Income Taxes - Deferred Tax Assets and Liabilites (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Operating Loss Carryforwards [Line Items]
Federal operating loss carryforwards, net of valuation allowances	$ 0.6
Deferred Tax Assets, Operating Loss Carryforwards, Foreign	2.6	$ 4.2
Deferred Tax Assets, Tax Credit Carryforwards, Foreign	5.0	6.5
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost	3.3	2.8
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation	3.6	1.4
Deferred Tax Assets, Operating Loss Carryforwards, State and Local	9.2	8.7
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	1.1	0.0	$ 21.9
Deferred Tax Assets, Intangible Assets	4.8	1.8
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Allowance for Doubtful Accounts	1.3	1.5
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	0.5	0.0
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits	2.2	3.4
Deferred Tax Assets, Other	1.1	0.9
Deferred Tax Assets, Gross	34.7	31.2
Deferred Tax Assets, Valuation Allowance	(8.5)	(3.5)
Deferred Tax Assets, Net of Valuation Allowance	26.2	27.7
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	3.2	2.6
Deferred Tax Liabilities, Prepaid Expenses	2.2	1.8
Deferred Tax Liabilities, Property, Plant and Equipment	2.6	3.1
Deferred Tax Liabilities, Pension Liability	0.2	0.4
Deferred Tax Liabilities, Gross	8.2	7.9
Deferred tax assets, Net	$ 18.0	19.8
SINGAPORE
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards, Foreign		2.9
Tax Period [Domain]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Tax Credit Carryforwards, Foreign		$ 5.1