Condensed Parent Only Financial Statements - Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2011	Sep. 30, 2012
Condensed Financial Statements, Captions [Line Items]
Depreciation and amortization	$ 7.2	$ 7.3	$ 8.0
Bad debts and impairments	7.3	5.5	0.8
Deferred Income Tax Expense (Benefit)	5.0	(5.7)	(7.7)
Amortization of Financing Costs and Discounts	0.9	1.0	1.2
Share-based Compensation	3.6	4.3	9.3
Notes receivable, net	(14.5)	(27.9)	66.6
Income taxes receivable	0.0	4.2	(0.2)
Financial instruments owned, at fair value	(565.0)	(42.6)	3.1
Other assets	(16.2)	0.1	4.3
Increase (Decrease) in Accounts Payable and Accrued Liabilities	23.2	1.9	(0.2)
Increase (Decrease) in Payables to Customers	332.1	191.5	24.0
Increase (Decrease) in Financial Instruments Sold, Not yet Purchased	177.5	84.1	4.6
Net Cash Provided by (Used in) Operating Activities	37.9	132.6	44.9
Payments to Acquire Property, Plant, and Equipment	(9.1)	(4.3)	(4.9)
Net Cash Provided by (Used in) Investing Activities	(15.5)	(4.3)	5.1
Net change in payable to lenders under loans	15.5	(38.5)	(157.2)
Proceeds from notes payable	4.0	0.0	0.0
Repayments of Notes Payable	(0.4)	0.0	0.0	$ 0.0
Proceeds from Issuance of Senior Long-term Debt	0.0	0.0	45.5
Payments related to earn-outs on acquisitions	(2.2)	(1.6)	(12.0)
Share repurchase	(4.7)	(9.7)	(3.9)
Debt issuance costs	(0.2)	(0.3)	(3.7)
Exercise of stock options	2.5	1.4	1.5
Excess Tax Benefit (Tax Deficiency) from Share-based Compensation, Financing Activities	0.5	(0.1)	0.1
Net cash provided by (used in) financing activities	15.0	(48.8)	(129.7)
Cash and Cash Equivalents, Period Increase (Decrease)	36.8	75.2	(80.2)
Cash and Cash Equivalents, at Carrying Value	268.1	231.3	156.1		$ 236.3
Cash paid for interest	15.8	9.6	8.9
Income taxes paid, net of cash refunds	15.3	3.0	10.2
Additional consideration payable related to acquisitions	1.9	(1.8)	8.2
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net loss	(27.7)	(26.2)	(24.6)
Depreciation and amortization	1.8	1.8	1.5
Bad debts and impairments	1.6	0.1	0.0
Deferred Income Tax Expense (Benefit)	4.6	(9.6)	(6.1)
Amortization of Financing Costs and Discounts	0.8	0.8	0.5
Share-based Compensation	3.6	4.3	9.3
Increase (Decrease) Due from Affiliates	33.2	84.6	(2.8)
Notes receivable, net	(7.8)	(12.8)	(17.1)
Income taxes receivable	(11.4)	4.6	(3.2)
Financial instruments owned, at fair value	(3.0)	0.0	1.5
Other assets	(3.9)	(1.1)	1.7
Increase (Decrease) in Accounts Payable and Accrued Liabilities	12.6	(1.1)	1.4
Increase (Decrease) in Payables to Customers	4.9	7.1	18.0
Increase (Decrease) in Financial Instruments Sold, Not yet Purchased	0.0	(0.6)	(24.7)
Net Cash Provided by (Used in) Operating Activities	9.3	51.9	(44.6)
Payments for Advance to Affiliate	(22.4)	(0.5)	(11.5)
Proceeds from Contributions from Affiliates	7.8	0.0	0.0
Payments to Acquire Property, Plant, and Equipment	(7.8)	(1.8)	(0.8)
Net Cash Provided by (Used in) Investing Activities	(22.4)	(2.3)	(12.3)
Net change in payable to lenders under loans	13.0	(40.0)	7.0
Proceeds from Issuance of Senior Long-term Debt	0.0	0.0	45.5
Payments related to earn-outs on acquisitions	(2.2)	(1.1)	0.0
Share repurchase	(4.7)	(9.7)	(4.0)
Debt issuance costs	(0.1)	(0.2)	(3.2)
Exercise of stock options	2.5	1.4	1.5
Excess Tax Benefit (Tax Deficiency) from Share-based Compensation, Financing Activities	0.5	(0.1)	0.1
Net cash provided by (used in) financing activities	12.6	(49.7)	46.9
Cash and Cash Equivalents, Period Increase (Decrease)	(0.5)	(0.1)	(10.0)
Cash and Cash Equivalents, at Carrying Value	2.5	3.0	3.1		$ 13.1
Cash paid for interest	11.9	6.9	3.0
Income taxes paid, net of cash refunds	(12.9)	(5.3)	(1.6)
Additional consideration payable related to acquisitions	$ 1.9	$ (3.0)	$ 5.6