Condensed Parent Only Financial Statements - Income Statement (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Condensed Financial Statements, Captions [Line Items]
Trading gains, net									$ 328.6	$ 244.5	$ 244.0
Consulting fees									192.5	180.7	173.3
Interest income									39.4	8.0	8.9
Dividend income from subsidiaries									0.3	0.7	0.9
Operating revenues	$ 178.7	$ 151.6	$ 156.5	$ 137.5	$ 130.6	$ 118.2	$ 129.2	$ 112.9	624.3	490.9	468.2
Interest expense	5.0	4.9	4.5	2.7	2.5	2.5	2.8	2.7	17.1	10.5	7.9
Net operating revenues	127.3	103.4	107.9	93.2	86.7	76.0	85.9	73.4	431.8	322.0	309.7
Non-interest expenses:
Compensation and benefits									251.1	201.9	198.7
Clearing and related expenses	31.3	30.2	31.8	29.4	27.5	28.1	27.7	25.2	122.7	108.5	110.1
Introducing broker commissions	15.1	13.1	12.3	12.2	13.9	11.6	12.8	11.6	52.7	49.9	40.5
Communication and data services									28.1	25.8	23.1
Occupancy and equipment rental									13.5	12.3	12.0
Professional fees									12.5	14.9	12.4
Travel and business development									10.5	9.9	10.4
Depreciation and amortization									7.2	7.3	8.0
Bad debts and impairments									7.3	5.5	0.8
Other									23.5	18.4	23.1
Total non-interest expenses	98.1	86.2	89.8	79.6	78.9	72.0	75.6	69.5	353.7	296.0	288.5
Loss from continuing operations, before tax	(29.2)	(17.2)	(18.1)	(13.6)	(7.8)	(4.0)	(10.3)	(3.9)	(78.1)	(26.0)	(21.2)
Income tax benefit	$ (8.1)	$ (5.0)	$ (5.1)	$ (4.2)	$ (2.0)	$ (0.3)	$ (2.6)	$ (1.5)	(22.4)	(6.4)	(2.6)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Management fees from affiliates									26.6	9.5	9.2
Trading gains, net									3.2	0.0	0.0
Consulting fees									2.1	1.6	1.6
Interest income									4.6	4.3	3.6
Dividend income from subsidiaries									6.0	0.0	0.0
Operating revenues									42.5	15.4	14.4
Interest expense									12.7	10.6	7.1
Net operating revenues									29.8	4.8	7.3
Non-interest expenses:
Compensation and benefits									43.5	29.8	30.5
Clearing and related expenses									1.2	0.3	0.5
Introducing broker commissions									0.5	0.3	0.5
Communication and data services									5.7	1.3	0.9
Occupancy and equipment rental									2.1	2.0	1.4
Professional fees									4.6	5.0	2.3
Travel and business development									1.4	1.1	1.2
Depreciation and amortization									1.8	1.8	1.5
Bad debts and impairments									1.6	0.1	0.0
Management services fee to affiliates									4.3	2.9	2.7
Other									10.2	3.5	3.6
Total non-interest expenses									76.9	48.1	45.1
Loss from continuing operations, before tax									47.1	43.3	37.8
Income tax benefit									19.4	17.1	13.2
Net loss									(27.7)	(26.2)	(24.6)
Adjustment to Investment in Subs for Equity Method Accounting									$ 83.4	$ 45.5	$ 43.9