Investments - Summary of Short-term, Long-term Investments and Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2023	Sep. 30, 2022
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Short-term investments, Available-for-sale, Adjusted Basis	$ 295,699
Short-term investments, Available-for-sale, Gross Unrealized Gains	0
Short-term investments, Available-for-sale, Gross Unrealized Losses	(2,964)
Short-term investments, Available-for-sale, Fair Value	292,735	$ 0
Marketable debt securities, Adjusted Basis	295,700
Net unrealized gain (loss) on held-to-maturity securities	(3,000)
Short-term investments, Held to maturity, Adjusted Bases	0	268,391
Short-term investments, Held to maturity, Gross Unrealized Gains		0
Short-term investments, Held to maturity, Gross Unrealized Losses		(3,661)
Short-term investments, Held to maturity, Fair Value		264,730
Long-term investments, Held to maturity, Adjusted Basis	0	105,872
Long-term investments, Held to maturity, Gross Unrealized Gains		0
Long-term investments, Held to maturity, Gross Unrealized Losses		(5,569)
Long-term investments, Held to maturity, Fair Value		100,303
Debt securities
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Short-term investments, Available-for-sale, Adjusted Basis	295,699
Short-term investments, Available-for-sale, Gross Unrealized Gains	0
Short-term investments, Available-for-sale, Gross Unrealized Losses	(2,964)
Short-term investments, Available-for-sale, Fair Value	$ 292,735
Short-term investments, Held to maturity, Adjusted Bases		218,391
Short-term investments, Held to maturity, Gross Unrealized Gains		0
Short-term investments, Held to maturity, Gross Unrealized Losses		(3,661)
Short-term investments, Held to maturity, Fair Value		214,730
Long-term investments, Held to maturity, Adjusted Basis		105,872
Long-term investments, Held to maturity, Gross Unrealized Gains		0
Long-term investments, Held to maturity, Gross Unrealized Losses		(5,569)
Long-term investments, Held to maturity, Fair Value		100,303
Certificate of deposits
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Short-term investments, Held to maturity, Adjusted Bases		50,000
Short-term investments, Held to maturity, Gross Unrealized Gains		0
Short-term investments, Held to maturity, Gross Unrealized Losses		0
Short-term investments, Held to maturity, Fair Value		$ 50,000

Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, Current

No definition available.

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

- Definition

Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, Current

+ References

No definition available.

+ Details

Name:	arwr_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossCurrent
Namespace Prefix:	arwr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt Securities, Held-to-Maturity, Accumulated Unrecognized Gain, Current

+ References

No definition available.

+ Details

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

- Definition

Debt Securities, Held-to-Maturity, Accumulated Unrecognized Gain, Noncurrent

+ References

No definition available.

+ Details

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

- Definition

Debt Securities, Held-to-Maturity, Accumulated Unrecognized Loss, Current

+ References

No definition available.

+ Details

Name:	arwr_DebtSecuritiesHeldToMaturityAccumulatedUnrecognizedLossCurrent
Namespace Prefix:	arwr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt Securities, Held-to-Maturity, Accumulated Unrecognized Loss, Noncurrent

+ References

No definition available.

+ Details

Name:	arwr_DebtSecuritiesHeldToMaturityAccumulatedUnrecognizedLossNoncurrent
Namespace Prefix:	arwr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Schedule of held to maturity securities and marketable securities.

+ References

No definition available.

+ Details

Name:	arwr_ScheduleOfHeldToMaturitySecuritiesAndMarketableSecuritiesLineItems
Namespace Prefix:	arwr_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481830/320-10-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), classified as current.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityFairValueCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), classified as noncurrent.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityFairValueNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity), classified as current.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-5

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481830/320-10-45-2

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity), classified as noncurrent.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before tax and adjustment, of unrealized gain (loss) on investment in debt security measured at amortized cost (held-to-maturity) from transfer to investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-10

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482790/220-10-45-11

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CertificatesOfDepositMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: