Derivatives - Details of Derivatives Held to Manage Foreign Currency Risk (Detail)		Sep. 30, 2020 USD ($)	Sep. 30, 2020 EUR (€)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 EUR (€)	Sep. 30, 2016
3.4% Notes [Member]
Derivative [Line Items]
Interest rate on borrowing		3.40%	3.40%	3.40%	3.40%
3.4% Notes [Member] | Fixed Rate Debt [Member]
Derivative [Line Items]
Interest rate on borrowing						3.40%
Net Investment Hedge [Member] | Cross Currency Swaps [Member]
Derivative [Line Items]
Notional Amount		$ 250,000,000
Net Investment Hedge [Member] | 3.4% Notes [Member] | Cross Currency Swaps [Member]
Derivative [Line Items]
Notional Amount		$ 250,000,000	€ 233,000,000	$ 250,000,000	€ 223,000,000
Net Investment Hedge [Member] | 3.4% Notes [Member] | Fixed Rate Debt [Member] | Cross Currency Swaps [Member]
Derivative [Line Items]
Interest rate on borrowing		3.40%	3.40%	3.40%	3.40%
Not Designated as Hedging Instrument [Member] | Forward Foreign Currency Contracts [Member]
Derivative [Line Items]
Notional Amount	[1]	$ 54,000,000		$ 54,000,000

[1] As of September 30, 2020, Cabot’s forward foreign exchange contracts were denominated in Canadian dollar, Indonesian rupiah and Czech koruna. As of September 30, 2019, Cabot’s forward foreign exchange contracts were denominated in British pound, Canadian dollar, Indonesian rupiah and Czech koruna.