CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Capital In Excess of Par	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjustment	Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of the adoption of the stranded tax effect accounting standard	$ 666,692		$ 36	$ 622,498	$ 471,673		$ 4,539		$ (432,054)
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 common stock in connection with acquisition of KMG Chemicals, Inc.	331,048		3	331,045
Issuance of Cabot Microelectronics restricted stock under Deposit Share Program	75			75
Net income	13,443				13,443
Dividends	(11,598)				(11,598)
Effect of the adoption of the stranded tax effect accounting standard	1,008,167		39	964,885	472,585		6,713		(436,055)
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends (in dollars per share)	$ 0.40
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	59,458
Effect of the adoption of the stranded tax effect accounting standard	1,025,333		39	979,940	494,009		(6,847)		(441,808)
Foreign currency translation adjustment	4,132
Cash flow hedges	(15,518)
Balance at end of period at Jun. 30, 2019	1,025,333		39	979,940	494,009		(6,847)		(441,808)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of the adoption of the stranded tax effect accounting standard	1,008,167		39	964,885	472,585		6,713		(436,055)
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)
Effect of the adoption of the stranded tax effect accounting standard	1,025,867		39	975,612	487,467		(502)		(436,749)
Foreign currency translation adjustment	(992)						(992)
Cash flow hedges	(6,474)						(6,474)
Minimum pension liability adjustment	251						251
Balance at end of period at Mar. 31, 2019	$ 1,025,867		39	975,612	487,467		(502)		(436,749)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends (in dollars per share)	$ 0.42
Effect of the adoption of the stranded tax effect accounting standard	$ 1,025,867		39	975,612	487,467		(502)		(436,749)
Share-based compensation expense	3,340			3,340
Repurchases of common stock under Share Repurchase Program, at cost	(5,001)								(5,001)
Repurchases of common stock - other, at cost	(58)								(58)
Exercise of stock options	988			988
Net income	18,878				18,878
Dividends	(12,336)				(12,336)
Effect of the adoption of the stranded tax effect accounting standard	1,025,333		39	979,940	494,009		(502)		(441,808)
Foreign currency translation adjustment	2,699						2,699
Cash flow hedges	(9,044)
Cash flow hedges	(9,044)						(9,044)
Balance at end of period at Jun. 30, 2019	$ 1,025,333		39	979,940	494,009		(6,847)		(441,808)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends (in dollars per share)	$ 0.42
Effect of the adoption of the stranded tax effect accounting standard	$ 1,025,333		39	979,940	494,009		(6,847)		(441,808)
Effect of the adoption of the stranded tax effect accounting standard	980,377	$ 0	40	988,980	461,501	$ 488	(23,238)	$ (488)	(446,906)
Balance at beginning of period at Sep. 30, 2019	980,377	0	40	988,980	461,501	488	(23,238)	(488)	(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 standard	1,029,999	0	40	995,191	488,187	488	(3,616)	(488)	(449,803)
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends (in dollars per share)	$ 0.42
Balance at beginning of period at Sep. 30, 2019	$ 980,377	0	40	988,980	461,501	488	(23,238)	(488)	(446,906)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	105,973
Effect of the adoption of the stranded tax effect accounting standard	980,377	$ 0	40	1,012,131	529,786	$ 488	(31,896)	$ (488)	(485,027)
Foreign currency translation adjustment	5,212
Cash flow hedges	(13,382)
Balance at end of period at Jun. 30, 2020	1,025,034		40	1,012,131	529,786		(31,896)		(485,027)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Effect of the adoption of the stranded tax effect accounting standard	1,029,999		40	995,191	488,187		(3,616)		(449,803)
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)
Effect of the adoption of the stranded tax effect accounting standard	1,009,410		40	1,008,311	508,126		(40,648)		(466,419)
Foreign currency translation adjustment	(19,725)						(19,725)
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends (in dollars per share)	$ 0.44
Effect of the adoption of the stranded tax effect accounting standard	$ 1,009,410		40	1,008,311	508,126		(40,648)		(466,419)
Share-based compensation expense	3,194			3,194
Repurchases of common stock under Share Repurchase Program, at cost	(18,595)								(18,595)
Repurchases of common stock - other, at cost	(13)								(13)
Exercise of stock options	626			626
Net income	34,525				34,525
Dividends	(12,865)				(12,865)
Effect of the adoption of the stranded tax effect accounting standard	1,009,410		40	1,012,131	529,786		(31,896)		(485,027)
Foreign currency translation adjustment	9,086						9,086
Cash flow hedges	(334)						(334)
Balance at end of period at Jun. 30, 2020	$ 1,025,034		40	1,012,131	529,786		(31,896)		(485,027)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends (in dollars per share)	$ 0.44
Effect of the adoption of the stranded tax effect accounting standard	$ 1,025,034		$ 40	$ 1,012,131	$ 529,786		$ (31,896)		$ (485,027)