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

No definition available.

Name:	arwr_AdditionalAmountOfCommonStockSharesAgreedToSellUponExerciseOfOption
Namespace Prefix:	arwr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Agreement date.

+ References

No definition available.

+ Details

Name:	arwr_AgreementDate
Namespace Prefix:	arwr_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Cash received as due under collaboration agreement.

+ References

No definition available.

+ Details

Name:	arwr_CashReceivedAsDueUnderCollaborationAgreement
Namespace Prefix:	arwr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of agreements.

+ References

No definition available.

+ Details

Name:	arwr_NumberOfAgreements
Namespace Prefix:	arwr_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	arwr_OptionPaymentsAndDevelopmentRegulatoryAndSalesMilestonesPayment
Namespace Prefix:	arwr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	arwr_TradingPeriodUsedToCalculateSharePriceOfCommonStockSurroundingOptionExerciseDate
Namespace Prefix:	arwr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Trading period used to calculate weighted average price of common stock listed in stock market.

+ References

No definition available.

+ Details

Name:	arwr_TradingPeriodUsedToCalculateWeightedAveragePriceOfCommonStockListedInStockMarket
Namespace Prefix:	arwr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 13.A.3)
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 13.A.4(a).Q1)
-URI http://asc.fasb.org/extlink&oid=65893310&loc=d3e214044-122780

+ Details

Name:	us-gaap_DeferredRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of revenue recognized that was previously reported as deferred or unearned revenue.

+ References

+ Details

Name:	us-gaap_DeferredRevenueRevenueRecognized1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Revenue from multiple-deliverable arrangements that include licensing fees and services revenue. Licensing revenue is consideration received from another party for the right to use, but not own, certain of the entity's intangible assets. Licensing arrangements include, but are not limited to, rights to use a patent, copyright, technology, manufacturing process, software or trademark. Licensing fees are generally, but not always, fixed as to amount and not dependent upon the revenue generated by the licensing party. An entity may receive licensing fees for licenses that also generate royalty payments to the entity. Services revenue may be derived by providing other, nonspecified, services during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(d),(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_LicenseAndServicesRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfCommonStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of shares of stock issued as of the balance sheet date, including shares that had been issued and were previously outstanding but which are now held in the treasury.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644

+ Details

Name:	us-gaap_SharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Per share or per unit amount of equity securities issued.

+ References

No definition available.

+ Details

Name:	us-gaap_SharesIssuedPricePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	dei_LegalEntityAxis=arwr_AmgenIncorporatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=us-gaap_CollaborativeArrangementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=arwr_AROAMG1AgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=arwr_CollaborationAndLicenseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=arwr_AmgenIncorporatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=arwr_AROLPAAMG890AgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=arwr_CommonStockPurchaseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_EntityListingsExchangeAxis=exch_XNAS
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)				12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Nov. 18, 2016 USD ($)	Sep. 28, 2016 USD ($) Agreement	Sep. 30, 2017 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock				$ 12,419,119	$ 52,231,433
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share \| $ / shares	$ 7.16			$ 7.16	$ 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
Deferred revenue	$ 5,000,000			$ 5,000,000
Deferred revenue amortized into revenue				2,500,000
Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement			$ 35,000,000
Collaboration and License agreements \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option payments, and development, regulatory and sales milestone payments.			617,000,000
Collaboration and License agreements \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock			$ 21,500,000
ARO-LPA (AMG 890) Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 30,000,000			30,000,000
Deferred revenue amortized into revenue				27,300,000
Revenue generated from work orders under services agreement				$ 1,500,000
Common Stock Purchase Agreement \| Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Shares issued \| shares	3,002,793			3,002,793
Stock issued, price per share \| $ / shares	$ 7.16			$ 7.16
Proceeds from the issuance of common stock		$ 12,500,000
Common Stock Purchase Agreement \| Amgen \| NASDAQ
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of trading days, used to calculate weighted average price of Common Stock, listed in stock market	30 days