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Income Taxes - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Operating Loss Carryforwards [Line Items] |  |  |
Federal statutory rate effect of: | 21.00% | 21.00% | 24.50%
Effective income tax rate reconciliation, Change in enacted tax rate, amount |  |  | $ 8.6
Remeasurement of deferred tax | 0.00% | 0.00% | 8.50%
Effective income tax rate reconciliation, Repatriation of foreign earnings, amount |  |  | $ 11.2
Repatriation transition tax | 0.00% | 0.00% | 11.00%
Operating loss carryforwards | $ 6.9 | $ 7.1 |
State and local net operating loss carryforwards of net of valuation allowance | 4.7 |  |
Federal operating loss carryforwards, net of valuation allowances | 0.6 |  |
Foreign tax credit carryforwards | 2.4 | 5.0 | $ 5.1
Valuation allowance | (12.4) | (8.5) |
Net change in total valuation allowance | 3.9 |  |
Foreign earnings repatriated | 30.0 | $ 13.0 |
Gain Capital Holdings Inc [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Foreign tax credit carryforwards | 1.1 |  |
Foreign tax credits, net | 1.3 |  |
Gain Capital U.K. Ltd [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Operating loss carryforwards | $ 0.9 |  |

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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible foreign tax credit carryforwards. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319 + Details
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