v3.26.1
Derivative Financial Instruments - Schedule of Effects of Derivative Instruments on Other Comprehensive Income ('OCI') and Earnings (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Derivative Instruments, Gain (Loss) [Line Items]        
Total $ 0.3 $ (0.5) $ (0.9) $ (0.9)
Cash flow hedges, amount of gain (loss) recognized in OCI (1.6) 2.2 (2.7) 3.9
Cash flow hedges, amount of (gain) loss reclassified from accumulated OCI into income 0.8 (0.1) 1.5 0.3
Net investment hedges, amount of gain (loss) recognized in OCI 1.4 (3.5) 1.8 (6.3)
Net investment hedges, amount of (gain) loss reclassified from accumulated OCI into income 0.0 0.0 0.0 0.0
Fair Value Hedging        
Derivative Instruments, Gain (Loss) [Line Items]        
Fair value hedges, amount of gain (loss) recognized in OCI (0.1) 0.4 0.2 0.8
Fair value hedges, amount of (gain) loss reclassified from accumulated OCI into income 0.0 0.0 0.0 0.0
Foreign currency hedge contracts | Fair Value Hedging        
Derivative Instruments, Gain (Loss) [Line Items]        
Fair value hedges, amount of gain (loss) recognized in OCI (0.1) 0.4 0.2 0.8
Fair value hedges, amount of (gain) loss reclassified from accumulated OCI into income 0.0 0.0 0.0 0.0
Cross-currency swap        
Derivative Instruments, Gain (Loss) [Line Items]        
Net investment hedges, amount of gain (loss) recognized in OCI 1.4 (3.5) 1.8 (6.3)
Net investment hedges, amount of (gain) loss reclassified from accumulated OCI into income 0.0 0.0 0.0 0.0
Forward treasury locks        
Derivative Instruments, Gain (Loss) [Line Items]        
Cash flow hedges, amount of gain (loss) recognized in OCI     0.0 0.0
Cash flow hedges, amount of (gain) loss reclassified from accumulated OCI into income     0.0 0.0
Income Statement Location [Axis]: us-gaap:CostOfGoodsAndServicesSold        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (loss) on derivative instruments, net, pretax 0.7 (0.4) 1.1 0.2
Income Statement Location [Axis]: us-gaap:CostOfGoodsAndServicesSold | Foreign currency hedge contracts        
Derivative Instruments, Gain (Loss) [Line Items]        
Cash flow hedges, amount of gain (loss) recognized in OCI (1.3) 4.1 (2.6) 6.5
Cash flow hedges, amount of (gain) loss reclassified from accumulated OCI into income 0.7 (0.4) 1.1 0.2
Income Statement Location [Axis]: us-gaap:InterestExpenseNonoperating        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (loss) on derivative instruments, net, pretax 0.0 0.0 0.0 0.0
Income Statement Location [Axis]: us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax        
Derivative Instruments, Gain (Loss) [Line Items]        
Gain (loss) on derivative instruments, net, pretax 0.1 0.3 0.4 0.1
Income Statement Location [Axis]: us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax | Foreign currency hedge contracts        
Derivative Instruments, Gain (Loss) [Line Items]        
Cash flow hedges, amount of gain (loss) recognized in OCI (0.3) (1.9) (0.1) (2.6)
Cash flow hedges, amount of (gain) loss reclassified from accumulated OCI into income 0.1 0.3 0.4 0.1
Income Statement Location [Axis]: wst:OtherIncomeExpense        
Derivative Instruments, Gain (Loss) [Line Items]        
Hedged item (intercompany loan) (0.4) 0.1 (4.9) 3.4
Derivative designated as hedging instrument 0.4 (0.1) 4.9 (3.4)
Amount excluded from effectiveness testing $ 0.3 $ (0.5) $ (0.9) $ (0.9)