Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The average strike price on the group of price risk option contracts such as put options or call options.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeAveragePriceRiskOptionStrikePrice
Namespace Prefix:	us-gaap_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) from the increase (decrease) in fair value of foreign currency derivatives and nonderivative instruments designated as fair value hedging instruments which were recognized in earnings, net of offsets by the gain (loss) on the hedged item to the extent that the fair value hedge was determined to be effective.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

+ Details

Name:	us-gaap_GainLossOnForeignCurrencyFairValueHedgeDerivatives
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in earnings in the period from the increase (decrease) in fair value of price risk derivatives not designated as hedging instruments.

+ References

+ Details

Name:	us-gaap_GainLossOnPriceRiskDerivativeInstrumentsNotDesignatedAsHedgingInstruments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Nominal or face amount used to calculate payments on a nonderivative instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_NotionalAmountOfNonderivativeInstruments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after tax, of cumulative translation adjustment attributable to gain (loss) on foreign currency derivatives that are designated as, and are effective as, economic hedges of a net investment in a foreign entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section 45
-Paragraph 20
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109240241&loc=d3e32211-110900

+ Details

Name:	us-gaap_TranslationAdjustmentForNetInvestmentHedgeNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Pre-tax cumulative adjustment resulting from gains and losses on foreign currency derivatives that are designated as, and are effective as, hedges of a net investment in a foreign entity, commencing as of the designation date. Adjustment reported in equity as part of accumulated other comprehensive income.

+ References

No definition available.

+ Details

Name:	wst_TranslationAdjustmentForNetInvestmentHedgePreTax
Namespace Prefix:	wst_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_DebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=wst_DebtYenDenominatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_OptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_EUR
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_SGD
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_USD
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PositionAxis=us-gaap_LongMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CurrencyAxis=currency_JPY
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PositionAxis=us-gaap_ShortMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Financial Instruments (Textuals) (Details) € in Millions, ¥ in Millions, $ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 EUR (€) MBbls $ / bbl	Mar. 31, 2019 JPY (¥)	Mar. 31, 2019 USD ($)	Mar. 31, 2019 SGD ($) MBbls $ / bbl	Mar. 31, 2019 JPY (¥) MBbls $ / bbl	Mar. 31, 2019 USD ($) MBbls $ / bbl	Jan. 31, 2019 MBbls	Dec. 31, 2018 EUR (€)	Dec. 31, 2018 SGD ($)	Dec. 31, 2018 USD ($)	Apr. 24, 2018 MBbls	Nov. 22, 2017 MBbls
Derivative [Line Items]
Gain (Loss) on Foreign Currency Fair Value Hedge Derivatives			$ 4.8
Debt EUR Denominated [Member]
Derivative [Line Items]
Notional amount, nonderivative instruments	€ 21.0		23.6
Cumulative foreign currency translation loss						$ 0.1
Cumulative foreign currency translation loss, net of tax						0.1
Debt Yen Denominated [Member]
Derivative [Line Items]
Notional amount, nonderivative instruments		¥ 500.0	4.5
Cumulative foreign currency translation loss						0.4
Cumulative foreign currency translation loss, net of tax						$ 0.3
Options Held [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount \| MBbls	108,891			108,891	108,891	108,891	81,459				30,612	125,166
Gain (Loss) related to call options			$ 0.1
Derivative, Average Price Risk Option Strike Price \| $ / bbl	66.22			66.22	66.22	66.22
Euro Member Countries, Euro \| Forward Contracts [Member]
Derivative [Line Items]
Notional amount \| €								€ 10.0
Singapore, Dollars \| Forward Contracts [Member]
Derivative [Line Items]
Notional amount				$ 601.5					$ 601.5
United States of America, Dollars \| Forward Contracts [Member]
Derivative [Line Items]
Notional amount						$ 13.4				$ 13.4
Long [Member] \| Designated as Hedging Instrument [Member] \| Singapore, Dollars \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount				$ 44.0
Long [Member] \| Designated as Hedging Instrument [Member] \| United States of America, Dollars \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount						31.5
Long [Member] \| Designated as Hedging Instrument [Member] \| Japan, Yen \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount \| ¥					¥ 5,232.0
Short [Member] \| Designated as Hedging Instrument [Member] \| Singapore, Dollars \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount	€ 6.4					25.1
Short [Member] \| Designated as Hedging Instrument [Member] \| United States of America, Dollars \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount	26.8					0.0
Short [Member] \| Designated as Hedging Instrument [Member] \| Japan, Yen \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount	€ 20.5					$ 24.4