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Stock-Based Compensation (Stock Appreciation Rights Award Activity) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
SARs, Weighted Average Exercise Price [Roll Forward] |  |  |
Stock-Settled SARs, Outstanding | 203,540 wst_StockSettledStockAppreciationRightsOutstanding |  |
Cash-Settled SARs, Outstanding | 94,174 wst_CashSettledStockAppreciationRIghtsOutstanding |  |
Stock Appreciation Rights (SARs) [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Awards, expiration period | 10 years |  |
SARs Activity [Roll Forward] |  |  |
Outstanding, Beginning | 375,104 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | 389,686 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | 320,336 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember
Granted | 7,733 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | 132,566 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | 145,018 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember
Exercised | (85,123)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | (147,148)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | (75,668)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember
Outstanding, Ending | 297,714 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | 375,104 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | 389,686 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember
Exercisable | 88,751 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | 56,938 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | 110,292 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableNumber / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember
SARs, Weighted Average Exercise Price [Roll Forward] |  |  |
Oustanding, Beginning (in usd per share) | $ 24.03 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 20.81 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 20.17 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember
Granted (in usd per share) | $ 47.74 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGrantsInPeriodWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 29.56 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGrantsInPeriodWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 21.22 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGrantsInPeriodWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember
Exercised (in usd per share) | $ 22.09 wst_ShareBasedCompensationArrangementsByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisesInPeriodWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 20.47 wst_ShareBasedCompensationArrangementsByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisesInPeriodWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 18.91 wst_ShareBasedCompensationArrangementsByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisesInPeriodWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember
Oustanding, Ending (in usd per share) | $ 25.20 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 24.03 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 20.81 wst_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember
Exercisable (in usd per share) | $ 23.15 wst_ShareBasedCompensationArrangementsByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 20.95 wst_ShareBasedCompensationArrangementsByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember | $ 20.70 wst_ShareBasedCompensationArrangementsByShareBasedPaymentAwardNonOptionEquityInstrumentsExercisableWeightedAverageExercisePrice / us-gaap_AwardTypeAxis = us-gaap_StockAppreciationRightsSARSMember

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- Definition Period from grant date that an equity-based award expires, 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 (a)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of non-option equity instruments exercised by participants. + 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 (c)(1)(iv)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Net number of non-option equity instruments granted to participants. + 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 (c)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of equity instruments other than options outstanding, including both vested and non-vested instruments. + 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 (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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