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Assets and Liabilities, at Fair Value - Schedule of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Sep. 30, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 309.0 $ 51.8
Securities and other assets segregated under federal and other regulations 3,156.6 2,841.2
Financial instruments owned 6,918.1 6,767.1
Physical commodities inventory 861.4 681.1
Accounts payable and other accrued liabilities 441.8 522.1
Financial instruments sold, not yet purchased 3,541.6 2,853.3
Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 45.8 49.2
Securities and other assets segregated under federal and other regulations 309.0 51.8
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net 4,781.3 3,287.5
Receivables from clients, net - Derivatives (2.8) (8.4)
Financial instruments owned 6,918.1 6,767.1
Total assets at fair value 12,528.1 10,523.8
Accounts payable and other accrued liabilities 2.9  
Payables to clients - Derivatives 371.1 265.9
Total liabilities at fair value 3,953.1 3,120.1
Fair Value, Recurring | To be announced and forward settling securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 15.9 1.5
Fair Value, Recurring | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 21.6 (2.9)
Fair Value, Recurring | Payable to broker-dealers, clearing organizations and counterparties    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 37.5 (1.4)
Fair Value, Recurring | Accounts payable and other accrued liabilities - contingent liabilities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities   2.3
Fair Value, Recurring | Physical commodities inventory    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Physical commodities inventory 476.7 376.6
Fair Value, Recurring | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 623.7 1,088.6
Financial instruments sold, not yet purchased 2,583.2 2,139.3
Fair Value, Recurring | Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.8 41.4
Financial instruments sold, not yet purchased 0.8 41.0
Fair Value, Recurring | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 226.0 228.1
Financial instruments sold, not yet purchased 288.9 264.8
Fair Value, Recurring | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 540.3 378.9
Financial instruments sold, not yet purchased 429.8 224.8
Fair Value, Recurring | Corporate and municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 475.6 322.1
Financial instruments sold, not yet purchased 230.3 154.6
Fair Value, Recurring | U.S. government agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 488.7 531.0
Financial instruments sold, not yet purchased 0.2 0.2
Fair Value, Recurring | Agency mortgage-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 4,197.1 3,837.2
Financial instruments sold, not yet purchased 1.7 23.6
Fair Value, Recurring | Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 265.5 223.5
Financial instruments sold, not yet purchased 5.0 3.9
Fair Value, Recurring | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 61.0 67.8
Fair Value, Recurring | Exchange firm common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 13.9 13.2
Fair Value, Recurring | Cash flow hedges    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 1.2 9.6
Financial instruments sold, not yet purchased 0.9 0.2
Fair Value, Recurring | Mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 24.3 25.7
Fair Value, Recurring | Other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments sold, not yet purchased 0.8 0.9
Fair Value, Recurring | To be announced and forward settling securities    
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 49.3 7.8
Fair Value, Recurring | Foreign government obligations    
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 16.7 18.3
Fair Value, Recurring | U.S. Treasury obligations    
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 4,241.5 2,933.2
Fair Value, Recurring | Derivatives    
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 473.8 328.2
Fair Value, Recurring | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 9.1 51.8
Fair Value, Recurring | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 299.9  
Fair Value, Recurring | Netting    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0.0 0.0
Securities and other assets segregated under federal and other regulations 0.0 0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net (6,047.5) (5,758.4)
Receivables from clients, net - Derivatives (495.3) (537.0)
Financial instruments owned (593.2) (375.2)
Total assets at fair value (7,136.0) (6,670.6)
Accounts payable and other accrued liabilities 0.0  
Payables to clients - Derivatives (3,271.1) (3,540.1)
Financial instruments sold, not yet purchased (513.2) (314.3)
Total liabilities at fair value (7,026.4) (6,614.6)
Fair Value, Recurring | Netting | To be announced and forward settling securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties (15.0) (22.9)
Fair Value, Recurring | Netting | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties (3,227.1) (2,737.3)
Fair Value, Recurring | Netting | Payable to broker-dealers, clearing organizations and counterparties    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties (3,242.1) (2,760.2)
Fair Value, Recurring | Netting | Accounts payable and other accrued liabilities - contingent liabilities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities   0.0
Fair Value, Recurring | Netting | Physical commodities inventory    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Physical commodities inventory 0.0 0.0
Fair Value, Recurring | Netting | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Fair Value, Recurring | Netting | Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Fair Value, Recurring | Netting | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned (593.2) (375.2)
Financial instruments sold, not yet purchased (513.2) (314.3)
Fair Value, Recurring | Netting | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Fair Value, Recurring | Netting | Corporate and municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Fair Value, Recurring | Netting | U.S. government agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Fair Value, Recurring | Netting | Agency mortgage-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Fair Value, Recurring | Netting | Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Fair Value, Recurring | Netting | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Fair Value, Recurring | Netting | Exchange firm common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Fair Value, Recurring | Netting | Cash flow hedges    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Fair Value, Recurring | Netting | Mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Fair Value, Recurring | Netting | Other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments sold, not yet purchased 0.0 0.0
Fair Value, Recurring | Netting | To be announced and forward settling securities    
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 (24.7) (18.3)
Fair Value, Recurring | Netting | Foreign government obligations    
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, Recurring | Netting | U.S. Treasury obligations    
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, Recurring | Netting | Derivatives    
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 (6,022.8) (5,740.1)
Fair Value, Recurring | Netting | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 0.0 0.0
Fair Value, Recurring | Netting | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 0.0  
Level 1 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 45.8 49.2
Securities and other assets segregated under federal and other regulations 309.0 51.8
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net 7,965.1 6,851.6
Receivables from clients, net - Derivatives (7.0) 22.4
Financial instruments owned 1,237.5 1,598.6
Total assets at fair value 9,751.4 8,781.2
Accounts payable and other accrued liabilities 0.0  
Payables to clients - Derivatives 3,401.7 3,577.1
Financial instruments sold, not yet purchased 3,007.6 2,408.2
Total liabilities at fair value 6,688.6 6,363.3
Level 1 | Fair Value, Recurring | To be announced and forward settling securities    
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
Level 1 | Fair Value, Recurring | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 279.3 378.0
Level 1 | Fair Value, Recurring | Payable to broker-dealers, clearing organizations and counterparties    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 279.3 378.0
Level 1 | Fair Value, Recurring | Accounts payable and other accrued liabilities - contingent liabilities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities   0.0
Level 1 | Fair Value, Recurring | Physical commodities inventory    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Physical commodities inventory 201.0 207.6
Level 1 | Fair Value, Recurring | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 623.7 1,088.6
Financial instruments sold, not yet purchased 2,583.2 2,139.3
Level 1 | Fair Value, Recurring | Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.8 41.4
Financial instruments sold, not yet purchased 0.8 41.0
Level 1 | Fair Value, Recurring | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.1
Financial instruments sold, not yet purchased 3.3 8.1
Level 1 | Fair Value, Recurring | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 515.9 363.9
Financial instruments sold, not yet purchased 420.3 219.8
Level 1 | Fair Value, Recurring | Corporate and municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 1 | Fair Value, Recurring | U.S. government agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 1 | Fair Value, Recurring | Agency mortgage-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 1 | Fair Value, Recurring | Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 1 | Fair Value, Recurring | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 61.0 67.8
Level 1 | Fair Value, Recurring | Exchange firm common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 13.9 13.2
Level 1 | Fair Value, Recurring | Cash flow hedges    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 1 | Fair Value, Recurring | Mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 22.2 23.6
Level 1 | Fair Value, Recurring | Other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments sold, not yet purchased 0.0 0.0
Level 1 | Fair Value, Recurring | To be announced and forward settling securities    
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
Level 1 | Fair Value, Recurring | Foreign government obligations    
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 16.7 18.3
Level 1 | Fair Value, Recurring | U.S. Treasury obligations    
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 4,241.5 2,933.2
Level 1 | Fair Value, Recurring | Derivatives    
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 3,706.9 3,900.1
Level 1 | Fair Value, Recurring | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 9.1 51.8
Level 1 | Fair Value, Recurring | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 299.9  
Level 2 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0.0 0.0
Securities and other assets segregated under federal and other regulations 0.0 0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net 2,863.7 2,194.3
Receivables from clients, net - Derivatives 499.5 506.2
Financial instruments owned 6,271.7 5,541.6
Total assets at fair value 9,910.6 8,411.1
Accounts payable and other accrued liabilities 0.0  
Payables to clients - Derivatives 240.5 228.9
Financial instruments sold, not yet purchased 1,046.4 758.5
Total liabilities at fair value 4,287.2 3,368.2
Level 2 | Fair Value, Recurring | To be announced and forward settling securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 30.9 24.4
Level 2 | Fair Value, Recurring | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 2,969.4 2,356.4
Level 2 | Fair Value, Recurring | Payable to broker-dealers, clearing organizations and counterparties    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 3,000.3 2,380.8
Level 2 | Fair Value, Recurring | Accounts payable and other accrued liabilities - contingent liabilities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities   0.0
Level 2 | Fair Value, Recurring | Physical commodities inventory    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Physical commodities inventory 275.7 169.0
Level 2 | Fair Value, Recurring | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 2 | Fair Value, Recurring | Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 2 | Fair Value, Recurring | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 819.2 603.2
Financial instruments sold, not yet purchased 798.8 571.0
Level 2 | Fair Value, Recurring | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 24.4 15.0
Financial instruments sold, not yet purchased 9.5 5.0
Level 2 | Fair Value, Recurring | Corporate and municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 475.6 322.1
Financial instruments sold, not yet purchased 230.3 154.6
Level 2 | Fair Value, Recurring | U.S. government agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 488.7 531.0
Financial instruments sold, not yet purchased 0.2 0.2
Level 2 | Fair Value, Recurring | Agency mortgage-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 4,197.1 3,837.2
Financial instruments sold, not yet purchased 1.7 23.6
Level 2 | Fair Value, Recurring | Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 265.5 223.5
Financial instruments sold, not yet purchased 5.0 3.9
Level 2 | Fair Value, Recurring | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Level 2 | Fair Value, Recurring | Exchange firm common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Level 2 | Fair Value, Recurring | Cash flow hedges    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 1.2 9.6
Financial instruments sold, not yet purchased 0.9 0.2
Level 2 | Fair Value, Recurring | Mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Level 2 | Fair Value, Recurring | Other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments sold, not yet purchased 0.0 0.0
Level 2 | Fair Value, Recurring | To be announced and forward settling securities    
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 74.0 26.1
Level 2 | Fair Value, Recurring | Foreign government obligations    
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
Level 2 | Fair Value, Recurring | U.S. Treasury obligations    
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
Level 2 | Fair Value, Recurring | Derivatives    
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 2,789.7 2,168.2
Level 2 | Fair Value, Recurring | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 0.0 0.0
Level 2 | Fair Value, Recurring | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 0.0  
Level 3 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0.0 0.0
Securities and other assets 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 0.0
Financial instruments owned 2.1 2.1
Total assets at fair value 2.1 2.1
Accounts payable and other accrued liabilities 2.9  
Payables to clients - Derivatives 0.0 0.0
Financial instruments sold, not yet purchased 0.8 0.9
Total liabilities at fair value 3.7 3.2
Level 3 | Fair Value, Recurring | To be announced and forward settling securities    
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
Level 3 | Fair Value, Recurring | Derivatives    
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
Level 3 | Fair Value, Recurring | Payable to broker-dealers, clearing organizations and counterparties    
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
Level 3 | Fair Value, Recurring | Accounts payable and other accrued liabilities - contingent liabilities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities   2.3
Level 3 | Fair Value, Recurring | Physical commodities inventory    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Physical commodities inventory 0.0 0.0
Level 3 | Fair Value, Recurring | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 3 | Fair Value, Recurring | Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 3 | Fair Value, Recurring | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 3 | Fair Value, Recurring | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 3 | Fair Value, Recurring | Corporate and municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 3 | Fair Value, Recurring | U.S. government agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 3 | Fair Value, Recurring | Agency mortgage-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 3 | Fair Value, Recurring | Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 3 | Fair Value, Recurring | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Level 3 | Fair Value, Recurring | Exchange firm common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Level 3 | Fair Value, Recurring | Cash flow hedges    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Financial instruments sold, not yet purchased 0.0 0.0
Level 3 | Fair Value, Recurring | Mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 2.1 2.1
Level 3 | Fair Value, Recurring | Other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments sold, not yet purchased 0.8 0.9
Level 3 | Fair Value, Recurring | To be announced and forward settling securities    
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
Level 3 | Fair Value, Recurring | Foreign government obligations    
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
Level 3 | Fair Value, Recurring | U.S. Treasury obligations    
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
Level 3 | Fair Value, Recurring | Derivatives    
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
Level 3 | Fair Value, Recurring | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 0.0 0.0
Level 3 | Fair Value, Recurring | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 0.0  
Certificates of deposit | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets 10.2 13.9
Certificates of deposit | Fair Value, Recurring | Netting    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets 0.0 0.0
Certificates of deposit | Level 1 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets 10.2 13.9
Certificates of deposit | Level 2 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets 0.0 0.0
Certificates of deposit | Level 3 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets 0.0 0.0
Money market mutual funds | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets 35.6 35.3
Money market mutual funds | Fair Value, Recurring | Netting    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets 0.0 0.0
Money market mutual funds | Level 1 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets 35.6 35.3
Money market mutual funds | Level 2 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets 0.0 0.0
Money market mutual funds | Level 3 | Fair Value, Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets $ 0.0 $ 0.0