Condensed Parent Only Financial Statements - Income Statement (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Interest income									$ 198.9	$ 123.3	$ 69.7
Dividend income from subsidiaries									85.7	41.9	52.7
Operating revenues	$ 286.9	$ 283.4	$ 271.1	$ 264.7	$ 243.2	$ 259.8	$ 260.2	$ 212.6	1,106.1	975.8	784.0
Interest expense	40.8	42.5	38.4	33.0	25.3	22.1	19.0	14.3	154.7	80.7	42.1
Net operating revenues	173.4	165.6	165.2	149.0	142.4	154.6	154.3	130.3	653.2	581.6	492.6
Non-interest expenses:
Compensation and benefits	105.2	100.9	97.9	89.1	85.4	86.9	88.2	77.2	393.1	337.7	295.7
Clearing and related expenses	45.0	45.7	42.7	50.1	43.1	49.0	50.7	36.9	183.5	179.7	136.3
Introducing broker commissions	27.7	29.6	24.8	32.6	32.4	34.1	36.2	31.1	114.7	133.8	113.0
Trade systems and market information									38.8	34.7	34.4
Occupancy and equipment rental									19.4	16.5	15.2
Professional fees									21.0	18.1	15.2
Travel and business development									16.2	13.8	13.3
Depreciation and amortization									14.0	11.6	9.8
Communications									6.6	5.4	5.0
Bad debts	1.0	0.5	0.7	0.3	1.2	1.6	0.2	0.1
Other	43.2	42.6	41.1	37.6	35.3	35.2	36.4	33.4	28.4	26.3	25.9
Total non-interest expenses	139.4	144.0	139.7	124.6	121.9	123.7	124.8	111.7	547.7	482.1	477.4
Other gains	0.1	0.0	5.4	0.0	0.0	2.0	0.0	0.0	5.5	2.0	0.0
Loss from continuing operations, before tax	(34.1)	(21.6)	(30.9)	(24.4)	(20.5)	(32.9)	(29.5)	(18.6)	(111.0)	(101.5)	(15.2)
Income tax benefit	$ (6.9)	$ (5.3)	$ (7.5)	$ (6.2)	$ (4.8)	$ (8.9)	$ (6.8)	$ (25.5)	(25.9)	(46.0)	(8.8)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Management fees from affiliates									43.2	40.4	39.1
Trading gains, net									0.0	0.0	(1.0)
Fees and Commissions (Deprecated 2018-01-31)									0.1	0.0	0.0
Interest income									1.5	2.3	1.2
Operating revenues									130.5	84.6	92.0
Interest expense									19.7	15.7	14.4
Net operating revenues									110.8	68.9	77.6
Non-interest expenses:
Compensation and benefits									79.7	73.0	60.3
Clearing and related expenses									0.9	1.1	1.2
Trade systems and market information									6.4	5.8	6.4
Occupancy and equipment rental									3.4	2.6	2.5
Professional fees									7.3	6.7	3.7
Travel and business development									2.9	2.6	2.7
Non-trading technology and support									12.5	9.1	7.4
Depreciation and amortization									5.2	4.8	3.3
Communications									0.8	0.9	0.9
Management services fee to affiliates									0.5	0.0	0.0
Other									5.8	6.9	5.6
Total non-interest expenses									125.4	113.5	94.0
Other gains									5.3	0.0	0.0
Loss from continuing operations, before tax									9.3	44.6	16.4
Income tax benefit									(24.6)	(7.4)	(26.8)
Net loss									15.3	(37.2)	10.4
Adjustment to Investment in Subs for Equity Method Accounting									$ 69.8	$ 92.7	$ (4.0)