Effects of Derivative Instruments on OCI and Earnings (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Cash Flow Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI	$ 0.3	$ (3.4)	$ 1.5	$ (2.0)
Amount of (gain) loss reclassified from accumulated OCI into income	0.4	0.5	1.4	0.8
Net Investment Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI	1.2	(0.9)	0.5	(1.2)
Amount of (gain) loss reclassified from accumulated OCI into income	0.0	0.0	0.0	0.0
Foreign Currency Hedge Contracts [Member] \| Net sales [Member] \| Cash Flow Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI	0.5	(1.2)	0.1	(0.9)
Amount of (gain) loss reclassified from accumulated OCI into income	0.3	0.2	0.8	0.3
Foreign Currency Hedge Contracts [Member] \| Cost of goods and services sold [Member] \| Cash Flow Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI	(0.2)	(2.2)	1.4	(1.1)
Amount of (gain) loss reclassified from accumulated OCI into income	0.1	0.1	0.5	0.1
Interest Rate Swap Contracts [Member] \| Interest expense [Member] \| Cash Flow Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI	0.0	0.0	0.0	0.0
Amount of (gain) loss reclassified from accumulated OCI into income	0.0	0.2	0.0	0.3
Forward Treasury Locks [Member] \| Interest expense [Member] \| Cash Flow Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI			0.0	0.0
Amount of (gain) loss reclassified from accumulated OCI into income			0.1	0.1
Foreign Currency - Denominated Debt [Member] \| Other expense [Member] \| Net Investment Hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI	1.2	(0.9)	0.5	(1.2)
Amount of (gain) loss reclassified from accumulated OCI into income	$ 0.0	$ 0.0	$ 0.0	$ 0.0

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.

No definition available.

+ Details

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

- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying 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 (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959

+ Details

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

- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

+ Details

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

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_NetInvestmentHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_IncomeStatementLocationAxis=wst_NetSalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_CostOfSalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_InterestExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_ForeignCurrencyGainLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: