Investments - Summary of Investments by Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 39,553	$ 34,075
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(72)	(16)
Fair Market Value	39,481	34,059
U.S. Treasury, short-term \| Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Cost	7,356	3,897
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(16)	(3)
Fair Market Value	7,340	3,894
U.S. Treasury, short-term \| Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Cost	3,678
Gross Unrealized Gains	0
Gross Unrealized Losses	(13)
Fair Market Value	3,665
Corporate debt securities \| Short-term investments
Schedule Of Available For Sale Securities [Line Items]
Cost	24,689	26,393
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(30)	(8)
Fair Market Value	24,659	26,385
Corporate debt securities \| Long-term investments
Schedule Of Available For Sale Securities [Line Items]
Cost	3,830	3,785
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13)	(5)
Fair Market Value	$ 3,817	$ 3,780

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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