Document and Entity Information - shares	3 Months Ended
	Dec. 31, 2018	Jan. 31, 2019
Document and Entity Information [Abstract]
Entity Registrant Name	CABOT MICROELECTRONICS CORP
Entity Central Index Key	0001102934
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		28,955,343
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1