Income Taxes - Unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Activity within the reserve for unrecognized tax benefits
Balance at the beginning of the period	$ 77.9	$ 38.9	$ 33.9
Additions for tax positions related to the current year	6.2	31.6	0.7
Additions for tax positions of prior years	9.2	2.5	0.6
Reductions for tax positions of prior years	(32.3)	(0.8)	(2.9)
Settlements	(0.4)	(0.1)	(1.7)
Balance attributable to purchase accounting	0.0	5.8	8.3
Balance attributable to divestiture	(13.6)	0.0	0.0
Balance at the end of the period	47.0	77.9	$ 38.9
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	47.0	77.9
Additional amounts recognized (released) for interest and penalties	1.4	2.6
Other amounts accrued for interest and penalties	$ 7.8	7.5
Amount by which the unrecognized tax benefits could increase or decrease over the next 12 months		$ 0.5