Description of Business and Significant Accounting Policies - Cash, Cash Equivalents, Securities (Details) - USD ($)	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Cash Held at Banks and Money Market Funds - Segregated	$ 743,900,000	$ 765,400,000
US Government Obligations and other securities	299,800,000	600,400,000
Commodities Warehouse Receipts	6,200,000	42,900,000
Cash and Cash Equivalents - Deposits and Receivables	1,600,000,000	1,500,000,000
US Government Securities and Other Securities - Deposits and Receivables	$ 603,814,800	$ 800,000,000