Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 1,075.6	$ 952.6	$ 465.2
Cash and cash equivalents	96.1	2.6
Cash and securities segregated under federal and other regulation	2,325.5	1,920.2
Financial instruments owned	3,467.2	2,727.7
Physical commodities inventory	484.8	281.1
Accounts payable and other accrued liabilities - contingent liabilities	302.7	272.6
Payable to broker-dealers, clearing organizations and counterparties	375.5	537.5
Financial instruments sold, not yet purchased	1,764.2	686.0
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	20.2	16.0
Cash and securities segregated under federal and other regulation	96.1	2.6
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	1,028.8	1,775.8
Receivables from clients - Derivatives	5.1	1.5
Financial instruments owned	3,467.2	2,727.7
Total assets at fair value	4,993.7	4,739.3
Payables to clients - Derivatives	(495.7)	(222.7)
Total liabilities at fair value	1,281.3	479.5
Fair Value, 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	12.4	10.2
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	0.0	4.5
Fair Value, 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	12.4	14.7
Fair Value, Recurring | Contingent Consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities - contingent liabilities	0.4	1.5
Fair Value, Recurring | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	376.3	215.7
Fair Value, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	502.3	264.3
Financial instruments sold, not yet purchased	404.3	232.9
Fair Value, Recurring | Corporate and municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	150.5	66.9
Financial instruments sold, not yet purchased	44.4	22.5
Fair Value, Recurring | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	442.7	419.9
Financial instruments sold, not yet purchased	823.1	247.5
Fair Value, Recurring | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	274.9	293.4
Financial instruments sold, not yet purchased	2.5	0.1
Fair Value, Recurring | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	3.7	2.5
Fair Value, Recurring | Agency mortgage-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,738.8	1,384.6
Financial instruments sold, not yet purchased	11.7	5.1
Fair Value, Recurring | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	105.9	33.0
Fair Value, Recurring | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	198.0	117.4
Financial instruments sold, not yet purchased	477.1	176.8
Fair Value, Recurring | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	22.1	24.9
Financial instruments sold, not yet purchased	1.1	1.1
Fair Value, Recurring | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5.7	103.2
Fair Value, Recurring | Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	12.8	10.1
Fair Value, Recurring | Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9.8	7.5
Fair Value, 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	21.6	19.2
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	13.2	8.0
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	749.5	1,941.3
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	244.5	(192.7)
Fair Value, 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 regulation	18.8	2.4
Fair Value, 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 regulation	77.3	0.2
Fair Value, Recurring | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	5.3	3.2
Fair Value, Recurring | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	14.9	12.8
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
Cash and securities segregated under federal and other regulation	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	(4,120.0)	(2,549.3)
Receivables from clients - Derivatives	(335.4)	(234.1)
Financial instruments owned	(715.4)	(535.6)
Total assets at fair value	(5,170.8)	(3,319.0)
Payables to clients - Derivatives	(4,659.3)	(2,399.9)
Financial instruments sold, not yet purchased	(592.4)	(386.8)
Total liabilities at fair value	(5,308.7)	(3,100.7)
Fair Value, 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	(4.1)	(11.8)
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	(52.9)	(302.2)
Fair Value, 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	(57.0)	(314.0)
Fair Value, Recurring | 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	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 | 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. 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 | 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 | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	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
Fair Value, Recurring | Netting | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	(715.4)	(535.6)
Financial instruments sold, not yet purchased	(592.4)	(386.8)
Fair Value, Recurring | Netting | Commodities leases
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 | 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 | 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	(11.8)
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	(4,120.0)	(2,537.5)
Fair Value, 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 regulation	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]
Cash and securities segregated under federal and other regulation	0.0	0.0
Fair Value, Recurring | Netting | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Fair Value, Recurring | Netting | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Fair Value, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	20.2	16.0
Cash and securities segregated under federal and other regulation	96.1	2.6
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	4,878.1	3,898.3
Receivables from clients - Derivatives	0.0	0.0
Financial instruments owned	959.5	798.8
Total assets at fair value	6,089.0	4,742.5
Payables to clients - Derivatives	3,858.6	2,000.8
Financial instruments sold, not yet purchased	1,232.0	465.5
Total liabilities at fair value	5,090.6	2,466.3
Fair Value, 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, 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	0.0	0.0
Fair Value, 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	0.0	0.0
Fair Value, Recurring | 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	0.0
Fair Value, Recurring | Level 1 | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	135.1	26.8
Fair Value, Recurring | Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	484.1	254.9
Financial instruments sold, not yet purchased	397.7	218.0
Fair Value, Recurring | 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
Fair Value, Recurring | Level 1 | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	442.7	419.9
Financial instruments sold, not yet purchased	823.1	247.5
Fair Value, Recurring | 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	0.0
Fair Value, Recurring | Level 1 | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	3.7	2.5
Fair Value, Recurring | 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	0.2	0.0
Fair Value, Recurring | Level 1 | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.2	0.0
Fair Value, Recurring | Level 1 | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.5	0.7
Financial instruments sold, not yet purchased	11.0	0.0
Fair Value, Recurring | Level 1 | Commodities leases
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 | Level 1 | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5.7	103.2
Fair Value, Recurring | Level 1 | Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	12.8	10.1
Fair Value, Recurring | Level 1 | Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9.8	7.5
Fair Value, 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, 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	13.2	8.0
Fair Value, 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	749.5	1,941.3
Fair Value, 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	4,115.4	1,949.0
Fair Value, 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 regulation	18.8	2.4
Fair Value, 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 regulation	77.3	0.2
Fair Value, Recurring | Level 1 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	5.3	3.2
Fair Value, Recurring | Level 1 | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	14.9	12.8
Fair Value, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Cash and securities segregated under federal and other regulation	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	270.7	426.8
Receivables from clients - Derivatives	340.5	235.6
Financial instruments owned	3,223.1	2,464.5
Total assets at fair value	4,075.5	3,315.8
Payables to clients - Derivatives	305.0	176.4
Financial instruments sold, not yet purchased	1,124.6	607.3
Total liabilities at fair value	1,499.0	1,112.4
Fair Value, 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	16.5	22.0
Fair Value, 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	52.9	306.7
Fair Value, 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	69.4	328.7
Fair Value, Recurring | 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	0.0
Fair Value, Recurring | Level 2 | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	241.2	188.9
Fair Value, Recurring | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	18.2	9.4
Financial instruments sold, not yet purchased	6.6	14.9
Fair Value, Recurring | Level 2 | Corporate and municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	150.5	66.9
Financial instruments sold, not yet purchased	44.4	22.5
Fair Value, Recurring | 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
Fair Value, Recurring | Level 2 | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	274.9	293.4
Financial instruments sold, not yet purchased	2.5	0.1
Fair Value, Recurring | Level 2 | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Fair Value, Recurring | Level 2 | Agency mortgage-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,738.8	1,384.6
Financial instruments sold, not yet purchased	11.5	5.1
Fair Value, Recurring | Level 2 | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	105.7	33.0
Fair Value, Recurring | Level 2 | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	912.9	652.3
Financial instruments sold, not yet purchased	1,058.5	563.6
Fair Value, Recurring | Level 2 | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	22.1	24.9
Financial instruments sold, not yet purchased	1.1	1.1
Fair Value, Recurring | 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
Fair Value, Recurring | 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
Fair Value, Recurring | 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
Fair Value, 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	21.6	31.0
Fair Value, 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, 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, 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	249.1	395.8
Fair Value, 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 regulation	0.0	0.0
Fair Value, 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 regulation	0.0	0.0
Fair Value, Recurring | Level 2 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Fair Value, Recurring | Level 2 | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Fair Value, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Cash and securities segregated under federal and other regulation	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	0.0	0.0
Receivables from clients - Derivatives
Financial instruments owned	0.0	0.0
Total assets at fair value	0.0	0.0
Payables to clients - Derivatives
Financial instruments sold, not yet purchased	0.0	0.0
Total liabilities at fair value	0.4	1.5
Fair Value, 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, 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, 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, Recurring | 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	0.4	1.5
Fair Value, 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, Recurring | 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
Fair Value, Recurring | 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
Fair Value, Recurring | 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
Fair Value, Recurring | 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	0.0
Fair Value, Recurring | 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
Fair Value, Recurring | 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
Fair Value, Recurring | 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
Fair Value, Recurring | 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
Fair Value, Recurring | Level 3 | Commodities leases
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 | 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
Fair Value, Recurring | 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
Fair Value, Recurring | Level 3 | 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 | 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, 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, 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, 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, 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 regulation	0.0	0.0
Fair Value, 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 regulation	0.0	0.0
Fair Value, Recurring | Level 3 | Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Fair Value, Recurring | Level 3 | Money market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 0.0	$ 0.0