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Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions | Mar. 31, 2017 | Sep. 30, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and Securities Segregated under Federal and Other Regulations | $ 1,085.5 | $ 1,136.3
Receivables from Clearing Organizations | 1,679.4 | 1,524.4
Receivables from Brokers-Dealers and Clearing Organizations | 170.3 | 237.0
Financial Instruments, Owned, at Fair Value | 1,906.7 | 1,606.1
Physical commodities inventory | 181.7 | 123.8
Assets, Fair Value Disclosure | 2,946.7 | 3,156.5
Accounts Payable and Other Accrued Liabilities | 147.4 | 161.3
Payables to Broker-Dealers and Clearing Organizations | 178.2 | 260.1
Financial Instruments Sold, Not yet Purchased, at Fair Value | 1,081.2 | 839.4
Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and Securities Segregated under Federal and Other Regulations | 247.6 | 618.8
Receivables from Clearing Organizations | 636.5 | 868.5
Receivables from Brokers-Dealers and Clearing Organizations | 34.7 | (15.2)
Financial Instruments, Owned, at Fair Value | 1,906.7 | 1,606.1
Assets, Fair Value Disclosure | 2,946.7 | 3,156.5
Payables to Broker-Dealers and Clearing Organizations | 9.8 | 3.5
Financial Instruments Sold, Not yet Purchased, at Fair Value | 1,081.2 | 839.4
Liabilities, Fair Value Disclosure | 1,091.9 | 843.7
Fair Value, Measurements, Recurring [Member] | TBA and forward settling securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payables to Broker-Dealers and Clearing Organizations | 9.9 | 3.5
Fair Value, Measurements, Recurring [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payables to Broker-Dealers and Clearing Organizations | (0.1) | 0.0
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 | 0.9 | 0.8
Fair Value, Measurements, Recurring [Member] | Physical commodities inventory - precious metals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Physical commodities inventory | 117.8 | 71.2
Fair Value, Measurements, Recurring [Member] | Common stock and American Depositary Receipts (ADRs) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 30.7 | 36.5
Financial Instruments Sold, Not yet Purchased, at Fair Value | 45.9 | 23.9
Fair Value, Measurements, Recurring [Member] | Exchangeable foreign ordinary equities and ADRs [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 31.7 | 25.7
Financial Instruments Sold, Not yet Purchased, at Fair Value | 31.9 | 25.8
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 6.2 | 40.8
Financial Instruments Sold, Not yet Purchased, at Fair Value | 1.2 | 6.9
Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 625.4 | 514.9
Financial Instruments Sold, Not yet Purchased, at Fair Value | 699.9 | 509.8
Fair Value, Measurements, Recurring [Member] | Foreign Government Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 49.4 | 14.6
Financial Instruments Sold, Not yet Purchased, at Fair Value | 10.1 |
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 926.6 | 747.5
Financial Instruments Sold, Not yet Purchased, at Fair Value | 4.7 |
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 | 198.4 | 193.9
Financial Instruments Sold, Not yet Purchased, at Fair Value | 229.6 | 210.9
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 | 17.2 | 8.1
Financial Instruments Sold, Not yet Purchased, at Fair Value | 57.9 | 62.1
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 | 8.0 | 8.9
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 | 7.1 | 6.4
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 | 6.0 | 8.8
Fair Value, Measurements, Recurring [Member] | TBA and forward settling securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Brokers-Dealers and Clearing Organizations | 5.3 | 0.3
Fair Value, Measurements, Recurring [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Brokers-Dealers and Clearing Organizations | 29.4 | (15.5)
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | 376.0 | 512.7
Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | 281.8 | 472.1
Fair Value, Measurements, Recurring [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | (21.3) | (116.3)
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 | 21.9 | 23.3
Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and Securities Segregated under Federal and Other Regulations | 225.7 | 595.5
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and Cash Equivalents, Fair Value Disclosure | 3.4 | 7.1
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [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
Receivables from Clearing Organizations | (1,820.3) | (2,266.2)
Receivables from Brokers-Dealers and Clearing Organizations | (60.7) | (23.5)
Financial Instruments, Owned, at Fair Value | (1,587.0) | (1,492.9)
Assets, Fair Value Disclosure | (3,468.0) | (3,782.6)
Payables to Broker-Dealers and Clearing Organizations | (1,664.8) | (2,058.3)
Financial Instruments Sold, Not yet Purchased, at Fair Value | (1,714.2) | (1,453.5)
Liabilities, Fair Value Disclosure | (3,379.0) | (3,511.8)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | TBA and forward settling securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payables to Broker-Dealers and Clearing Organizations | (0.9) | 0.9
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payables to Broker-Dealers and Clearing Organizations | (1,663.9) | (2,059.2)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Physical commodities inventory - precious metals [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Common stock and American Depositary Receipts (ADRs) [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Exchangeable foreign ordinary equities and ADRs [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Corporate Debt 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] | Counterparty And Cash Collateral Netting Adjustment [Member] | US Government Debt 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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Foreign Government Debt 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 |
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Collateralized Mortgage Backed 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 |
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | (1,487.0) | (1,363.8)
Financial Instruments Sold, Not yet Purchased, at Fair Value | (1,607.3) | (1,307.8)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Commodities leases [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | (100.0) | (129.1)
Financial Instruments Sold, Not yet Purchased, at Fair Value | (106.9) | (145.7)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | TBA and forward settling securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Brokers-Dealers and Clearing Organizations | (0.9) | 0.0
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Brokers-Dealers and Clearing Organizations | (59.8) | (23.5)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Money Market Funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | US Government Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | (1,820.3) | (2,266.2)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | US Government Debt Securities [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and Cash Equivalents, Fair Value Disclosure | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and Securities Segregated under Federal and Other Regulations | 21.9 | 23.3
Receivables from Clearing Organizations | 2,175.0 | 2,662.6
Receivables from Brokers-Dealers and Clearing Organizations | 0.0 | 0.0
Financial Instruments, Owned, at Fair Value | 352.2 | 327.7
Assets, Fair Value Disclosure | 2,670.3 | 3,091.9
Payables to Broker-Dealers and Clearing Organizations | 1,560.8 | 1,961.7
Financial Instruments Sold, Not yet Purchased, at Fair Value | 339.2 | 255.1
Liabilities, Fair Value Disclosure | 1,900.0 | 2,216.8
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | TBA and forward settling securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payables to Broker-Dealers and Clearing Organizations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payables to Broker-Dealers and Clearing Organizations | 1,560.8 | 1,961.7
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, Level 1 [Member] | Physical commodities inventory - precious metals [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Physical commodities inventory | 117.8 | 71.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Common stock and American Depositary Receipts (ADRs) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 28.5 | 34.6
Financial Instruments Sold, Not yet Purchased, at Fair Value | 45.4 | 23.5
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Exchangeable foreign ordinary equities and ADRs [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 31.3 | 25.2
Financial Instruments Sold, Not yet Purchased, at Fair Value | 31.6 | 25.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 4.9 | 36.9
Financial Instruments Sold, Not yet Purchased, at Fair Value | 1.2 | 6.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Debt 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] | Fair Value, Inputs, Level 1 [Member] | Foreign Government Debt 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 |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Collateralized Mortgage Backed 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 |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 266.4 | 206.9
Financial Instruments Sold, Not yet Purchased, at Fair Value | 261.0 | 199.4
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, 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 | 8.0 | 8.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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 | 7.1 | 6.4
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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 | 6.0 | 8.8
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | TBA and forward settling securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Brokers-Dealers and Clearing Organizations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Brokers-Dealers and Clearing Organizations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | 376.0 | 512.7
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | 0.0 | 0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | 1,799.0 | 2,149.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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 | 21.9 | 23.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Debt Securities [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] | Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and Cash Equivalents, Fair Value Disclosure | 3.4 | 7.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and Securities Segregated under Federal and Other Regulations | 225.7 | 595.5
Receivables from Clearing Organizations | 281.8 | 472.1
Receivables from Brokers-Dealers and Clearing Organizations | 95.4 | 8.3
Financial Instruments, Owned, at Fair Value | 3,141.3 | 2,768.1
Assets, Fair Value Disclosure | 3,744.2 | 3,844.0
Payables to Broker-Dealers and Clearing Organizations | 113.8 | 100.1
Financial Instruments Sold, Not yet Purchased, at Fair Value | 2,456.2 | 2,037.8
Liabilities, Fair Value Disclosure | 2,570.0 | 2,137.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | TBA and forward settling securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payables to Broker-Dealers and Clearing Organizations | 10.8 | 2.6
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payables to Broker-Dealers and Clearing Organizations | 103.0 | 97.5
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, Level 2 [Member] | Physical commodities inventory - precious metals [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] | Fair Value, Inputs, Level 2 [Member] | Common stock and American Depositary Receipts (ADRs) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 2.0 | 1.7
Financial Instruments Sold, Not yet Purchased, at Fair Value | 0.5 | 0.4
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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 | 49.4 | 14.6
Financial Instruments Sold, Not yet Purchased, at Fair Value | 10.1 |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 926.6 | 747.5
Financial Instruments Sold, Not yet Purchased, at Fair Value | 4.7 |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 117.2 | 137.2
Financial Instruments Sold, Not yet Purchased, at Fair Value | 164.8 | 207.8
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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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] |  |
Receivables from Brokers-Dealers and Clearing Organizations | 6.2 | 0.3
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | 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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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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Cash and Securities Segregated under Federal and Other Regulations | 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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Assets, Fair Value Disclosure | 0.2 | 3.2
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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