Employee Benefit Plans - Defined Benefit Pension Plans Weighted-Average Asset Allocations (Detail) - Foreign Plans [Member]		Sep. 30, 2023	Sep. 30, 2022
Defined Benefit Plan Disclosure [Line Items]
Equity securities		19.00%	20.00%
Debt securities		31.00%	68.00%
Real estate		10.00%	7.00%
Cash and other securities	[1]	40.00%	5.00%
Total		100.00%	100.00%

[1] The asset allocation is temporarily changed as a result of the insurance contracts Cabot Carbon Limited Pension Plan and Carbon Plastics Pension Plan purchased with plan assets in anticipation of the settlement discussed above.