v3.26.1
Assets and Liabilities, at Fair Value - Schedule of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Sep. 30, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 76.6 $ 105.1
Securities and other assets segregated under federal and other regulations 888.5 950.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net 5,414.0 6,442.9
Receivables from clients, net - Derivatives 70.3 58.1
Financial instruments owned 11,135.0 8,604.4
Physical commodities inventory 1,165.5 917.5
Total assets at fair value 17,997.9 16,631.6
Accounts payable and other accrued liabilities - contingent liabilities 57.5  
Payables to clients - Derivatives 1,033.2 530.7
Financial instruments sold, not yet purchased 3,797.3 2,919.8
Total liabilities at fair value 4,915.0 3,521.2
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 34.4 49.9
Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties (7.4) (11.5)
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 27.0 38.4
Contingent Consideration    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities - contingent liabilities   32.3
Physical commodities inventory    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Physical commodities inventory 413.5 471.1
Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 1,077.8 562.7
Financial instruments sold, not yet purchased 659.7 371.7
Corporate and municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 586.9 485.7
Financial instruments sold, not yet purchased 202.9 264.1
U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 1,162.5 678.8
Financial instruments sold, not yet purchased 2,492.7 1,983.2
U.S. government agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 836.1 701.9
Financial instruments sold, not yet purchased   1.3
Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 4.6 4.1
Agency mortgage-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 6,739.9 5,378.9
Financial instruments sold, not yet purchased 0.1 1.1
Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 175.4 373.4
Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 235.1 190.4
Financial instruments sold, not yet purchased 420.0 297.3
Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 223.2 144.6
Exchange firm common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 25.4 47.0
Cash flow hedges    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 1.3 9.5
Financial instruments sold, not yet purchased 21.8 1.0
Mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 66.8 27.4
Other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments sold, not yet purchased 0.1 0.1
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 11.2 4.8
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 23.4 21.2
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 3,117.7 5,210.9
U.S. government agency 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 1,712.2 1,103.1
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 549.5 102.9
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 19.6 145.4
U.S. government agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 110.8 110.7
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 758.1 693.9
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 (6,284.6) (6,644.1)
Receivables from clients, net - Derivatives (1,132.8) (509.6)
Financial instruments owned (677.7) (468.0)
Total assets at fair value (8,095.1) (7,621.7)
Accounts payable and other accrued liabilities - contingent liabilities 0.0  
Payables to clients - Derivatives (4,844.1) (3,486.5)
Financial instruments sold, not yet purchased (627.1) (391.6)
Total liabilities at fair value (8,051.6) (7,594.3)
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 (37.9) (26.1)
Netting | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties (2,542.5) (3,690.1)
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 (2,580.4) (3,716.2)
Netting | Contingent Consideration    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities - contingent liabilities   0.0
Netting | Physical commodities inventory    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Physical commodities inventory 0.0 0.0
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
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
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
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
Netting | Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
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
Netting | Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Netting | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned (677.7) (468.0)
Financial instruments sold, not yet purchased (627.1) (391.6)
Netting | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
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
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
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
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
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 (36.5) (30.1)
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
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
Netting | U.S. government agency 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
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 (6,248.1) (6,614.0)
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
Netting | U.S. government agency 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 0.0
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 0.0
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 76.6 105.1
Securities and other assets segregated under federal and other regulations 777.7 839.3
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net 8,194.2 8,440.3
Receivables from clients, net - Derivatives 589.8 61.2
Financial instruments owned 2,541.7 1,451.5
Total assets at fair value 12,367.9 11,119.2
Accounts payable and other accrued liabilities - contingent liabilities 0.0  
Payables to clients - Derivatives 4,695.2 3,113.1
Financial instruments sold, not yet purchased 3,143.0 2,356.8
Total liabilities at fair value 8,669.8 5,667.4
Level 1 | 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 | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 831.6 197.5
Level 1 | 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 831.6 197.5
Level 1 | Contingent Consideration    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities - contingent liabilities   0.0
Level 1 | Physical commodities inventory    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Physical commodities inventory 187.9 221.8
Level 1 | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 1,066.1 556.0
Financial instruments sold, not yet purchased 647.2 367.3
Level 1 | 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 | U.S. Treasury obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 1,162.5 678.8
Financial instruments sold, not yet purchased 2,492.7 1,983.2
Level 1 | 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
Level 1 | Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Level 1 | 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 | Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Level 1 | 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 3.1 6.3
Level 1 | Commodities warehouse receipts    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 223.2 144.6
Level 1 | Exchange firm common stock    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 25.4 47.0
Level 1 | 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 | Mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 64.5 25.1
Level 1 | 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 | 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 | 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 23.4 21.2
Level 1 | 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 3,117.7 5,210.9
Level 1 | U.S. government agency 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 1 | 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 5,053.1 3,208.2
Level 1 | 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 19.6 145.4
Level 1 | U.S. government agency 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 0.0
Level 1 | 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 758.1 693.9
Level 2    
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 110.8 110.7
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net 3,504.4 4,646.7
Receivables from clients, net - Derivatives 613.3 506.5
Financial instruments owned 9,268.7 7,618.6
Total assets at fair value 13,722.8 13,131.8
Accounts payable and other accrued liabilities - contingent liabilities 0.0  
Payables to clients - Derivatives 1,182.1 904.1
Financial instruments sold, not yet purchased 1,281.3 954.5
Total liabilities at fair value 4,239.2 5,415.7
Level 2 | 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 72.3 76.0
Level 2 | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Payable to broker-dealers, clearing organizations and counterparties 1,703.5 3,481.1
Level 2 | 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 1,775.8 3,557.1
Level 2 | Contingent Consideration    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities - contingent liabilities   0.0
Level 2 | Physical commodities inventory    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Physical commodities inventory 225.6 249.3
Level 2 | Equity securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 11.7 6.7
Financial instruments sold, not yet purchased 12.5 4.4
Level 2 | Corporate and municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 586.9 485.7
Financial instruments sold, not yet purchased 202.9 264.1
Level 2 | 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 | U.S. government agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 836.1 701.9
Financial instruments sold, not yet purchased   1.3
Level 2 | Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 4.6 4.1
Level 2 | Agency mortgage-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 6,739.9 5,378.9
Financial instruments sold, not yet purchased 0.1 1.1
Level 2 | Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 175.4 373.4
Level 2 | Derivatives    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 912.8 658.4
Financial instruments sold, not yet purchased 1,044.0 682.6
Level 2 | 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 | 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 | Cash flow hedges    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 1.3 9.5
Financial instruments sold, not yet purchased 21.8 1.0
Level 2 | 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 | 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 | 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 47.7 34.9
Level 2 | 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 | 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 | U.S. government agency 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 1,712.2 1,103.1
Level 2 | 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 1,744.5 3,508.7
Level 2 | 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 | U.S. government agency obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities and other assets segregated under federal and other regulations 110.8 110.7
Level 2 | 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 0.0
Level 3    
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 2.3 2.3
Total assets at fair value 2.3 2.3
Accounts payable and other accrued liabilities - contingent liabilities 57.5  
Payables to clients - Derivatives 0.0 0.0
Financial instruments sold, not yet purchased 0.1 0.1
Total liabilities at fair value 57.6 32.4
Level 3 | 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 | 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 | 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 | Contingent Consideration    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Accounts payable and other accrued liabilities - contingent liabilities   32.3
Level 3 | 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 | 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 | 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 | 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 | 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
Level 3 | Foreign government obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Level 3 | 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 | Asset-backed obligations    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 0.0 0.0
Level 3 | 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 | 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 | 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 | 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 | Mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments owned 2.3 2.3
Level 3 | Other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial instruments sold, not yet purchased 0.1 0.1
Level 3 | 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 | 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 | 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 | U.S. government agency 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 | 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 | 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 | U.S. government agency 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 0.0
Level 3 | 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 0.0
Certificates of deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets: 22.1 10.3
Certificates of deposit | 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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets: 22.1 10.3
Certificates of deposit | Level 2    
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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets: 0.0 0.0
Money market mutual funds and other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets: 54.5 94.8
Money market mutual funds and other | Netting    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets: 0.0 0.0
Money market mutual funds and other | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets: 54.5 94.8
Money market mutual funds and other | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets: 0.0 0.0
Money market mutual funds and other | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Assets: $ 0.0 $ 0.0