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100780 - Disclosure - Employee Benefit Plans - Assumptions Used to Determine Pension Benefit Obligations and Periodic Benefit Costs, Postretirement Benefit Obligations and Net Costs (Detail)
(http://www.cabot-corp.com/20160930/taxonomy/role/DisclosureEmployeeBenefitPlansAssumptionsUsedToDeterminePensionBenefitObligationsAndPeriodicBenefitCostsPostretirementBenefitObligationsAndNetCostsDetail)
TableSchedule Of Defined Benefit Plans Disclosures [Table]
Slicers (applies to each fact value in each table cell)
Reporting Entity [Axis]0000016040 (http://www.sec.gov/CIK)
Defined Benefit Plan Disclosure [Line Items]Period [Axis]
2015-10-01 - 2016-09-30
2014-10-01 - 2015-09-30
2013-10-01 - 2014-09-30
Defined Benefit Plans and Other Postretirement Benefit PlansDefined Benefit Plans and Other Postretirement Benefit PlansDefined Benefit Plans and Other Postretirement Benefit Plans
U.S. Pension Benefits [Member]Foreign Pension Benefits [Member]U. S. Postretirement Benefits [Member]Foreign Postretirement Benefits [Member]U.S. Pension Benefits [Member]Foreign Pension Benefits [Member]U. S. Postretirement Benefits [Member]Foreign Postretirement Benefits [Member]U.S. Pension Benefits [Member]Foreign Pension Benefits [Member]U. S. Postretirement Benefits [Member]Foreign Postretirement Benefits [Member]
Discount rate
3.40%  
1.80%  
3.00%  
2.80%  
4.20%  
2.90%  
3.70%  
3.90%  
4.00%  
3.00%  
3.80%  
3.90%  
Initial health care cost trend rate
 
2.80%  
7.00%  
6.10%  
 
2.80%  
6.50%  
6.80%  
 
2.80%  
7.00%  
7.10%  
Discount rate - benefit obligation
4.20%  
2.90%  
3.70%  
3.90%  
4.00%  
3.00%  
3.80%  
3.90%  
4.50%  
3.80%  
4.00%  
4.40%  
Discount rate - service cost
 
2.80%  
3.40%  
4.10%  
 
3.00%  
3.80%  
3.90%  
 
3.80%  
4.00%  
4.40%  
Discount rate - interest cost
3.30%  
2.40%  
2.80%  
3.70%  
4.00%  
3.00%  
3.80%  
3.90%  
4.50%  
3.80%  
4.00%  
4.40%  
Expected long-term rate of return on plan assets
7.50%  
5.10%  
  
7.50%  
5.40%  
  
7.80%  
5.30%  
  
Initial health care cost trend rate
 
2.80%  
6.50%  
6.80%  
 
2.80%  
7.00%  
7.10%  
3.00%  
3.10%  
7.50%  
7.50%