CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Common Stock [Member]	Capital In Excess Of Par [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Treasury Stock [Member]	Total
Balance at beginning 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		5,881				5,881
Repurchases of common stock under share repurchase plans, at cost					(1,591)	(1,591)
Repurchases of common stock - other, at cost					(3,160)	(3,160)
Exercise of stock options		6,139				6,139
Issuance of Cabot Microelectronics stock under Employee Stock Purchase Plan		300				300
Net income			(3,083)			(3,083)
Dividends			(5,153)			(5,153)
Foreign currency translation adjustment				7,144		7,144
Interest rate swaps				199		199
Unrealized loss in short term available-for-sale securities				(46)		(46)
Minimum pension liability adjustment						0
Balance at end of period at Dec. 31, 2017	35	593,258	389,645	11,246	(392,517)	601,667
Balance at beginning of period at Sep. 30, 2018	36	622,498	471,673	4,539	(432,054)	666,692
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense		8,170				8,170
Repurchases of common stock - other, at cost					(4,001)	(4,001)
Exercise of stock options		3,097				3,097
Issuance of common stock in connection with acquisition of KMG Chemicals, Inc.	3	331,045				331,048
Issuance of Cabot Microelectronics restricted stock under Deposit Share Program		75				75
Net income			13,443			13,443
Dividends			(11,598)			(11,598)
Foreign currency translation adjustment				2,425		2,425
Unrealized loss in short term available-for-sale securities						0
Minimum pension liability adjustment				(251)		(251)
Balance at end of period at Dec. 31, 2018	$ 39	$ 964,885	472,585	$ 6,713	$ (436,055)	1,008,167
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of the adoption of the revenue recognition accounting standards			$ (933)			$ (933)