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Commitments and Contingencies - Additional Information (Detail) - USD ($) | 3 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Other Commitments [Line Items] |  |
Contingent liabilities | $ 0 |
Future commitments | 100,200,000 |
Commitments expected to be incurred in remaining fiscal year 2021 | 85,000,000.0 |
Technology License Commitments |  |
Other Commitments [Line Items] |  |
Milestone payments | $ 0 | $ 0

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- Definition Milestone payments. + References No definition available. + Details
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Namespace Prefix: | arwr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of loss contingency liability. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_OtherCommitmentsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier. + References No definition available. + Details
Name: | us-gaap_PurchaseObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of purchase arrangement to be paid in remainder of current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_OtherCommitmentsAxis=arwr_TechnologyLicenseCommitmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |