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Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Sep. 30, 2019
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and securities segregated under federal and other regulations | $ 1,920.2 | $ 1,049.9
Financial instruments, owned, at fair value | 2,727.7 | 2,175.2
Physical commodities inventory | 281.1 | 229.3
Accounts payable and other accrued liabilities | 272.6 | 157.5
Payable to broker-dealers, clearing organizations and counterparties | 537.5 | 266.2
Financial instruments sold, not yet purchased, at fair value | 686.0 | 714.8
Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 16.0 |
Cash and securities segregated under federal and other regulations | 2.6 | 306.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 1,775.8 | 626.9
Receivables from clients, net - Derivatives | 1.5 |
Financial instruments, owned, at fair value | 2,727.7 | 2,175.2
Total assets at fair value | 4,739.3 | 3,273.8
Payables to clients - Derivatives | (222.7) |
Liabilities, fair value disclosure | 479.5 | 722.2
Fair Value, Measurements, Recurring [Member] | TBA and Forward Settling Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 10.2 | 5.3
Fair Value, Measurements, Recurring [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 4.5 | 0.3
Fair Value, Measurements, Recurring [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 14.7 | 5.6
Fair Value, Measurements, Recurring [Member] | Contingent Consideration [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Accounts payable and other accrued liabilities | 1.5 | 1.8
Fair Value, Measurements, Recurring [Member] | Physical commodities inventory [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Physical commodities inventory | 215.7 | 151.9
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 264.3 | 168.5
Financial instruments sold, not yet purchased, at fair value | 232.9 | 158.1
Fair Value, Measurements, Recurring [Member] | Corporate and Municipal Bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 66.9 | 80.0
Financial instruments sold, not yet purchased, at fair value | 22.5 | 39.2
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 419.9 | 248.7
Financial instruments sold, not yet purchased, at fair value | 247.5 | 272.3
Fair Value, Measurements, Recurring [Member] | U.S. Government Agency Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 293.4 | 447.1
Financial instruments sold, not yet purchased, at fair value | 0.1 | 43.8
Fair Value, Measurements, Recurring [Member] | Foreign Government Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 2.5 | 0.5
Fair Value, Measurements, Recurring [Member] | Agency Mortgage-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 1,384.6 | 1,045.0
Financial instruments sold, not yet purchased, at fair value | 5.1 | 29.6
Fair Value, Measurements, Recurring [Member] | Asset-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 33.0 | 29.1
Fair Value, Measurements, Recurring [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 117.4 | 66.5
Financial instruments sold, not yet purchased, at fair value | 176.8 | 58.1
Fair Value, Measurements, Recurring [Member] | Commodities Leases[Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 24.9 | 28.6
Financial instruments sold, not yet purchased, at fair value | 1.1 | 113.7
Fair Value, Measurements, Recurring [Member] | Commodities Warehouse Receipts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 103.2 | 48.4
Fair Value, Measurements, Recurring [Member] | Exchange firm Common Stock [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 10.1 | 12.7
Fair Value, Measurements, Recurring [Member] | Mutual Funds and Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 7.5 | 0.1
Fair Value, Measurements, Recurring [Member] | TBA and Forward Settling Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 19.2 | 8.3
Fair Value, Measurements, Recurring [Member] | Foreign Government Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 8.0 | 9.9
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 1,941.3 | 593.9
Fair Value, Measurements, Recurring [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | (192.7) | 14.8
Fair Value, Measurements, Recurring [Member] | Commodities Warehouse Receipts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and securities segregated under federal and other regulations | 2.4 | 6.2
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and securities segregated under federal and other regulations | 0.2 | 299.8
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 3.2 | 4.9
Cash and cash equivalents |  | 13.8
Fair Value, Measurements, Recurring [Member] | Money Market Mutual Funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 12.8 | 8.9
Fair Value, Measurements, Recurring [Member] | Netting [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 0.0 |
Cash and securities segregated under federal and other regulations | 0.0 | 0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | (2,549.3) | (3,161.1)
Receivables from clients, net - Derivatives | (234.1) |
Financial instruments, owned, at fair value | (535.6) | (420.8)
Total assets at fair value | (3,319.0) | (3,581.9)
Payables to clients - Derivatives | (2,399.9) |
Financial instruments sold, not yet purchased, at fair value | (386.8) | (422.2)
Liabilities, fair value disclosure | (3,100.7) | (3,540.8)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | TBA and Forward Settling Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | (11.8) | (1.5)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | (302.2) | (3,117.1)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | (314.0) | (3,118.6)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Contingent Consideration [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Accounts payable and other accrued liabilities | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Physical commodities inventory [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Physical commodities inventory | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Corporate and Municipal Bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | U.S. Government Agency Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Foreign Government Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Agency Mortgage-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Asset-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | (535.6) | (420.8)
Financial instruments sold, not yet purchased, at fair value | (386.8) | (422.2)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Commodities Leases[Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Commodities Warehouse Receipts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Exchange firm Common Stock [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Mutual Funds and Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | TBA and Forward Settling Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | (11.8) | (1.5)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Foreign Government Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | (2,537.5) | (3,159.6)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Commodities Warehouse Receipts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and securities segregated under federal and other regulations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and securities segregated under federal and other regulations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 0.0 | 0.0
Cash and cash equivalents |  | 0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Money Market Mutual Funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 16.0 | 13.8
Cash and securities segregated under federal and other regulations | 2.6 | 306.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 3,898.3 | 3,735.0
Receivables from clients, net - Derivatives | 0.0 |
Financial instruments, owned, at fair value | 798.8 | 470.9
Total assets at fair value | 4,742.5 | 4,532.8
Payables to clients - Derivatives | 2,000.8 |
Financial instruments sold, not yet purchased, at fair value | 465.5 | 419.6
Liabilities, fair value disclosure | 2,466.3 | 3,498.7
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | TBA and Forward Settling Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 0.0 | 3,079.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 0.0 | 3,079.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Contingent Consideration [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Accounts payable and other accrued liabilities | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Physical commodities inventory [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Physical commodities inventory | 26.8 | 7.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 254.9 | 159.5
Financial instruments sold, not yet purchased, at fair value | 218.0 | 147.3
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate and Municipal Bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 419.9 | 248.7
Financial instruments sold, not yet purchased, at fair value | 247.5 | 272.3
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Government Agency Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Foreign Government Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 2.5 | 0.5
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Agency Mortgage-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Asset-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.7 | 1.0
Financial instruments sold, not yet purchased, at fair value |  | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodities Leases[Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodities Warehouse Receipts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 103.2 | 48.4
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Exchange firm Common Stock [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 10.1 | 12.7
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Mutual Funds and Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 7.5 | 0.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | TBA and Forward Settling Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Foreign Government Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 8.0 | 9.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 1,941.3 | 593.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 1,949.0 | 3,131.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodities Warehouse Receipts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and securities segregated under federal and other regulations | 2.4 | 6.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and securities segregated under federal and other regulations | 0.2 | 299.8
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 3.2 | 4.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money Market Mutual Funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 12.8 | 8.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 0.0 |
Cash and securities segregated under federal and other regulations | 0.0 | 0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 426.8 | 53.0
Receivables from clients, net - Derivatives | 235.6 |
Financial instruments, owned, at fair value | 2,464.5 | 2,125.1
Total assets at fair value | 3,315.8 | 2,322.9
Payables to clients - Derivatives | 176.4 |
Financial instruments sold, not yet purchased, at fair value | 607.3 | 717.4
Liabilities, fair value disclosure | 1,112.4 | 762.5
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | TBA and Forward Settling Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 22.0 | 6.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 306.7 | 38.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 328.7 | 45.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Contingent Consideration [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Accounts payable and other accrued liabilities | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Physical commodities inventory [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Physical commodities inventory | 188.9 | 144.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 9.4 | 9.0
Financial instruments sold, not yet purchased, at fair value | 14.9 | 10.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate and Municipal Bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 66.9 | 80.0
Financial instruments sold, not yet purchased, at fair value | 22.5 | 39.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Government Agency Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 293.4 | 447.1
Financial instruments sold, not yet purchased, at fair value | 0.1 | 43.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Government Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Agency Mortgage-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 1,384.6 | 1,045.0
Financial instruments sold, not yet purchased, at fair value | 5.1 | 29.6
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Asset-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 33.0 | 29.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 652.3 | 486.3
Financial instruments sold, not yet purchased, at fair value | 563.6 | 480.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodities Leases[Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 24.9 | 28.6
Financial instruments sold, not yet purchased, at fair value | 1.1 | 113.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodities Warehouse Receipts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Exchange firm Common Stock [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Mutual Funds and Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | TBA and Forward Settling Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 31.0 | 9.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Government Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 395.8 | 43.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodities Warehouse Receipts [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and securities segregated under federal and other regulations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and securities segregated under federal and other regulations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 0.0 | 0.0
Cash and cash equivalents |  | 0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Money Market Mutual Funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 0.0 |
Cash and securities segregated under federal and other regulations | 0.0 | 0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
Receivables from clients, net - Derivatives | 0.0 |
Financial instruments, owned, at fair value | 0.0 | 0.0
Total assets at fair value | 0.0 | 0.0
Payables to clients - Derivatives | 0.0 |
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Liabilities, fair value disclosure | 1.5 | 1.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | TBA and Forward Settling Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payable to broker-dealers, clearing organizations and counterparties | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Contingent Consideration [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Accounts payable and other accrued liabilities | 1.5 | 1.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Physical commodities inventory [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Physical commodities inventory | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Equity Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate and Municipal Bonds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Treasury Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Government Agency Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Foreign Government Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Agency Mortgage-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Asset-backed Obligations [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
Financial instruments sold, not yet purchased, at fair value | 0.0 | 0.0
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial instruments, owned, at fair value | 0.0 | 0.0
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Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
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Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net | 0.0 | 0.0
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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Cash and cash equivalents |  | 0.0
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets | $ 0.0 | $ 0.0

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