Investments - Summary of Short-term and Long-term Investments (Detail) - USD ($)	Jun. 30, 2020	Sep. 30, 2019
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	$ 245,239,144	$ 81,075,887
Gross Unrealized Gains	7,033,593	1,097,842
Gross Unrealized Losses	(14,833)
Fair Value	252,257,904	82,173,729
Commercial Notes Due Within One Year
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	67,709,263	36,899,894
Gross Unrealized Gains	1,188,416	222,584
Fair Value	68,897,679	37,122,478
Commercial Notes Due Within One Through Three Years
Schedule Of Held To Maturity Securities [Line Items]
Amortized Cost	177,529,881	44,175,993
Gross Unrealized Gains	5,845,177	875,258
Gross Unrealized Losses	(14,833)
Fair Value	$ 183,360,225	$ 45,051,251

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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