Assets and Liabilities, at Fair Value - Schedule of Financial and Non-Financial Assets and Liabilities Accounted at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 950.0	$ 51.8
Securities and other assets segregated under federal and other regulations	5,271.0	2,841.2
Financial instruments owned	8,604.4	6,767.1
Physical commodities inventory	917.5	681.1
Accounts payable and other accrued liabilities - contingent liabilities	769.1	522.1
Financial instruments sold, not yet purchased	2,919.8	2,853.3
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	105.1	49.2
Securities and other assets segregated under federal and other regulations	950.0	51.8
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	6,442.9	3,287.5
Receivable from clients, net - Derivatives	58.1	(8.4)
Financial instruments owned	8,604.4	6,767.1
Total assets at fair value	16,631.6	10,523.8
Payables to clients - Derivatives	530.7	265.9
Financial instruments sold, not yet purchased	2,919.8	2,853.3
Total liabilities at fair value	3,521.2	3,120.1
Fair Value, Measurements, Recurring \| Accounts payable and other accrued liabilities - contingent liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities - contingent liabilities	32.3	2.3
Fair Value, Measurements, 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	49.9	1.5
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	(11.5)	(2.9)
Fair Value, Measurements, 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	38.4	(1.4)
Fair Value, Measurements, Recurring \| Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	471.1	376.6
Fair Value, Measurements, Recurring \| Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	562.7	378.9
Financial instruments sold, not yet purchased	371.7	224.8
Fair Value, Measurements, Recurring \| Corporate and municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	485.7	322.1
Financial instruments sold, not yet purchased	264.1	154.6
Fair Value, Measurements, Recurring \| U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	678.8	1,088.6
Financial instruments sold, not yet purchased	1,983.2	2,139.3
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	701.9	531.0
Financial instruments sold, not yet purchased	1.3	0.2
Fair Value, Measurements, Recurring \| Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	4.1	41.4
Financial instruments sold, not yet purchased		41.0
Fair Value, Measurements, Recurring \| Agency mortgage-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,378.9	3,837.2
Financial instruments sold, not yet purchased	1.1	23.6
Fair Value, Measurements, Recurring \| Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	373.4	223.5
Financial instruments sold, not yet purchased		3.9
Fair Value, Measurements, Recurring \| Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	190.4	228.1
Financial instruments sold, not yet purchased	297.3	264.8
Fair Value, Measurements, Recurring \| Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	144.6	67.8
Fair Value, Measurements, Recurring \| Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	47.0	13.2
Fair Value, Measurements, Recurring \| Cash flow hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9.5	9.6
Financial instruments sold, not yet purchased	1.0	0.2
Fair Value, Measurements, Recurring \| Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	27.4	25.7
Fair Value, Measurements, Recurring \| Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased	0.1	0.9
Fair Value, Measurements, 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	4.8	7.8
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	21.2	18.3
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	5,210.9	2,933.2
Fair Value, Measurements, 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	1,103.1
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	102.9	328.2
Fair Value, Measurements, 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	145.4	51.8
Fair Value, Measurements, Recurring \| U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	110.7
Fair Value, Measurements, Recurring \| U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	693.9
Fair Value, Measurements, Recurring \| Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	10.3	13.9
Fair Value, Measurements, Recurring \| Money market mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	94.8	35.3
Fair Value, Measurements, Recurring \| Netting
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Securities and other assets segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	(6,644.1)	(5,758.4)
Receivable from clients, net - Derivatives	(509.6)	(537.0)
Financial instruments owned	(468.0)	(375.2)
Total assets at fair value	(7,621.7)	(6,670.6)
Payables to clients - Derivatives	(3,486.5)	(3,540.1)
Financial instruments sold, not yet purchased	(391.6)	(314.3)
Total liabilities at fair value	(7,594.3)	(6,614.6)
Fair Value, Measurements, Recurring \| Netting \| Accounts payable and other accrued liabilities - contingent liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities - contingent liabilities	0.0	0.0
Fair Value, Measurements, 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	(26.1)	(22.9)
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	(3,690.1)	(2,737.3)
Fair Value, Measurements, 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	(3,716.2)	(2,760.2)
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	0.0	0.0
Financial instruments sold, not yet purchased	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	0.0	0.0
Financial instruments sold, not yet purchased	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	0.0	0.0
Financial instruments sold, not yet purchased	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	0.0	0.0
Financial instruments sold, not yet purchased	0.0	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	0.0	0.0
Financial instruments sold, not yet purchased		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	0.0	0.0
Financial instruments sold, not yet purchased	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	0.0	0.0
Financial instruments sold, not yet purchased		0.0
Fair Value, Measurements, Recurring \| Netting \| Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	(468.0)	(375.2)
Financial instruments sold, not yet purchased	(391.6)	(314.3)
Fair Value, Measurements, 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, Measurements, 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, Measurements, Recurring \| Netting \| Cash flow hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Financial instruments sold, not yet purchased	0.0	0.0
Fair Value, Measurements, 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, Measurements, Recurring \| Netting \| Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased	0.0	0.0
Fair Value, Measurements, 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	(30.1)	(18.3)
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 \| 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, 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	(6,614.0)	(5,740.1)
Fair Value, Measurements, 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, Measurements, Recurring \| Netting \| U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	0.0
Fair Value, Measurements, Recurring \| Netting \| U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	0.0
Fair Value, Measurements, 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, Measurements, Recurring \| Netting \| Money market mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	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	105.1	49.2
Securities and other assets segregated under federal and other regulations	839.3	51.8
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	8,440.3	6,851.6
Receivable from clients, net - Derivatives	61.2	22.4
Financial instruments owned	1,451.5	1,598.6
Total assets at fair value	11,119.2	8,781.2
Payables to clients - Derivatives	3,113.1	3,577.1
Financial instruments sold, not yet purchased	2,356.8	2,408.2
Total liabilities at fair value	5,667.4	6,363.3
Fair Value, Measurements, Recurring \| Level 1 \| Accounts payable and other accrued liabilities - contingent liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities - contingent liabilities	0.0	0.0
Fair Value, Measurements, 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, 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	197.5	378.0
Fair Value, Measurements, 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	197.5	378.0
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	221.8	207.6
Fair Value, Measurements, Recurring \| Level 1 \| Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	556.0	363.9
Financial instruments sold, not yet purchased	367.3	219.8
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	0.0	0.0
Financial instruments sold, not yet purchased	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	678.8	1,088.6
Financial instruments sold, not yet purchased	1,983.2	2,139.3
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	0.0	0.0
Financial instruments sold, not yet purchased	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]
Financial instruments owned	0.0	41.4
Financial instruments sold, not yet purchased		41.0
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	0.0	0.0
Financial instruments sold, not yet purchased	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	0.0	0.0
Financial instruments sold, not yet purchased		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	0.0	0.1
Financial instruments sold, not yet purchased	6.3	8.1
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	144.6	67.8
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	47.0	13.2
Fair Value, Measurements, 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	0.0	0.0
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	25.1	23.6
Fair Value, Measurements, Recurring \| Level 1 \| Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased	0.0	0.0
Fair Value, Measurements, 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, 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	21.2	18.3
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	5,210.9	2,933.2
Fair Value, Measurements, 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, 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	3,208.2	3,900.1
Fair Value, Measurements, 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	145.4	51.8
Fair Value, Measurements, Recurring \| Level 1 \| U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	0.0
Fair Value, Measurements, Recurring \| Level 1 \| U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	693.9
Fair Value, Measurements, Recurring \| Level 1 \| Certificates of deposit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	10.3	13.9
Fair Value, Measurements, Recurring \| Level 1 \| Money market mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	94.8	35.3
Fair Value, Measurements, 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	110.7	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	4,646.7	2,194.3
Receivable from clients, net - Derivatives	506.5	506.2
Financial instruments owned	7,618.6	5,541.6
Total assets at fair value	13,131.8	8,411.1
Payables to clients - Derivatives	904.1	228.9
Financial instruments sold, not yet purchased	954.5	758.5
Total liabilities at fair value	5,415.7	3,368.2
Fair Value, Measurements, Recurring \| Level 2 \| Accounts payable and other accrued liabilities - contingent liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities - contingent liabilities	0.0	0.0
Fair Value, Measurements, 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	76.0	24.4
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	3,481.1	2,356.4
Fair Value, Measurements, 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	3,557.1	2,380.8
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	249.3	169.0
Fair Value, Measurements, Recurring \| Level 2 \| Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	6.7	15.0
Financial instruments sold, not yet purchased	4.4	5.0
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	485.7	322.1
Financial instruments sold, not yet purchased	264.1	154.6
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	0.0	0.0
Financial instruments sold, not yet purchased	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	701.9	531.0
Financial instruments sold, not yet purchased	1.3	0.2
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	4.1	0.0
Financial instruments sold, not yet purchased		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	5,378.9	3,837.2
Financial instruments sold, not yet purchased	1.1	23.6
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	373.4	223.5
Financial instruments sold, not yet purchased		3.9
Fair Value, Measurements, Recurring \| Level 2 \| Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	658.4	603.2
Financial instruments sold, not yet purchased	682.6	571.0
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	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	0.0	0.0
Fair Value, Measurements, Recurring \| Level 2 \| Cash flow hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9.5	9.6
Financial instruments sold, not yet purchased	1.0	0.2
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	0.0	0.0
Fair Value, Measurements, Recurring \| Level 2 \| Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased	0.0	0.0
Fair Value, Measurements, 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	34.9	26.1
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 \| U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	1,103.1
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	3,508.7	2,168.2
Fair Value, Measurements, 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, Measurements, Recurring \| Level 2 \| U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	110.7
Fair Value, Measurements, Recurring \| Level 2 \| U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities and other assets segregated under federal and other regulations	0.0
Fair Value, Measurements, 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, Measurements, Recurring \| Level 2 \| Money market mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	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	0.0	0.0
Securities and other assets segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	0.0	0.0
Receivable from clients, net - Derivatives		0.0
Financial instruments owned	2.3	2.1
Total assets at fair value	2.3	2.1
Payables to clients - Derivatives	0.0	0.0
Financial instruments sold, not yet purchased	0.1	0.9
Total liabilities at fair value	32.4	3.2
Fair Value, Measurements, Recurring \| Level 3 \| Accounts payable and other accrued liabilities - contingent liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities - contingent liabilities	32.3	2.3
Fair Value, Measurements, Recurring \| Level 3 \| To be announced and forward settling securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	0.0	0.0
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 \| Payable to broker-dealers, clearing organizations and counterparties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	0.0	0.0
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	0.0	0.0
Financial instruments sold, not yet purchased	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	0.0	0.0
Financial instruments sold, not yet purchased	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	0.0	0.0
Financial instruments sold, not yet purchased	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	0.0	0.0
Financial instruments sold, not yet purchased	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]
Financial instruments owned	0.0	0.0
Financial instruments sold, not yet purchased		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	0.0	0.0
Financial instruments sold, not yet purchased	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	0.0	0.0
Financial instruments sold, not yet purchased		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	0.0	0.0
Financial instruments sold, not yet purchased	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	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	0.0	0.0
Fair Value, Measurements, Recurring \| Level 3 \| Cash flow hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Financial instruments sold, not yet purchased	0.0	0.0
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	2.3	2.1
Fair Value, Measurements, Recurring \| Level 3 \| Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased	0.1	0.9
Fair Value, Measurements, Recurring \| Level 3 \| To be announced and forward settling securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	0.0	0.0

Assets and Liabilities, at Fair Value - Schedule of Financial and Non-Financial Assets and Liabilities Accounted at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions