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Related Party Transactions - Additional Information (Detail) (USD $) | 1 Months Ended | 12 Months Ended
--- | --- | ---
Aug. 31, 2010 | Sep. 30, 2012 | Sep. 30, 2011
Related Party Transaction [Line Items] |  |  |
Monthly remuneration of employee | $ 10,000 |  |
Remuneration of Vincent Anzalone |  | $ 120,000 | $ 120,000
Nanotope, Inc [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Shares owned by Arrowhead's President and CEO |  | 1,395,900 |
Percentage of outstanding voting securities owned by Arrowhead's President and CEO |  | 14.20% |

X
- Definition Monthly remuneration of employee. + References No definition available. + Details
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Namespace Prefix: | arwr_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage shares owned by employee. + References No definition available. + Details
Name: | arwr_PercentageSharesOwnedByEmployee
Namespace Prefix: | arwr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Remuneration. + References No definition available. + Details
Name: | arwr_RemunerationPaid
Namespace Prefix: | arwr_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Shares owned employee. + References No definition available. + Details
Name: | arwr_SharesOwnedByEmployee
Namespace Prefix: | arwr_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration