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

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

+ 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 30
-Section 45
-Paragraph 20
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=64808268&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:	us-gaap_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:	us-gaap_CurrencyAxis=currency_SGD
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_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:	us-gaap_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, 2018 EUR (€) MBbls	Mar. 31, 2018 JPY (¥)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 SGD ($) MBbls	Mar. 31, 2018 JPY (¥) MBbls	Mar. 31, 2018 USD ($) MBbls	Dec. 31, 2017 EUR (€) MBbls	Dec. 31, 2017 SGD ($) MBbls	Dec. 31, 2017 USD ($) MBbls
Debt EUR Denominated [Member]
Derivative [Line Items]
Notional amount, nonderivative instruments	€ 21.0		$ 25.9
Cumulative foreign currency translation loss						$ 2.0
Cumulative foreign currency translation loss, net of tax						1.6
Debt Yen Denominated [Member]
Derivative [Line Items]
Notional amount, nonderivative instruments		¥ 500.0	4.7
Cumulative foreign currency translation loss						0.5
Cumulative foreign currency translation loss, net of tax						$ 0.4
Options Held [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount \| MBbls	96,771			96,771	96,771	96,771	125,166	125,166	125,166
Loss related to call options			$ 0.1
Euro Member Countries, Euro \| Forward Contracts [Member]
Derivative [Line Items]
Notional amount \| €	€ 10.0						€ 12.0
Singapore, Dollars \| Forward Contracts [Member]
Derivative [Line Items]
Notional amount				$ 171.0				$ 171.0
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				$ 45.2
Long [Member] \| Designated as Hedging Instrument [Member] \| United States of America, Dollars \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount						39.7
Long [Member] \| Designated as Hedging Instrument [Member] \| Japan, Yen \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount \| ¥					¥ 7,155.1
Short [Member] \| Designated as Hedging Instrument [Member] \| Singapore, Dollars \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount	10.1					21.6
Short [Member] \| Designated as Hedging Instrument [Member] \| United States of America, Dollars \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount	33.5					0.0
Short [Member] \| Designated as Hedging Instrument [Member] \| Japan, Yen \| Foreign Exchange Forward [Member]
Derivative [Line Items]
Notional amount	€ 23.9					$ 37.3