| Fair Value, Assets Measured on Recurring Basis [Table Text Block] |
The following tables set forth the Company’s financial and nonfinancial assets and liabilities accounted for at fair value, on a recurring basis, as of September 30, 2019 and September 30, 2018 by level in the fair value hierarchy. Except as disclosed in Note 7, there were no assets or liabilities that were measured at fair value on a nonrecurring basis as of September 30, 2019 and 2018. | | | | | | | | | | | | | | | | | | | | | | September 30, 2019 | (in millions) | Level 1 | | Level 2 | | Level 3 | | Netting (1) | | Total | Assets: | | | | | | | | | | Certificates of deposit | $ | 4.9 |
| | $ | — |
| | $ | — |
| | $ | — |
| | $ | 4.9 |
| Money market mutual funds | 8.9 |
| | — |
| | — |
| | — |
| | 8.9 |
| Cash and cash equivalents | 13.8 |
| | — |
| | — |
| | — |
| | 13.8 |
| Commodities warehouse receipts | 6.2 |
| | — |
| | — |
| | — |
| | 6.2 |
| U.S. Treasury obligations | 299.8 |
| | — |
| | — |
| | — |
| | 299.8 |
| Securities and other assets segregated under federal and other regulations | 306.0 |
| | — |
| | — |
| | — |
| | 306.0 |
| U.S. Treasury obligations | 593.9 |
| | — |
| | — |
| | — |
| | 593.9 |
| "To be announced" (TBA) and forward settling securities | — |
| | 9.8 |
| | — |
| | (1.5 | ) | | 8.3 |
| Foreign government obligations | 9.9 |
| | — |
| | — |
| | — |
| | 9.9 |
| Derivatives | 3,131.2 |
| | 43.2 |
| | — |
| | (3,159.6 | ) | | 14.8 |
| Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 3,735.0 |
| | 53.0 |
| | — |
| | (3,161.1 | ) | | 626.9 |
| Equity securities | 159.5 |
| | 9.0 |
| | — |
| | — |
| | 168.5 |
| Corporate and municipal bonds | — |
| | 80.0 |
| | — |
| | — |
| | 80.0 |
| U.S. Treasury obligations | 248.7 |
| | — |
| | — |
| | — |
| | 248.7 |
| U.S. government agency obligations | — |
| | 447.1 |
| | — |
| | — |
| | 447.1 |
| Foreign government obligations | 0.5 |
| | — |
| | — |
| | — |
| | 0.5 |
| Agency mortgage-backed obligations | — |
| | 1,045.0 |
| | — |
| | — |
| | 1,045.0 |
| Asset-backed obligations | — |
| | 29.1 |
| | — |
| | — |
| | 29.1 |
| Derivatives | 1.0 |
| | 486.3 |
| | — |
| | (420.8 | ) | | 66.5 |
| Commodities leases | — |
| | 28.6 |
| | — |
| | — |
| | 28.6 |
| Commodities warehouse receipts | 48.4 |
| | — |
| | — |
| | — |
| | 48.4 |
| Exchange firm common stock | 12.7 |
| | — |
| | — |
| | — |
| | 12.7 |
| Mutual funds and other | 0.1 |
| | — |
| | — |
| | — |
| | 0.1 |
| Financial instruments owned | 470.9 |
| | 2,125.1 |
| | — |
| | (420.8 | ) | | 2,175.2 |
| Physical commodities inventory | 7.1 |
| | 144.8 |
| | — |
| | — |
| | 151.9 |
| Total assets at fair value | $ | 4,532.8 |
| | $ | 2,322.9 |
| | $ | — |
| | $ | (3,581.9 | ) | | $ | 3,273.8 |
| Liabilities: | | | | | | | | | | Accounts payable and other accrued liabilities - contingent liabilities | $ | — |
| | $ | — |
| | $ | 1.8 |
| | $ | — |
| | $ | 1.8 |
| TBA and forward settling securities | — |
| | 6.8 |
| | — |
| | (1.5 | ) | | 5.3 |
| Derivatives | 3,079.1 |
| | 38.3 |
| | — |
| | (3,117.1 | ) | | 0.3 |
| Payable to broker-dealers, clearing organizations and counterparties | 3,079.1 |
| | 45.1 |
| | — |
| | (3,118.6 | ) | | 5.6 |
| Equity securities | 147.3 |
| | 10.8 |
| | — |
| | — |
| | 158.1 |
| Foreign government obligations | — |
| | — |
| | — |
| | — |
| | — |
| Corporate and municipal bonds | — |
| | 39.2 |
| | — |
| | — |
| | 39.2 |
| U.S. Treasury obligations | 272.3 |
| | — |
| | — |
| | — |
| | 272.3 |
| U.S. government agency obligations | — |
| | 43.8 |
| | — |
| | — |
| | 43.8 |
| Agency mortgage-backed obligations | — |
| | 29.6 |
| | — |
| | — |
| | 29.6 |
| Derivatives | | | 480.3 |
| | — |
| | (422.2 | ) | | 58.1 |
| Commodities leases | $ | — |
| | $ | 113.7 |
| | $ | — |
| | $ | — |
| | $ | 113.7 |
| Financial instruments sold, not yet purchased | 419.6 |
| | 717.4 |
| | — |
| | (422.2 | ) | | 714.8 |
| Total liabilities at fair value | $ | 3,498.7 |
| | $ | 762.5 |
| | $ | 1.8 |
| | $ | (3,540.8 | ) | | $ | 722.2 |
|
| | (1) | Represents cash collateral and the impact of netting across the levels of the fair value hierarchy. Netting among positions classified within the same level are included in that level. |
| | | | | | | | | | | | | | | | | | | | | | September 30, 2018 | (in millions) | Level 1 | | Level 2 | | Level 3 | | Netting (1) | | Total | Assets: | | | | | | | | | | Cash and cash equivalents - certificaes of deposit | $ | 3.8 |
| | $ | — |
| | $ | — |
| | $ | — |
| | $ | 3.8 |
| Commodities warehouse receipts | 42.9 |
| | — |
| | — |
| | — |
| | 42.9 |
| U.S. Treasury obligations | 600.4 |
| | — |
| | — |
| | — |
| | 600.4 |
| Securities and other assets segregated under federal and other regulations | 643.3 |
| | — |
| | — |
| | — |
| | 643.3 |
| U.S. Treasury obligations | 778.4 |
| | — |
| | — |
| | — |
| | 778.4 |
| TBA and forward settling securities | — |
| | 5.0 |
| | — |
| | (2.1 | ) | | 2.9 |
| Foreign government obligations | 7.7 |
| | — |
| | — |
| | — |
| | 7.7 |
| Derivatives | 7,495.9 |
| | 19.6 |
| | — |
| | (7,787.1 | ) | | (271.6 | ) | Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 8,282.0 |
| | 24.6 |
| | — |
| | (7,789.2 | ) | | 517.4 |
| Equity securities | 71.2 |
| | 3.0 |
| | — |
| | — |
| | 74.2 |
| Corporate and municipal bonds | — |
| | 79.1 |
| | — |
| | — |
| | 79.1 |
| U.S. Treasury obligations | 120.1 |
| | — |
| | — |
| | — |
| | 120.1 |
| U.S. government agency obligations | — |
| | 472.9 |
| | — |
| | — |
| | 472.9 |
| Foreign government obligations | 6.4 |
| | — |
| | — |
| | — |
| | 6.4 |
| Agency mortgage-backed obligations | — |
| | 1,022.5 |
| | — |
| | — |
| | 1,022.5 |
| Asset-backed obligations | — |
| | 42.9 |
| | — |
| | — |
| | 42.9 |
| Derivatives | 0.8 |
| | 514.6 |
| | — |
| | (329.3 | ) | | 186.1 |
| Commodities leases | — |
| | 29.5 |
| | — |
| | (11.8 | ) | | 17.7 |
| Commodities warehouse receipts | 16.4 |
| | — |
| | — |
| | — |
| | 16.4 |
| Exchange firm common stock | 10.2 |
| | — |
| | — |
| | — |
| | 10.2 |
| Mutual funds and other | 6.3 |
| | — |
| | — |
| | — |
| | 6.3 |
| Financial instruments owned | 231.4 |
| | 2,164.5 |
| | — |
| | (341.1 | ) | | 2,054.8 |
| Physical commodities inventory | 42.1 |
| | 114.8 |
| | — |
| | — |
| | 156.9 |
| Total assets at fair value | $ | 9,202.6 |
| | $ | 2,303.9 |
| | $ | — |
| | $ | (8,130.3 | ) | | $ | 3,376.2 |
| Liabilities: | | | | | | | | | | TBA and forward settling securities | $ | — |
| | $ | 2.1 |
| | $ | — |
| | $ | (2.1 | ) | | $ | — |
| Derivatives | 7,809.3 |
| | 11.6 |
| | — |
| | (7,820.9 | ) | | — |
| Payable to broker-dealers, clearing organizations and counterparties | 7,809.3 |
| | 13.7 |
| | — |
| | (7,823.0 | ) | | — |
| Equity securities | 51.1 |
| | 0.4 |
| | — |
| | — |
| | 51.5 |
| Corporate and municipal bonds | — |
| | 20.1 |
| | — |
| | — |
| | 20.1 |
| U.S. Treasury obligations | 484.8 |
| | — |
| | — |
| | — |
| | 484.8 |
| U.S. government agency obligations | — |
| | 57.2 |
| | — |
| | — |
| | 57.2 |
| Agency mortgage-backed obligations | — |
| | 0.2 |
| | — |
| | — |
| | 0.2 |
| Derivatives | | | 688.0 |
| | — |
| | (494.6 | ) | | 193.4 |
| Commodities leases | — |
| | 75.5 |
| | — |
| | (16.2 | ) | | 59.3 |
| Financial instruments sold, not yet purchased | 535.9 |
| | 841.4 |
| | — |
| | (510.8 | ) | | 866.5 |
| Total liabilities at fair value | $ | 8,345.2 |
| | $ | 855.1 |
| | $ | — |
| | $ | (8,333.8 | ) | | $ | 866.5 |
|
| | (1) | Represents cash collateral and the impact of netting across the levels of the fair value hierarchy. Netting among positions classified within the same level are included in that level. |
|
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] |
The following tables set forth a summary of changes in the fair value of the Company’s Level 3 financial assets and liabilities during the years ended September 30, 2019 and 2018, including a summary of unrealized gains (losses) during the fiscal year ended on the Company’s Level 3 financial assets and liabilities held during the periods. | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Level 3 Financial Liabilities
For the Year Ended September 30, 2019 | (in millions) | Balances at
beginning of
period | | Realized (gains)
losses during
period | | Remeasurement
(gains) losses
during period | | Acquisitions | | Settlements | | Transfers in
or (out) of
Level 3 | | Balances at
end of period | Liabilities: | | | | | | | | | | | | | | Contingent liabilities | $ | — |
| | $ | — |
| | $ | — |
| | $ | 1.8 |
| | $ | — |
| | $ | — |
| | $ | 1.8 |
|
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Level 3 Financial Assets and Financial Liabilities
For the Year Ended September 30, 2018 | (in millions) | Balances at
beginning of
period | | Realized gains
(losses) during
period | | Unrealized
gains (losses)
during period | | Purchases/
issuances | | Settlements | | Transfers in
or (out) of
Level 3 | | Balances at
end of period | Assets: | | | | | | | | | | | | | | Equity securities | $ | 0.1 |
| | $ | — |
| | $ | (0.1 | ) | | $ | — |
| | $ | — |
| | $ | — |
| | $ | — |
| | | | | | | | | | | | | | | (in millions) | Balances at
beginning of
period | | Realized (gains)
losses during
period | | Remeasurement
(gains) losses
during period | | Acquisitions | | Settlements | | Transfers in
or (out) of
Level 3 | | Balances at
end of period | Liabilities: | | | | | | | | | | | | | | Contingent liabilities | $ | 1.0 |
| | $ | — |
| | $ | — |
| | $ | — |
| | $ | (1.0 | ) | | $ | — |
| | $ | — |
|
|