Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2018
Accounting Policies [Abstract]
New Accounting Pronouncements, Policy [Policy Text Block]	Accounting Guidance Implemented Income Taxes In March 2018, the Financial Accounting Standards Board (“FASB”) issued guidance relative to Income Taxes (Topic 740) that adds various Securities and Exchange Commission (“SEC”) paragraphs pursuant to the issuance of the December 2017 SEC Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”), which was effective immediately. The SEC issued SAB 118 to address concerns about reporting entities’ ability to timely comply with the accounting requirements to recognize all of the effects of the Tax Cuts and Jobs Act (the “Tax Act”) in the period of enactment. SAB 118 allows disclosure that timely determination of some or all of the income tax effects from the Tax Cuts and Jobs Act are incomplete by the due date of the financial statements and if possible, to provide a reasonable estimate. We have accounted for the tax effects of the Tax Act under the guidance of SAB 118, on a provisional basis. Our accounting for certain income tax effects is incomplete, but we have determined reasonable estimates for those effects and have recorded provisional amounts in our Consolidated Financial Statements as of September 30, 2018. Goodwill Impairment In January 2017, the FASB issued guidance simplifying the test for goodwill impairment, which removes certain steps from the goodwill impairment test. The guidance is effective for annual periods beginning after December 15, 2019, including interim periods within those periods, with early adoption permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The company adopted the standard effective October 1, 2017. The adoption did not have a material impact on the Consolidated Financial Statements. Measurement of Inventory In July 2015, the FASB issued guidance to simplify the measurement of inventory recorded using either the first-in, first-out (“FIFO”) or average cost basis by changing the subsequent measurement guidance from lower of cost or market to the lower of cost or net realizable value. Inventory measured using last-in, first-out (“LIFO”) is not impacted. The company adopted the standard effective October 1, 2017. This guidance did not have a significant impact on our Consolidated Financial Statements. New Accounting Guidance to be Implemented Collaborative Arrangements In November 2018, the FASB issued guidance on how to assess whether certain transactions between collaborative arrangement participants should be accounted for within the revenue recognition standard. The company is required to adopt this guidance beginning October 1, 2020. Early adoption is permitted. The company is currently assessing the impact of the guidance on its consolidated financial statements and related disclosures. Internal Use Software In August 2018, FASB issued guidance which requires a customer in a cloud computing arrangement that is a service contract to follow the internal-use software guidance in Accounting Standards Codification 350-40 to determine which implementation costs to defer and recognize as an asset. The new guidance generally aligns the guidance on recognizing implementation costs incurred in a cloud computing arrangement that is a service contract with that for implementation costs incurred to develop or obtain internal-use software, including hosting arrangements that include an internal-use software license. The company is required to adopt this guidance beginning October 1, 2020. Early adoption is permitted. The company is currently assessing the impact of the guidance on its consolidated financial statements and related disclosures. Defined Benefit Plan Disclosures In August 2018, the FASB issued guidance which eliminates requirements for certain disclosures that are not considered cost beneficial, clarifies certain required disclosures and adds additional disclosures under defined benefit pension plans and other post-retirement plans. The company is required to adopt this guidance beginning October 1, 2021. Early adoption is permitted. The amendments in the guidance would need to be applied on a retrospective basis. The company is currently evaluating the potential impact of the adoption of this guidance on our disclosures. Net Periodic Pension Costs In March 2017, the FASB issued guidance which requires an entity to report the service cost component of pension expense in the same line item as other compensation costs. The other components of net (benefit) cost will be required to be presented in the income statement separately from the service cost component and outside a subtotal of income from operations. This standard is effective for interim and annual reporting periods beginning after December 15, 2017. The components of the net (benefit) cost are shown in Note 15, “Retirement Benefits”. The company will adopt the standard effective October 1, 2018. The adoption is not expected to have a material impact on the Consolidated Financial Statements. Business Combinations In January 2017, the FASB issued guidance on the definition of a business in business combinations. The guidance clarifies the definition of a business with the objective of adding guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The definition of a business affects many areas of accounting including acquisitions, disposals, goodwill, and consolidation. The guidance is effective for annual periods beginning after December 15, 2017, including interim periods within those periods. The company will adopt the standard effective October 1, 2018. The adoption is not expected to have a material impact on the Annual Consolidated Financial Statements. Revenue Recognition In May 2014, the FASB issued guidance based on the principle that revenue is recognized in an amount expected to be collected and to which the entity expects to be entitled in exchange for the transfer of goods or services. In August 2015, the FASB deferred the effective date by one year, while providing the option to early adopt the standard on the original effective date. In December 2016 there were further updates to the original guidance that did not revise the effective date. The guidance can be adopted either retrospectively or as a cumulative-effect adjustment as of the date of adoption. The company will adopt the standard effective October 1, 2018. The company will adopt the standard using the modified retrospective method, with a cumulative-effect adjustment recorded in retained earnings as of the beginning of the period of adoption. The company has determined that the adoption of this new standard will not have a material impact to our consolidated financial statements. In addition, we do not expect major changes to the company’s existing accounting systems or internal controls as a result of the adoption. The company is finalizing the disclosures which will be included in its consolidated financial statements upon adoption of the new standard. Leases In February 2016, the FASB issued guidance which requires lessees to recognize a right of use asset and lease liability on the balance sheet for all leases, including operating leases, with a term in excess of 12 months. The guidance also expands the quantitative and qualitative disclosure requirements. The guidance is effective in fiscal year 2020, with early adoption permitted, and must be applied using a modified retrospective approach. In July 2018, the FASB issued updates to the lease standard making transition requirements less burdensome. The update provides an option to apply the transition provisions of the new standard at its adoption date instead of at the earliest comparative period presented in the company’s financial statements. The company is currently the lessee under various agreements for distribution equipment and vehicles that are currently accounted for as operating leases as discussed in Note 12, “Leases”. The new guidance requires the lessee to record operating leases on the balance sheet with a right-of-use asset and corresponding liability for future payment obligations. The company will adopt the standard effective October 1, 2019. We are currently evaluating the impact of adopting this new guidance on our Consolidated Financial Statements. Cash Flow Statement Classification In August 2016, the FASB issued guidance to reduce diversity in practice on how certain cash receipts and cash payments are classified in the statement of cash flows. The guidance is effective beginning fiscal year 2019, with early adoption permitted, and should be applied retrospectively. The company will adopt the standard effective October 1, 2018. The adoption is not expected to have a material impact on the Consolidated Financial Statements.
Basis of Accounting, Policy [Policy Text Block]	the accompanying Annual Consolidated Financial Statements of Versum, were prepared on a stand-alone basis and were derived from Air Products’ consolidated financial statements and accounting records where Versum was a division of Air Products. For all periods presented, the Annual Consolidated Financial Statements were prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and include the accounts of Versum. All significant intercompany transactions and balances have been eliminated in the preparation of the accompanying Annual Consolidated Financial Statements. Prior to the Separation, transactions between Versum and Air Products were reflected in the annual consolidated statements of cash flows as a financing activity in “Net transfers to Air Products.”
Allocation of Shared Costs [Policy Text Block]	Air Products provided us with centrally managed services and corporate functions. Accordingly, certain shared costs including but not limited to administrative expenses for information technology, general services, human resources, legal, accounting and other services, had been allocated to us and were reflected as expenses in the Annual Consolidated Financial Statements. Expenses had been allocated on the basis of direct usage when identifiable, with the remainder allocated on the basis of fixed costs, revenue, operating income or headcount. We consider the expense allocation methodology and results to be reasonable and consistently applied for all periods presented prior to the Separation.
Estimates and Assumptions	Estimates and Assumptions The preparation of the Annual Consolidated Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect amounts reported in the Annual Consolidated Financial Statements and accompanying notes, including allocations of costs during the reporting period. Management’s estimates are based on historical experience, facts and circumstances available at the time, and various other assumptions that are believed to be reasonable. Actual results could differ from those estimates.
Revenue Recognition	Revenue Recognition Revenue from product sales is recognized as risk and title to the product transfer to the customer, the sales price is fixed or determinable, and collectability is reasonably assured. Sales returns and allowances are generally not a business practice in the industry. We use both the completed contract and percentage-of-completion methods to record revenue from equipment sale contracts. The completed contract method is used in circumstances in which financial position and results of operations are not materially different from those resulting from use of the percentage-of-completion method, e.g., certain short-term contracts. We use the percentage-of-completion method when we can make reasonably dependable estimates of progress toward completion and performance is expected. Under the percentage-of-completion method, revenue from the sale of major equipment is recognized primarily based on costs incurred to date compared with total estimated costs. Under the completed contract method, revenue and cost is recognized when the equipment is completed and transferred to the customer. Changes to estimated labor hours under the percentage-of-completion method or anticipated losses under either method, if any, are recognized in the period determined. Revenue from on-site services are generally fixed monthly fee arrangements for which we recognize revenue as the services are performed. Amounts billed for shipping and handling fees are classified as sales in the consolidated income statements. Amounts billed for sales and use taxes, value-added taxes, and certain excise and other specific transactional taxes imposed on revenue-producing transactions are presented on a net basis and excluded from sales in the consolidated income statements. We record a liability until remitted to the respective taxing authority.
Cost of Sales	Cost of Sales Cost of sales predominantly represents the cost of tangible products sold. These costs include labor, raw materials, depreciation, production supplies, and materials packaging costs. Costs incurred for shipping and handling are also included in cost of sales.
Depreciation	Depreciation Depreciation is recorded using the straight-line method, which deducts equal amounts of the cost of each asset from earnings every year over its expected economic useful life.
Selling and Administrative	Selling and Administrative The principal components of selling and administrative expenses are compensation, advertising, and promotional costs.
Research and Development	Research and Development Research and development costs are expensed as incurred. Research and development expenses include employee costs, materials, contract services, research agreements, and other external spending related to the discovery and development of new products, enhancement of existing products and regulatory approval of new and existing products.
Postemployment Benefits	Post-employment Benefits We provide termination benefits to employees as part of ongoing benefit arrangements and record a liability for termination benefits when probable and estimable. These criteria are met when management, with the appropriate level of authority, approves and commits to its plan of action for termination; the plan identifies the employees to be terminated and their related benefits; and the plan is to be completed within one year. We typically do not provide one-time benefit arrangements of significance.
Fair Value Measurements	Fair Value Measurements We are required to measure certain assets and liabilities at fair value, either upon initial measurement or for subsequent accounting or reporting. For example, fair value is used in the initial measurement of net assets acquired in a business combination; on a recurring basis in the measurement of derivative financial instruments; and on a nonrecurring basis when long-lived assets are written down to fair value when held for sale or determined to be impaired.
Financial Instruments	Financial Instruments We address certain financial exposures through a controlled program of risk management that includes the use of derivative financial instruments. If the circumstances qualify and we designate hedge accounting, the hedging relationship between the underlying financial exposures and the related hedging instrument would be documented on the date the derivative is entered into with the counterparty. We have not designated hedge accounting for any of the forward exchange contracts.
Foreign Currency	Foreign Currency Since we do business in many foreign countries, fluctuations in currency exchange rates affect our financial position and results of operations. In certain of our foreign operations, the local currency is considered the functional currency. These foreign subsidiaries translate their assets and liabilities into U.S. dollars at current exchange rates in effect at the end of the fiscal period. The gains or losses that result from this process are shown as translation adjustments in accumulated other comprehensive income (loss) in the stockholders’ equity section of the annual consolidated balance sheets. The revenue and expense accounts of these foreign subsidiaries are translated into U.S. dollars at the average exchange rates that prevail during the period. Therefore, the U.S. dollar value of these items on the income statement fluctuates from period to period, depending on the value of the dollar against foreign currencies. Some transactions are made in currencies different from an entity’s functional currency. Gains and losses from these foreign currency transactions are generally included in other income (expense), net on our consolidated income statements as they occur.
Litigation	Litigation Accruals for litigation are made when the information available indicates that it is probable that a liability has been incurred and the amount can be reasonably estimated. When only a range of possible loss can be established, the most probable amount in the range is accrued. If no amount within this range is a better estimate than any other amount within the range, the minimum amount in the range is accrued. The accrual for a litigation loss contingency includes estimates of potential damages and other directly related costs expected to be incurred. Litigation liabilities and expenditures included in the Annual Consolidated Financial Statements were not material for the periods presented.
Share-Based Compensation	Share-Based Compensation Prior to the Separation, our employees participated in Air Products’ share-based compensation plans, which included stock options, deferred stock units, and restricted stock. Prior to the Separation, the Annual Consolidated Financial Statements included share-based compensation expense associated with our employees and Air Products’ costs that had been allocated to us based on awards and terms previously granted. In accordance with the Employee Matters Agreement entered into between Versum and Air Products on September 29, 2016 in connection with the Separation, all share-based compensation awards previously granted to Versum employees under Air Products’ Long-Term Incentive Plan that were outstanding on October 1, 2016, other than restricted stock, were adjusted and converted into Versum equity with substantially the same terms and conditions as the original Air Products awards. The grant-date fair value of awards is expensed over the vesting period during which employees perform related services. Expense recognition is accelerated for retirement-eligible individuals who would meet the requirements for vesting of awards upon their retirement. The Black Scholes model is utilized to value new stock option awards. The grant date fair value of the deferred stock units tied to a market condition is estimated using a Monte Carlo simulation model for new awards.
Income Taxes	Income Taxes We account for income taxes under the liability method. Under this method, deferred tax assets and liabilities are recognized for the tax effects of temporary differences between the financial reporting and tax bases of assets and liabilities measured using enacted tax rates. The cumulative impact of a change in tax rates or regulations is included in income tax expense in the period that includes the enactment date. For the fiscal year ended September 30, 2016, certain of our operations included in our Annual Consolidated Financial Statements were divisions of legal entities included in Air Products consolidated U.S. federal and state income tax returns, or tax returns of non-U.S. subsidiaries of Air Products. The provision for income taxes has been prepared and presented in the Annual Consolidated Financial Statements based on a stand-alone basis separate from Air Products. Differences between our separate return income tax provision and cash flows attributable to income taxes for operations that were divisions of legal entities have been recognized as capital contributions from, or dividends to, Air Products, within Air Products’ net investment. For the fiscal year ended September 30, 2016, the majority of the accrued U.S. federal, state, and foreign income tax balances were treated as settled with Air Products as of the end of the year. Therefore, this is included in Air Products’ net investment in the Annual Consolidated Financial Statements, for such periods.
Non-controlling Interests	Non-controlling Interests We consolidate investments that we control but do not wholly own. The Annual Consolidated Financial Statements include all assets, liabilities, revenues, and expenses of our joint venture in Taiwan for which we have a 74% ownership interest during the periods presented in the Annual Consolidated Financial Statements. The ownership interests held by third party non-controlling partners are presented as non-controlling interests in our annual consolidated balance sheets. Any net income or loss attributed to the non-controlling partners is presented as such in the consolidated income statements. The activity for non-controlling interests for the years ended September 30, 2018, 2017 and 2016 is presented in the annual consolidated statements of stockholders’ equity.
Cash and Cash Items	Cash and Cash Items Cash and cash items generally include cash, time deposits, and certificates of deposit acquired with an original maturity of three months or less for our foreign entities.
Trade Receivables, net	Trade Receivables, net Trade receivables comprise amounts owed to us through our operating activities and are presented net of allowances for doubtful accounts. The allowances for doubtful accounts represent estimated uncollectible receivables associated with potential customer defaults on contractual obligations. A provision for customer defaults is made on a general formula basis when it is determined that the risk of some default is probable and estimable but cannot yet be associated with specific customers. The assessment of the likelihood of customer defaults is based on various factors, including the length of time the receivables are past due, historical experience, and existing economic conditions. The allowance also includes amounts for certain customers where a risk of default has been specifically identified, considering factors such as the financial condition of the customer and customer disputes over contractual terms and conditions.
Inventories	Inventories Inventories are stated at the lower of cost or net realizable value. We write down our inventories for estimated obsolescence or unmarketable inventory based upon assumptions about future demand and market conditions.
Equity Investments	Equity Investments The equity method of accounting is used when we exercise significant influence but do not have operating control, generally assumed to be 20% to 50% ownership. Under the equity method, original investments are recorded at cost and adjusted by our share of undistributed earnings or losses of these companies. Equity investments are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the investment may not be recoverable. The Annual Consolidated Financial Statements include our investment in and proportionate share of the income from our 20% owned equity affiliate, Daido Air Products Electronics, Inc. During the first quarter of 2016, we sold our investment in this affiliate. Refer to Note 6, “Sale of Equity Affiliate”, for additional information.
Plant and Equipment	Plant and Equipment Plant and equipment is stated at cost less accumulated depreciation. Construction costs, labor, and applicable overhead related to installations are capitalized. Expenditures for additions and improvements that extend the lives or increase the capacity of plant assets are capitalized. The costs of maintenance and repairs of plant and equipment are charged to expense as incurred. Fully depreciated assets are retained in the gross plant and equipment and accumulated depreciation accounts until they are removed from service. In the case of disposals, assets and related depreciation are removed from the accounts, and the net amounts, less proceeds from disposal, are included in income.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets are grouped for impairment testing at the lowest level for which there are identifiable cash flows that are largely independent of the cash flows of other assets and liabilities and are evaluated for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset group may not be recoverable. We assess recoverability by comparing the carrying amount of the asset group to estimated undiscounted future cash flows expected to be generated by the asset group. If an asset group is considered impaired, the impairment loss to be recognized is measured as the amount by which the asset group’s carrying amount exceeds its fair value. Long-lived assets held for sale are reported at the lower of carrying amount or fair value less cost to sell.
Goodwill	Goodwill The purchase price is allocated to the assets acquired and liabilities assumed based on their estimated fair market values. Any excess purchase price over the fair market value of the net assets acquired, including identified intangibles, is recorded as goodwill. Preliminary purchase price allocations are made at the date of acquisition and finalized when information needed to affirm underlying estimates is obtained, within a maximum allocation period of one year. Goodwill is subject to impairment testing at least annually. In addition, goodwill is tested more frequently if a change in circumstances or the occurrence of events indicates that potential impairment exists. We conducted impairment tests during the fourth quarter of 2018 and determined that there was no goodwill impairment.
Intangible Assets	Intangible Assets Intangible assets with determinable lives primarily consist of customer relationships and purchased patents and technology. The cost of intangible assets with determinable lives is amortized on a straight-line basis over the estimated period of economic benefit. Amortizable lives are adjusted whenever there is a change in the estimated period of economic benefit. No residual value is estimated for intangible assets.
Retirement Benefits	Retirement Benefits For periods prior to October 1, 2016, Air Products sponsored defined benefit pension plans, defined contribution plans, and other post-employment benefit plans that are shared amongst its businesses. Prior to the Separation on October 1, 2016, participation of our employees in these plans was reflected in the Annual Consolidated Financial Statements as though Versum participated in a multi-employer plan with Air Products. A proportionate share of cost was reflected in these Annual Consolidated Financial Statements, primarily within selling and administrative expenses.
Air Products' Net Investment	Air Products’ Net Investment Prior to the Separation, Air Products’ net investment in our business was presented as “Air Products’ net investment” in lieu of stockholders’ equity, as a stand-alone legal and capital structure did not exist for the historical periods presented.
Earnings Per Share	Earnings Per Share Prior to the Separation, Versum earnings per share for 2016 were calculated using the shares that were distributed to Air Products stockholders immediately following the Separation. For periods prior to the Separation it is assumed that there are no dilutive equity instruments as there were no equity awards in Versum outstanding prior to the Separation.