v3.26.1
Derivative Financial Instruments - Schedule of Derivative Instruments Not Designated as Hedges (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]        
Derivative instruments not designated as hedging instruments, gain (loss), net $ (1.0) $ (0.2) $ 0.9 $ (0.4)
Not Designated as Hedging Instrument        
Derivative Instruments, Gain (Loss) [Line Items]        
Derivative instruments not designated as hedging instruments, gain (loss), net (1.2) 0.6 2.2 0.4
Commodity call options | Not Designated as Hedging Instrument | Income Statement Location [Axis]: wst:OtherIncomeExpense        
Derivative Instruments, Gain (Loss) [Line Items]        
Derivative instruments not designated as hedging instruments, gain (loss), net (0.9) (0.2) 1.0 (0.4)
Currency Forwards | Not Designated as Hedging Instrument | Income Statement Location [Axis]: wst:OtherIncomeExpense        
Derivative Instruments, Gain (Loss) [Line Items]        
Derivative instruments not designated as hedging instruments, gain (loss), net $ (0.3) $ 0.8 $ 1.2 $ 0.8