Consolidated Statements of Comprehensive Income Statement - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 85.1	$ 55.5	$ 6.4
Other comprehensive income (loss), net of tax [Abstract]
Foreign currency translation adjustment	(0.8)	(9.0)	(1.4)
Pension liabilities adjustment	(0.8)	0.3	1.2
Reclassification of adjustment for losses (gains) included in net income:
Periodic pension costs (included in compensation and benefits)	0.1	0.1	0.4
Foreign currency gains realized upon dissolution of subsidiaries (included in principal gains, net)	(0.2)	0.0	0.0
Income tax expense from reclassification adjustments (included in income tax expense)	0.0	0.0	(0.1)
Reclassification adjustment for losses (gains) included in net income	(0.1)	0.1	0.3
Other comprehensive income (loss)	(1.7)	(8.6)	0.1
Comprehensive income	$ 83.4	$ 46.9	$ 6.5