Summary of Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation: The condensed consolidated financial statements included in this report are unaudited and have been prepared in accordance with United States (“U.S.”) generally accepted accounting principles (“U.S. GAAP”) for interim financial reporting and U.S. Securities and Exchange Commission (“SEC”) regulations. The year-end condensed consolidated balance sheet data was derived from audited financial statements, but does not include all disclosures required by U.S. GAAP. Certain information and footnote disclosures normally included in annual financial statements prepared in accordance with U.S. GAAP have been condensed or omitted. In the opinion of management, these financial statements include all adjustments, which are of a normal recurring nature, necessary for a fair statement of the financial position, results of operations, cash flows and the change in equity for the periods presented. The condensed consolidated financial statements for the three and nine months ended September 30, 2018 should be read in conjunction with the consolidated financial statements and notes thereto of West Pharmaceutical Services, Inc. and its majority-owned subsidiaries (which may be referred to as “West”, the “Company”, “we”, “us” or “our”) appearing in our Annual Report on Form 10-K for the year ended December 31, 2017 (the “2017 Annual Report”). The results of operations for any interim period are not necessarily indicative of results for the full year. As of April 1, 2017, our consolidated financial statements exclude the results of our Venezuelan subsidiary. Please refer to Note 13, Other (Income) Expense, for further discussion.
New Accounting Standards	Recently Adopted Standards In March 2018, the Financial Accounting Standards Board (“FASB”) issued guidance which updates the income tax accounting in U.S. GAAP to reflect the SEC’s interpretive guidance released on December 22, 2017, when the Tax Cuts and Jobs Act (the “2017 Tax Act”) was signed into law. This guidance was effective immediately upon issuance. Please refer to Note 15, Income Taxes, to the consolidated financial statements in our 2017 Annual Report for additional information. In May 2017, the FASB issued guidance which amends the scope of modification accounting for share-based payment arrangements. The guidance focuses on changes to the terms or conditions of share-based payment awards that would require the application of modification accounting and specifies that an entity would not apply modification accounting if its fair value, vesting conditions and classification of the awards are the same immediately before and after the modification. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption is permitted. We adopted this guidance as of January 1, 2018, on a prospective basis. The adoption did not have a material impact on our financial statements. In March 2017, the FASB issued guidance on the presentation of net periodic pension and postretirement benefit cost (net benefit cost). The guidance requires the bifurcation of net benefit cost. The service cost component will be presented with other employee compensation costs in operating income (or capitalized in assets) and the other components will be reported separately outside of operations, and will not be eligible for capitalization. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption is permitted. We adopted this guidance as of January 1, 2018, on a retrospective basis. As a result of this adoption, we reclassified net benefit cost components other than service cost from operating income to outside of operations. Net periodic benefit cost for the three months ended September 30, 2018 and 2017 was $0.8 million and $1.4 million, respectively, of which $2.6 million and $2.5 million, respectively, related to service cost and $1.8 million and $1.1 million, respectively, related to net benefit cost components other than service cost. Net periodic benefit cost for the nine months ended September 30, 2018 and 2017 was $3.0 million and $5.3 million, respectively, of which $8.1 million and $7.8 million, respectively, related to service cost and $5.1 million and $2.5 million, respectively, related to net benefit cost components other than service cost. The adoption of this guidance had no impact on net income. In November 2016, the FASB issued guidance on the classification and presentation of restricted cash in the statement of cash flows. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption is permitted. We adopted this guidance as of January 1, 2018, on a retrospective basis. As of September 30, 2018 and December 31, 2017, we had no restricted cash. In August 2016, the FASB issued guidance to reduce the diversity in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption is permitted. We adopted this guidance as of January 1, 2018, on a retrospective basis. The adoption did not have a material impact on our financial statements. In January 2016, the FASB issued guidance that addresses certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. We adopted this guidance as of January 1, 2018, on a prospective basis. The adoption did not have a material impact on our financial statements. In May 2014, the FASB issued guidance on the accounting for revenue from contracts with customers, Accounting Standards Codification (“ASC”) Topic 606 (“ASC 606”), that supersedes most existing revenue recognition guidance, including industry-specific guidance. The core principle requires an entity to recognize revenue to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In addition, ASC 606 requires enhanced disclosures regarding the nature, timing and uncertainty of revenue and cash flows arising from an entity’s contracts with customers. The FASB subsequently issued additional clarifying standards to address issues arising from implementation of ASC 606. We adopted ASC 606 as of January 1, 2018, on a modified retrospective basis. Please refer to Note 3, Revenue, for additional information. Standards Issued Not Yet Adopted In August 2018, the FASB issued guidance to align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by this update. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019. Early adoption is permitted, including adoption in any interim period. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements. In August 2018, the FASB issued guidance which modifies the disclosure requirements for defined benefit pension plans and other postretirement plans. The guidance removes disclosures that no longer are considered cost beneficial, clarifies the specific requirements of disclosures, and adds disclosure requirements identified as relevant. This guidance is effective for fiscal years ending after December 15, 2020. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements. In August 2018, the FASB issued guidance which modifies the disclosure requirements on fair value measurements by removing, modifying, or adding certain disclosures. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements. In June 2018, the FASB issued guidance which expands the scope of accounting for share-based payment arrangements to include share-based payment transactions for acquiring goods and services from nonemployees. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements. In February 2018, the FASB issued guidance to address a specific consequence of the 2017 Tax Act by allowing a reclassification from accumulated other comprehensive income (loss) to retained earnings for stranded tax effects resulting from the 2017 Tax Act’s reduction of the U.S. federal corporate income tax rate. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements. In August 2017, the FASB issued guidance which expands and refines hedge accounting for both nonfinancial and financial risk components and aligns the recognition and presentation of the effects of the hedging instrument and the hedged item in the financial statements. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance will have on our financial statements. In February 2016, the FASB issued guidance on the accounting for leases. This guidance requires lessees to recognize lease assets and lease liabilities on the balance sheet and to expand disclosures about leasing arrangements, both qualitative and quantitative. In terms of transition, the guidance requires adoption based upon a modified retrospective approach. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018. We are currently evaluating the impact that this guidance will have on our financial statements. As of September 30, 2018 and December 31, 2017, future minimum rental payments under non-cancelable operating leases were $67.0 million and $79.1 million, respectively.

Recently Adopted Standards

In March 2018, the Financial Accounting Standards Board (“FASB”) issued guidance which updates the income tax accounting in U.S. GAAP to reflect the SEC’s interpretive guidance released on December 22, 2017, when the Tax Cuts and Jobs Act (the “2017 Tax Act”) was signed into law. This guidance was effective immediately upon issuance. Please refer to Note 15, Income Taxes, to the consolidated financial statements in our 2017 Annual Report for additional information.

In May 2017, the FASB issued guidance which amends the scope of modification accounting for share-based payment arrangements. The guidance focuses on changes to the terms or conditions of share-based payment awards that would require the application of modification accounting and specifies that an entity would not apply modification accounting if its fair value, vesting conditions and classification of the awards are the same immediately before and after the modification. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption is permitted. We adopted this guidance as of January 1, 2018, on a prospective basis. The adoption did not have a material impact on our financial statements.

In March 2017, the FASB issued guidance on the presentation of net periodic pension and postretirement benefit cost (net benefit cost). The guidance requires the bifurcation of net benefit cost. The service cost component will be presented with other employee compensation costs in operating income (or capitalized in assets) and the other components will be reported separately outside of operations, and will not be eligible for capitalization. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption is permitted. We adopted this guidance as of January 1, 2018, on a retrospective basis. As a result of this adoption, we reclassified net benefit cost components other than service cost from operating income to outside of operations. Net periodic benefit cost for the three months ended September 30, 2018 and 2017 was $0.8 million and $1.4 million, respectively, of which $2.6 million and $2.5 million, respectively, related to service cost and $1.8 million and $1.1 million, respectively, related to net benefit cost components other than service cost. Net periodic benefit cost for the nine months ended September 30, 2018 and 2017 was $3.0 million and $5.3 million, respectively, of which $8.1 million and $7.8 million, respectively, related to service cost and $5.1 million and $2.5 million, respectively, related to net benefit cost components other than service cost. The adoption of this guidance had no impact on net income.

In November 2016, the FASB issued guidance on the classification and presentation of restricted cash in the statement of cash flows. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption is permitted. We adopted this guidance as of January 1, 2018, on a retrospective basis. As of September 30, 2018 and December 31, 2017, we had no restricted cash.

In August 2016, the FASB issued guidance to reduce the diversity in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption is permitted. We adopted this guidance as of January 1, 2018, on a retrospective basis. The adoption did not have a material impact on our financial statements.

In January 2016, the FASB issued guidance that addresses certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. We adopted this guidance as of January 1, 2018, on a prospective basis. The adoption did not have a material impact on our financial statements.

In May 2014, the FASB issued guidance on the accounting for revenue from contracts with customers, Accounting Standards Codification (“ASC”) Topic 606 (“ASC 606”), that supersedes most existing revenue recognition guidance, including industry-specific guidance. The core principle requires an entity to recognize revenue to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In addition, ASC 606 requires enhanced disclosures regarding the nature, timing and uncertainty of revenue and cash flows arising from an entity’s contracts with customers. The FASB subsequently issued additional clarifying standards to address issues arising from implementation of ASC 606. We adopted ASC 606 as of January 1, 2018, on a modified retrospective basis. Please refer to Note 3, Revenue, for additional information.

Standards Issued Not Yet Adopted

In August 2018, the FASB issued guidance to align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by this update. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019. Early adoption is permitted, including adoption in any interim period. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements.

In August 2018, the FASB issued guidance which modifies the disclosure requirements for defined benefit pension plans and other postretirement plans. The guidance removes disclosures that no longer are considered cost beneficial, clarifies the specific requirements of disclosures, and adds disclosure requirements identified as relevant. This guidance is effective for fiscal years ending after December 15, 2020. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements.

In August 2018, the FASB issued guidance which modifies the disclosure requirements on fair value measurements by removing, modifying, or adding certain disclosures. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements.

In June 2018, the FASB issued guidance which expands the scope of accounting for share-based payment arrangements to include share-based payment transactions for acquiring goods and services from nonemployees. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements.

In February 2018, the FASB issued guidance to address a specific consequence of the 2017 Tax Act by allowing a reclassification from accumulated other comprehensive income (loss) to retained earnings for stranded tax effects resulting from the 2017 Tax Act’s reduction of the U.S. federal corporate income tax rate. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance may have on our financial statements.

In August 2017, the FASB issued guidance which expands and refines hedge accounting for both nonfinancial and financial risk components and aligns the recognition and presentation of the effects of the hedging instrument and the hedged item in the financial statements. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018. Early adoption is permitted. We are currently evaluating our adoption timing and the impact that this guidance will have on our financial statements.

In February 2016, the FASB issued guidance on the accounting for leases. This guidance requires lessees to recognize lease assets and lease liabilities on the balance sheet and to expand disclosures about leasing arrangements, both qualitative and quantitative. In terms of transition, the guidance requires adoption based upon a modified retrospective approach. This guidance is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018. We are currently evaluating the impact that this guidance will have on our financial statements. As of September 30, 2018 and December 31, 2017, future minimum rental payments under non-cancelable operating leases were $67.0 million and $79.1 million, respectively.