Consolidated Statements of Comprehensive Income Statement - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 6.4	$ 54.7	$ 55.7
Other comprehensive income (loss), net of tax [Abstract]
Foreign currency translation adjustment	(1.4)	(7.4)	(4.0)
Pension liabilities adjustment	1.2	(0.2)	(1.5)
Net unrealized gain on available-for-sale securities	0.0	0.0	2.7
Reclassification of adjustment for losses (gains) included in net income:
Periodic pension costs (included in compensation and benefits)	0.4	0.5	0.3
Realized gain on available-for-sale securities (included in trading gains, net and interest income)	0.0	0.0	(5.4)
Income tax expense from reclassification adjustments (included in income tax expense)	(0.1)	0.0	2.0
Reclassification adjustment for losses (gains) included in net income	0.3	0.5	(3.1)
Other comprehensive income (loss)	0.1	(7.1)	(5.9)
Comprehensive income	$ 6.5	$ 47.6	$ 49.8