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Investments - Summary of Short and Long-term Investments (Detail) (USD $) | Dec. 31, 2014 | Sep. 30, 2014
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Schedule Of Investments [Line Items] |  |
Gross Unrealized Losses | $ (579,833)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss | $ (407,523)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Fair Value | 40,759,068 us-gaap_InvestmentsFairValueDisclosure | 44,333,855 us-gaap_InvestmentsFairValueDisclosure
Commercial notes (due within one year), amortized cost | 23,532,402 us-gaap_MarketableSecuritiesCurrent | 21,653,032 us-gaap_MarketableSecuritiesCurrent
Commercial notes (due after one year through two years), amortized cost | 17,806,499 us-gaap_MarketableSecuritiesNoncurrent | 23,088,346 us-gaap_MarketableSecuritiesNoncurrent
Total | 41,338,901 us-gaap_MarketableSecurities | 44,741,378 us-gaap_MarketableSecurities
Commercial Notes Due Within One Year |  |
Schedule Of Investments [Line Items] |  |
Gross Unrealized Losses | (313,407)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = arwr_CommercialNotesDueWithinOneYearMember | (189,830)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = arwr_CommercialNotesDueWithinOneYearMember
Fair Value | 23,218,995 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = arwr_CommercialNotesDueWithinOneYearMember | 21,463,202 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = arwr_CommercialNotesDueWithinOneYearMember
Commercial Notes Due After One Year Through Two Years |  |
Schedule Of Investments [Line Items] |  |
Gross Unrealized Losses | (266,426)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = arwr_CommercialNotesDueAfterOneYearThroughTwoYearsMember | (217,693)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = arwr_CommercialNotesDueAfterOneYearThroughTwoYearsMember
Fair Value | $ 17,540,073 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = arwr_CommercialNotesDueAfterOneYearThroughTwoYearsMember | $ 22,870,653 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = arwr_CommercialNotesDueAfterOneYearThroughTwoYearsMember

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- Definition Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 + Details
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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 + Details
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- Definition Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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