Condensed Parent Only Financial Statements - Income Statement (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Condensed Financial Statements, Captions [Line Items]
Trading gains, net									$ 321.2	$ 328.6	$ 244.5
Consulting fees									224.3	192.5	180.7
Interest income									55.2	39.4	8.0
Dividend income from subsidiaries									31.0	6.0	0.0
Operating revenues	$ 178.6	$ 175.0	$ 166.1	$ 151.3	$ 178.7	$ 151.6	$ 156.5	$ 137.5	671.0	624.3	490.9
Interest expense	7.5	7.7	7.1	6.0	5.0	4.9	4.5	2.7	28.3	17.1	10.5
Net operating revenues	111.0	117.3	112.9	102.7	127.3	103.4	107.9	93.2	443.9	431.8	322.0
Non-interest expenses:
Compensation and benefits									263.9	251.1	201.9
Clearing and related expenses	32.0	35.2	32.9	29.8	31.3	30.2	31.8	29.4	129.9	122.7	108.5
Introducing broker commissions	28.1	14.8	13.2	12.8	15.1	13.1	12.3	12.2	68.9	52.7	49.9
Communication and data services									32.7	28.1	25.8
Occupancy and equipment rental									13.3	13.5	12.3
Professional fees									14.0	12.5	14.9
Travel and business development									11.5	10.5	9.9
Depreciation and amortization									8.2	7.2	7.3
Bad debts and impairments									4.4	7.3	5.5
Other									29.4	23.5	18.4
Total non-interest expenses	98.0	95.9	92.9	90.6	98.1	86.2	89.8	79.6	377.4	353.7	296.0
Gain on acquisition	6.2	0.0	0.0	0.0					6.2	0.0	0.0
Loss from continuing operations, before tax	(19.2)	(21.4)	(20.0)	(12.1)	(29.2)	(17.2)	(18.1)	(13.6)	(72.7)	(78.1)	(26.0)
Income tax benefit	$ (2.4)	$ (6.8)	$ (5.5)	$ (3.3)	$ (8.1)	$ (5.0)	$ (5.1)	$ (4.2)	(18.0)	(22.4)	(6.4)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Management fees from affiliates									30.1	26.6	9.5
Trading gains, net									0.7	3.2	0.0
Consulting fees									2.2	2.1	1.6
Interest income									1.8	4.6	4.3
Operating revenues									65.8	42.5	15.4
Interest expense									13.4	12.7	10.6
Net operating revenues									52.4	29.8	4.8
Non-interest expenses:
Compensation and benefits									52.8	43.5	29.8
Clearing and related expenses									1.7	1.2	0.3
Introducing broker commissions									0.6	0.5	0.3
Communication and data services									6.7	5.7	1.3
Occupancy and equipment rental									2.8	2.1	2.0
Professional fees									4.8	4.6	5.0
Travel and business development									1.7	1.4	1.1
Depreciation and amortization									2.5	1.8	1.8
Bad debts and impairments									0.2	1.6	0.1
Management services fee to affiliates									1.2	4.3	2.9
Other									11.7	10.2	3.5
Total non-interest expenses									86.7	76.9	48.1
Gain on acquisition									6.2	0.0	0.0
Loss from continuing operations, before tax									28.1	47.1	43.3
Income tax benefit									24.7	19.4	17.1
Net loss									(3.4)	(27.7)	(26.2)
Adjustment to Investment in Subs for Equity Method Accounting									$ 58.1	$ 83.4	$ 45.5