages in an unspecified amount. The Company believes it has meritorious defenses and intends to vigorously defend itself in these matters. The Company makes provisions for liabilities when it is both probable that a liability has been incurred and the amount can be reasonably estimated. No such liability has been recorded related to these matters. The Company cannot predict the ultimate outcome of this matter and cannot accurately estimate any potential liability the Company may incur or the impact of the results of this matter on the Company. With regard to legal fees, such as attorney fees related to these matters or any other legal matters, the Company’s recognizes such costs as incurred.

On November 18, 2016, the Company and Amgen received Hart-Scott-Rodino clearance with regard to the Second Collaboration and License Agreement discussed in Note 2 above. Based on the terms of this agreement, and the Common Stock Purchase Agreement, the Company issued 1,745,810 shares of Common Stock to Amgen, and received proceeds of approximately $12.5 million. Additionally, the Company received a $30 million upfront payment due under the Second Collaboration and License Agreement discussed above.

On November 29, 2016, the Company announced that it would be discontinuing its clinical programs ARC-520, ARC-AAT and ARC-521, and redeploying its resources and focus toward utilizing the Company’s new proprietary subcutaneous and extra-hepatic delivery systems. Each of these clinical candidates utilized the intravenously administered DPC_iv, or EX1, delivery vehicle. The decision to discontinue development of EX1-containing programs was based primarily on two factors. First, during ongoing discussions with regulatory agencies and outside experts, it became apparent that there would be substantial delays in all clinical programs that utilize EX1, while the Company further explored the cause of deaths in a non-clinical toxicology study in non-human primates. Second, the Company has made substantial advances in RNA chemistry and targeting resulting in large potency gains for subcutaneous administered and extra-hepatic RNAi-based development programs. In preclinical studies with the subcutaneous platform, the Company has obtained depth and duration of target gene knockdown approaching that of intravenously administered EX1-containing candidates, at lower doses and with good safety margins. ARO-HBV and ARO-AAT are the Company’s subcutaneous administered preclinical candidates for chronic hepatitis B virus and liver disease associated with AATD, respectively. Because of the discontinuation of its existing clinical programs, the Company has also reduced its workforce by approximately 30%, while maintaining resourcing necessary to support current and potential future partner-based programs and the Company’s pipeline.