Condensed Parent Only Financial Statements - Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Condensed Financial Statements, Captions [Line Items]
Depreciation and amortization					$ 8.2	$ 7.2	$ 7.3
Bad debts and impairments					4.4	7.3	5.5
Deferred income taxes					(0.8)	5.0	(5.7)
Amortization of debt issuance costs and debt discount					1.1	0.9	1.0
Amortization of share-based compensation expense					5.1	3.6	4.3
Gain on acquisition	$ (6.2)	$ 0.0	$ 0.0	$ 0.0	(6.2)	0.0	0.0
Deposits and receivables from broker-dealers, clearing organizations, and counterparties					135.7	(150.2)	1.1
Receivables from subsidiaries, net					(3.1)	0.0	0.0
Notes receivable, net					59.5	(14.5)	(27.9)
Income taxes receivable					8.2	0.0	4.2
Financial instruments owned, at fair value					(192.9)	(565.0)	(42.6)
Other assets					(17.4)	(16.2)	0.1
Accounts payable and other accrued liabilities					7.5	23.2	1.9
Payable to customers					172.2	332.1	191.5
Financial instruments sold, not yet purchased, at fair value					273.9	177.5	84.1
Net cash (used in) provided by operating activities					(27.8)	37.9	132.6
Purchase of property and equipment					(15.4)	(9.1)	(4.3)
Net cash used in investing activities					(35.5)	(15.5)	(4.3)
Payable to lenders under loans					142.0	15.5	(38.5)
Proceeds from notes payable					0.0	4.0	0.0
Payments of notes payable					(0.8)	(0.4)	0.0
Payments related to earn-outs on acquisitions					(2.9)	(2.2)	(1.6)
Share repurchase					(19.5)	(4.7)	(9.7)
Debt issuance costs					(2.1)	(0.2)	(0.3)
Exercise of stock options					3.5	2.5	1.4
Income tax benefit on stock options and awards					0.8	0.5	(0.1)
Net cash provided by (used in) financing activities					121.0	15.0	(48.8)
Net (decrease) increase in cash and cash equivalents					48.1	36.8	75.2
Cash and cash equivalents at beginning of period				268.1	268.1	231.3	156.1
Cash and cash equivalents at end of period	316.2				316.2	268.1	231.3
Cash paid for interest					26.0	15.8	9.6
Income taxes (received) paid, net of cash refunds					8.5	15.3	3.0
Additional consideration payable related to acquisitions					(0.4)	1.9	(1.8)
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net loss					(3.4)	(27.7)	(26.2)
Depreciation and amortization					2.5	1.8	1.8
Bad debts and impairments					0.2	1.6	0.1
Deferred income taxes					(3.3)	4.6	(9.6)
Amortization of debt issuance costs and debt discount					1.0	0.8	0.8
Amortization of share-based compensation expense					5.1	3.6	4.3
Gain on acquisition					(6.2)	0.0	0.0
Deposits and receivables from broker-dealers, clearing organizations, and counterparties					2.8	0.0	0.0
Due to/from subsidiaries					(86.6)	33.2	84.6
Notes receivable, net					39.1	(7.8)	(12.8)
Income taxes receivable					10.3	(11.4)	4.6
Financial instruments owned, at fair value					1.7	(3.0)	0.0
Other assets					0.3	(3.9)	(1.1)
Accounts payable and other accrued liabilities					0.4	12.6	(1.1)
Payable to customers					(26.1)	4.9	7.1
Financial instruments sold, not yet purchased, at fair value					35.9	0.0	(0.6)
Net cash (used in) provided by operating activities					(35.0)	9.3	51.9
Capital contribution in affiliates					(48.4)	(22.4)	(0.5)
Capital withdrawals from affiliates					0.0	7.8	0.0
Purchase of property and equipment					(5.5)	(7.8)	(1.8)
Net cash used in investing activities					(53.9)	(22.4)	(2.3)
Payable to lenders under loans					108.5	13.0	(40.0)
Payments related to earn-outs on acquisitions					(2.9)	(2.2)	(1.1)
Share repurchase					(19.5)	(4.7)	(9.7)
Debt issuance costs					(1.9)	(0.1)	(0.2)
Exercise of stock options					3.5	2.5	1.4
Income tax benefit on stock options and awards					0.8	0.5	(0.1)
Net cash provided by (used in) financing activities					87.7	12.6	(49.7)
Net (decrease) increase in cash and cash equivalents					(1.2)	(0.5)	(0.1)
Cash and cash equivalents at beginning of period				$ 2.5	2.5	3.0	3.1
Cash and cash equivalents at end of period	$ 1.3				1.3	2.5	3.0
Cash paid for interest					9.0	11.9	6.9
Income taxes (received) paid, net of cash refunds					(33.8)	(12.9)	(5.3)
Additional consideration payable related to acquisitions					$ (0.4)	$ 1.9	$ (3.0)