Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value	$ 14.1	$ 2.6
Cash and securities segregated under federal and other regulations	2,274.4	1,920.2
Financial instruments, owned, at fair value	4,354.6	2,727.7
Physical commodities inventory	447.5	281.1
Accounts payable and other accrued liabilities	305.1	272.6
Payable to broker-dealers, clearing organizations and counterparties	613.5	537.5
Financial instruments sold, not yet purchased, at fair value	1,771.2	686.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value	20.8	16.0
Cash and securities segregated under federal and other regulations	14.1	2.6
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	1,070.6	1,775.8
Receivables from clients, net - Derivatives	2.6	1.5
Financial instruments, owned, at fair value	4,354.6	2,727.7
Total assets at fair value	5,822.6	4,739.3
Payables to clients - Derivatives	291.5	(222.7)
Liabilities, fair value disclosure	2,078.2	479.5
Fair Value, Measurements, Recurring | TBA 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	12.7	10.2
Fair Value, Measurements, 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	4.5
Fair Value, Measurements, Recurring | Payables 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	12.7	14.7
Fair Value, Measurements, Recurring | Contingent Consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities	2.8	1.5
Fair Value, Measurements, Recurring | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	359.9	215.7
Fair Value, Measurements, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	527.0	264.3
Financial instruments sold, not yet purchased, at fair value	436.1	232.9
Fair Value, Measurements, Recurring | Corporate and municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	150.8	66.9
Financial instruments sold, not yet purchased, at fair value	51.1	22.5
Fair Value, Measurements, Recurring | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	433.1	419.9
Financial instruments sold, not yet purchased, at fair value	851.2	247.5
Fair Value, Measurements, Recurring | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	327.6	293.4
Financial instruments sold, not yet purchased, at fair value		0.1
Fair Value, Measurements, Recurring | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value		2.5
Fair Value, Measurements, Recurring | Agency mortgage-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	2,599.8	1,384.6
Financial instruments sold, not yet purchased, at fair value	63.4	5.1
Fair Value, Measurements, Recurring | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	110.4	33.0
Fair Value, Measurements, Recurring | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	144.3	117.4
Financial instruments sold, not yet purchased, at fair value	368.5	176.8
Fair Value, Measurements, Recurring | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	18.1	24.9
Financial instruments sold, not yet purchased, at fair value	0.9	1.1
Fair Value, Measurements, Recurring | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	21.4	103.2
Fair Value, Measurements, Recurring | Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	11.6	10.1
Fair Value, Measurements, Recurring | Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	10.5	7.5
Fair Value, Measurements, Recurring | TBA 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	19.0	19.2
Fair Value, Measurements, 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	12.2	8.0
Fair Value, Measurements, 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	798.5	1,941.3
Fair Value, Measurements, 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	240.9	(192.7)
Fair Value, Measurements, Recurring | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	13.9	2.4
Fair Value, Measurements, Recurring | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.2	0.2
Fair Value, Measurements, Recurring | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	7.9	3.2
Fair Value, Measurements, Recurring | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	12.9	12.8
Fair Value, Measurements, Recurring | Netting
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value	0.0	0.0
Cash and securities segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	(4,442.4)	(2,549.3)
Receivables from clients, net - Derivatives	(411.1)	(234.1)
Financial instruments, owned, at fair value	(500.4)	(535.6)
Total assets at fair value	(5,353.9)	(3,319.0)
Payables to clients - Derivatives	(4,278.4)	(2,399.9)
Financial instruments sold, not yet purchased, at fair value	(335.0)	(386.8)
Liabilities, fair value disclosure	(4,955.4)	(3,100.7)
Fair Value, Measurements, Recurring | Netting | TBA 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	(39.5)	(11.8)
Fair Value, Measurements, Recurring | Netting | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	(302.5)	(302.2)
Fair Value, Measurements, Recurring | Netting | Payables 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	(342.0)	(314.0)
Fair Value, Measurements, Recurring | Netting | Contingent Consideration
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 | 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, Measurements, Recurring | Netting | Equity securities
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 | Netting | Corporate and municipal bonds
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 | Netting | U.S. Treasury obligations
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 | Netting | U.S. government agency obligations
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 | Netting | Foreign government obligations
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 | Netting | Agency mortgage-backed obligations
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 | Netting | Asset-backed obligations
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 | Netting | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	(500.4)	(535.6)
Financial instruments sold, not yet purchased, at fair value	(335.0)	(386.8)
Fair Value, Measurements, Recurring | Netting | Commodities leases
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 | Netting | Commodities warehouse receipts
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 | Netting | Exchange firm common stock
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 | Netting | Mutual funds and other
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 | Netting | TBA 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	(40.1)	(11.8)
Fair Value, Measurements, 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, Measurements, 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, Measurements, 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	(4,402.3)	(2,537.5)
Fair Value, Measurements, Recurring | Netting | Commodities warehouse receipts
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 | Netting | U.S. Treasury obligations
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 | Netting | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Netting | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value	20.8	16.0
Cash and securities segregated under federal and other regulations	14.1	2.6
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	5,286.5	3,898.3
Receivables from clients, net - Derivatives	0.0	0.0
Financial instruments, owned, at fair value	989.6	798.8
Total assets at fair value	6,422.2	4,742.5
Payables to clients - Derivatives	4,413.8	2,000.8
Financial instruments sold, not yet purchased, at fair value	1,284.3	465.5
Liabilities, fair value disclosure	5,761.2	2,466.3
Fair Value, Measurements, Recurring | Level 1 | TBA 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
Fair Value, Measurements, Recurring | Level 1 | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	63.1	0.0
Fair Value, Measurements, Recurring | Level 1 | Payables 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	63.1	0.0
Fair Value, Measurements, Recurring | Level 1 | Contingent Consideration
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 | Level 1 | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	111.2	26.8
Fair Value, Measurements, Recurring | Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	512.4	254.9
Financial instruments sold, not yet purchased, at fair value	429.9	218.0
Fair Value, Measurements, Recurring | Level 1 | Corporate and municipal bonds
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 | Level 1 | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	433.1	419.9
Financial instruments sold, not yet purchased, at fair value	851.2	247.5
Fair Value, Measurements, Recurring | Level 1 | U.S. government agency obligations
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 | Level 1 | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value		2.5
Fair Value, Measurements, Recurring | Level 1 | Agency mortgage-backed obligations
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 | Level 1 | Asset-backed obligations
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 | Level 1 | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.6	0.7
Financial instruments sold, not yet purchased, at fair value	3.2	0.0
Fair Value, Measurements, Recurring | Level 1 | Commodities leases
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 | Level 1 | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	21.4	103.2
Fair Value, Measurements, Recurring | Level 1 | Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	11.6	10.1
Fair Value, Measurements, Recurring | Level 1 | Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	10.5	7.5
Fair Value, Measurements, Recurring | Level 1 | TBA 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
Fair Value, Measurements, Recurring | 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	12.2	8.0
Fair Value, Measurements, Recurring | 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	798.5	1,941.3
Fair Value, Measurements, Recurring | 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	4,475.8	1,949.0
Fair Value, Measurements, Recurring | Level 1 | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	13.9	2.4
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.2	0.2
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	7.9	3.2
Fair Value, Measurements, Recurring | Level 1 | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	12.9	12.8
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value	0.0	0.0
Cash and securities segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	226.5	426.8
Receivables from clients, net - Derivatives	413.7	235.6
Financial instruments, owned, at fair value	3,865.4	2,464.5
Total assets at fair value	4,754.3	3,315.8
Payables to clients - Derivatives	156.1	176.4
Financial instruments sold, not yet purchased, at fair value	821.9	607.3
Liabilities, fair value disclosure	1,269.6	1,112.4
Fair Value, Measurements, Recurring | Level 2 | TBA 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	52.2	22.0
Fair Value, Measurements, Recurring | Level 2 | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	239.4	306.7
Fair Value, Measurements, Recurring | Level 2 | Payables 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	291.6	328.7
Fair Value, Measurements, Recurring | Level 2 | Contingent Consideration
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 | Level 2 | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	248.7	188.9
Fair Value, Measurements, Recurring | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	14.6	9.4
Financial instruments sold, not yet purchased, at fair value	6.2	14.9
Fair Value, Measurements, Recurring | Level 2 | Corporate and municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	150.8	66.9
Financial instruments sold, not yet purchased, at fair value	51.1	22.5
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury obligations
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 | Level 2 | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	327.6	293.4
Financial instruments sold, not yet purchased, at fair value		0.1
Fair Value, Measurements, Recurring | Level 2 | Foreign government obligations
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 | Level 2 | Agency mortgage-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	2,599.8	1,384.6
Financial instruments sold, not yet purchased, at fair value	63.4	5.1
Fair Value, Measurements, Recurring | Level 2 | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	110.4	33.0
Fair Value, Measurements, Recurring | Level 2 | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	644.1	652.3
Financial instruments sold, not yet purchased, at fair value	700.3	563.6
Fair Value, Measurements, Recurring | Level 2 | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	18.1	24.9
Financial instruments sold, not yet purchased, at fair value	0.9	1.1
Fair Value, Measurements, Recurring | Level 2 | Commodities warehouse receipts
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 | Level 2 | Exchange firm common stock
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 | Level 2 | Mutual funds and other
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 | Level 2 | TBA 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	59.1	31.0
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | 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	167.4	395.8
Fair Value, Measurements, Recurring | Level 2 | Commodities warehouse receipts
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 | Level 2 | U.S. Treasury obligations
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 | Level 2 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Level 2 | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value	0.0	0.0
Cash and securities segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	0.0	0.0
Receivables from clients, net - Derivatives		0.0
Financial instruments, owned, at fair value	0.0	0.0
Total assets at fair value	0.0	0.0
Payables to clients - Derivatives	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Liabilities, fair value disclosure	2.8	1.5
Fair Value, Measurements, Recurring | Level 3 | TBA 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
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | Level 3 | Payables 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
Fair Value, Measurements, Recurring | Level 3 | Contingent Consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities	2.8	1.5
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | Level 3 | Equity securities
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 | Level 3 | Corporate and municipal bonds
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 | Level 3 | U.S. Treasury obligations
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 | Level 3 | U.S. government agency obligations
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 | Level 3 | Foreign government obligations
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 | Level 3 | Agency mortgage-backed obligations
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 | Level 3 | Asset-backed obligations
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 | Level 3 | Derivatives
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 | Level 3 | Commodities leases
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 | Level 3 | Commodities warehouse receipts
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 | Level 3 | Exchange firm common stock
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 | Level 3 | Mutual funds and other
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 | Level 3 | TBA 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
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | 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
Fair Value, Measurements, Recurring | Level 3 | Commodities warehouse receipts
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 | Level 3 | U.S. Treasury obligations
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 | Level 3 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Fair Value, Measurements, Recurring | Level 3 | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 0.0	$ 0.0