Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	$ 1,920.2	$ 1,049.9
Financial instruments, owned, at fair value	2,727.7	2,175.2
Physical commodities inventory	281.1	229.3
Accounts payable and other accrued liabilities	272.6	157.5
Payable to broker-dealers, clearing organizations and counterparties	537.5	266.2
Financial instruments sold, not yet purchased, at fair value	686.0	714.8
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	16.0
Cash and securities segregated under federal and other regulations	2.6	306.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	1,775.8	626.9
Receivables from clients, net - Derivatives	1.5
Financial instruments, owned, at fair value	2,727.7	2,175.2
Total assets at fair value	4,739.3	3,273.8
Payables to clients - Derivatives	(222.7)
Liabilities, fair value disclosure	479.5	722.2
Fair Value, Measurements, Recurring [Member] | TBA and Forward Settling Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	10.2	5.3
Fair Value, Measurements, Recurring [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	4.5	0.3
Fair Value, Measurements, Recurring [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	14.7	5.6
Fair Value, Measurements, Recurring [Member] | Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities	1.5	1.8
Fair Value, Measurements, Recurring [Member] | Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	215.7	151.9
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	264.3	168.5
Financial instruments sold, not yet purchased, at fair value	232.9	158.1
Fair Value, Measurements, Recurring [Member] | Corporate and Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	66.9	80.0
Financial instruments sold, not yet purchased, at fair value	22.5	39.2
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	419.9	248.7
Financial instruments sold, not yet purchased, at fair value	247.5	272.3
Fair Value, Measurements, Recurring [Member] | U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	293.4	447.1
Financial instruments sold, not yet purchased, at fair value	0.1	43.8
Fair Value, Measurements, Recurring [Member] | Foreign Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	2.5	0.5
Fair Value, Measurements, Recurring [Member] | Agency Mortgage-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	1,384.6	1,045.0
Financial instruments sold, not yet purchased, at fair value	5.1	29.6
Fair Value, Measurements, Recurring [Member] | Asset-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	33.0	29.1
Fair Value, Measurements, Recurring [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	117.4	66.5
Financial instruments sold, not yet purchased, at fair value	176.8	58.1
Fair Value, Measurements, Recurring [Member] | Commodities Leases[Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	24.9	28.6
Financial instruments sold, not yet purchased, at fair value	1.1	113.7
Fair Value, Measurements, Recurring [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	103.2	48.4
Fair Value, Measurements, Recurring [Member] | Exchange firm Common Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	10.1	12.7
Fair Value, Measurements, Recurring [Member] | Mutual Funds and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	7.5	0.1
Fair Value, Measurements, Recurring [Member] | TBA and Forward Settling Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	19.2	8.3
Fair Value, Measurements, Recurring [Member] | Foreign Government Obligations [Member]
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	8.0	9.9
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Obligations [Member]
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	1,941.3	593.9
Fair Value, Measurements, Recurring [Member] | Derivative [Member]
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	(192.7)	14.8
Fair Value, Measurements, Recurring [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	2.4	6.2
Fair Value, Measurements, Recurring [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.2	299.8
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	3.2	4.9
Cash and cash equivalents		13.8
Fair Value, Measurements, Recurring [Member] | Money Market Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	12.8	8.9
Fair Value, Measurements, Recurring [Member] | Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0
Cash and securities segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	(2,549.3)	(3,161.1)
Receivables from clients, net - Derivatives	(234.1)
Financial instruments, owned, at fair value	(535.6)	(420.8)
Total assets at fair value	(3,319.0)	(3,581.9)
Payables to clients - Derivatives	(2,399.9)
Financial instruments sold, not yet purchased, at fair value	(386.8)	(422.2)
Liabilities, fair value disclosure	(3,100.7)	(3,540.8)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | TBA and Forward Settling Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	(11.8)	(1.5)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	(302.2)	(3,117.1)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	(314.0)	(3,118.6)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Corporate and Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Foreign Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Agency Mortgage-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Asset-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	(535.6)	(420.8)
Financial instruments sold, not yet purchased, at fair value	(386.8)	(422.2)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Commodities Leases[Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Exchange firm Common Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Mutual Funds and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | TBA and Forward Settling Securities [Member]
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	(11.8)	(1.5)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Foreign Government Obligations [Member]
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 [Member] | Netting [Member] | U.S. Treasury Obligations [Member]
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 [Member] | Netting [Member] | Derivative [Member]
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	(2,537.5)	(3,159.6)
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Cash and cash equivalents		0.0
Fair Value, Measurements, Recurring [Member] | Netting [Member] | Money Market Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	16.0	13.8
Cash and securities segregated under federal and other regulations	2.6	306.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	3,898.3	3,735.0
Receivables from clients, net - Derivatives	0.0
Financial instruments, owned, at fair value	798.8	470.9
Total assets at fair value	4,742.5	4,532.8
Payables to clients - Derivatives	2,000.8
Financial instruments sold, not yet purchased, at fair value	465.5	419.6
Liabilities, fair value disclosure	2,466.3	3,498.7
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | TBA and Forward Settling Securities [Member]
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 [Member] | Level 1 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	0.0	3,079.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	0.0	3,079.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	26.8	7.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	254.9	159.5
Financial instruments sold, not yet purchased, at fair value	218.0	147.3
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate and Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	419.9	248.7
Financial instruments sold, not yet purchased, at fair value	247.5	272.3
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Foreign Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	2.5	0.5
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Agency Mortgage-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Asset-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.7	1.0
Financial instruments sold, not yet purchased, at fair value		0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodities Leases[Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	103.2	48.4
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Exchange firm Common Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	10.1	12.7
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Mutual Funds and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	7.5	0.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | TBA and Forward Settling Securities [Member]
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 [Member] | Level 1 [Member] | Foreign Government Obligations [Member]
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	8.0	9.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury Obligations [Member]
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	1,941.3	593.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Derivative [Member]
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	1,949.0	3,131.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	2.4	6.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.2	299.8
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	3.2	4.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money Market Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	12.8	8.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0
Cash and securities segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	426.8	53.0
Receivables from clients, net - Derivatives	235.6
Financial instruments, owned, at fair value	2,464.5	2,125.1
Total assets at fair value	3,315.8	2,322.9
Payables to clients - Derivatives	176.4
Financial instruments sold, not yet purchased, at fair value	607.3	717.4
Liabilities, fair value disclosure	1,112.4	762.5
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | TBA and Forward Settling Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	22.0	6.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	306.7	38.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payable to broker-dealers, clearing organizations and counterparties	328.7	45.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	188.9	144.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	9.4	9.0
Financial instruments sold, not yet purchased, at fair value	14.9	10.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate and Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	66.9	80.0
Financial instruments sold, not yet purchased, at fair value	22.5	39.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	293.4	447.1
Financial instruments sold, not yet purchased, at fair value	0.1	43.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Agency Mortgage-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	1,384.6	1,045.0
Financial instruments sold, not yet purchased, at fair value	5.1	29.6
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Asset-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	33.0	29.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	652.3	486.3
Financial instruments sold, not yet purchased, at fair value	563.6	480.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodities Leases[Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	24.9	28.6
Financial instruments sold, not yet purchased, at fair value	1.1	113.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Exchange firm Common Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Mutual Funds and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | TBA and Forward Settling Securities [Member]
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	31.0	9.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Government Obligations [Member]
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 [Member] | Level 2 [Member] | U.S. Treasury Obligations [Member]
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 [Member] | Level 2 [Member] | Derivative [Member]
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	395.8	43.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Cash and cash equivalents		0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Money Market Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0.0
Cash and securities segregated under federal and other regulations	0.0	0.0
Deposits with and receivables from broker-dealers, clearing organizations and counterparties, net	0.0	0.0
Receivables from clients, net - Derivatives	0.0
Financial instruments, owned, at fair value	0.0	0.0
Total assets at fair value	0.0	0.0
Payables to clients - Derivatives	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Liabilities, fair value disclosure	1.5	1.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | TBA and Forward Settling Securities [Member]
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 [Member] | Level 3 [Member] | Derivative [Member]
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 [Member] | Level 3 [Member] | Payables to Broker-dealers, Clearing Organizations and Counterparties [Member]
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 [Member] | Level 3 [Member] | Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities	1.5	1.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Physical commodities inventory [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate and Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Government Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Foreign Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Agency Mortgage-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Asset-backed Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commodities Leases[Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Financial instruments sold, not yet purchased, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Exchange firm Common Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Mutual Funds and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments, owned, at fair value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | TBA and Forward Settling Securities [Member]
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 [Member] | Level 3 [Member] | Foreign Government Obligations [Member]
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 [Member] | Level 3 [Member] | U.S. Treasury Obligations [Member]
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 [Member] | Level 3 [Member] | Derivative [Member]
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 [Member] | Level 3 [Member] | Commodities Warehouse Receipts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Treasury Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and securities segregated under federal and other regulations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	0.0	0.0
Cash and cash equivalents		0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Money Market Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 0.0	$ 0.0