Summary of Significant Accounting Policies - Intangible assets (Details) $ in Millions	12 Months Ended
	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)
Future amortization expense
2017	$ 91.3
2018	91.3
2019	91.3
2020	90.3
2021	90.0
Components of intangible assets and the related accumulated amortization
Cost	1,549.1	$ 1,614.7
Accumulated amortization	(588.4)	(526.2)
Other Finite-Lived Intangible Assets, Gross	1,724.4	1,812.1
Advanced Process Solutions
Components of intangible assets and the related accumulated amortization
Trade names, indefinite lives	114.8	112.3
Finite-Lived Intangible Assets, Net	638.1	693.0
Molding Technology Solutions
Components of intangible assets and the related accumulated amortization
Trade names, indefinite lives	60.5	85.1
Finite-Lived Intangible Assets, Net	$ 306.2	388.3
Minimum
Components of intangible assets and the related accumulated amortization
Intangible assets amortization period	3 years
Maximum
Components of intangible assets and the related accumulated amortization
Intangible assets amortization period	21 years
Trade Names
Components of intangible assets and the related accumulated amortization
Impairment of Intangible Assets, Indefinite-lived (Excluding Goodwill)	$ 1.2	27.0
Trade names, indefinite lives	$ 175.3	197.4
Trade Names | Minimum | Measurement Input, Royalty Rate [Member]
Components of intangible assets and the related accumulated amortization
Intangible Asset, Measurement Input (percent)	0.015
Trade Names | Maximum | Measurement Input, Royalty Rate [Member]
Components of intangible assets and the related accumulated amortization
Intangible Asset, Measurement Input (percent)	0.030
Trade Names
Components of intangible assets and the related accumulated amortization
Cost	$ 53.9	52.5
Accumulated amortization	(16.7)	(10.6)
Customer relationships
Components of intangible assets and the related accumulated amortization
Cost	1,239.7	1,316.9
Accumulated amortization	(419.1)	(379.4)
Technology, including patents
Components of intangible assets and the related accumulated amortization
Cost	189.0	192.3
Accumulated amortization	(113.4)	(100.9)
Software
Components of intangible assets and the related accumulated amortization
Cost	66.5	53.0
Accumulated amortization	$ (39.2)	$ (35.3)