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Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions | Jun. 30, 2016 | Sep. 30, 2015
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and Securities Segregated under Federal and Other Regulations | $ 974.0 | $ 756.9
Receivables from Clearing Organizations | 1,483.5 | 1,533.5
Receivables from Brokers-Dealers and Clearing Organizations | 206.6 | 277.6
Financial Instruments, Owned, at Fair Value | 1,977.2 | 1,421.9
Physical commodities inventory | 106.6 | 32.8
Assets, Fair Value Disclosure | 3,720.4 | 2,910.4
Accounts Payable and Other Accrued Liabilities | 137.2 | 144.8
Payables to Broker-Dealers and Clearing Organizations | 254.0 | 262.9
Financial Instruments Sold, Not yet Purchased, at Fair Value | 871.6 | 568.3
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 | 631.3 | 515.5
Receivables from Clearing Organizations | 1,082.0 | 1,009.4
Receivables from Brokers-Dealers and Clearing Organizations | (14.8) | (52.9)
Financial Instruments, Owned, at Fair Value | 1,977.2 | 1,421.9
Assets, Fair Value Disclosure | 3,720.4 | 2,910.4
Payables to Broker-Dealers and Clearing Organizations | 3.5 | 1.6
Financial Instruments Sold, Not yet Purchased, at Fair Value | 871.6 | 568.3
Liabilities, Fair Value Disclosure | 875.9 | 573.2
Fair Value, Measurements, Recurring [Member] | TBA and forward settling securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Payables to Broker-Dealers and Clearing Organizations | 3.5 | 1.6
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 | 0.0 | 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 | 0.8 | 3.3
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 | 40.6 | 15.2
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 | 32.1 | 26.1
Financial Instruments Sold, Not yet Purchased, at Fair Value | 21.4 | 18.6
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 | 19.4 | 89.5
Financial Instruments Sold, Not yet Purchased, at Fair Value | 19.6 | 90.0
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 | 34.6 | 31.3
Financial Instruments Sold, Not yet Purchased, at Fair Value | 0.1 |
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 | 689.9 | 513.4
Financial Instruments Sold, Not yet Purchased, at Fair Value | 544.9 | 341.0
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 | 12.4 | 12.1
Financial Instruments Sold, Not yet Purchased, at Fair Value |  | 6.4
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 | 907.4 | 699.5
Financial Instruments Sold, Not yet Purchased, at Fair Value |  | 2.8
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 | 236.2 | 30.6
Financial Instruments Sold, Not yet Purchased, at Fair Value | 225.5 | 54.1
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 | 30.4 | 7.6
Financial Instruments Sold, Not yet Purchased, at Fair Value | 60.1 | 55.4
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 |  | 2.8
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 | 5.9 | 5.6
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 | 8.9 | 3.4
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 | 1.5 | 0.2
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 | (16.3) | (53.1)
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 | 713.9 | 431.8
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 | 320.1 | 501.4
Fair Value, Measurements, Recurring [Member] | Derivative [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Clearing Organizations | 48.0 | 76.2
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 | 33.9 | 22.1
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 | 597.4 | 493.4
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 | 4.1 | 1.3
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 | (2,643.9) | (3,539.7)
Receivables from Brokers-Dealers and Clearing Organizations | (364.0) | (592.1)
Financial Instruments, Owned, at Fair Value | (3,474.1) | (2,006.9)
Assets, Fair Value Disclosure | (6,482.0) | (6,138.7)
Payables to Broker-Dealers and Clearing Organizations | (2,953.9) | (4,021.0)
Financial Instruments Sold, Not yet Purchased, at Fair Value | (3,231.0) | (1,977.1)
Liabilities, Fair Value Disclosure | (6,184.9) | (5,998.1)
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 | (1.9) | (1.0)
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] |  |
Payables to Broker-Dealers and Clearing Organizations | (2,952.0) | (4,020.0)
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 |
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 | (3,307.4) | (1,949.9)
Financial Instruments Sold, Not yet Purchased, at Fair Value | (3,130.3) | (1,933.4)
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 | (166.7) | (57.0)
Financial Instruments Sold, Not yet Purchased, at Fair Value | (100.7) | (43.7)
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
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 | (1.9) | (1.0)
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 | (362.1) | (591.1)
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 | (2,643.9) | (3,539.7)
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] |  |
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 | 33.9 | 22.1
Receivables from Clearing Organizations | 3,405.8 | 4,047.7
Receivables from Brokers-Dealers and Clearing Organizations | 0.0 | 0.1
Financial Instruments, Owned, at Fair Value | 456.5 | 423.0
Assets, Fair Value Disclosure | 3,940.9 | 4,509.4
Payables to Broker-Dealers and Clearing Organizations | 2,572.5 | 3,491.3
Financial Instruments Sold, Not yet Purchased, at Fair Value | 388.2 | 371.0
Liabilities, Fair Value Disclosure | 2,960.7 | 3,862.3
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 | 2,572.5 | 3,491.3
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 | 40.6 | 15.2
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 | 29.4 | 23.7
Financial Instruments Sold, Not yet Purchased, at Fair Value | 21.2 | 18.0
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 | 17.5 | 82.9
Financial Instruments Sold, Not yet Purchased, at Fair Value | 18.2 | 89.0
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 | 30.7 | 26.1
Financial Instruments Sold, Not yet Purchased, at Fair Value |  |
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 | 364.1 | 278.5
Financial Instruments Sold, Not yet Purchased, at Fair Value | 348.8 | 264.0
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 |  | 2.8
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 | 5.9 | 5.6
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 | 8.9 | 3.4
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 | 0.1
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 | 713.9 | 431.8
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 | 2,691.9 | 3,615.9
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 | 33.9 | 22.1
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 | 4.1 | 1.3
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 | 597.4 | 493.4
Receivables from Clearing Organizations | 320.1 | 501.4
Receivables from Brokers-Dealers and Clearing Organizations | 349.2 | 539.1
Financial Instruments, Owned, at Fair Value | 4,991.5 | 3,002.1
Assets, Fair Value Disclosure | 6,258.2 | 4,536.0
Payables to Broker-Dealers and Clearing Organizations | 384.9 | 531.3
Financial Instruments Sold, Not yet Purchased, at Fair Value | 3,714.4 | 2,174.4
Liabilities, Fair Value Disclosure | 4,099.3 | 2,705.7
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 | 5.4 | 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 | 379.5 | 528.7
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 | 2.4 | 1.9
Financial Instruments Sold, Not yet Purchased, at Fair Value | 0.2 | 0.6
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Financial Instruments Sold, Not yet Purchased, at Fair Value | 1.4 | 1.0
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Financial Instruments, Owned, at Fair Value | 0.9 | 2.0
Financial Instruments Sold, Not yet Purchased, at Fair Value | 0.1 |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 689.9 | 513.4
Financial Instruments Sold, Not yet Purchased, at Fair Value | 544.9 | 341.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [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 | 12.4 | 12.1
Financial Instruments Sold, Not yet Purchased, at Fair Value |  | 6.4
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 907.4 | 699.5
Financial Instruments Sold, Not yet Purchased, at Fair Value |  | 2.8
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Financial Instruments, Owned, at Fair Value | 3,179.5 | 1,702.0
Financial Instruments Sold, Not yet Purchased, at Fair Value | 3,007.0 | 1,723.5
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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Financial Instruments Sold, Not yet Purchased, at Fair Value | 160.8 | 99.1
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Financial Instruments, Owned, at Fair Value |  | 0.0
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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Receivables from Brokers-Dealers and Clearing Organizations | 3.4 | 1.2
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Receivables from Brokers-Dealers and Clearing Organizations | 345.8 | 537.9
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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] |  |
Cash and Securities Segregated under Federal and Other Regulations | 597.4 | 493.4
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Receivables from Clearing Organizations | 0.0 | 0.0
Receivables from Brokers-Dealers and Clearing Organizations | 0.0 | 0.0
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Assets, Fair Value Disclosure | 3.3 | 3.7
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