Income Taxes - Unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Activity within the reserve for unrecognized tax benefits
Balance at the beginning of the period	$ 40.5	$ 35.7	$ 9.7
Additions for tax positions related to the current year	0.0	6.5	0.6
Additions for tax positions of prior years	1.0	1.6	0.7
Reductions for tax positions of prior years	(6.9)	(3.3)	(4.4)
Settlements	(0.7)	0.0	(0.1)
Balance at the end of the period	33.9	40.5	35.7
Unrecognized Tax Benefits, Increase Resulting from Acquisition	0.0	0.0	$ 29.2
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	33.9	40.5
Additional amounts recognized (released) for interest and penalties	0.5	1.2
Other amounts accrued for interest and penalties	$ 3.8	3.7
Amount by which the unrecognized tax benefits could increase or decrease over the next 12 months		$ 0.5