Description of Business and Significant Accounting Policies - Basis of Presentation and Consolidation (Details) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2018
Memberships in Exchanges Owned	$ 12,000,000		$ 11,400,000
Fair Value of Exchange Memberships	10,200,000		10,300,000
Cash and Cash Equivalents - Deposits and Receivables	$ 2,300,000,000		$ 1,500,000,000
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	$ 300,000,000		$ 785,753,569
Non-Trading Technology and Support [Member]
Prior Period Reclassification Adjustment	11,600,000	$ 7,100,000
communications [Member]
Prior Period Reclassification Adjustment	$ 5,000,000	$ 4,700,000