Retirement and Other Benefit Plans - Schedule Of Net periodic benefit cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Retirement and Other Benefit Plans
Service cost	$ 0.0	$ 0.0	$ 0.0
Interest cost	3.0	3.7	3.4
Expected return on plan assets	(4.2)	(4.4)	(3.8)
Settlements	53.6	0.0	0.0
Net actuarial loss	2.8	2.1	2.3
Net periodic benefit cost	55.2	1.4	1.9
Defined contribution plans	7.4	6.8	6.6
Total	$ 62.6	$ 8.2	$ 8.5