Segment Information - Schedule of Segment Financial Information Footnote (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026 USD ($) program	Jun. 30, 2025 USD ($) program	Jun. 30, 2026 USD ($) program	Jun. 30, 2025 USD ($) program
Segment Reporting [Line Items]
Number of restructuring program \| program	2	2	2	2
Charges related to other expense (income)	$ 12.8	$ 5.2	$ 16.8	$ 25.8
Selling, general and administrative expenses	117.6	95.9	217.1	183.9
Cost-method investment impairment charges	3.5	0.0	3.5	0.0
Disposal Group, Held-for-Sale, Not Discontinued Operations \| SmartDose® 3.5mL On-Body Delivery System And Associated Facilities
Segment Reporting [Line Items]
Charges related to other expense (income)	1.3		2.2
Selling, general and administrative expenses	5.1		6.1
Corporate And Reconciling Items
Segment Reporting [Line Items]
Restructuring and related charges	1.5	1.6	2.9	19.4
SmartDose® 3.5mL sale	6.4	0.0	8.3	0.0
Cost-method investment impairment charges	3.5	0.0	3.5	0.0
Amortization of acquisition-related intangible assets		0.0	0.0	0.2
Corporate And Reconciling Items \| Affiliated Entity
Segment Reporting [Line Items]
Cost-method investment impairment charges	3.5		3.5
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Segment Reporting [Line Items]
Restructuring and related charges	0.5	1.4	1.0	2.8
Corporate And Reconciling Items \| Income Statement Location [Axis]: wst:OtherIncomeExpense
Segment Reporting [Line Items]
Restructuring and related charges	$ 1.0	$ 0.2	$ 1.9	$ 16.6

Amount of amortization expense for finite-lived intangible asset. Excludes goodwill and capitalized cost for software to be sold, leased, or marketed.

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