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Fair Value Measurements - Assumptions Used in Valuing Derivative Liabilities (Detail) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014
2010 Warrants |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |
Risk free interest rate | 0.25%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandTenWarrantMember | 0.13%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandTenWarrantMember
Expected life | 1 year | 1 year 2 months 12 days
Dividend yield |  |
Volatility | 75.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandTenWarrantMember | 69.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandTenWarrantMember
2012 Warrants |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |
Risk free interest rate | 1.10%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandTwelveWarrantMember | 1.07%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandTwelveWarrantMember
Expected life | 3 years | 3 years 2 months 12 days
Dividend yield |  |
Volatility | 75.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandTwelveWarrantMember | 69.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandTwelveWarrantMember
2013 Warrants |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |
Risk free interest rate | 1.10%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandThirteenWarrantMember | 1.07%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandThirteenWarrantMember
Expected life | 3 years 1 month 6 days | 3 years 3 months 18 days
Dividend yield |  |
Volatility | 75.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandThirteenWarrantMember | 69.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = arwr_TwoThousandThirteenWarrantMember
Exchange rights |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |
Risk free interest rate | 1.27%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember | 1.07%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember
Expected life | 3 years | 3 years 3 months 18 days
Volatility | 75.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember | 100.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember

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- Definition Expected dividends to be paid to holders of the underlying shares or financial instruments (expressed as a percentage of the share or instrument's price). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Period the instrument, asset or liability is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Risk-free interest rate assumption used in valuing an instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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