Assets and Liabilities, at Fair Value Level 4 - Schedule of Financial Instruments Owned and Sold, Not Yet Purchased (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Sep. 30, 2011
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	$ 176.5	$ 390.9
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	130.1	223.1
Derivative Liabilities	43.1
Common stock and American Depositary Receipts (ADRs) [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	13.7	23.4
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	17.1	46.9
Exchangeable foreign ordinary equities and ADRs [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	5.1	23.8
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	7.9	9.8
Corporate Debt Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	0	0
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	9.5	8.7
US Government Debt Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	0	0
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	0.8	0.8
Foreign Government Debt Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	0	0
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	10.1	6.7
Derivative [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased		122.9
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	47.8	101.9
Commodities leases and unpriced positions [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	114.6	220.8
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	17.9	26.1
Commodities warehouse receipts [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	0	0
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	5.3	16.2
Exchange firm common stock [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	0	0
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	12.0	3.7
Mutual funds and other [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	0	0
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	1.7	1.0
Investment in managed funds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Security Owned and Sold, Not yet Purchased Fair Value, Security Sold, Not yet Purchased	0	0
Security Owned and Sold, Not yet Purchased Fair Value, Security Owned	$ 0	$ 1.3