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Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2024	Sep. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 48.4	$ 5.8
Securities and other assets segregated under federal and other regulations	2,750.4	2,426.3
Financial instruments owned	6,155.4	5,044.8
Physical commodities inventory	587.4	537.3
Accounts payable and other accrued liabilities	544.9	533.0
Financial instruments sold, not yet purchased, at fair value	3,569.8	3,085.6
Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	48.0	66.5
Securities and other assets segregated under federal and other regulations	48.4	5.8
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	3,740.4	4,248.3
Receivables from clients, net - Derivatives	(0.5)	(7.9)
Financial instruments owned	6,155.4	5,044.8
Total assets at fair value	10,336.2	9,744.0
Accounts payable and other accrued liabilities	1.7
Payables to clients - Derivatives	387.3	79.8
Total liabilities at fair value	3,962.4	3,177.1
Fair Value, Recurring | Contingent Consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities		1.5
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	6.0	16.1
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	(2.4)	(5.9)
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	3.6	10.2
Fair Value, Recurring | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	344.5	386.5
Fair Value, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	456.7	334.3
Financial instruments sold, not yet purchased, at fair value	277.1	236.1
Fair Value, Recurring | Corporate and municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	361.1	284.2
Financial instruments sold, not yet purchased, at fair value	141.3	81.6
Fair Value, Recurring | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	721.8	531.7
Financial instruments sold, not yet purchased, at fair value	2,662.3	2,409.3
Fair Value, Recurring | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	373.9	451.7
Financial instruments sold, not yet purchased, at fair value	13.7	5.1
Fair Value, Recurring | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	150.5	43.3
Financial instruments sold, not yet purchased, at fair value	130.6	21.5
Fair Value, Recurring | Agency mortgage-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	3,469.7	2,865.8
Financial instruments sold, not yet purchased, at fair value	12.1	31.7
Fair Value, Recurring | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	219.4	138.8
Financial instruments sold, not yet purchased, at fair value	4.0
Fair Value, Recurring | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	316.7	268.5
Financial instruments sold, not yet purchased, at fair value	325.5	261.2
Fair Value, Recurring | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		16.0
Fair Value, Recurring | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	43.0	54.7
Fair Value, Recurring | Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	11.8	12.0
Fair Value, Recurring | Cash flow hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5.5	1.7
Financial instruments sold, not yet purchased, at fair value	2.1	27.1
Fair Value, Recurring | Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	25.3	42.1
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	1.1	12.0
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	4.0	41.8
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	17.3	17.8
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	3,227.7	4,023.8
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	491.4	164.9
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	48.4	5.8
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	(5,989.3)	(6,500.2)
Receivables from clients, net - Derivatives	(692.0)	(630.9)
Financial instruments owned	(416.2)	(600.2)
Total assets at fair value	(7,097.5)	(7,731.3)
Accounts payable and other accrued liabilities	0.0
Payables to clients - Derivatives	(4,323.7)	(5,577.1)
Financial instruments sold, not yet purchased, at fair value	(340.1)	(510.4)
Total liabilities at fair value	(7,021.8)	(7,639.0)
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		0.0
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	(20.2)	(31.4)
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	(2,337.8)	(1,520.1)
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	(2,358.0)	(1,551.5)
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	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
Financial instruments sold, not yet purchased, at fair value	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, 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
Fair Value, Recurring | Netting | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	(416.2)	(600.2)
Financial instruments sold, not yet purchased, at fair value	(340.1)	(510.4)
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	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 | 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	(21.4)	(31.7)
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	(5,967.9)	(6,468.5)
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
Level 1 | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	48.0	66.5
Securities and other assets segregated under federal and other regulations	48.4	5.8
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	7,955.5	9,539.1
Receivables from clients, net - Derivatives	76.7	61.7
Financial instruments owned	1,390.0	1,005.6
Total assets at fair value	9,712.4	10,919.0
Accounts payable and other accrued liabilities	0.0
Payables to clients - Derivatives	4,418.5	5,430.7
Financial instruments sold, not yet purchased, at fair value	3,070.7	2,663.8
Total liabilities at fair value	7,860.8	8,206.7
Level 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		0.0
Level 1 | 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	0.0	0.0
Level 1 | 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	371.6	112.2
Level 1 | 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	371.6	112.2
Level 1 | Fair Value, Recurring | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	193.8	240.3
Level 1 | Fair Value, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	440.3	324.0
Financial instruments sold, not yet purchased, at fair value	272.0	230.6
Level 1 | Fair Value, Recurring | 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
Level 1 | Fair Value, Recurring | U.S. Treasury obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	721.8	531.7
Financial instruments sold, not yet purchased, at fair value	2,662.3	2,409.3
Level 1 | Fair Value, Recurring | 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	0.0
Level 1 | Fair Value, Recurring | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	150.5	43.3
Financial instruments sold, not yet purchased, at fair value	130.6	21.5
Level 1 | Fair Value, Recurring | 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
Level 1 | Fair Value, Recurring | 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
Level 1 | Fair Value, Recurring | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.1	0.6
Financial instruments sold, not yet purchased, at fair value	5.8	2.4
Level 1 | Fair Value, Recurring | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		0.0
Level 1 | Fair Value, Recurring | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	43.0	54.7
Level 1 | Fair Value, Recurring | Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	11.8	12.0
Level 1 | Fair Value, Recurring | 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	0.0
Level 1 | Fair Value, Recurring | Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	22.5	39.3
Level 1 | 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	0.0	0.0
Level 1 | 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	0.0	0.0
Level 1 | 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	17.3	17.8
Level 1 | 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	3,227.7	4,023.8
Level 1 | 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	4,710.5	5,497.5
Level 1 | 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	48.4	5.8
Level 2 | Fair Value, Recurring
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	1,774.2	1,209.4
Receivables from clients, net - Derivatives	614.8	561.3
Financial instruments owned	5,178.8	4,636.6
Total assets at fair value	7,718.5	6,553.5
Accounts payable and other accrued liabilities	0.0
Payables to clients - Derivatives	292.5	226.2
Financial instruments sold, not yet purchased, at fair value	838.1	931.1
Total liabilities at fair value	3,120.6	2,606.8
Level 2 | Fair Value, Recurring | Contingent Consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities		0.0
Level 2 | 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	26.2	47.5
Level 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	1,963.8	1,402.0
Level 2 | 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	1,990.0	1,449.5
Level 2 | Fair Value, Recurring | Physical commodities inventory
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	150.7	146.2
Level 2 | Fair Value, Recurring | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	16.4	10.3
Financial instruments sold, not yet purchased, at fair value	5.1	5.5
Level 2 | Fair Value, Recurring | Corporate and municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	361.1	284.2
Financial instruments sold, not yet purchased, at fair value	141.3	81.6
Level 2 | Fair Value, Recurring | 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
Level 2 | Fair Value, Recurring | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	373.9	451.7
Financial instruments sold, not yet purchased, at fair value	13.7	5.1
Level 2 | Fair Value, Recurring | 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, at fair value	0.0	0.0
Level 2 | Fair Value, Recurring | Agency mortgage-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	3,469.7	2,865.8
Financial instruments sold, not yet purchased, at fair value	12.1	31.7
Level 2 | Fair Value, Recurring | Asset-backed obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	219.4	138.8
Financial instruments sold, not yet purchased, at fair value	4.0
Level 2 | Fair Value, Recurring | Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	732.8	868.1
Financial instruments sold, not yet purchased, at fair value	659.8	769.2
Level 2 | Fair Value, Recurring | Commodities leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		16.0
Level 2 | Fair Value, Recurring | Commodities warehouse receipts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Level 2 | Fair Value, Recurring | Exchange firm common stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Level 2 | Fair Value, Recurring | Cash flow hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5.5	1.7
Financial instruments sold, not yet purchased, at fair value	2.1	27.1
Level 2 | Fair Value, Recurring | Mutual funds and other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	0.0	0.0
Level 2 | 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	0.0	10.9
Level 2 | 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	25.4	73.5
Level 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	0.0	0.0
Level 2 | 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	0.0	0.0
Level 2 | 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	1,748.8	1,135.9
Level 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	0.0	0.0
Level 3 | Fair Value, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0	0.0
Securities and other assets segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	0.0	0.0
Receivables from clients, net - Derivatives	0.0
Financial instruments owned	2.8	2.8
Total assets at fair value	2.8	2.8
Accounts payable and other accrued liabilities	1.7
Payables to clients - Derivatives	0.0
Financial instruments sold, not yet purchased, at fair value	1.1	1.1
Total liabilities at fair value	2.8	2.6
Level 3 | Fair Value, Recurring | Contingent Consideration
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities		1.5
Level 3 | 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	0.0	0.0
Level 3 | Fair Value, Recurring | Derivatives
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