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Leases - Additional Information (Detail) (USD $)	6 Months Ended		119 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013
Component of Operating Other Cost and Expense [Line Items]
Rental expense			$ 4,407,000
Rental expenses include common area maintenance and real estate taxes	13,000
Utilities costs per month	16,000
Increase in total monthly expenditures	72,000
Facility and equipment rent expense	281,000	214,000
Pasadena, California [Member]
Component of Operating Other Cost and Expense [Line Items]
Rental expense	13,000
Roche [Member]
Component of Operating Other Cost and Expense [Line Items]
Rental expense	22,000
Monthly payments under capital lease	$ 21,000

X

- Definition

Facility and equipment rent expense.

+ References

No definition available.

+ Details

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

X

- Definition

Increase in monthly expenditure due to utilities costs.

+ References

No definition available.

+ Details

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

X

- Definition

Other rental expenses.

+ References

No definition available.

+ Details

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

X

- Definition

Payments under capital lease.

+ References

No definition available.

+ Details

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

X

- Details

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

X

- Definition

Rental expense incurred for leased assets including furniture and equipment which has not been recognized in costs and expenses applicable to sales and revenues; for example, cost of goods sold or other operating costs and expenses.

+ References

No definition available.

+ Details

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

X

- Definition

Utilities costs incurred during the reporting period for services, such as water, sewer, gas, electricity and telephone required to operate a building.

+ 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 210.5-03.2)

-URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688

+ Details

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