Income Taxes - Current and Deferred Taxes (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Current taxes:
Total current taxes									$ 33.0	$ 22.2	$ 23.7
Deferred taxes									4.1	3.7	22.3
Income tax expense	$ 2.5	$ 12.4	$ 16.8	$ 5.4	$ 6.9	$ 5.3	$ 7.5	$ 6.2	37.1	25.9	46.0
United States [Member]
Current taxes:
U.S. federal									(0.6)	(1.9)	0.8
U.S. State and local									2.3	(0.8)	0.5
Non-US [Member]
Current taxes:
International									$ 31.3	$ 24.9	$ 22.4