CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Capital in Excess of Par	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	[1]	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjustment	[1]	Treasury Stock
Beginning Balance (in shares) at Sep. 30, 2018		35,862								10,356
Beginning balance at Sep. 30, 2018		$ 36	$ 622,498	$ 471,673			$ 4,539			$ (432,054)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		313
Exercise of stock options		$ 1	13,194
Share-based compensation expense			18,227
Repurchase of common stock under Share Repurchase Program (in shares)										88
Repurchases of common stock under Share Repurchase Program										$ (10,002)
Repurchase of common stock - other (in shares)										47
Repurchases of common stock - other										$ (4,850)
Common stock in connection with acquisition of KMG Chemicals, Inc. (in shares)		3,237
Common stock in connection with acquisition of KMG Chemicals, Inc.		$ 3	331,045
Issuance of restricted stock under Deposit Share Program (in shares)		131
Issuance of restricted stock under Deposit Share Program			75
Common stock under Employee Stock Purchase Plan (in shares)		49
Common stock under Employee Stock Purchase Plan			3,941
Net (loss) income	$ 39,215			39,215
Dividends				(48,454)
Total foreign currency translation adjustment, net of tax	(8,548)						(8,548)
Cash flow hedges	(18,780)						(18,780)
Minimum pension liability adjustment	(449)						(449)
Ending Balance (in shares) at Sep. 30, 2019		39,592								10,491
Ending Balance at Sep. 30, 2019	980,377	$ 40	988,980	461,501	$ (933)		(23,238)			$ (446,906)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		181
Exercise of stock options			9,491
Share-based compensation expense			16,396
Repurchase of common stock under Share Repurchase Program (in shares)										318
Repurchases of common stock under Share Repurchase Program										$ (35,009)
Repurchase of common stock - other (in shares)										25
Repurchases of common stock - other										$ (3,229)
Issuance of restricted stock under Deposit Share Program (in shares)		95
Issuance of restricted stock under Deposit Share Program			150
Common stock under Employee Stock Purchase Plan (in shares)		46
Common stock under Employee Stock Purchase Plan			4,786
Net (loss) income	142,828			142,828
Dividends				(51,099)
Total foreign currency translation adjustment, net of tax	19,286						19,286
Cash flow hedges	(10,711)						(10,711)
Minimum pension liability adjustment	1,047						1,047
Ending Balance (in shares) at Sep. 30, 2020		39,914								10,834
Ending Balance at Sep. 30, 2020	1,074,313	$ 40	1,019,803	553,718	$ 488		(14,104)	$ (488)		$ (485,144)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options (in shares)		133
Exercise of stock options			7,151
Share-based compensation expense			19,678
Repurchase of common stock under Share Repurchase Program (in shares)										924
Repurchases of common stock under Share Repurchase Program										$ (120,027)
Repurchase of common stock - other (in shares)										37
Repurchases of common stock - other										$ (5,560)
Issuance of restricted stock under Deposit Share Program (in shares)		123
Issuance of restricted stock under Deposit Share Program			215
Common stock under Employee Stock Purchase Plan (in shares)		51
Common stock under Employee Stock Purchase Plan			6,022
Net (loss) income	(68,577)			(68,577)
Dividends				(53,173)
Total foreign currency translation adjustment, net of tax	2,517						2,517
Cash flow hedges	16,541						16,541
Minimum pension liability adjustment	(163)						(163)
Ending Balance (in shares) at Sep. 30, 2021		40,221								11,795
Ending Balance at Sep. 30, 2021	$ 878,937	$ 40	$ 1,052,869	$ 431,968			$ 4,791			$ (610,731)

[1] Impact due to the adoption of ASU No. 2018-02 in fiscal 2020.