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Available-for-Sale Securities - Interest-bearing Investments in Available-for-Sale Securities (Parenthetical) (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | $ 559,385 | $ 69,370
Short Term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Maturity period of Available-for-sale securities | longer than 90 days | longer than 90 days
Available-for-sale securities | $ 752 |
Cash and Cash Equivalents [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Maturity period of Available-for-sale securities | 90 days or less | 90 days or less
Available-for-sale securities | $ 558,633 |

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- Definition Maturity period of short term interest bearing securities included in cash and cash equivalents. + References No definition available. + Details
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- Definition Amount 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 -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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