Subsequent Events	12 Months Ended
Sep. 30, 2021
Subsequent Events [Abstract]
Subsequent Events	Note V. Subsequent Events Acquisition of Tokai Manufacturing Facility On November 15, 2021, Cabot entered into an agreement with Tokai Carbon Group to purchase its carbon black manufacturing facility in Tianjin, China for approximately $9 million, which is subject to customary closing adjustments and is expected to close in the second quarter of fiscal 2022. Sale of Purification Solutions Business On November 25, 2021 Cabot and an affiliate of funds advised by One Equity Partners (“OEP”) entered into a Share Purchase Agreement (the “Agreement”) for the sale of Cabot’s Purification Solutions business (the “Business”), subject to the satisfaction or waiver of the conditions set forth in the Agreement. Under the terms of the Agreement, OEP will acquire the Business on a cash-free and debt-free basis in a transaction valued at approximately $111 million, subject to certain debt-like and other closing adjustments, including a customary working capital adjustment and costs related to the closure of the Business’s former lignite mine as disclosed in Note D. The net cash proceeds from the transaction are expected to be approximately $80 million and are to be paid when the transaction is closed. The transaction is expected to close in the second quarter of fiscal 2022. The Company will begin to account for the assets and liabilities of the Business as held for sale for the quarter ending December 31, 2021. Based on the carrying value of the Business as of September 30, 2021 and an estimate of the net cash proceeds from the sale, Cabot estimates a pre-tax impairment charge in the range of $155 million to $165 million to be recorded in the first quarter of fiscal 2022.

Subsequent Events

12 Months Ended

Sep. 30, 2021

Note V. Subsequent Events

Acquisition of Tokai Manufacturing Facility

On November 15, 2021, Cabot entered into an agreement with Tokai Carbon Group to purchase its carbon black manufacturing facility in Tianjin, China for approximately $9 million, which is subject to customary closing adjustments and is expected to close in the second quarter of fiscal 2022.

Sale of Purification Solutions Business

On November 25, 2021 Cabot and an affiliate of funds advised by One Equity Partners (“OEP”) entered into a Share Purchase Agreement (the “Agreement”) for the sale of Cabot’s Purification Solutions business (the “Business”), subject to the satisfaction or waiver of the conditions set forth in the Agreement.

Under the terms of the Agreement, OEP will acquire the Business on a cash-free and debt-free basis in a transaction valued at approximately $111 million, subject to certain debt-like and other closing adjustments, including a customary working capital adjustment and costs related to the closure of the Business’s former lignite mine as disclosed in Note D. The net cash proceeds from the transaction are expected to be approximately $80 million and are to be paid when the transaction is closed. The transaction is expected to close in the second quarter of fiscal 2022.

The Company will begin to account for the assets and liabilities of the Business as held for sale for the quarter ending December 31, 2021. Based on the carrying value of the Business as of September 30, 2021 and an estimate of the net cash proceeds from the sale, Cabot estimates a pre-tax impairment charge in the range of $155 million to $165 million to be recorded in the first quarter of fiscal 2022.