Short-term Investments - Additional Information (Details) - USD ($)	12 Months Ended
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

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.

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

Name:	us-gaap_RealizedInvestmentGainsLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration