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Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2017
Jan. 31, 2018
Jun. 30, 2017
Document and Entity Information [Abstract]      
Entity Registrant Name WEST PHARMACEUTICAL SERVICES INC    
Entity Central Index Key 0000105770    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 6,994,711,952
Entity Common Stock, Shares Outstanding   73,986,496  
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2017