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Description of Business and Significant Accounting Policies - Cash, Cash Equivalents, Securities (Details) - USD ($) | Sep. 30, 2019 | Sep. 30, 2018
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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

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