Description of Business and Significant Accounting Policies - Basis of Presentation and Consolidation (Details) - USD ($)	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Amount Recognized in Income Due to Inflationary Accounting	$ 3,900,000	$ 3,400,000
Net assets denominated in Argentine Pesos	5,500,000	11,600,000
Cash denominated in Argentine Pesos	100,000	1,400,000
Fair Value of Exchange Memberships	$ 6,000,000	$ 6,400,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01
Preferred Stock, Shares Outstanding	0	0	0
US Government Securities and Other Securities - Deposits and Receivables	$ 603,814,800	$ 800,000,000
Non-Trading Technology and Support [Member]
Prior Period Reclassification Adjustment			$ 11,600,000
communications [Member]
Prior Period Reclassification Adjustment			$ 5,000,000
Exchange memberships not representing an ownership interest [Member]
Memberships in Exchanges Owned	5,800,000
Exchange memberships representing an ownership interest [Member]
Memberships in Exchanges Owned	$ 5,600,000