Other Finance Income (Costs) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Sep. 30, 2018
Other Nonoperating Income (Expense) [Abstract]
Pension interest cost	$ (0.7)	$ (2.7)	$ (2.9)
Expected return on pension plan assets	0.9	3.5	(3.7)
Foreign currency translation on senior bank debt	(2.9)	(0.8)	3.4
Foreign currency remeasurement on hedging instrument	2.0	3.2
Other finance income (costs)	$ (0.7)	$ 3.2	$ 4.1