Description of Business and Significant Accounting Policies - Basis of Presentation and Consolidation (Details) - USD ($)	Sep. 30, 2017	Sep. 30, 2016
Accounting Policies [Abstract]
Memberships in Exchanges Owned	$ 12,000,000	$ 12,100,000
Fair Value of Exchange Memberships	10,200,000	9,100,000
Cash and Cash Equivalents - Deposits and Receivables	$ 2,300,000,000	$ 900,000,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Preferred Stock, Shares Outstanding	0	0
US Government Securities and Other Securities - Deposits and Receivables	$ 251,354,305	$ 500,000,000

Cash and Cash Equivalents - Deposits and Receivables

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- Definition

Fair Value of Exchange Memberships

+ References

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- Definition

US Government Securities and Other Securities - Deposits and Receivables

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- References

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- Definition

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.

+ References

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

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- Definition

Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

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