Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Jun. 30, 2018
Basis of Presentation	Basis of presentation The accompanying unaudited Condensed Consolidated Financial Statements have been prepared in accordance with accounting principles generally accepted in the United States for interim financial reporting and Securities and Exchange Commission (SEC) regulations. In the opinion of management, all adjustments considered necessary for a fair presentation have been included. Additionally, certain prior period data, primarily related to discontinued operations, have been reclassified in the Consolidated Financial Statements and accompanying notes to conform to the current period presentation, as further described in this section. These statements omit certain information and footnote disclosures required for complete annual financial statements and, therefore, should be read in conjunction with Ashland’s Annual Report on Form 10-K for the fiscal year ended September 30, 2017. Results of operations for the period ended June 30, 2018 are not necessarily indicative of the expected results for the remaining quarter in the fiscal year. On May 12, 2017, Ashland completed the distribution of its remaining 170 million shares of common stock of Valvoline Inc. which represented approximately 83% of the total outstanding shares of Valvoline Inc.'s common stock. This separation from Valvoline represented a strategic shift in Ashland's business and qualified as a discontinued operation. Accordingly, Valvoline's operating results and cash flows for the three and nine months ended June 30, 2017 have been classified as discontinued operations within the Condensed Consolidated Financial Statements. See Note B for additional information on the separation of Valvoline Inc. Subsequent to completing the separation from Valvoline Inc., Ashland's operations are managed within the following three reportable segments: Specialty Ingredients, Composites and Intermediates and Solvents.
Use of Estimates, Risk and Uncertainties	Use of estimates, risks and uncertainties The preparation of Ashland’s Condensed Consolidated Financial Statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and the disclosures of contingent assets and liabilities. Significant items that are subject to such estimates and assumptions include, but are not limited to, long-lived assets (including goodwill and other intangible assets), income taxes and liabilities and receivables associated with asbestos litigation and environmental remediation. Although management bases its estimates on historical experience and various other assumptions that are believed to be reasonable under the circumstances, actual results could differ significantly from the estimates under different assumptions or conditions. Ashland’s results are affected by domestic and international economic, political, legislative, regulatory and legal actions. Economic conditions, such as recessionary trends, inflation, interest and monetary exchange rates, government fiscal policies and changes in the prices of certain key raw materials, can have a significant effect on operations. While Ashland maintains reserves for anticipated liabilities and carries various levels of insurance, Ashland could be affected by civil, criminal, regulatory or administrative actions, claims or proceedings relating to asbestos, environmental remediation or other matters.
New Accounting Standards	New accounting standards A description of new U.S. GAAP accounting standards issued or adopted during the current year is required in interim financial reporting. A detailed listing of new accounting standards relevant to Ashland is included in the Annual Report on Form 10-K for the fiscal year ended September 30, 2017. The following standards relevant to Ashland were either issued or adopted in the current period, or will become effective in a subsequent period. In May 2014, the FASB issued accounting guidance outlining a single comprehensive five step model for entities to use in accounting for revenue arising from contracts with customers (ASC 606 Revenue from Contracts with Customers). The new guidance supersedes most current revenue recognition guidance, in an effort to converge the revenue recognition principles within U.S. GAAP. This new guidance also requires entities to disclose certain quantitative and qualitative information regarding the nature, amount, timing and uncertainty of qualifying revenue and cash flows arising from contracts with customers. Ashland has formed an implementation team that is continuing to evaluate the impact of the new standard on the Condensed Consolidated Financial Statements and the adoption method options available, as well as the overall impact the new guidance will have on the organization. The assessment process consists of categorizing Ashland’s revenue streams and reviewing the current internal accounting policies and practices to determine potential differences that would result from applying the requirements of the new standard to revenue contracts. Additional discussions and meetings with each revenue stream team have occurred to solicit input and to identify potential impacts and appropriate changes to Ashland’s business processes, systems and controls to support the revenue recognition and disclosure requirements under the new standard. Based on various preliminary assessments conducted to date, Ashland has identified agreements with distributors and customers that are subject to rebate and incentive programs that could contain elements of material rights and/or variable consideration. Ashland does not currently believe that these elements would result in a material change to how revenue would be recognized for these agreements. Ashland is currently completing the last phase of its implementation, which includes final assessment documentation and internal control design for the new standard. Ashland intends to adopt this standard using the modified retrospective approach and does not believe the impact will be material to the Condensed Consolidated Financial Statements but does expect there to be significant additional disclosures within the Notes to Condensed Consolidated Financial Statements. This guidance becomes effective for Ashland on October 1, 2018. In February 2018, the FASB issued guidance which permits entities to reclassify tax effects stranded in accumulated other comprehensive income (AOCI) as a result of U.S. tax reform legislation to retained earnings. Additionally, this guidance requires entities to disclose whether they made an election to reclassify the tax effects and to disclose their accounting policy for releasing income tax effects from AOCI. This guidance becomes effective for Ashland on October 1, 2019. Ashland is currently evaluating the impact this guidance may have on Ashland’s Condensed Consolidated Financial Statements. In March 2017, the FASB issued accounting guidance that will change how employers who sponsor defined benefit pension and/or postretirement benefit plans present the net periodic benefit cost in the Statement of Consolidated Comprehensive Income (Loss). This guidance requires employers to present the service cost component of net periodic benefit cost in the same caption within the Statement of Consolidated Comprehensive Income (Loss) as other employee compensation costs from services rendered during the period. All other components of the net periodic benefit cost will be presented separately outside of the operating income caption. This guidance must be applied retrospectively. Ashland elected to early adopt this guidance on October 1, 2017, which resulted in a reclassification of $3 million in income from the selling, general and administrative expense and cost of sales captions to the other net periodic benefit income caption in the Statement of Consolidated Comprehensive Income (Loss) for the nine months ended June 30, 2017. The components of net periodic benefits income (costs) reclassified primarily relate to interest cost, expected return on assets, curtailments, settlements and actuarial gains and losses. Ashland did not have to adjust the classification of service cost since it previously was recorded within the caption required by the new guidance. See Note J for additional information on net periodic benefit costs. In March 2016, the FASB issued new accounting guidance for certain aspects of share-based payments to employees. This guidance requires all excess tax benefits and tax deficiencies related to share-based payments to be recognized as income tax expense in the Statements of Consolidated Comprehensive Income (Loss) instead of additional paid-in capital, and changes the classification of excess tax benefits from a financing activity to an operating activity within the Statements of Condensed Consolidated Cash Flows. This guidance also allows entities to make an accounting policy election to either estimate the number of awards that are expected to vest or account for forfeitures when they occur. In addition, this guidance increases the amount an employer can withhold to cover income taxes on awards and still qualify for equity classification and requires that cash paid by an employer when directly withholding shares for tax-withholding purposes be classified as a financing activity within the Statements of Condensed Consolidated Cash Flows. The guidance became effective for Ashland and was adopted on October 1, 2017. The guidance specifically related to the Statements of Consolidated Comprehensive Income (Loss) was adopted prospectively while the guidance related to the Statements of Condensed Consolidated Cash Flows was adopted retrospectively, as required by the guidance. Additionally, Ashland made a policy election to account for forfeitures when they occur across all share-based awards. Upon adoption, the overall impact on Ashland's Condensed Consolidated Financial Statements was not significant.
Fair Value of Financial Instruments Policy	As required by U.S. GAAP, Ashland uses applicable guidance for defining fair value, the initial recording and periodic remeasurement of certain assets and liabilities measured at fair value and related disclosures for instruments measured at fair value. Fair value accounting guidance establishes a fair value hierarchy, which prioritizes the inputs to valuation techniques used to measure fair value into three broad levels. The fair value hierarchy gives the highest priority to quoted prices in active markets for identical assets or liabilities (Level 1) and the lowest priority to unobservable inputs (Level 3). An instrument’s categorization within the fair value hierarchy is based upon the lowest level of input that is significant to the instrument’s fair value measurement. The three levels within the fair value hierarchy are described as follows. Level 1 – Observable inputs such as unadjusted quoted prices in active markets for identical assets or liabilities. Level 2 – Inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. These include quoted prices for similar assets or liabilities in active markets and quoted prices for identical or similar assets or liabilities in markets that are not active. Level 3 – Unobservable inputs for the asset or liability for which there is little, if any, market activity at the measurement date. Unobservable inputs reflect Ashland’s own assumptions about what market participants would use to price the asset or liability. The inputs are developed based on the best information available in the circumstances, which might include Ashland’s own financial data such as internally developed pricing models, discounted cash flow methodologies, as well as instruments for which the fair value determination requires significant management judgment. For assets that are measured using quoted prices in active markets (Level 1), the total fair value is the published market price per unit multiplied by the number of units held without consideration of transaction costs. Assets and liabilities that are measured using significant other observable inputs (Level 2) are primarily valued by reference to quoted prices of similar assets or liabilities in active markets, adjusted for any terms specific to that asset or liability. For all other assets and liabilities for which unobservable inputs are used (Level 3), fair value is derived using fair value models, such as a discounted cash flow model or other standard pricing models that Ashland deems reasonable.
Derivatives, Methods of Accounting, Derivatives Not Designated or Qualifying as Hedges, Policy	Ashland conducts business in a variety of foreign currencies. Accordingly, Ashland regularly uses foreign currency derivative instruments to manage exposure on certain transactions denominated in foreign currencies to curtail potential earnings volatility effects on certain assets and liabilities, including short-term inter-company loans, denominated in currencies other than Ashland’s functional currency of an entity. These derivative contracts generally require exchange of one foreign currency for another at a fixed rate at a future date and generally have maturities of less than twelve months. All contracts are valued at fair value with net changes in fair value recorded within the selling, general and administrative expense caption. The impacts of these contracts were largely offset by gains and losses resulting from the impact of changes in exchange rates on transactions denominated in non-functional currencies.
Inventories	Inventories are carried at the lower of cost or net realizable value. Inventories are primarily stated at cost using the weighted-average cost method. In addition, certain inventories are valued at cost using the last-in, first-out (LIFO) method.
Goodwill and Intangible Assets, Goodwill	Ashland reviews goodwill and indefinite-lived intangible assets for impairment annually or when events and circumstances indicate an impairment may have occurred. This annual assessment is performed as of July 1 and consists of Ashland determining each reporting unit’s current fair value compared to its current carrying value. For its July 1, 2017 assessment, Ashland determined that its reporting units for the allocation of goodwill are its three reportable segments: Specialty Ingredients, Composites and Intermediates and Solvents. At that time, Ashland determined no additional impairment existed.
Finite-Lived Intangible Asset	Intangible assets principally consist of trademarks and trade names, intellectual property and customer and supplier relationships. Intangible assets classified as finite are amortized on a straight-line basis over their estimated useful lives. The cost of trademarks and trade names is amortized principally over 3 to 25 years, intellectual property over 5 to 25 years, and customer and supplier relationships over 3 to 24 years.
Goodwill and Intangible Assets, Intangible Assets, Indefinite-Lived	Ashland annually reviews indefinite-lived intangible assets for possible impairment or whenever events or changes in circumstances indicate that carrying amounts may not be recoverable.
Commitments and Contingencies Policy	Ashland and Hercules have liabilities from claims alleging personal injury caused by exposure to asbestos. To assist in developing and annually updating independent reserve estimates for future asbestos claims and related costs given various assumptions, Ashland retained third party actuarial experts. Prior to 2018, Ashland retained Hamilton, Rabinovitz & Associates, Inc. (HR&A). During 2018, Ashland transitioned these responsibilities to Nathan Associates, Inc. (Nathan). The processes and methodologies used by Nathan are similar to the ones historically employed by HR&A. The methodology used by Nathan to project future asbestos costs is based largely on recent experience, including claim-filing and settlement rates, disease mix, enacted legislation, open claims and litigation defense. The claim experience of Ashland and Hercules are separately compared to the results of previously conducted third party epidemiological studies estimating the number of people likely to develop asbestos-related diseases. Those studies were undertaken in connection with national analyses of the population expected to have been exposed to asbestos. Using that information, Nathan estimates a range of the number of future claims that may be filed, as well as the related costs that may be incurred in resolving those claims. Changes in asbestos-related liabilities and receivables are recorded on an after-tax basis within the discontinued operations caption in the Statements of Consolidated Comprehensive Income (Loss).
Environmental Cost Policy	The total reserves for environmental remediation reflect Ashland’s estimates of the most likely costs that will be incurred over an extended period to remediate identified conditions for which the costs are reasonably estimable, without regard to any third-party recoveries. Engineering studies, probability techniques, historical experience and other factors are used to identify and evaluate remediation alternatives and their related costs in determining the estimated reserves for environmental remediation. Ashland continues to discount certain environmental sites and regularly adjusts its reserves as environmental remediation continues. Ashland has estimated the value of its probable insurance recoveries associated with its environmental reserve based on management’s interpretations and estimates surrounding the available or applicable insurance coverage.
Earnings Per Share	The following is the computation of basic and diluted earnings per share (EPS) from continuing operations attributable to Ashland. Stock appreciation rights (SARs), stock options and warrants available to purchase shares outstanding for each reporting period whose grant price was greater than the average market price of Ashland Common Stock for each applicable period were not included in the computation of income from continuing operations per diluted share because the effect of these instruments would be antidilutive. The total number of these shares outstanding was approximately 0.7 million and 1.4 million at June 30, 2018 and 2017, respectively. Earnings per share is reported under the treasury stock method.
Stock Incentive Plan Policy	Ashland has stock incentive plans under which key employees or directors are granted stock appreciation rights (SARs), performance awards or nonvested stock awards. Each program is typically a long-term incentive plan designed to link employee compensation with increased shareholder value or reward superior performance and encourage continued employment with Ashland. Ashland recognizes compensation expense for the grant date fair value of stock-based awards over the applicable vesting period.
Segment Reporting	Ashland determines its reportable segments based on how operations are managed internally for the products and services sold to customers, including how the results are reviewed by the chief operating decision maker, which includes determining resource allocation methodologies used for reportable segments. Operating income is the primary measure on the Statements of Consolidated Comprehensive Income (Loss) that is reviewed by the chief operating decision maker in assessing each reportable segment's financial performance. Ashland does not aggregate operating segments to arrive at these reportable segments.
Available-for-sale Securities [Member]
Marketable Securities, Policy	During 2015, Ashland diversified the restricted investments received from the January 2015 asbestos insurance settlement into primarily equity and corporate bond mutual funds that are designated as available-for-sale securities, classified as Level 1 measurements within the fair value hierarchy. Available-for-sale securities are reported at fair value with unrealized gains and losses, net of related deferred income taxes, included in the stockholders' equity section of the Condensed Consolidated Balance Sheets as a component of accumulated other comprehensive income (AOCI). Investment income and realized gains and losses on the available-for-sale securities are reported in the net interest and other financing expense caption in the Statements of Consolidated Comprehensive Income (Loss).
Trading Securities [Member]
Marketable Securities, Policy	Gains and losses related to deferred compensation investments are immediately recognized within the Statements of Consolidated Comprehensive Income (Loss).