Hyperinflationary Economies	9 Months Ended
Jun. 30, 2019
Equity Method Investments And Joint Ventures [Abstract]
Hyperinflationary Economies	O. Hyperinflationary Economies Argentina Cabot owns 100% of a carbon black operating entity in Argentina. Effective July 1, 2018, the operating entity was considered to be functioning in a highly inflationary economy and began using Cabot’s reporting currency, the U.S. dollar, as its functional currency. Since the conversion, all impacts of foreign exchange changes between the reporting currency and Argentine peso are reflected in earnings in the Consolidated Statements of Operations. The Company’s income from operations has not been significantly impacted from this change because the operating entity’s sales and a portion of its raw material purchases were already denominated in U.S. dollars. For the three and nine months ended June 30, 2019, the operating entity’s net revenue represented approximately 3% and 2%, respectively, of Cabot’s total net revenue. The operating entity’s assets and liabilities held in local currency, which consist primarily of cash and cash equivalents, inventories, property, plant and equipment and accounts payable and accrued liabilities, made up less than 2% of Cabot’s total assets and total liabilities as of both June 30, 2019 and September 30, 2018. Changes in the Argentine peso exchange rate will result in foreign currency exchange gains or losses on the operating entity’s peso-denominated monetary assets and liabilities. For the three and nine months ended June 30, 2019, the Company recorded a net gain of less than $1 million and a net loss of $1 million, respectively, within Other (income) expense in the Consolidated Statements of Operations, which reflects the remeasurement of the operating entity’s net monetary liabilities denominated in Argentine pesos using an exchange rate of 42.7 Argentine pesos to the U.S. dollar at June 30, 2019. The Company will continue to monitor the developments in Argentina and their potential impact on the operating entity’s operations or carrying value. Venezuela Cabot owns 49% of a carbon black operating affiliate in Venezuela, which is accounted for as an equity affiliate, through wholly-owned subsidiaries that carry the investment and receive its dividends. While the operating entity has historically been profitable, it has not been operational in recent periods due to a lack of available raw materials, which is expected to continue for the foreseeable future. As such, in the second quarter of fiscal 2019, the Company performed an impairment analysis and determined that the decrease in fair value of the Venezuelan equity investment is other-than-temporary and that the investment is fully impaired. The Company recorded an impairment charge of $11 million in the second quarter of fiscal 2019, which is included in Other income (expense) within the Consolidated Statement of Operations.

Hyperinflationary Economies

9 Months Ended

Jun. 30, 2019

Equity Method Investments And Joint Ventures [Abstract]

O. Hyperinflationary Economies

Argentina

Cabot owns 100% of a carbon black operating entity in Argentina. Effective July 1, 2018, the operating entity was considered to be functioning in a highly inflationary economy and began using Cabot’s reporting currency, the U.S. dollar, as its functional currency. Since the conversion, all impacts of foreign exchange changes between the reporting currency and Argentine peso are reflected in earnings in the Consolidated Statements of Operations.

The Company’s income from operations has not been significantly impacted from this change because the operating entity’s sales and a portion of its raw material purchases were already denominated in U.S. dollars. For the three and nine months ended June 30, 2019, the operating entity’s net revenue represented approximately 3% and 2%, respectively, of Cabot’s total net revenue.

The operating entity’s assets and liabilities held in local currency, which consist primarily of cash and cash equivalents, inventories, property, plant and equipment and accounts payable and accrued liabilities, made up less than 2% of Cabot’s total assets and total liabilities as of both June 30, 2019 and September 30, 2018. Changes in the Argentine peso exchange rate will result in foreign currency exchange gains or losses on the operating entity’s peso-denominated monetary assets and liabilities. For the three and nine months ended June 30, 2019, the Company recorded a net gain of less than $1 million and a net loss of $1 million, respectively, within Other (income) expense in the Consolidated Statements of Operations, which reflects the remeasurement of the operating entity’s net monetary liabilities denominated in Argentine pesos using an exchange rate of 42.7 Argentine pesos to the U.S. dollar at June 30, 2019.

The Company will continue to monitor the developments in Argentina and their potential impact on the operating entity’s operations or carrying value.

Venezuela

Cabot owns 49% of a carbon black operating affiliate in Venezuela, which is accounted for as an equity affiliate, through wholly-owned subsidiaries that carry the investment and receive its dividends. While the operating entity has historically been profitable, it has not been operational in recent periods due to a lack of available raw materials, which is expected to continue for the foreseeable future. As such, in the second quarter of fiscal 2019, the Company performed an impairment analysis and determined that the decrease in fair value of the Venezuelan equity investment is other-than-temporary and that the investment is fully impaired. The Company recorded an impairment charge of $11 million in the second quarter of fiscal 2019, which is included in Other income (expense) within the Consolidated Statement of Operations.