Description of Business and Significant Accounting Policies - Cash, Cash Equivalents, Securities (Details) - USD ($)	Sep. 30, 2018	Sep. 30, 2017
Accounting Policies [Abstract]
Cash Held at Banks and Money Market Funds - Segregated	$ 765,400,000	$ 464,300,000
US Government Obligations and other securities	600,400,000	33,500,000
Commodities Warehouse Receipts	42,900,000	21,000,000
Cash and Cash Equivalents - Deposits and Receivables	1,500,000,000	2,300,000,000
US Government Securities and Other Securities - Deposits and Receivables	785,753,569	300,000,000
Memberships in Exchanges Owned	11,400,000	12,000,000
Fair Value of Exchange Memberships	$ 10,300,000	$ 10,200,000

Carrying amount as of the balance sheet date of the capitalized costs of obtaining memberships that provide the entity, typically a broker-dealer, with the right to do business on trading exchanges (such as stock or commodity exchanges) in which the entity or its employees are members. Memberships are usually registered in the names of individuals who are affiliated with the broker-dealer. A membership held in the name of an individual is considered to be an asset of the broker-dealer if it is held by the broker-dealer under an agreement (such as an ABC agreement) that would require the member, upon leaving the broker-dealer, to do one of the following: a) Pay the member organization the amount necessary to purchase another membership (if the named individual wished to retain the membership); b) Sell the membership and pay the proceeds over to the member organization; or, c) Transfer the membership for a nominal consideration to a person designated by the member organization and satisfactory to the exchange.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

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