Derivatives - Details of Derivatives Held to Manage Foreign Currency Risk (Detail)		Jun. 30, 2021 USD ($)	Jun. 30, 2021 EUR (€)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 EUR (€)
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	€ 223,000,000	$ 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]	$ 37,000,000		$ 54,000,000

[1] As of June 30, 2021, Cabot’s forward foreign exchange contracts were denominated in Indonesian rupiah and Czech koruna. As of September 30, 2020, Cabot’s forward foreign exchange contracts were denominated in Canadian dollar, Indonesian rupiah and Czech koruna. At both June 30, 2021 and September 30, 2020 the fair value of these derivative instruments were a nominal amount.