Condensed Parent Only Financial Statements - Income Statement (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Condensed Financial Statements, Captions [Line Items]
Trading gains, net									$ 332.2	$ 321.2	$ 328.6
Consulting fees									283.4	224.3	192.5
Interest income									69.7	55.2	39.4
Dividend income from subsidiaries									52.7	31.0	6.0
Operating revenues	$ 205.1	$ 197.6	$ 195.8	$ 185.5	$ 178.6	$ 175.0	$ 166.1	$ 151.3	784.0	671.0	624.3
Interest expense	12.0	11.2	10.0	8.9	7.5	7.7	7.1	6.0	42.1	28.3	17.1
Net operating revenues	131.1	123.3	123.9	114.3	111.0	117.3	112.9	102.7	492.6	443.9	431.8
Non-interest expenses:
Compensation and benefits	73.0	75.5	76.6	70.6	66.2	69.4	65.2	63.1	295.7	263.9	251.1
Clearing and related expenses	35.1	33.9	33.7	33.6	32.0	35.2	32.9	29.8	136.3	129.9	122.7
Introducing broker commissions	26.9	29.2	28.2	28.7	28.1	14.8	13.2	12.8	113.0	68.9	52.7
Communication and data services									39.4	32.7	28.1
Occupancy and equipment rental									15.2	13.3	13.5
Professional fees									15.2	14.0	12.5
Travel and business development									13.3	11.5	10.5
Depreciation and amortization									9.8	8.2	7.2
Bad debts	0.4	0.1	1.3	2.5	(0.2)	0.0	2.6	2.0	4.3	4.4	7.3
Other	33.2	32.7	31.7	32.8	32.0	26.5	25.1	25.5	37.5	29.4	23.5
Total non-interest expenses	153.6	108.3	109.6	105.9	98.0	95.9	92.9	90.6	477.4	377.4	353.7
Gain on acquisition					6.2	0.0	0.0	0.0	0.0	6.2	0.0
Loss from continuing operations, before tax	22.5	(15.0)	(14.3)	(8.4)	(19.2)	(21.4)	(20.0)	(12.1)	(15.2)	(72.7)	(78.1)
Income tax benefit	$ (1.1)	$ (2.3)	$ (3.3)	$ (2.1)	$ (2.4)	$ (6.8)	$ (5.5)	$ (3.3)	(8.8)	(18.0)	(22.4)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Management fees from affiliates									39.1	30.1	26.6
Trading gains, net									(1.0)	0.7	3.2
Consulting fees									0.0	2.2	2.1
Interest income									1.2	1.8	4.6
Operating revenues									92.0	65.8	42.5
Interest expense									14.4	13.4	12.7
Net operating revenues									77.6	52.4	29.8
Non-interest expenses:
Compensation and benefits									60.3	52.8	43.5
Clearing and related expenses									1.2	1.7	1.2
Introducing broker commissions									0.0	0.6	0.5
Communication and data services									7.3	6.7	5.7
Occupancy and equipment rental									2.5	2.8	2.1
Professional fees									3.7	4.8	4.6
Travel and business development									2.7	1.7	1.4
Depreciation and amortization									3.3	2.5	1.8
Bad debts									0.0	0.2	1.6
Management services fee to affiliates									0.0	1.2	4.3
Other									13.0	11.7	10.2
Total non-interest expenses									94.0	86.7	76.9
Gain on acquisition									0.0	6.2	0.0
Loss from continuing operations, before tax									16.4	28.1	47.1
Income tax benefit									26.8	24.7	19.4
Net loss									10.4	(3.4)	(27.7)
Adjustment to Investment in Subs for Equity Method Accounting									$ 4.0	$ 58.1	$ 83.4