Derivatives - Details of Derivatives Held to Manage Foreign Currency Risk (Detail)		Dec. 31, 2020 USD ($)	Dec. 31, 2020 EUR (€)	Sep. 30, 2020 USD ($)	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]	$ 39,000,000		$ 54,000,000

[1] As of December 31, 2020, 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 December 31, 2020 and September 30, 2020 the fair value of these derivative instruments were a nominal amount.