Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	May 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying Unaudited Condensed Consolidated Financial Statements of Schnitzer Steel Industries, Inc. (the “Company”) have been prepared pursuant to generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial information and the rules and regulations of the United States Securities and Exchange Commission (the “SEC”) for Form 10-Q, including Article 10 of Regulation S-X. 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 note disclosures normally included in annual financial statements have been condensed or omitted pursuant to the rules and regulations of the SEC. In the opinion of management, all normal, recurring adjustments considered necessary for a fair statement have been included. Management suggests that these Unaudited Condensed Consolidated Financial Statements be read in conjunction with the financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the fiscal year ended August 31, 2017. The results for the three and nine months ended May 31, 2018 and 2017 are not necessarily indicative of the results of operations for the entire fiscal year.
Accounting Changes	Accounting Changes In July 2015, an accounting standards update was issued that requires an entity to measure certain types of inventory, including inventory that is measured using the first-in, first out (“FIFO”) or average cost method, at the lower of cost and net realizable value. The accounting standard in effect at the time of issuance of the update required an entity to measure inventory at the lower of cost or market, whereby market could be replacement cost, net realizable value, or net realizable value less an approximately normal profit margin. The amendments do not apply to inventory that is measured using the last-in, first-out (“LIFO”) or retail inventory method. The Company adopted the new requirement, which is to be applied prospectively, as of the beginning of the first quarter of fiscal 2018 with no impact to the Unaudited Condensed Consolidated Financial Statements. In March 2016, an accounting standards update was issued that amends several aspects of the accounting for share-based payments, including accounting for income taxes, forfeitures and statutory tax withholding requirements, and classification within the statement of cash flows. The Company adopted the new requirements as of the beginning of the first quarter of fiscal 2018 with no impact to the Unaudited Condensed Consolidated Financial Statements, including no cumulative-effect adjustments to retained earnings, as of the date of adoption. On a prospective basis beginning with the date of adoption, the Company records all of the tax effects related to share-based payments through the income statement, subject to normal valuation allowance considerations, and all tax-related cash flows resulting from share-based payments are reported as operating activities in the statement of cash flows. Cash payments to taxing authorities made on behalf of Company employees for withheld shares are reported as financing activities in the statement of cash flows, consistent with the Company’s practice prior to adopting the new requirements. The Company has elected to continue the practice of estimating the forfeiture rate for the purpose of recognizing estimated compensation cost over the requisite service period. In February 2018, an accounting standards update was issued that allows for a reclassification from accumulated other comprehensive income (“AOCI”) to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act (“Tax Act”) enacted on December 22, 2017. Stranded tax effects result from adjusting deferred tax liabilities and assets for the effect of a change in tax laws or rates to income from continuing operations, as required under existing accounting guidance, even in situations in which the adjustments relate to income tax effects reported within AOCI. If an entity elects to reclassify the stranded tax effects of the Tax Act, the amount of that reclassification shall include the effect of the change in the U.S. federal corporate income tax rate on the gross deferred tax amounts and related valuation allowances, if any, at the date of enactment of the Tax Act related to items remaining in AOCI, and other income tax effects of the Tax Act on items remaining in AOCI that an entity elects to reclassify. The Company early-adopted the foregoing accounting standard update in the second quarter of fiscal 2018 and elected to reclassify to retained earnings the effect of the change in the U.S. federal corporate income tax rate on items remaining in AOCI at the date of enactment of the Tax Act. The resulting aggregate reclassification from AOCI to retained earnings recorded in the second quarter of fiscal 2018 was $1 million. See Note 7 - Changes in Equity and Note 8 - Accumulated Other Comprehensive Loss for further detail.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include short-term securities that are not restricted by third parties and have an original maturity date of 90 days or less. Included in accounts payable are book overdrafts representing outstanding checks in excess of funds on deposit of $29 million and $21 million as of May 31, 2018 and August 31, 2017, respectively.
Assets Held for Sale	Assets Held for Sale During the third quarter of fiscal 2017, the Company sold equipment assets that had been classified as held for sale prior to being fully impaired in fiscal 2015. The Company recorded a gain on the sale of $1 million, which is reported within other asset impairment charges (recoveries), net in the Unaudited Condensed Consolidated Statements of Income.
Investments in Joint Ventures	Investments in Joint Ventures During the nine months ended May 31, 2017, one of the Company’s joint venture interests sold real estate resulting in recognition of a $6 million gain by the joint venture, $3 million of which was attributable to the Company’s investment. The Company’s share of the gain is reported within (income) from joint ventures in the Unaudited Condensed Consolidated Statements of Income for the nine months ended May 31, 2017.
Cost Method Investment	Cost Method Investment During the nine months ended May 31, 2017, the Company invested $6 million in a privately-held waste and recycling entity. The Company’s influence over the operating and financial policies of the entity is not significant and, thus, the investment is accounted for under the cost method. Under the cost method, the investment is carried at cost and adjusted only for other-than-temporary impairments, certain distributions and additional investments. The investment is presented as part of the Auto and Metals Recycling (“AMR”) reportable segment and reported within other assets in the Unaudited Condensed Consolidated Balance Sheets. The Company does not hold any other cost-method investments. The carrying value of the investment was $6 million as of May 31, 2018 and August 31, 2017. As of May 31, 2018, the Company had not identified any events or changes in circumstances that may have a significant adverse effect on the fair value of the investment or indicators of other-than-temporary impairment.
Long-Lived Assets	Long-Lived Assets Changes in circumstances may merit a change in the estimated useful lives or salvage values of individual long-lived assets, which are accounted for prospectively in the period of change. For such assets, the useful life is shortened based on the Company’s plans to dispose of or abandon the asset before the end of its original useful life and depreciation is accelerated beginning when that determination is made. During the nine months ended May 31, 2018 and 2017, the Company recognized accelerated depreciation of $1 million and less than $1 million, respectively, due to shortening the useful lives of decommissioned machinery and equipment assets in the Cascade Steel and Scrap (“CSS”) reportable segment, which are reported within other asset impairment charges (recoveries), net in the Unaudited Condensed Consolidated Statements of Income. Also, during the three and nine months ended May 31, 2018, the Company sold previously impaired assets consisting primarily of machinery and equipment, recognizing a gain of $2 million and $3 million, respectively, which are reported within other asset impairment charges (recoveries), net in the Unaudited Condensed Consolidated Statements of Income.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash and cash equivalents, accounts receivable, and notes and other contractual receivables from suppliers. The majority of cash and cash equivalents is maintained with major financial institutions. Balances with these and certain other institutions exceeded the Federal Deposit Insurance Corporation insured amount of $250,000 as of May 31, 2018. Concentration of credit risk with respect to accounts receivable is limited because a large number of geographically diverse customers make up the Company’s customer base. The Company controls credit risk through credit approvals, limits, insurance, letters of credit or other collateral, cash deposits and monitoring procedures. The Company is exposed to a residual credit risk with respect to open letters of credit by virtue of the possibility of the failure of a bank providing a letter of credit. The Company had $81 million and $48 million of open letters of credit as of May 31, 2018 and August 31, 2017, respectively.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, an accounting standards update was issued that clarifies the principles for recognizing revenue from contracts with customers. The update supersedes the existing standard for recognizing revenue. Additional updates have been issued since May 2014 amending aspects of the initial update and providing implementation guidance. The guidance is applicable to all contracts with customers regardless of industry-specific or transaction-specific fact patterns. Further, the guidance requires improved disclosures to help users of financial statements better understand the nature, amount, timing, and uncertainty of revenue that is recognized. The standard is effective for the Company beginning in fiscal 2019, including interim periods within that fiscal year. The Company plans to adopt the standard using the modified retrospective approach with the cumulative effect of the initial application, if any, recognized as an adjustment to the opening balance of retained earnings. The Company has nearly completed the examination of its current revenue streams and significant contracts with customers under the requirements of the new standard and, based on the progress of this examination to date, does not believe the standard will have a material impact on its financial position, net income or cash flows. The Company has not identified any significant changes to its processes, systems and internal controls required to support the new standard. The Company has identified certain scrap metal purchase and sale arrangements for which it currently recognizes revenue for the gross amount of consideration it expects to be entitled from the customer (as principal), but for which under the new revenue standard it recognizes revenue as the net amount of consideration that it expects to retain after paying the scrap metal supplier (as agent). This change in the classification of the cost of scrap metal purchased under such arrangements will have the effect of reducing the amount of revenues reported in the financial statements, while having no impact on net income. Based on analysis performed to date and recent historical levels of such scrap metal purchase and sale arrangements, the Company estimates the decrease in its annual revenues as a result of implementing this change in fiscal 2019 will be between $30 million and $40 million with no impact on net income. In January 2016, an accounting standards update was issued that amends certain aspects of the reporting model for financial instruments. Most prominent among the amendments is the requirement for equity investments, with certain exceptions including those accounted for under the equity method of accounting, to be measured at fair value with changes in fair value recognized in net income. An entity may choose to measure equity investments that do not have readily determinable fair values, such as certain cost method investments, at cost minus impairment, plus or minus changes resulting from observable price changes. The amendments also require a qualitative assessment to identify impairment of equity investments without readily determinable fair values. The standard is effective for the Company beginning in fiscal 2019, including interim periods within that fiscal year. The Company is evaluating the impact of adopting this standard on its consolidated financial position, results of operations and cash flows. In February 2016, an accounting standard was issued that will supersede the existing lease standard and require a lessee to recognize a lease liability and a lease asset on its balance sheet for all leases, including those classified as operating leases under the existing lease standard. The update also expands the required quantitative and qualitative disclosures surrounding leases. This standard is effective for the Company beginning in fiscal 2020, including interim periods within that fiscal year. This standard will be applied using a modified retrospective transition approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. The Company is in the process of identifying its population of leases within the scope of the new accounting standard and documenting salient lease terms to support the initial and subsequent measurement of lease liabilities and lease assets. The Company is evaluating the impact of adopting this standard on its financial position, results of operations, cash flows and disclosures.