BACKGROUND AND BASIS OF PRESENTATION	9 Months Ended
Jun. 30, 2018
BACKGROUND AND BASIS OF PRESENTATION [Abstract]
BACKGROUND AND BASIS OF PRESENTATION	1. BACKGROUND AND BASIS OF PRESENTATION Cabot Microelectronics Corporation ("Cabot Microelectronics'', "the Company'', "us'', "we'', or "our'') supplies high-performance polishing slurries and pads used in the manufacture of advanced integrated circuit (IC) devices within the semiconductor industry, in a process called chemical mechanical planarization (CMP). CMP polishes surfaces at an atomic level, thereby helping to enable IC device manufacturers to produce smaller, faster and more complex IC devices with fewer defects. We develop, produce and sell CMP slurries for polishing many of the conducting and insulating materials used in IC devices. We develop, manufacture and sell CMP polishing pads, which are used in conjunction with slurries in the CMP process. We also develop and provide products for demanding surface modification applications in other industries through our Engineered Surface Finishes (ESF) business. For additional information, refer to Part 1, Item 1, "Business", in our Annual Report on Form 10-K for the fiscal year ended September 30, 2017. The unaudited Consolidated Financial Statements have been prepared by Cabot Microelectronics pursuant to the rules of the Securities and Exchange Commission (SEC) and accounting principles generally accepted in the United States of America (U.S. GAAP). In the opinion of management, these unaudited Consolidated Financial Statements include all adjustments, consisting of normal recurring adjustments, necessary for the fair statement of Cabot Microelectronics' financial position as of June 30, 2018, cash flows for the nine months ended June 30, 2018 and June 30, 2017, and results of operations for the three and nine months ended June 30, 2018 and June 30, 2017. The Consolidated Balance Sheets as of September 30, 2017 were derived from audited financial statements. The results of operations for the three and nine months ended June 30, 2018 may not be indicative of results to be expected for future periods, including the fiscal year ending September 30, 2018. This Report on Form 10-Q does not contain all of the footnote disclosures from the annual financial statements and should be read in conjunction with the Consolidated Financial Statements and related notes thereto included in Cabot Microelectronics' Annual Report on Form 10-K for the fiscal year ended September 30, 2017. The Consolidated Financial Statements include the accounts of Cabot Microelectronics and its subsidiaries. All intercompany transactions and balances between the companies have been eliminated as of June 30, 2018. USE OF ESTIMATES The preparation of financial statements and related disclosures in conformity with U.S. GAAP requires management to make judgments, assumptions and estimates that affect the amounts reported in the Consolidated Financial Statements and accompanying notes. The accounting estimates that require management's most difficult and subjective judgments include, but are not limited to, those estimates related to bad debt expense, inventory valuation, valuation and classification of auction rate securities, impairment of long-lived assets and investments, business combinations, goodwill, other intangible assets, interest rate swaps, share-based compensation, income taxes and contingencies. We base our estimates on historical experience, current conditions and on various other assumptions that we believe are reasonable under the circumstances. However, future events are subject to change and estimates and judgments routinely require adjustment. Actual results may differ from these estimates under different assumptions or conditions. The results of operations for the quarter ended December 31, 2017 and nine months ended June 30, 2018 include a correction to prior period amounts, which we determined to be immaterial to the prior periods to which they relate and are expected to be immaterial to our fiscal 2018 results. The adjustments, relating primarily to accumulated earnings taxes of a foreign operation, increased the income tax expense for the first quarter of fiscal 2018 by $2,071. Separately, in Note 14 of this Report on Form 10-Q, we discuss the effects of the Tax Cuts and Jobs Act ("Tax Act") on our financial statements.

1. BACKGROUND AND BASIS OF PRESENTATION

Cabot Microelectronics Corporation ("Cabot Microelectronics'', "the Company'', "us'', "we'', or "our'') supplies high-performance polishing slurries and pads used in the manufacture of advanced integrated circuit (IC) devices within the semiconductor industry, in a process called chemical mechanical planarization (CMP). CMP polishes surfaces at an atomic level, thereby helping to enable IC device manufacturers to produce smaller, faster and more complex IC devices with fewer defects. We develop, produce and sell CMP slurries for polishing many of the conducting and insulating materials used in IC devices. We develop, manufacture and sell CMP polishing pads, which are used in conjunction with slurries in the CMP process. We also develop and provide products for demanding surface modification applications in other industries through our Engineered Surface Finishes (ESF) business. For additional information, refer to Part 1, Item 1, "Business", in our Annual Report on Form 10-K for the fiscal year ended September 30, 2017.

The unaudited Consolidated Financial Statements have been prepared by Cabot Microelectronics pursuant to the rules of the Securities and Exchange Commission (SEC) and accounting principles generally accepted in the United States of America (U.S. GAAP). In the opinion of management, these unaudited Consolidated Financial Statements include all adjustments, consisting of normal recurring adjustments, necessary for the fair statement of Cabot Microelectronics' financial position as of June 30, 2018, cash flows for the nine months ended June 30, 2018 and June 30, 2017, and results of operations for the three and nine months ended June 30, 2018 and June 30, 2017. The Consolidated Balance Sheets as of September 30, 2017 were derived from audited financial statements. The results of operations for the three and nine months ended June 30, 2018 may not be indicative of results to be expected for future periods, including the fiscal year ending September 30, 2018. This Report on Form 10-Q does not contain all of the footnote disclosures from the annual financial statements and should be read in conjunction with the Consolidated Financial Statements and related notes thereto included in Cabot Microelectronics' Annual Report on Form 10-K for the fiscal year ended September 30, 2017.

The Consolidated Financial Statements include the accounts of Cabot Microelectronics and its subsidiaries. All intercompany transactions and balances between the companies have been eliminated as of June 30, 2018.

USE OF ESTIMATES

The preparation of financial statements and related disclosures in conformity with U.S. GAAP requires management to make judgments, assumptions and estimates that affect the amounts reported in the Consolidated Financial Statements and accompanying notes. The accounting estimates that require management's most difficult and subjective judgments include, but are not limited to, those estimates related to bad debt expense, inventory valuation, valuation and classification of auction rate securities, impairment of long-lived assets and investments, business combinations, goodwill, other intangible assets, interest rate swaps, share-based compensation, income taxes and contingencies. We base our estimates on historical experience, current conditions and on various other assumptions that we believe are reasonable under the circumstances. However, future events are subject to change and estimates and judgments routinely require adjustment. Actual results may differ from these estimates under different assumptions or conditions.

The results of operations for the quarter ended December 31, 2017 and nine months ended June 30, 2018 include a correction to prior period amounts, which we determined to be immaterial to the prior periods to which they relate and are expected to be immaterial to our fiscal 2018 results. The adjustments, relating primarily to accumulated earnings taxes of a foreign operation, increased the income tax expense for the first quarter of fiscal 2018 by $2,071. Separately, in Note 14 of this Report on Form 10-Q, we discuss the effects of the Tax Cuts and Jobs Act ("Tax Act") on our financial statements.