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Investments - Summary of Investments by Type of Security (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Sep. 30, 2017
--- | --- | ---
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | $ 9,419 | $ 34,075
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (42) | (16)
Fair Market Value | 9,377 | 34,059
U.S. Treasury | Short-term investments |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 3,688 | 3,897
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (15) | (3)
Fair Market Value | 3,673 | 3,894
Corporate debt securities | Short-term investments |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost | 5,731 | 26,393
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (27) | (8)
Fair Market Value | $ 5,704 | 26,385
Corporate debt securities | Long-term investments |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Cost |  | 3,785
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | (5)
Fair Market Value |  | $ 3,780

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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