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Investments - Summary of Short-term and Long-term Investments (Detail) - USD ($) | Jun. 30, 2020 | Sep. 30, 2019
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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

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