Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2018
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation: The consolidated financial statements include the accounts of West after the elimination of intercompany transactions. We have no participation or other rights in variable interest entities. As of April 1, 2017, our consolidated financial statements exclude the results of our Venezuelan subsidiary. Please refer to Note 15, Other Expense, for further discussion.
Use of Estimates	Use of Estimates: The financial statements are prepared in conformity with U.S. GAAP. These principles require management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingencies in the financial statements. Actual amounts realized may differ from these estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents: Cash equivalents include time deposits, certificates of deposit and all highly liquid debt instruments with maturities of three months or less at the time of purchase.
Accounts Receivable	Accounts Receivable: Our accounts receivable balance was net of an allowance for doubtful accounts of $2.0 million and $0.5 million at December 31, 2018 and 2017, respectively. We record the allowance based on a specific identification methodology.
Inventories	Inventories: Inventories are valued at the lower of cost (on a first-in, first-out basis) and net realizable value.
Property, Plant and Equipment	Property, Plant and Equipment: Property, plant and equipment assets are carried at cost. Maintenance and minor repairs and renewals are charged to expense as incurred. Costs incurred for computer software developed or obtained for internal use are capitalized for application development activities and immediately expensed for preliminary project activities or post-implementation activities. Upon sale or retirement of depreciable assets, costs and related accumulated depreciation are eliminated, and gains or losses are recognized in other (income) expense. Depreciation and amortization are computed principally using the straight-line method over the estimated useful lives of the assets, or the remaining term of the lease, if shorter.
Impairment of Goodwill and Other Intangible Assets
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets: Long-lived assets, including property, plant and equipment, are tested for impairment whenever circumstances indicate that the carrying value of these assets may not be recoverable. An asset is considered impaired if the carrying value of the asset exceeds the sum of the future expected undiscounted cash flows to be derived from the asset. Once an asset is considered impaired, an impairment loss is recorded within other (income) expense for the difference between the asset’s carrying value and its fair value. For assets held and used in the business, management determines fair value using estimated future cash flows to be derived from the asset, discounted to a net present value using an appropriate discount rate. For assets held for sale or for investment purposes, management determines fair value by estimating the proceeds to be received upon sale of the asset, less disposition costs. During 2018, as part of our 2018 restructuring plan, we recorded within other expense a $2.2 million non-cash asset write-down associated with the discontinued use of certain equipment. During 2016, as part of our 2016 restructuring plan, we recorded within other expense a $4.5 million non-cash asset write-down associated with the discontinued use of certain equipment.
Employee Benefits	Employee Benefits: The measurement of the obligations under our defined benefit pension and postretirement medical plans are subject to a number of assumptions. These include the rate of return on plan assets (for funded plans) and the rate at which the future obligations are discounted to present value. For our funded plans, we consider the current and expected asset allocations of our plan assets, as well as historical and expected rates of return, in estimating the long-term rate of return on plan assets. U.S. GAAP requires the recognition of an asset or liability for the funded status of a defined benefit postretirement plan, as measured by the difference between the fair value of plan assets, if any, and the benefit obligation. For a pension plan, the benefit obligation is the projected benefit obligation; for any other postretirement plan, such as a retiree health plan, the benefit obligation is the accumulated postretirement benefit obligation. Please refer to Note 14, Benefit Plans, for a more detailed discussion of our pension and other retirement plans.
Financial Instruments	Financial Instruments: All derivatives are recognized as either assets or liabilities in the balance sheet and recorded at their fair value. For a derivative designated as hedging the exposure to variable cash flows of a forecasted transaction (referred to as a cash flow hedge), the effective portion of the derivative’s gain or loss is initially reported as a component of other comprehensive income (“OCI”), net of tax, and subsequently reclassified into earnings when the forecasted transaction affects earnings. For a derivative designated as hedging the exposure to changes in the fair value of a recognized asset or liability or a firm commitment (referred to as a fair value hedge), the derivative’s gain or loss is recognized in earnings in the period of change together with the offsetting loss or gain on the hedged item. For a derivative designated as hedging the foreign currency exposure of a net investment in a foreign operation, the gain or loss is reported in OCI, net of tax, as part of the cumulative translation adjustment. The ineffective portion of any derivative used in a hedging transaction is recognized immediately into earnings. Derivative financial instruments that are not designated as hedges are also recorded at fair value, with the change in fair value recognized immediately into earnings. We do not purchase or hold any derivative financial instrument for investment or trading purposes.
Foreign Currency Translation	Foreign Currency Translation: Foreign currency transaction gains and losses are recognized in the determination of net income. Foreign currency translation adjustments of subsidiaries and affiliates operating outside of the U.S. are accumulated in other comprehensive loss, a separate component of equity.
Revenue Recognition	Revenue Recognition: Our revenue results from the sale of goods or services and reflects the consideration to which we expect to be entitled in exchange for those goods or services. Revenue is recognized based on a five-step model, in accordance with ASC 606. Following the identification of a contract with a customer, we identify the performance obligations (goods or services) in the contract, determine the transaction price, allocate the transaction price to the performance obligations in the contract, and recognize the revenue when (or as) we satisfy the performance obligations by transferring the promised goods or services to our customers. A good or service is transferred when (or as) the customer obtains control of that good or service. Some customers receive pricing rebates upon attaining established sales volumes. We record rebate costs when sales occur based on our assessment of the likelihood that the required volumes will be attained. We also maintain an allowance for product returns, as we believe that we are able to reasonably estimate the amount of returns based on our substantial historical experience. Please refer to Note 3, Revenue, for additional information.
Shipping and Handling Costs	Shipping and Handling Costs: Shipping and handling costs are included in cost of goods and services sold. Shipping and handling costs billed to customers in connection with the sale are included in net sales.
Research and Development	Research and Development: Research and development expenditures are for the creation, engineering and application of new or improved products and processes. Expenditures include primarily salaries and outside services for those directly involved in research and development activities and are expensed as incurred.
Environmental Remediation and Compliance Costs	Environmental Remediation and Compliance Costs: Environmental remediation costs are accrued when such costs are probable and reasonable estimates are determinable. Cost estimates include investigation, cleanup and monitoring activities; such estimates are adjusted, if necessary, based on additional findings. Environmental compliance costs are expensed as incurred as part of normal operations.
Litigation	Litigation: From time to time, we are involved in legal proceedings, investigations and claims generally incidental to our normal business activities. In accordance with U.S. GAAP, we accrue for loss contingencies when it is probable that a liability has been incurred and the amount of the loss can be reasonably estimated. These estimates are based on an analysis made by internal and external legal counsel considering information known at the time. Legal costs in connection with loss contingencies are expensed as incurred.
Income Taxes	Income Taxes: Deferred income taxes are recognized by applying enacted statutory tax rates, applicable to future years, to temporary differences between the tax basis and financial statement carrying values of our assets and liabilities. Valuation allowances are established when it is more likely than not that all or a portion of a deferred tax asset will not be realized. In response to the 2017 Tax Act, we reevaluated our position regarding permanent reinvestment of foreign subsidiary earnings and profits through 2017 (with the exception of China and Mexico) and decided that those profits were no longer permanently reinvested. As of January 1, 2018, we reasserted indefinite reinvestment related to all post-2017 unremitted earnings in all of our foreign subsidiaries. Please refer to Note 16, Income Taxes, for additional information. We recognize interest costs related to income taxes in interest expense and penalties within other (income) expense. The tax law ordering approach is used for purposes of determining whether an excess tax benefit has been realized during the year.
Stock-based Compensation	Stock-Based Compensation: Under the fair value provisions of U.S. GAAP, stock-based compensation cost is measured at the grant date based on the value of the award and is recognized as expense over the vesting period. In order to determine the fair value of stock options on the grant date, we use the Black-Scholes valuation model.
Net Income Per Share	Net Income Per Share: Basic net income per share is computed by dividing net income attributable to common shareholders by the weighted average number of shares of common stock outstanding during each period. Net income per share assuming dilution considers the dilutive effect of outstanding stock options and other stock awards based on the treasury stock method. The treasury stock method assumes the use of exercise proceeds to repurchase common stock at the average fair market value in the period.
New Accounting Standards	Recently Adopted Standards In March 2018, the 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 2017 Tax Act was signed into law. This guidance was effective immediately upon issuance. Please refer to Note 16, Income Taxes, 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 was effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption was 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 was effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption was 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 year ended December 31, 2018 and 2017 was $4.1 million and $7.3 million, respectively, of which $10.8 million and $10.4 million, respectively, related to service cost and $6.7 million and $3.1 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 was effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption was permitted. We adopted this guidance as of January 1, 2018, on a retrospective basis. As of December 31, 2018 and 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 was effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. Early adoption was 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 was 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, 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 believe that the adoption of this guidance will not have a material impact 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 believe that the adoption of this guidance will not have a material impact 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 was permitted. We believe that the adoption of this guidance will not have a material impact 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 was 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 was permitted. We believe that the adoption of this guidance will not have a material impact 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 continue to review the impact that the adoption of this guidance will have on our financial statement disclosures, accounting policies, business processes, and internal controls. As of December 31, 2018 and 2017, future minimum rental payments under non-cancelable operating leases were $81.5 million and $79.1 million, respectively.