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Income Taxes - Narrative (Details) - USD ($) | 12 Months Ended
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Sep. 30, 2021 | Sep. 30, 2020
Operating Loss Carryforwards [Line Items] |  |
Operating loss carryforwards | $ 6,200,000 | $ 6,900,000
State and local net operating loss carryforwards of net of valuation allowance | 3,600,000 |
Federal operating loss carryforwards, net of valuation allowances | 600,000 |
Foreign operating loss carryforwards, net of valuation allowances | 2,000,000 |
Foreign tax credits, net | 0 |
Valuation allowance | 15,000,000.0 | 12,400,000
Net change in total valuation allowance | 2,600,000 |
Foreign tax credit carryforwards | 2,000,000.0 | 2,400,000
Foreign earnings repatriated | 300,600,000 | $ 30,000,000
Gain Capital Holdings Inc |  |
Operating Loss Carryforwards [Line Items] |  |
Foreign tax credit carryforwards | $ 800,000 |

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