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Investments - Summary of Short-term, Long-term Investments and Marketable Securities (Detail) - USD ($) | Sep. 30, 2020 | Sep. 30, 2019
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Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Held to Maturity, Amortized Cost | $ 224,376,634 | $ 81,075,887
Held to Maturity, Gross Unrealized Gains | 6,163,837 | 1,097,842
Held to Maturity, Gross Unrealized Losses | (79,471) |
Held to Maturity, Fair Value | 230,461,000 | 82,173,729
Marketable Securities, Amortized Cost | 85,000,000 |
Marketable Securities, Realized Gains/(Losses) | 94,734 |
Marketable Securities, Gross Unrealized Losses | (75,015) |
Marketable Securities, Fair Value | 85,019,719 |
Commercial Notes Due Within One Year |  |
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Held to Maturity, Amortized Cost | 86,889,751 | 36,899,894
Held to Maturity, Gross Unrealized Gains | 1,589,873 | 222,584
Held to Maturity, Fair Value | 88,479,624 | 37,122,478
Commercial Notes Due Within One Through Three Years |  |
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Held to Maturity, Amortized Cost | 137,486,883 | 44,175,993
Held to Maturity, Gross Unrealized Gains | 4,573,964 | 875,258
Held to Maturity, Gross Unrealized Losses | (79,471) |
Held to Maturity, Fair Value | 141,981,376 | $ 45,051,251
Marketable Securities |  |
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Marketable Securities, Amortized Cost | 85,000,000 |
Marketable Securities, Realized Gains/(Losses) | 94,734 |
Marketable Securities, Gross Unrealized Losses | (75,015) |
Marketable Securities, Fair Value | $ 85,019,719 |

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