Income Taxes (Notes)	3 Months Ended
Dec. 31, 2018
Income Taxes [Abstract]
Income Tax Disclosure [Text Block]	Income Taxes The income tax provision for interim periods is comprised of tax on ordinary income (loss) provided at the most recent estimated annual effective tax rate, adjusted for the tax effect of discrete items. Management uses an estimated annual effective tax rate based on the forecasted pretax income (loss) and statutory tax rates in the various jurisdictions in which it operates. The Company’s effective tax rate differs from the U.S. statutory rate primarily due to state and local taxes, global intangible low taxed income and differing statutory tax rates applied to the income of non-U.S. subsidiaries. The Company records the tax effect of certain discrete items, including the effects of changes in tax laws, tax rates and adjustments with respect to valuation allowances or other unusual or nonrecurring tax adjustments, in the interim period in which they occur, as an addition to, or reduction from, the income tax provision, rather than being included in the estimated effective annual income tax rate. In addition, jurisdictions with a projected loss for the year or a year-to-date loss where no tax benefit can be recognized are excluded from the estimated annual effective income tax rate. The Company is required to assess its deferred tax assets and the need for a valuation allowance at each reporting period. This assessment requires judgment on the part of management with respect to benefits that may be realized. The Company will record a valuation allowance against deferred tax assets when it is considered more likely than not that all or a portion of the deferred tax assets will not be realized. Effects of the Tax Cuts and Jobs Act On December 22, 2017, the President of the United States signed and enacted into law H.R. 1, the Tax Cuts and Jobs Act (“the Tax Reform”). Among the significant changes to the U.S. Internal Revenue Code, the Tax Reform lowered the U.S. federal corporate income tax rate from 35% to 21%, effective January 1, 2018. The Company will compute its income tax expense (benefit) for the September 30, 2019 tax year using a U.S. statutory tax rate of 21.0%. The Company computed income tax expense for the year ended September 30, 2018 using a U.S. statutory tax rate of 24.5%. The Tax Reform imposed a one-time mandatory repatriation transition tax on previously untaxed accumulated and current earnings and profits (“E&P”) of certain foreign subsidiaries. The Tax Reform also establishes new tax laws that will affect the fiscal year ending September 30, 2019, including, but not limited to, (1) elimination of the corporate alternative minimum tax, (2) a new provision designed to tax global intangible low-taxed income (“GILTI”), (3) limitations on the utilization of net operating losses incurred in tax years beginning after September 30, 2018 to 80% of taxable income per tax year, (4) the creation of the base erosion anti-abuse tax (“BEAT”), (5) a general elimination of U.S. federal income taxes on dividends from foreign subsidiaries, and (6) limitations on the deductibility of interest expense and certain executive compensation. The Company made the policy election to treat GILTI as a current period expense when incurred. The estimated impact of GILTI is included in the forecasted effective tax rate and increases the effective rate by approximately 2%. The SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”), which provides guidance on accounting for the tax effects of the Tax Reform. SAB 118 provides a provisional measurement period that should not extend beyond one year from the Tax Reform enactment date for companies to complete the accounting under ASC 740, Income Taxes. The Company’s income tax accounting provisional measurement period for the Tax Reform concluded during the three months ended December 31, 2018 with no adjustments to the provisional amounts previously recorded. For the three months ended December 31, 2017, the Company recorded tax expense of $8.9 million related to the remeasurement of deferred tax assets and liabilities based on the information available. As a result of additional information becoming available after December 31, 2017, the Company recorded a benefit of $0.3 million during the remainder of fiscal 2018 related to the remeasurement of deferred tax assets and liabilities, and as of September 30, 2018, the accounting for the remeasurement of the deferred tax assets and liabilities was complete. The Tax Reform also included a one-time mandatory repatriation transition tax on previously untaxed accumulated and current E&P of certain of the Company’s foreign subsidiaries. To determine the amount of the transition tax, the Company determined, in addition to other factors, the amount of post 1986 E&P of the relevant subsidiaries, as well as the amount of non-U.S. income taxes paid on such earnings. The Company made a reasonable estimate of the transition tax and recorded a provisional transition tax obligation of $12.0 million in the three months ended December 31, 2017. The Company recorded a benefit of $0.8 million during the remainder of fiscal 2018 due to revised E&P computations, refinement of the net operating loss carryforward available, and revised non-US income taxes paid. As of December 31, 2018, the accounting for the one-time mandatory repatriation transition tax on previously untaxed accumulated and current E&P of certain of the Company’s foreign subsidiaries is complete. Current and Prior Period Tax Expense Income tax expense of $6.2 million and $25.5 million for the three months ended December 31, 2018 and 2017, respectively, reflect estimated federal, foreign, state and local taxes. Due to the Tax Reform, the Company recorded discrete expense of $20.9 million during the three months ended December 31, 2017. Tax expense, excluding the discrete expense related to the Tax Reform, was $4.6 million for the three months ended December 31, 2017. For the three months ended December 31, 2018 and 2017, the Company’s effective tax rate was 25% and 137%, respectively. For the three months ended December 31, 2018, the effective rate was higher than the U.S. federal statutory rate of 21% due to U.S. state and local taxes, GILTI, and US and foreign permanent differences. The estimated GILTI tax expense increased the effective rate approximately 2%. The effective rate for the three months ended December 31, 2017 was 25%, excluding the impacts of Tax Reform, and was higher than the blended statutory rate of 24.5% due to U.S. state and local taxes and foreign permanent differences. For the three months ended December 31, 2017, the recorded discrete expense of $20.9 million related to Tax Reform, increased the effective tax rate by 112%. Further, the Company’s effective tax rate decreased 1.0% and 1.2% for the three months ended December 31, 2018 and 2017, respectively, due to excess tax benefits on share-based compensation recognized due to the adoption of ASU 2016-09. Deferred income tax balances reflect the effects of temporary differences between the carrying amounts of assets and liabilities and their tax bases and are stated at enacted tax rates expected to be in effect when taxes are actually paid or recovered. As of December 31, 2018 and September 30, 2018, the Company has net operating loss carryforwards for U.S. federal, state, local, and foreign income tax purposes of $9.4 million, net of valuation allowances, which are available to offset future taxable income in these jurisdictions. The state and local net operating loss carryforwards of $5.7 million, net of valuation allowance, begin to expire after September 2020. INTL Asia Pte. Ltd. has a Singapore net operating loss carryforward of $2.9 million. This Singapore net operating loss has an indefinite carryforward and, in the judgment of management, is more likely than not to be realized. As a result of Tax Reform, the alternative minimum tax (“AMT”) credit carryforward deferred tax asset has been reclassified to income taxes receivable. The Company can continue to utilize AMT credits to offset regular income tax liability in fiscal years 2019 through 2021. Any remaining amount is fully refundable by fiscal year 2022. In fiscal 2018, the Company generated $6.5 million in foreign tax credit carryforwards as part of the mandatory repatriation transition tax. These credits expire in fiscal year 2028. In the judgment of management, the Company believes that sufficient taxable income will be earned to utilize the foreign tax credit carryforwards within 10 years. The valuation allowance for deferred tax assets as of December 31, 2018 and September 30, 2018 was $3.5 million. The valuation allowances as of December 31, 2018 and September 30, 2018 were primarily related to acquired U.S. state and local net operating loss carryforwards and foreign net operating loss carryforwards that, in the judgment of management, are not more likely than not to be realized. When evaluating if U.S. federal, state, and local deferred taxes are realizable, the Company considers when deferred tax liabilities are scheduled to reverse as well as deferred tax liabilities associated with unrealized gains in securities which the Company could sell, if necessary. Furthermore, the Company considers its ability to implement business and tax planning strategies that would allow the remaining U.S. federal, state, and local deferred tax assets, net of valuation allowances, to be realized in less than 10 years. Based on the current and projected profitability of the Company, as well as tax planning strategies that can be implemented, management believes that it is more likely than not that the Company will realize the tax benefit of the deferred tax assets, net of the existing valuation allowances, in the future. As of December 31, 2018 and September 30, 2018, the Company has accumulated undistributed earnings generated by its foreign subsidiaries of approximately $360.1 million and $354.7 million, respectively. The Company recognized the mandatory repatriation tax related to these undistributed earnings as part of the Tax Reform and, as a result, repatriation of these amounts would not be subject to additional U.S. federal income tax but may be subject to applicable foreign withholding and state taxes in the relevant jurisdictions. With the exception of a $13.0 million dividend that was paid in the first quarter of fiscal year 2019, the Company intends to indefinitely reinvest these earnings outside of the United States as the Company expects future U.S. cash generation to be sufficient to meet future U.S. cash needs. Foreign withholding tax is not applicable to the first quarter distribution. The Company and its subsidiaries file income tax returns with the U.S. federal jurisdiction and various state, local and foreign jurisdictions. The Company has open tax years ranging from September 30, 2010 through September 30, 2018 with U.S. federal and state and local taxing authorities. The Company is currently under examination by the U.S. Internal Revenue Service for the 2016 fiscal tax year; however, no additional tax liability is expected. In the United Kingdom (“U.K.”), the Company has open tax years ending September 30, 2017 to September 30, 2018. In Brazil, the Company has open tax years ranging from December 31, 2013 through December 31, 2018. In Argentina, the Company has open tax years ranging from September 30, 2011 to September 30, 2018. In Singapore, the Company has open tax years ranging from September 30, 2014 to September 30, 2018.

Income Taxes

The income tax provision for interim periods is comprised of tax on ordinary income (loss) provided at the most recent estimated annual effective tax rate, adjusted for the tax effect of discrete items. Management uses an estimated annual effective tax rate based on the forecasted pretax income (loss) and statutory tax rates in the various jurisdictions in which it operates. The Company’s effective tax rate differs from the U.S. statutory rate primarily due to state and local taxes, global intangible low taxed income and differing statutory tax rates applied to the income of non-U.S. subsidiaries. The Company records the tax effect of certain discrete items, including the effects of changes in tax laws, tax rates and adjustments with respect to valuation allowances or other unusual or nonrecurring tax adjustments, in the interim period in which they occur, as an addition to, or reduction from, the income tax provision, rather than being included in the estimated effective annual income tax rate. In addition, jurisdictions with a projected loss for the year or a year-to-date loss where no tax benefit can be recognized are excluded from the estimated annual effective income tax rate.

The Company is required to assess its deferred tax assets and the need for a valuation allowance at each reporting period. This assessment requires judgment on the part of management with respect to benefits that may be realized. The Company will record a valuation allowance against deferred tax assets when it is considered more likely than not that all or a portion of the deferred tax assets will not be realized.

Effects of the Tax Cuts and Jobs Act

On December 22, 2017, the President of the United States signed and enacted into law H.R. 1, the Tax Cuts and Jobs Act (“the Tax Reform”). Among the significant changes to the U.S. Internal Revenue Code, the Tax Reform lowered the U.S. federal corporate income tax rate from 35% to 21%, effective January 1, 2018. The Company will compute its income tax expense (benefit) for the September 30, 2019 tax year using a U.S. statutory tax rate of 21.0%. The Company computed income tax expense for the year ended September 30, 2018 using a U.S. statutory tax rate of 24.5%. The Tax Reform imposed a one-time mandatory repatriation transition tax on previously untaxed accumulated and current earnings and profits (“E&P”) of certain foreign subsidiaries.

The Tax Reform also establishes new tax laws that will affect the fiscal year ending September 30, 2019, including, but not limited to, (1) elimination of the corporate alternative minimum tax, (2) a new provision designed to tax global intangible low-taxed income (“GILTI”), (3) limitations on the utilization of net operating losses incurred in tax years beginning after September 30, 2018 to 80% of taxable income per tax year, (4) the creation of the base erosion anti-abuse tax (“BEAT”), (5) a general elimination of U.S. federal income taxes on dividends from foreign subsidiaries, and (6) limitations on the deductibility of interest expense and certain executive compensation. The Company made the policy election to treat GILTI as a current period expense when incurred. The estimated impact of GILTI is included in the forecasted effective tax rate and increases the effective rate by approximately 2%.

The SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”), which provides guidance on accounting for the tax effects of the Tax Reform. SAB 118 provides a provisional measurement period that should not extend beyond one year from the Tax Reform enactment date for companies to complete the accounting under ASC 740, Income Taxes. The Company’s income tax accounting provisional measurement period for the Tax Reform concluded during the three months ended December 31, 2018 with no adjustments to the provisional amounts previously recorded.

For the three months ended December 31, 2017, the Company recorded tax expense of $8.9 million related to the remeasurement of deferred tax assets and liabilities based on the information available. As a result of additional information becoming available after December 31, 2017, the Company recorded a benefit of $0.3 million during the remainder of fiscal 2018 related to the remeasurement of deferred tax assets and liabilities, and as of September 30, 2018, the accounting for the remeasurement of the deferred tax assets and liabilities was complete.

The Tax Reform also included a one-time mandatory repatriation transition tax on previously untaxed accumulated and current E&P of certain of the Company’s foreign subsidiaries. To determine the amount of the transition tax, the Company determined, in addition to other factors, the amount of post 1986 E&P of the relevant subsidiaries, as well as the amount of non-U.S. income taxes paid on such earnings. The Company made a reasonable estimate of the transition tax and recorded a provisional transition tax obligation of $12.0 million in the three months ended December 31, 2017. The Company recorded a benefit of $0.8 million during the remainder of fiscal 2018 due to revised E&P computations, refinement of the net operating loss carryforward available, and revised non-US income taxes paid. As of December 31, 2018, the accounting for the one-time mandatory repatriation transition tax on previously untaxed accumulated and current E&P of certain of the Company’s foreign subsidiaries is complete.

Current and Prior Period Tax Expense

Income tax expense of $6.2 million and $25.5 million for the three months ended December 31, 2018 and 2017, respectively, reflect estimated federal, foreign, state and local taxes. Due to the Tax Reform, the Company recorded discrete expense of $20.9 million during the three months ended December 31, 2017. Tax expense, excluding the discrete expense related to the Tax Reform, was $4.6 million for the three months ended December 31, 2017.

For the three months ended December 31, 2018 and 2017, the Company’s effective tax rate was 25% and 137%, respectively. For the three months ended December 31, 2018, the effective rate was higher than the U.S. federal statutory rate of 21% due to U.S. state and local taxes, GILTI, and US and foreign permanent differences. The estimated GILTI tax expense increased the effective rate approximately 2%. The effective rate for the three months ended December 31, 2017 was 25%, excluding the impacts of Tax Reform, and was higher than the blended statutory rate of 24.5% due to U.S. state and local taxes and foreign permanent differences. For the three months ended December 31, 2017, the recorded discrete expense of $20.9 million related to Tax Reform, increased the effective tax rate by 112%. Further, the Company’s effective tax rate decreased 1.0% and 1.2% for the three months ended December 31, 2018 and 2017, respectively, due to excess tax benefits on share-based compensation recognized due to the adoption of ASU 2016-09.

Deferred income tax balances reflect the effects of temporary differences between the carrying amounts of assets and liabilities and their tax bases and are stated at enacted tax rates expected to be in effect when taxes are actually paid or recovered. As of December 31, 2018 and September 30, 2018, the Company has net operating loss carryforwards for U.S. federal, state, local, and foreign income tax purposes of $9.4 million, net of valuation allowances, which are available to offset future taxable income in these jurisdictions. The state and local net operating loss carryforwards of $5.7 million, net of valuation allowance, begin to expire after September 2020. INTL Asia Pte. Ltd. has a Singapore net operating loss carryforward of $2.9 million. This Singapore net operating loss has an indefinite carryforward and, in the judgment of management, is more likely than not to be realized. As a result of Tax Reform, the alternative minimum tax (“AMT”) credit carryforward deferred tax asset has been reclassified to income taxes receivable. The Company can continue to utilize AMT credits to offset regular income tax liability in fiscal years 2019 through 2021. Any remaining amount is fully refundable by fiscal year 2022. In fiscal 2018, the Company generated $6.5 million in foreign tax credit carryforwards as part of the mandatory repatriation transition tax. These credits expire in fiscal year 2028. In the judgment of management, the Company believes that sufficient taxable income will be earned to utilize the foreign tax credit carryforwards within 10 years.

The valuation allowance for deferred tax assets as of December 31, 2018 and September 30, 2018 was $3.5 million. The valuation allowances as of December 31, 2018 and September 30, 2018 were primarily related to acquired U.S. state and local net operating loss carryforwards and foreign net operating loss carryforwards that, in the judgment of management, are not more likely than not to be realized.

When evaluating if U.S. federal, state, and local deferred taxes are realizable, the Company considers when deferred tax liabilities are scheduled to reverse as well as deferred tax liabilities associated with unrealized gains in securities which the Company could sell, if necessary. Furthermore, the Company considers its ability to implement business and tax planning strategies that would allow the remaining U.S. federal, state, and local deferred tax assets, net of valuation allowances, to be realized in less than 10 years. Based on the current and projected profitability of the Company, as well as tax planning strategies that can be implemented, management believes that it is more likely than not that the Company will realize the tax benefit of the deferred tax assets, net of the existing valuation allowances, in the future.

As of December 31, 2018 and September 30, 2018, the Company has accumulated undistributed earnings generated by its foreign subsidiaries of approximately $360.1 million and $354.7 million, respectively. The Company recognized the mandatory repatriation tax related to these undistributed earnings as part of the Tax Reform and, as a result, repatriation of these amounts would not be subject to additional U.S. federal income tax but may be subject to applicable foreign withholding and state taxes in the relevant jurisdictions. With the exception of a $13.0 million dividend that was paid in the first quarter of fiscal year 2019, the Company intends to indefinitely reinvest these earnings outside of the United States as the Company expects future U.S. cash generation to be sufficient to meet future U.S. cash needs. Foreign withholding tax is not applicable to the first quarter distribution.

The Company and its subsidiaries file income tax returns with the U.S. federal jurisdiction and various state, local and foreign jurisdictions. The Company has open tax years ranging from September 30, 2010 through September 30, 2018 with U.S. federal and state and local taxing authorities. The Company is currently under examination by the U.S. Internal Revenue Service for the 2016 fiscal tax year; however, no additional tax liability is expected. In the United Kingdom (“U.K.”), the Company has open tax years ending September 30, 2017 to September 30, 2018. In Brazil, the Company has open tax years ranging from December 31, 2013 through December 31, 2018. In Argentina, the Company has open tax years ranging from September 30, 2011 to September 30, 2018. In Singapore, the Company has open tax years ranging from September 30, 2014 to September 30, 2018.