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Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	$ 678.7	$ 1,136.3
Receivables from Clearing Organizations	1,988.6	1,524.4
Receivables from Brokers-Dealers and Clearing Organizations	177.7	237.0
Financial Instruments, Owned, at Fair Value	1,779.1	1,606.1
Physical commodities inventory	172.5	123.8
Assets, Fair Value Disclosure	2,365.2	3,156.5
Accounts Payable and Other Accrued Liabilities	142.5	161.3
Payables to Broker-Dealers and Clearing Organizations	(182.3)	(260.1)
Financial Instruments Sold, Not yet Purchased, at Fair Value	742.9	839.4
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	241.8	618.8
Receivables from Clearing Organizations	232.5	868.5
Receivables from Brokers-Dealers and Clearing Organizations	22.5
Financial Instruments, Owned, at Fair Value	1,779.1	1,606.1
Assets, Fair Value Disclosure	2,365.2	3,156.5
Payables to Broker-Dealers and Clearing Organizations		(15.2)
Financial Instruments Sold, Not yet Purchased, at Fair Value	742.9
Liabilities, Fair Value Disclosure	747.2	843.7
Fair Value, Measurements, Recurring [Member] | TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	(3.4)	(3.5)
Fair Value, Measurements, Recurring [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Brokers-Dealers and Clearing Organizations	0.0
Payables to Broker-Dealers and Clearing Organizations		0.0
Fair Value, Measurements, Recurring [Member] | Payable to broker-dealers, clearing organizations and counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	(3.4)	(3.5)
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	0.9	0.8
Fair Value, Measurements, Recurring [Member] | Physical commodities inventory - precious metals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	84.4	71.2
Fair Value, Measurements, Recurring [Member] | Common stock and American Depositary Receipts (ADRs) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	27.0	36.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	40.5	23.9
Fair Value, Measurements, Recurring [Member] | Exchangeable foreign ordinary equities and ADRs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	10.5	25.7
Financial Instruments Sold, Not yet Purchased, at Fair Value	10.5	25.8
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	4.4	40.8
Financial Instruments Sold, Not yet Purchased, at Fair Value		6.9
Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	524.7	514.9
Financial Instruments Sold, Not yet Purchased, at Fair Value	348.8	509.8
Fair Value, Measurements, Recurring [Member] | Foreign Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	44.4	14.6
Financial Instruments Sold, Not yet Purchased, at Fair Value	11.2
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	932.0	747.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.4
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	203.8	193.9
Financial Instruments Sold, Not yet Purchased, at Fair Value	280.0
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	15.5	8.1
Financial Instruments Sold, Not yet Purchased, at Fair Value	51.5	62.1
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	3.5	8.9
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	7.5	6.4
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	5.8	8.8
Fair Value, Measurements, Recurring [Member] | TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Brokers-Dealers and Clearing Organizations	6.5	0.3
Fair Value, Measurements, Recurring [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Brokers-Dealers and Clearing Organizations	16.0
Payables to Broker-Dealers and Clearing Organizations		(15.5)
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations		512.7
Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	295.5	472.1
Fair Value, Measurements, Recurring [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	(63.0)	(116.3)
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	12.2	23.3
Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	229.6	595.5
Fair Value, Measurements, Recurring [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	4.9	7.1
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [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
Receivables from Clearing Organizations	(1,956.9)	(2,266.2)
Receivables from Brokers-Dealers and Clearing Organizations	(87.2)	(23.5)
Financial Instruments, Owned, at Fair Value	(2,184.9)	(1,492.9)
Assets, Fair Value Disclosure	(4,229.0)	(3,782.6)
Financial Instruments Sold, Not yet Purchased, at Fair Value	(2,401.1)	(1,453.5)
Liabilities, Fair Value Disclosure	(4,354.6)	(3,511.8)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	0.0	(0.9)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	1,953.5	2,059.2
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Payable to broker-dealers, clearing organizations and counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	1,953.5	2,058.3
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Physical commodities inventory - precious metals [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Common stock and American Depositary Receipts (ADRs) [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Exchangeable foreign ordinary equities and ADRs [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Corporate Debt 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
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | US Government Debt 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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Foreign Government Debt 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
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Collateralized Mortgage Backed 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
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	(2,077.7)	(1,363.8)
Financial Instruments Sold, Not yet Purchased, at Fair Value	(2,257.2)	(1,307.8)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Commodities leases [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	(107.2)	(129.1)
Financial Instruments Sold, Not yet Purchased, at Fair Value	(143.9)	(145.7)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Brokers-Dealers and Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Brokers-Dealers and Clearing Organizations	(87.2)	(23.5)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations		0.0
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	(1,956.9)	(2,266.2)
Fair Value, Measurements, Recurring [Member] | Counterparty And Cash Collateral Netting Adjustment [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | US Government Debt Securities [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] | Counterparty And Cash Collateral Netting Adjustment [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	12.2	23.3
Receivables from Clearing Organizations	1,893.9	2,662.6
Receivables from Brokers-Dealers and Clearing Organizations	0.0	0.0
Financial Instruments, Owned, at Fair Value	260.7	327.7
Assets, Fair Value Disclosure	2,256.1	3,091.9
Financial Instruments Sold, Not yet Purchased, at Fair Value	258.3	255.1
Liabilities, Fair Value Disclosure	2,116.8	2,216.8
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	(1,858.5)	(1,961.7)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Payable to broker-dealers, clearing organizations and counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	(1,858.5)	(1,961.7)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, Level 1 [Member] | Physical commodities inventory - precious metals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical commodities inventory	84.4	71.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Common stock and American Depositary Receipts (ADRs) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	24.3	34.6
Financial Instruments Sold, Not yet Purchased, at Fair Value	40.0	23.5
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Exchangeable foreign ordinary equities and ADRs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	10.4	25.2
Financial Instruments Sold, Not yet Purchased, at Fair Value	9.7	25.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	2.2	36.9
Financial Instruments Sold, Not yet Purchased, at Fair Value		6.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Debt 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] | Fair Value, Inputs, Level 1 [Member] | Foreign Government Debt 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
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Collateralized Mortgage Backed 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
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	207.0	206.9
Financial Instruments Sold, Not yet Purchased, at Fair Value	208.6	199.4
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, 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	3.5	8.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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	7.5	6.4
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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	5.8	8.8
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Brokers-Dealers and Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Brokers-Dealers and Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations		512.7
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Clearing Organizations	1,893.9	2,149.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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	12.2	23.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | US Government Debt Securities [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] | Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	4.9	7.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	229.6	595.5
Receivables from Clearing Organizations	295.5	472.1
Receivables from Brokers-Dealers and Clearing Organizations	109.7	8.3
Financial Instruments, Owned, at Fair Value	3,703.1	2,768.1
Assets, Fair Value Disclosure	4,337.9	3,844.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	2,885.7	2,037.8
Liabilities, Fair Value Disclosure	2,984.1	2,137.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	(3.4)	(2.6)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	(95.0)	(97.5)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Payable to broker-dealers, clearing organizations and counterparties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	(98.4)	(100.1)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, Level 2 [Member] | Physical commodities inventory - precious metals [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] | Fair Value, Inputs, Level 2 [Member] | Common stock and American Depositary Receipts (ADRs) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	2.5	1.7
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.5	0.4
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Exchangeable foreign ordinary equities and ADRs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	0.1	0.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.8	0.5
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments, Owned, at Fair Value	2.2	0.9
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