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Assets and Liabilities, at Fair Value - Schedule of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 13.4	$ 51.8
Securities and other assets segregated under federal and other regulations	2,924.3	2,841.2
Financial instruments owned	8,973.9	6,767.1
Physical commodities inventory	705.4	681.1
Accounts payable and other accrued liabilities	579.1	522.1
Financial instruments sold, not yet purchased, at fair value	3,707.7	2,853.3
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	57.9	49.2
Securities and other assets segregated under federal and other regulations	13.4	51.8
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	3,250.6	3,287.5
Receivables from clients, net - Derivatives	18.2	(8.4)
Financial instruments owned	8,973.9	6,767.1
Total assets at fair value	12,711.0	10,523.8
Accounts payable and other accrued liabilities	2.6
Payables to clients - Derivatives	(103.5)	265.9
Total liabilities at fair value	3,472.6	3,120.1
Fair Value, Recurring | Contingent Consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities		2.3
Fair Value, Recurring | To be announced and forward settling securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	64.6	1.5
Fair Value, Recurring | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	(198.8)	(2.9)
Fair Value, Recurring | Payable to broker-dealers, clearing organizations and counterparties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	(134.2)	(1.4)
Fair Value, Recurring | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	397.0	376.6
Fair Value, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	456.5	378.9
Financial instruments sold, not yet purchased, at fair value	430.2	224.8
Fair Value, Recurring | Corporate and municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	474.2	322.1
Financial instruments sold, not yet purchased, at fair value	268.5	154.6
Fair Value, Recurring | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,193.5	1,088.6
Financial instruments sold, not yet purchased, at fair value	2,791.6	2,139.3
Fair Value, Recurring | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	346.6	531.0
Financial instruments sold, not yet purchased, at fair value		0.2
Fair Value, Recurring | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		41.4
Financial instruments sold, not yet purchased, at fair value		41.0
Fair Value, Recurring | Agency mortgage-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,775.2	3,837.2
Financial instruments sold, not yet purchased, at fair value	7.0	23.6
Fair Value, Recurring | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	362.8	223.5
Financial instruments sold, not yet purchased, at fair value	0.7	3.9
Fair Value, Recurring | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	167.9	228.1
Financial instruments sold, not yet purchased, at fair value	207.7	264.8
Fair Value, Recurring | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		0.0
Fair Value, Recurring | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	134.2	67.8
Fair Value, Recurring | Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	16.5	13.2
Fair Value, Recurring | Cash flow hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	17.3	9.6
Financial instruments sold, not yet purchased, at fair value		0.2
Fair Value, Recurring | Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	29.2	25.7
Fair Value, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	2.0	0.9
Fair Value, Recurring | To be announced and forward settling securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	6.9	7.8
Fair Value, Recurring | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	20.1	18.3
Fair Value, Recurring | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	3,291.9	2,933.2
Fair Value, Recurring | 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	99.4
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	(167.7)	328.2
Fair Value, Recurring | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	13.4	51.8
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
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	(5,575.6)	(5,758.4)
Receivables from clients, net - Derivatives	(607.8)	(537.0)
Financial instruments owned	(506.4)	(375.2)
Total assets at fair value	(6,689.8)	(6,670.6)
Accounts payable and other accrued liabilities	0.0
Payables to clients - Derivatives	(3,084.9)	(3,540.1)
Financial instruments sold, not yet purchased, at fair value	(452.8)	(314.3)
Total liabilities at fair value	(6,659.6)	(6,614.6)
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		0.0
Fair Value, Recurring | Netting | To be announced and forward settling securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	(53.7)	(22.9)
Fair Value, Recurring | Netting | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	(3,068.2)	(2,737.3)
Fair Value, Recurring | Netting | Payable to broker-dealers, clearing organizations and counterparties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	(3,121.9)	(2,760.2)
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, at fair value	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, at fair value	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, at fair value	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, at fair value		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
Financial instruments sold, not yet purchased, at fair value		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, at fair value	0.0	0.0
Fair Value, Recurring | Netting | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	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	(506.4)	(375.2)
Financial instruments sold, not yet purchased, at fair value	(452.8)	(314.3)
Fair Value, Recurring | Netting | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		0.0
Fair Value, Recurring | Netting | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Fair Value, Recurring | Netting | Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Fair Value, Recurring | Netting | Cash flow hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Financial instruments sold, not yet purchased, at fair value		0.0
Fair Value, Recurring | Netting | Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Fair Value, Recurring | Netting | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Recurring | Netting | To be announced and forward settling securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	(54.6)	(18.3)
Fair Value, Recurring | Netting | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	0.0	0.0
Fair Value, Recurring | Netting | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	0.0	0.0
Fair Value, Recurring | Netting | 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
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	(5,521.0)	(5,740.1)
Fair Value, Recurring | Netting | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	0.0	0.0
Fair Value, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	57.9	49.2
Securities and other assets segregated under federal and other regulations	13.4	51.8
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	5,978.5	6,851.6
Receivables from clients, net - Derivatives	96.8	22.4
Financial instruments owned	1,811.3	1,598.6
Total assets at fair value	8,173.9	8,781.2
Accounts payable and other accrued liabilities	0.0
Payables to clients - Derivatives	2,616.4	3,577.1
Financial instruments sold, not yet purchased, at fair value	3,221.1	2,408.2
Total liabilities at fair value	6,027.2	6,363.3
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		0.0
Fair Value, Recurring | 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
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	189.7	378.0
Fair Value, Recurring | 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	189.7	378.0
Fair Value, Recurring | Level 1 | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	216.0	207.6
Fair Value, Recurring | Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	439.6	363.9
Financial instruments sold, not yet purchased, at fair value	425.0	219.8
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, at fair value	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	1,193.5	1,088.6
Financial instruments sold, not yet purchased, at fair value	2,791.6	2,139.3
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, at fair value		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		41.4
Financial instruments sold, not yet purchased, at fair value		41.0
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, at fair value	0.0	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	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Recurring | Level 1 | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.4	0.1
Financial instruments sold, not yet purchased, at fair value	4.5	8.1
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
Fair Value, Recurring | Level 1 | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	134.2	67.8
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	16.5	13.2
Fair Value, Recurring | 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, at fair value		0.0
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	27.1	23.6
Fair Value, Recurring | Level 1 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Recurring | 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
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	20.1	18.3
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	3,291.9	2,933.2
Fair Value, Recurring | 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
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	2,666.5	3,900.1
Fair Value, Recurring | 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	13.4	51.8
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
Securities and other assets segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	2,847.7	2,194.3
Receivables from clients, net - Derivatives	529.2	506.2
Financial instruments owned	7,666.9	5,541.6
Total assets at fair value	11,224.8	8,411.1
Accounts payable and other accrued liabilities	0.0
Payables to clients - Derivatives	365.0	228.9
Financial instruments sold, not yet purchased, at fair value	938.4	758.5
Total liabilities at fair value	4,101.4	3,368.2
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		0.0
Fair Value, Recurring | 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	118.3	24.4
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	2,679.7	2,356.4
Fair Value, Recurring | 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	2,798.0	2,380.8
Fair Value, Recurring | Level 2 | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	181.0	169.0
Fair Value, Recurring | Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	16.9	15.0
Financial instruments sold, not yet purchased, at fair value	5.2	5.0
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	474.2	322.1
Financial instruments sold, not yet purchased, at fair value	268.5	154.6
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, at fair value	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	346.6	531.0
Financial instruments sold, not yet purchased, at fair value		0.2
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
Financial instruments sold, not yet purchased, at fair value		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	5,775.2	3,837.2
Financial instruments sold, not yet purchased, at fair value	7.0	23.6
Fair Value, Recurring | Level 2 | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	362.8	223.5
Financial instruments sold, not yet purchased, at fair value	0.7	3.9
Fair Value, Recurring | Level 2 | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	673.9	603.2
Financial instruments sold, not yet purchased, at fair value	656.0	571.0
Fair Value, Recurring | Level 2 | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		0.0
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 | Cash flow hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	17.3	9.6
Financial instruments sold, not yet purchased, at fair value		0.2
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 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	1.0	0.0
Fair Value, Recurring | 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	61.5	26.1
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 | 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	99.4
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	2,686.8	2,168.2
Fair Value, Recurring | 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
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
Securities and other assets segregated under federal and other regulations	0.0	0.0
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