CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Common Stock [Member]	Capital in Excess Of Par [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Treasury Stock [Member]	Total
Balance at beginning of period at Sep. 30, 2015	$ 33	$ 495,673	$ 284,088	$ (6,090)	$ (344,740)	$ 428,964
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense		13,787				13,787
Repurchases of common stock under share repurchase plans, at cost					(25,980)	(25,980)
Repurchases of common stock - other, at cost					(2,838)	(2,838)
Exercise of stock options	1	16,623				16,624
Issuance of Cabot Microelectronics restricted stock under Deposit Share Program		52				52
Issuance of Cabot Microelectronics stock under Employee Stock Purchase Plan		2,837				2,837
Tax benefits from share-based compensation plans		1,868				1,868
Net income			59,849			59,849
Dividends paid			(13,161)			(13,161)
Foreign currency translation adjustments				15,996		15,996
Interest rate swaps				84		84
Minimum pension liability adjustment				(434)		(434)
Balance at end of period at Sep. 30, 2016	34	530,840	330,776	9,556	(373,558)	497,648
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense		13,004				13,004
Repurchases of common stock under share repurchase plans, at cost					(12,035)	(12,035)
Repurchases of common stock - other, at cost					(2,173)	(2,173)
Exercise of stock options	1	27,665				27,666
Issuance of Cabot Microelectronics stock under Employee Stock Purchase Plan		2,986				2,986
Tax benefits from share-based compensation plans		6,443				6,443
Net income			86,952			86,952
Dividends paid			(19,847)			(19,847)
Foreign currency translation adjustments				(6,746)		(6,746)
Interest rate swaps				863		863
Minimum pension liability adjustment				276		276
Balance at end of period at Sep. 30, 2017	35	580,938	397,881	3,949	(387,766)	595,037
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense		18,518				18,518
Repurchases of common stock under share repurchase plans, at cost					(40,726)	(40,726)
Repurchases of common stock - other, at cost					(3,562)	(3,562)
Exercise of stock options	1	19,278				19,279
Issuance of Cabot Microelectronics restricted stock under Deposit Share Program		300				300
Issuance of Cabot Microelectronics stock under Employee Stock Purchase Plan		3,464				3,464
Net income			110,043			110,043
Dividends paid			(36,251)			(36,251)
Foreign currency translation adjustments				679		679
Interest rate swaps				(63)		(63)
Minimum pension liability adjustment				(26)		(26)
Balance at end of period at Sep. 30, 2018	$ 36	$ 622,498	$ 471,673	$ 4,539	$ (432,054)	$ 666,692