CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Capital In Excess of Par	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Balance at beginning of period at Sep. 30, 2018	$ 666,692	$ 36	$ 622,498	$ 471,673	$ 4,539	$ (432,054)
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 Cabot Microelectronics restricted stock under Deposit Share Program	75		75
Issuance of common stock in connection with acquisition of KMG Chemicals, Inc.	331,048	3	331,045
Net income	13,443			13,443
Dividends	(11,598)			(11,598)
Foreign currency translation adjustment	2,425				2,425
Minimum pension liability adjustment	(251)				(251)
Balance at end of period at Dec. 31, 2018	1,008,167	39	964,885	472,585	6,713	(436,055)
Balance at beginning of period at Sep. 30, 2018	666,692	36	622,498	471,673	4,539	(432,054)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	40,580
Foreign currency translation adjustment	1,433
Minimum pension liability adjustment	0
Cash flow hedges	(6,474)
Balance at end of period at Mar. 31, 2019	1,025,867	39	975,612	487,467	(502)	(436,749)
Balance at beginning of period at Dec. 31, 2018	1,008,167	39	964,885	472,585	6,713	(436,055)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense	3,538		3,538
Repurchases of common stock - other, at cost	(694)					(694)
Exercise of stock options	5,080		5,080
Issuance of Cabot Microelectronics stock under Employee Stock Purchase Plan	2,109		2,109
Net income	27,137			27,137
Dividends	(12,255)			(12,255)
Foreign currency translation adjustment	(992)				(992)
Interest rate swap	(6,474)				(6,474)
Minimum pension liability adjustment	251				251
Cash flow hedges	(6,474)
Balance at end of period at Mar. 31, 2019	1,025,867	39	975,612	487,467	(502)	(436,749)
Balance at beginning of period at Sep. 30, 2019	980,377	40	988,980	461,501	(23,238)	(446,906)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense	4,763		4,763
Repurchases of common stock - other, at cost	(2,897)					(2,897)
Exercise of stock options	1,298		1,298
Issuance of Cabot Microelectronics restricted stock under Deposit Share Program	150		150
Net income	38,549			38,549
Dividends	(12,351)			(12,351)
Effect of the adoption of the stranded tax effect accounting standards	0			488	(488)
Foreign currency translation adjustment	15,851				15,851
Cash flow hedges	4,259				4,259
Balance at end of period at Dec. 31, 2019	1,029,999	40	995,191	488,187	(3,616)	(449,803)
Balance at beginning of period at Sep. 30, 2019	980,377	40	988,980	461,501	(23,238)	(446,906)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	71,448
Foreign currency translation adjustment	(3,874)
Minimum pension liability adjustment	0
Cash flow hedges	(13,048)
Balance at end of period at Mar. 31, 2020	1,009,410	40	1,008,311	508,126	(40,648)	(466,419)
Balance at beginning of period at Dec. 31, 2019	1,029,999	40	995,191	488,187	(3,616)	(449,803)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense	4,234		4,234
Repurchases of common stock under Share Repurchase Program, at cost	(16,414)					(16,414)
Repurchases of common stock - other, at cost	(202)					(202)
Exercise of stock options	6,350		6,350
Issuance of Cabot Microelectronics stock under Employee Stock Purchase Plan	2,536		2,536
Net income	32,899			32,899
Dividends	(12,960)			(12,960)
Foreign currency translation adjustment	(19,725)				(19,725)
Minimum pension liability adjustment	0
Cash flow hedges	(17,307)				(17,307)
Balance at end of period at Mar. 31, 2020	$ 1,009,410	$ 40	$ 1,008,311	$ 508,126	$ (40,648)	$ (466,419)