Additional amount of common stock shares agreed to sell upon exercise of option.

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Agreement date.

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Cash received as due under collaboration agreement.

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Number of agreements.

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Option payments, and development, regulatory and sales milestone payments.

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Trading Period used to calculate share price of common stock surrounding option exercise date.

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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.

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Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current.

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The cash inflow from the additional capital contribution to the entity.

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Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)		3 Months Ended
	Sep. 28, 2016 USD ($) Agreement $ / shares shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock		$ 60,521,729
Revenues		34,657,896	$ 3,509,821
Contract liabilities		$ 124,925,638		$ 600
Type of Revenue [Extensible List]		arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember	arwr:TechnologyLicensesCollaborativeResearchAndDevelopmentArrangementsResearchGrantsAndProductSalesMember
Common Stock Purchase Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Shares issued \| shares	3,002,793
Stock issued, price per share \| $ / shares	$ 7.16
Collaboration and License agreements \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Number of agreements \| Agreement	2
ARO-AMG1 Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional amount of common stock shares agreed to sell upon exercise of option	$ 5,000,000
Number of trading days, used to calculate share price of Common Stock, surrounding option exercise date	30 days
Collaboration and License agreements \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement	$ 35,000,000
Proceeds from the issuance of common stock	21,500,000
Collaboration and License agreements \| Maximum \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option payments, and development, regulatory and sales milestone payments.	$ 617,000,000
AMG 890 (ARO-LPA) and ARO-AMG1 Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues		$ 0	$ 3,500,000
Contract liabilities		100,000
Contract assets		$ 0
Type of Revenue [Extensible List]		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember