Hyperinflationary Economies	3 Months Ended
Dec. 31, 2018
Equity Method Investments And Joint Ventures [Abstract]
Hyperinflationary Economies	M. 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. The operating entity’s net revenue represented approximately 2% of Cabot’s total net revenue for the three months ended December 31, 2018. 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 December 31, 2018 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. The Company recorded a $1 million net loss within Other (income) expense in the Consolidated Statements of Operations for the three months ended December 31, 2018, which reflects the remeasurement of the operating entity’s net monetary liabilities denominated in Argentine peso using an exchange rate of 37.70 Argentine peso to the U.S. dollar at December 31, 2018. 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. As of December 31, 2018, these subsidiaries carried the operating affiliate investment of $12 million. During the three months ended December 31, 2018 and 2017, the Company received dividends in the amounts of $1 million and $2 million, respectively, which were paid in U.S. dollars. A significant portion of the Company’s operating affiliate’s sales are exports denominated in U.S. dollars. The Venezuelan government mandates that a certain percentage of the dollars collected from these sales be converted into bolivars. The exchange rate made available to the Company as of December 31, 2018 and September 30, 2018 was 564 and 62 bolivars soberano to the U.S. dollar, respectively. Due to a reduced level of export sales in recent periods, the exchange rate devaluation during the first three months of fiscal 2019 had an immaterial impact on the Company’s results. While the operating entity has historically been profitable, it has not been operational in recent months due to a lack of available raw materials. The Company continues to closely monitor developments in Venezuela and their potential impact on the recoverability of its equity affiliate investment. Any future change in the exchange rate made available to the Company could cause the Company to change the exchange rate it uses and result in gains or losses on the bolivar-denominated assets held by its operating affiliate.

Hyperinflationary Economies

3 Months Ended

Dec. 31, 2018

Equity Method Investments And Joint Ventures [Abstract]

M. 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. The operating entity’s net revenue represented approximately 2% of Cabot’s total net revenue for the three months ended December 31, 2018.

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 December 31, 2018 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. The Company recorded a $1 million net loss within Other (income) expense in the Consolidated Statements of Operations for the three months ended December 31, 2018, which reflects the remeasurement of the operating entity’s net monetary liabilities denominated in Argentine peso using an exchange rate of 37.70 Argentine peso to the U.S. dollar at December 31, 2018.

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. As of December 31, 2018, these subsidiaries carried the operating affiliate investment of $12 million.

During the three months ended December 31, 2018 and 2017, the Company received dividends in the amounts of $1 million and $2 million, respectively, which were paid in U.S. dollars.

A significant portion of the Company’s operating affiliate’s sales are exports denominated in U.S. dollars. The Venezuelan government mandates that a certain percentage of the dollars collected from these sales be converted into bolivars. The exchange rate made available to the Company as of December 31, 2018 and September 30, 2018 was 564 and 62 bolivars soberano to the U.S. dollar, respectively. Due to a reduced level of export sales in recent periods, the exchange rate devaluation during the first three months of fiscal 2019 had an immaterial impact on the Company’s results.

While the operating entity has historically been profitable, it has not been operational in recent months due to a lack of available raw materials. The Company continues to closely monitor developments in Venezuela and their potential impact on the recoverability of its equity affiliate investment. Any future change in the exchange rate made available to the Company could cause the Company to change the exchange rate it uses and result in gains or losses on the bolivar-denominated assets held by its operating affiliate.