Condensed Parent Only Financial Statements - Income Statement (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Condensed Financial Statements, Captions [Line Items]
Trading gains, net									$ 389.1	$ 332.2	$ 321.2
Consulting fees									356.8	283.4	224.3
Interest income									123.3	69.7	55.2
Dividend income from subsidiaries									41.9	52.7	31.0
Operating revenues	$ 243.2	$ 259.8	$ 260.2	$ 212.6	$ 205.1	$ 197.6	$ 195.8	$ 185.5	975.8	784.0	671.0
Interest expense	25.3	22.1	19.0	14.3	12.0	11.2	10.0	8.9	80.7	42.1	28.3
Net operating revenues	142.4	154.6	154.3	130.3	131.1	123.3	123.9	114.3	581.6	492.6	443.9
Non-interest expenses:
Compensation and benefits	85.4	86.9	88.2	77.2	73.0	75.5	76.6	70.6	337.7	295.7	263.9
Clearing and related expenses	43.1	49.0	50.7	36.9	35.1	33.9	33.7	33.6	179.7	136.3	129.9
Introducing broker commissions	32.4	34.1	36.2	31.1	26.9	29.2	28.2	28.7	133.8	113.0	68.9
Occupancy and equipment rental									16.5	15.2	13.3
Professional fees									18.1	15.2	14.0
Travel and business development									13.8	13.3	11.5
Non-trading technology and support									13.9	11.6	7.1
Depreciation and amortization									11.6	9.8	8.2
Communications									5.4	5.0	4.7
Bad debts	1.2	1.6	0.2	0.1	0.4	0.1	1.3	2.5	3.1	4.3	4.4
Other	35.3	35.2	36.4	33.4	33.2	32.7	31.7	32.8	26.3	25.9	22.3
Total non-interest expenses	121.9	123.7	124.8	111.7	153.6	108.3	109.6	105.9	482.1	477.4	377.4
Loss from continuing operations, before tax	(20.5)	(32.9)	(29.5)	(18.6)	22.5	(15.0)	(14.3)	(8.4)	(101.5)	(15.2)	(72.7)
Income tax benefit	$ (4.8)	$ (8.9)	$ (6.8)	$ (25.5)	$ (1.1)	$ (2.3)	$ (3.3)	$ (2.1)	(46.0)	(8.8)	(18.0)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Management fees from affiliates									40.4	39.1	30.1
Trading gains, net									0.0	(1.0)	0.7
Consulting fees									0.0	0.0	2.2
Interest income									2.3	1.2	1.8
Operating revenues									84.6	92.0	65.8
Interest expense									15.7	14.4	13.4
Net operating revenues									68.9	77.6	52.4
Non-interest expenses:
Compensation and benefits									73.0	60.3	52.8
Clearing and related expenses									1.1	1.2	1.7
Introducing broker commissions									0.0	0.0	0.6
Communication and data services									5.8	6.4	5.6
Occupancy and equipment rental									2.6	2.5	2.8
Professional fees									6.7	3.7	4.8
Travel and business development									2.6	2.7	1.7
Non-trading technology and support									9.1	7.4	4.7
Depreciation and amortization									4.8	3.3	2.5
Communications									0.9	0.9	1.1
Bad debts									0.0	0.0	0.2
Management services fee to affiliates									0.0	0.0	1.2
Other									6.9	5.6	7.0
Total non-interest expenses									113.5	94.0	86.7
Gain on acquisition									0.0	0.0	6.2
Loss from continuing operations, before tax									44.6	16.4	28.1
Income tax benefit									7.4	26.8	24.7
Net loss									(37.2)	10.4	(3.4)
Adjustment to Investment in Subs for Equity Method Accounting									$ 92.7	$ 4.0	$ 58.1