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Short-term Investments - Additional Information (Details) - USD ($) | 12 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Short Term Investments [Abstract] |  |  |
Realized investment losses | $ (11,000) | $ (3,000) | $ (5,000)
Short-term investments, contractual maturity period range start | 2018-11 |  |
Short-term investments, contractual maturity period range end | 2020-05 |  |

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- Definition Investment maturity month and year range end. + References No definition available. + Details
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Balance Type: | na
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- Definition Investment maturity month and year range start. + References No definition available. + Details
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- Definition The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- References No definition available. + Details
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