Commitments and Contingencies - Additional Information (Detail) - USD ($)	3 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Other Commitments [Line Items]
Contingent liabilities	$ 0
Future commitments	62,500,000
Commitments expected to be incurred in fiscal 2020	42,500,000
Commitments expected to be incurred beyond fiscal 2020	20,000,000.0
Technology License Commitments
Other Commitments [Line Items]
Milestone payments	$ 0	$ 0

Milestone payments.

No definition available.

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Namespace Prefix:	arwr_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- 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=118942415&loc=d3e14326-108349

+ Details

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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

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Period Type:	duration

- 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

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- Definition

Minimum amount of purchase arrangement maturing in the second fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

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- Definition

Minimum amount of purchase arrangement maturing in the remainder of the fiscal year following the latest fiscal year ended. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

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- Details

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