Contractual interest rate for funds borrowed, under the debt agreement.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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Debt - Cabot Senior Secured Notes (Detail) $ in Thousands			9 Months Ended
Debt - Cabot Senior Secured Notes (Detail) $ in Thousands	Mar. 27, 2014 USD ($)	Mar. 27, 2014 GBP (£)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 GBP (£)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Mar. 27, 2014 GBP (£)	Aug. 02, 2013 USD ($)	Aug. 02, 2013 GBP (£)	Jul. 25, 2013 USD ($)	Jul. 25, 2013 GBP (£)	Sep. 20, 2012 USD ($)	Sep. 20, 2012 GBP (£)
Debt Instrument [Line Items]
Senior secured notes			$ 32,500			$ 43,750
Repayment of senior credit facility			466,745		$ 878,883
Repayment of preferred equity certificates, net			0		$ 702
Cabot Senior Secured Notes [Member]
Debt Instrument [Line Items]
Senior secured notes			1,045,143			$ 1,076,952
Cabot Corporation [Member]
Debt Instrument [Line Items]
Repayment of senior credit facility			113,800	£ 75,000,000
Cabot 2019 Notes [Member]
Debt Instrument [Line Items]
Senior secured notes												$ 438,400	£ 265,000,000
Debt instrument, stated interest rate												10.375%	10.375%
Cabot 2020 Notes [Member]
Debt Instrument [Line Items]
Senior secured notes								$ 151,700	£ 100,000,000
Debt instrument, stated interest rate								8.375%	8.375%
J Bridge PECs [Member]
Debt Instrument [Line Items]
Repayment of preferred equity certificates, net			$ 37,900	£ 25,000,000.0
Cabot 2021 Notes [Member]
Debt Instrument [Line Items]
Senior secured notes	$ 291,800						£ 175,000,000.0
Debt instrument, stated interest rate	6.50%						6.50%
Repayment of senior credit facility	$ 174,800	£ 105,000,000
Debt issuance cost	$ 7,500
Senior Secured Notes Due 2020 [Member] \| Marlin Acquisition [Member]
Debt Instrument [Line Items]
Senior secured notes										$ 246,500	£ 150,000,000.0
Debt instrument, stated interest rate										10.50%	10.50%
Long-term debt, fair value										$ 284,200