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Derivatives - Details of Derivatives Held to Manage Foreign Currency Risk (Detail) - USD ($)
Mar. 31, 2016
Sep. 30, 2015
Not Designated as Hedging Instrument [Member] | Forward Foreign Currency Contracts [Member]    
Derivative [Line Items]    
Notional amount, net of forward foreign currency contract no designation hedge derivatives [1] $ 3,000,000 $ 2,000,000
[1] Cabot’s forward foreign exchange contracts are denominated primarily in the British pound sterling, Czech koruna, and Indonesian rupiah.