Reconciliation of Cash, Segregated Cash, Cash Equivalents, and Segregated Cash Equivalents (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Cash and cash equivalents	$ 471.3	$ 342.3
Cash, segregated cash, cash equivalents, and segregated cash equivalents shown in the consolidated statements of cash flows	2,451.3	2,287.6	$ 2,659.8	$ 1,637.6
Cash and Cash Equivalents [Member]
Cash and cash equivalents	471.3	342.3	314.9
Cash segregated under federal and other regulations [Member]
Cash segregated under federal and other regulations (1)	743.9	765.5	463.4
Deposits with and receivables from broker dealers, clearing organizations, and counterparties [Member]
Cash segregated and deposited with or pledged to exchange-clearing organizations and other futures commission merchants (“FCMs”)(2)	947.4	711.9	1,870.9
Securities segregated and pledged to exchange-clearing organizations(2)	$ 288.7	$ 467.9	$ 10.6

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of cash and cash equivalents, and cash and cash equivalents restricted to withdrawal or usage. Excludes amount for disposal group and discontinued operations. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Cash deposited in a special reserve account for the exclusive benefit of customers pursuant to the Commodity Exchange Act.

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Amount of cash restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits.

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