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Regulatory Requirements and Subsidiary Dividend Restrictions Regulatory Requirements and Subsidiary Dividend Restrictions - Customer Reserve Requirements (Details) $ in Millions | Sep. 30, 2019 USD ($)
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Customer Reserve Requirement [Member] [Member] |
Cash Reserve Deposit Required | $ 19.4
Cash Reserve Deposit Required and Made | 21.4
PAB Reserve Requirement [Member] |
Cash Reserve Deposit Required | 2.6
Cash Reserve Deposit Required and Made | $ 3.6

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- Definition cashreservedepositrequired + References No definition available. + Details
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Namespace Prefix: | intl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Cash deposited in a special reserve account for the exclusive benefit of customers pursuant to SEC Regulations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 305 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6478312&loc=d3e61473-112792 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |