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

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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