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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, 2017 | Sep. 30, 2016
Short Term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | $ 329,997 | $ 327,063
Maturity period of short term interest-bearing securities included in cash and cash equivalents | Longer than 90 days |
Cash and Cash Equivalents [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale securities | $ 55,200 | $ 453,319
Maturity period of short term interest-bearing securities included in cash and cash equivalents | 90 days or less |

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562 + Details
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