Condensed Parent Only Financial Statements [Schedule] Income Statement (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Condensed Financial Statements, Captions [Line Items]
Trading gains, net									$ 248.4	$ 205.7	$ 86.5
Commission and clearing fees									161.0	134.5	119.9
Interest income									10.3	10.3	7.0
Other Income									0.5	1.0	0.5
Gross Profit	118.0	123.8	119.6	96.3	108.1	105.4	113.0	96.7	457.7	423.2	269.0
Interest expense	3.3	2.6	3.6	2.1	2.2	2.0	3.3	3.8	11.6	11.3	9.9
Net revenues	114.7	121.2	116.0	94.2	105.9	103.4	109.7	92.9	446.1	411.9	259.1
Noninterest Expense [Abstract]
Compensation and benefits									202.4	176.6	104.2
Clearing and related expenses									107.2	77.4	68.2
Introducing broker commissions									31.0	24.0	18.9
Communication and data services									22.6	15.5	11.1
Occupancy and equipment rental									11.0	8.9	6.2
Professional Fees									12.9	10.6	8.1
Depreciation and amortization									7.2	4.7	1.6
Bad debts and impairments									1.5	6.2	5.8
Other									31.0	28.5	17.1
Total non-interest expenses	104.2	115.3	112.4	94.9	92.9	86.1	86.4	87.0	426.8	352.4	241.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	10.5	5.9	3.6	(0.7)	13.0	17.3	23.3	5.9	19.3	59.5	17.9
Income tax benefit	2.2	1.2	1.2	(0.2)	5.4	7.0	8.0	2.1	4.4	22.5	6.4
Net Income (Loss) Attributable to Parent	8.3	4.7	2.4	(0.4)	7.5	10.4	15.4	4.0	15.0	37.3	5.4
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Trading gains, net									7.1	3.1	2.5
Commission and clearing fees									0	0	0.9
Interest income									2.1	4.3	1.7
Other Income									0.3	0.4	13.9
Gross Profit									9.5	7.8	19.0
Interest expense									5.6	9.7	6.3
Net revenues									3.9	(1.9)	12.7
Noninterest Expense [Abstract]
Compensation and benefits									12.4	8.6	10.2
Clearing and related expenses									0.3	0.1	0.1
Introducing broker commissions									0	0.2	0.2
Communication and data services									0.6	0.5	0.5
Occupancy and equipment rental									1.0	1.2	0.8
Professional Fees									3.1	3.8	1.3
Depreciation and amortization									1.2	0.6	0
Bad debts and impairments									2.2	0	(1.1)
Other									6.8	(0.5)	3.1
Total non-interest expenses									27.6	14.5	15.1
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest									(23.7)	(16.4)	(2.4)
Income tax benefit									9.2	6.4	5.7
Net Income (Loss) Attributable to Parent									(14.5)	(10.0)	3.3
Adjustment to Investment in Subs for Equity Method Accounting									$ 29.5	$ 47.3	$ 2.1