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Stock-Based Compensation (Allocation of Share-based Compensation Costs) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition | 1 year 6 months |  |
Stock compensation expense | $ 18.6 us-gaap_AllocatedShareBasedCompensationExpense | $ 21.2 us-gaap_AllocatedShareBasedCompensationExpense | $ 15.5 us-gaap_AllocatedShareBasedCompensationExpense
Stock Option and Appreciation Rights [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock compensation expense | 7.6 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_StockOptionAndAppreciationRightsMember | 7.7 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_StockOptionAndAppreciationRightsMember | 5.3 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_StockOptionAndAppreciationRightsMember
Performance-Vesting Shares [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock compensation expense | 6.5 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 6.5 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 6.0 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Performance-Vesting Units [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock compensation expense | 1.9 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_PerformanceVestingUnitsMember | 2.4 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_PerformanceVestingUnitsMember | 0.9 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_PerformanceVestingUnitsMember
Performance-vesting Shares or Units Dividend Equivalents [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock compensation expense | 0.4 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_PerformanceVestingSharesOrUnitsDividendEquivalentsMember | 0.4 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_PerformanceVestingSharesOrUnitsDividendEquivalentsMember | 0.1 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_PerformanceVestingSharesOrUnitsDividendEquivalentsMember
Employee Stock Purchase Plan (ESPP) [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock compensation expense | 0.5 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_EmployeeStockPurchasePlanEsppMember | 0.4 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_EmployeeStockPurchasePlanEsppMember | 0.4 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = wst_EmployeeStockPurchasePlanEsppMember
Deferred Compensation Plans [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stock compensation expense | $ 1.7 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_DeferredCompensationShareBasedPaymentsMember | $ 3.8 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_DeferredCompensationShareBasedPaymentsMember | $ 2.8 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_DeferredCompensationShareBasedPaymentsMember

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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