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Fair Value Measurements - Reconciliation of Derivative Liability (Detail) (USD $) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |  |
Change in value | $ 2,179,651 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings | $ (3,507,496)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings |
Exchange rights |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |  |
Value, Beginning balance | 216,429 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember | 4,569 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember | 4,569 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember
Change in value | (202,491)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember |  | 211,860 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember
Net settlements | (1,242)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember |  |
Value, Ending balance | 12,696 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember |  | 216,429 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByLiabilityClassAxis = arwr_ExchangeRightsMember
Warrant |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items] |  |  |
Value, Beginning balance | 3,957,514 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FinancialInstrumentAxis = us-gaap_WarrantMember | 4,091,797 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FinancialInstrumentAxis = us-gaap_WarrantMember | 4,091,797 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FinancialInstrumentAxis = us-gaap_WarrantMember
Change in value | (2,179,651)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FinancialInstrumentAxis = us-gaap_WarrantMember |  | 5,821,796 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FinancialInstrumentAxis = us-gaap_WarrantMember
Net settlements |  |  | (5,956,079)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FinancialInstrumentAxis = us-gaap_WarrantMember
Value, Ending balance | $ 1,777,863 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FinancialInstrumentAxis = us-gaap_WarrantMember |  | $ 3,957,514 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FinancialInstrumentAxis = us-gaap_WarrantMember

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- Definition Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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 (c)(1) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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 (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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 (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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