Derivatives - Details of Derivatives Held to Manage Foreign Currency Risk (Detail)		Jun. 30, 2018 USD ($)	Jun. 30, 2018 EUR (€)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 EUR (€)
Net Investment [Member] | Cross Currency Swaps [Member]
Derivative [Line Items]
Notional Amount		$ 250,000,000
Net Investment [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 [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]	$ (6,000,000)		$ 5,000,000

[1] Cabot’s forward foreign exchange contracts are denominated in the Indonesian rupiah and Czech koruna.