Assets and Liabilities, at Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Securities Segregated under Federal and Other Regulations	$ 1,136.3	$ 756.9
Receivables from Exchange Clearing Organizations	1,524.4	1,533.5
Receivables from Brokers-Dealers and Clearing Organizations	237.0	277.6
Financial instruments owned, at fair value	1,606.1	1,421.9
Physical Commodities Inventory	123.8	32.8
Assets, Fair Value Disclosure	3,156.5
Accounts payable and other accrued liabilities	161.3	144.8
Payables to Broker-Dealers and Clearing Organizations	260.1	262.9
Financial Instruments Sold, Not yet Purchased, at Fair Value	839.4	568.3
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		1.3
Cash and Securities Segregated under Federal and Other Regulations	618.8	515.5
Receivables from Exchange Clearing Organizations	868.5	1,009.4
Receivables from Brokers-Dealers and Clearing Organizations	(15.2)	(52.9)
Financial instruments owned, at fair value	1,606.1	1,421.9
Assets, Fair Value Disclosure	3,156.5	2,910.4
Payables to Broker-Dealers and Clearing Organizations	3.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	839.4	568.3
Liabilities, Fair Value Disclosure	843.7	573.2
Fair Value, Measurements, Recurring [Member] | TBA and forward settling securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations	3.5	1.6
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	0.0	0.0
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	0.8	3.3
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	71.2	15.2
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	36.5	26.1
Financial Instruments Sold, Not yet Purchased, at Fair Value	23.9	18.6
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	25.7	89.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	25.8	90.0
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	40.8	31.3
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	514.9	513.4
Financial Instruments Sold, Not yet Purchased, at Fair Value	509.8	341.0
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	14.6	12.1
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	6.4
Fair Value, Measurements, Recurring [Member] | Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	747.5	699.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	2.8
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	193.9	30.6
Financial Instruments Sold, Not yet Purchased, at Fair Value	210.9	54.1
Fair Value, Measurements, Recurring [Member] | Commodities leases and unpriced positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	8.1	7.6
Financial Instruments Sold, Not yet Purchased, at Fair Value	62.1	55.4
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	8.9	2.8
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	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	8.8	3.4
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	0.3	0.2
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	(15.5)	(53.1)
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Exchange Clearing Organizations	512.7	431.8
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 Exchange Clearing Organizations	472.1	501.4
Fair Value, Measurements, Recurring [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Exchange Clearing Organizations	(116.3)	76.2
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	23.3	22.1
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	595.5	493.4
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	7.1
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 Cash Equivalents, Fair Value Disclosure		1.3
Cash and Securities Segregated under Federal and Other Regulations	23.3	22.1
Receivables from Exchange Clearing Organizations	2,662.6	4,047.7
Receivables from Brokers-Dealers and Clearing Organizations	0.0	0.1
Financial instruments owned, at fair value	327.7	423.0
Assets, Fair Value Disclosure	3,091.9	4,509.4
Payables to Broker-Dealers and Clearing Organizations	1,961.7
Financial Instruments Sold, Not yet Purchased, at Fair Value	255.1	371.0
Liabilities, Fair Value Disclosure	2,216.8	3,862.3
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	1,961.7	3,491.3
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[Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Physical Commodities Inventory	71.2	15.2
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	34.6	23.7
Financial Instruments Sold, Not yet Purchased, at Fair Value	23.5	18.0
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	25.2	82.9
Financial Instruments Sold, Not yet Purchased, at Fair Value	25.3	89.0
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	36.9	26.1
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	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | 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	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	206.9	278.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	199.4	264.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Commodities leases and unpriced positions [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	8.9	2.8
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	6.4	5.6
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	8.8	3.4
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	0.1
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 Exchange Clearing Organizations	512.7	431.8
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 Exchange 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 Exchange Clearing Organizations	2,149.9	3,615.9
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	23.3	22.1
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	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 Cash Equivalents, Fair Value Disclosure		0.0
Cash and Securities Segregated under Federal and Other Regulations	595.5	493.4
Receivables from Exchange Clearing Organizations	472.1	501.4
Receivables from Brokers-Dealers and Clearing Organizations	8.3	539.1
Financial instruments owned, at fair value	2,768.1	3,002.1
Assets, Fair Value Disclosure	3,844.0	4,536.0
Payables to Broker-Dealers and Clearing Organizations	100.1
Financial Instruments Sold, Not yet Purchased, at Fair Value	2,037.8	2,174.4
Liabilities, Fair Value Disclosure	2,137.9	2,705.7
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	2.6	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	97.5	528.7
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[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	1.7	1.9
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.4	0.6
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	0.5	6.6
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.5	1.0
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	0.9	2.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [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	514.9	513.4
Financial Instruments Sold, Not yet Purchased, at Fair Value	509.8	341.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [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	14.6	12.1
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	6.4
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	747.5	699.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	2.8
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]
Financial instruments owned, at fair value	1,350.8	1,702.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	1,319.3	1,723.5
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commodities leases and unpriced positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	137.2	64.6
Financial Instruments Sold, Not yet Purchased, at Fair Value	207.8	99.1
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, 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] | Fair Value, Inputs, 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] | 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]
Receivables from Brokers-Dealers and Clearing Organizations	0.3	1.2
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]
Receivables from Brokers-Dealers and Clearing Organizations	8.0	537.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Exchange Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Exchange Clearing Organizations	472.1	501.4
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]
Receivables from Exchange Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, Level 2 [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	595.5	493.4
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [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
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		0.0
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Receivables from Exchange Clearing Organizations	0.0	0.0
Receivables from Brokers-Dealers and Clearing Organizations	0.0	0.0
Financial instruments owned, at fair value	3.2	3.7
Assets, Fair Value Disclosure	3.2	3.7
Payables to Broker-Dealers and Clearing Organizations	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Liabilities, Fair Value Disclosure	0.8	3.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [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 3 [Member] | Derivative [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 3 [Member] | Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accounts payable and other accrued liabilities	0.8	3.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, Level 3 [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	0.2	0.5
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [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] | Fair Value, Inputs, Level 3 [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	3.0	3.2
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [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 3 [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	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | 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	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, Level 3 [Member] | Commodities leases and unpriced positions [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 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] | Fair Value, Inputs, 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] | Fair Value, Inputs, 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] | Fair Value, Inputs, Level 3 [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 3 [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 3 [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Exchange Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | US Government Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Exchange Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Exchange Clearing Organizations	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, 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] | Fair Value, Inputs, Level 3 [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 3 [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
Derivative [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations		1.6
Derivative [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations		3,491.3
Derivative [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations		531.3
Derivative [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations		0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		0.0
Cash and Securities Segregated under Federal and Other Regulations	0.0	0.0
Receivables from Exchange Clearing Organizations	(2,266.2)	(3,539.7)
Receivables from Brokers-Dealers and Clearing Organizations	(23.5)	(592.1)
Financial instruments owned, at fair value	(1,492.9)	(2,006.9)
Assets, Fair Value Disclosure	(3,782.6)	(6,138.7)
Payables to Broker-Dealers and Clearing Organizations	(2,058.3)
Financial Instruments Sold, Not yet Purchased, at Fair Value	(1,453.5)	(1,977.1)
Liabilities, Fair Value Disclosure	(3,511.8)	(5,998.1)
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.9	(1.0)
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	(2,059.2)	(4,020.0)
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Financial Instruments Sold, Not yet Purchased, at Fair Value	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member] | 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	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	(1,363.8)	(1,949.9)
Financial Instruments Sold, Not yet Purchased, at Fair Value	(1,307.8)	(1,933.4)
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member] | Commodities leases and unpriced positions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	(129.1)	(57.0)
Financial Instruments Sold, Not yet Purchased, at Fair Value	(145.7)	(43.7)
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	(1.0)
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	(23.5)	(591.1)
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Exchange Clearing Organizations	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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 Exchange Clearing Organizations	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | Fair Value, Measurements, Recurring [Member] | Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Receivables from Exchange Clearing Organizations	(2,266.2)	(3,539.7)
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	0.0	0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | 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	$ 0.0
Counterparty And Cash Collateral Netting Adjustment [Member] | Derivative [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Payables to Broker-Dealers and Clearing Organizations		$ (4,021.0)