CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Restricted Stock	Common Stock	Common Stock	Common Stock Restricted Stock	Common Stock Common Stock	Treasury Stock	Capital In Excess of Par	Capital In Excess of Par Restricted Stock	Capital In Excess of Par Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance (in shares) at Sep. 30, 2017				35,231,000
Beginning Balance, Treasury Stock (in shares) at Sep. 30, 2017							9,948,000
Beginning Balance at Sep. 30, 2017	$ 595,037			$ 35			$ (387,766)	$ 580,938			$ 397,881	$ 3,949
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense	18,518							18,518
Repurchase of common stock under Share Repurchase Program (in shares)							370,000
Repurchases of common stock under Share Repurchase Program	(40,726)						$ (40,726)
Repurchase of common stock - other (in shares)							38,000
Repurchases of common stock - other	(3,562)						$ (3,562)
Exercise of stock options (in shares)				488,000
Exercise of stock options	19,279			$ 1				19,278
Issuance of restricted stock under Deposit Share Program (in shares)						50,000
Issuance of restricted stock under Deposit Share Program			$ 3,464							$ 3,464
Issuance of stock under Employee Stock Purchase Program (in shares)					93,000
Issuance of restricted stock under Deposit Share Program		$ 300							$ 300
Issuance of stock under Employee Stock Purchase Plan		300							300
Net income	110,043										110,043
Dividends paid	(36,251)										(36,251)
Foreign currency translation adjustments	679											679
Cash flow hedges	(63)											(63)
Cash flow hedges	(63)
Minimum pension liability adjustment	(26)											(26)
Ending Balance (in shares) at Sep. 30, 2018				35,862,000
Ending Balance, Treasure Stock (in shares) at Sep. 30, 2018							10,356,000
Ending Balance at Sep. 30, 2018	666,692			$ 36			$ (432,054)	622,498			471,673	4,539
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense	18,227							18,227
Repurchase of common stock under Share Repurchase Program (in shares)							88,000
Repurchases of common stock under Share Repurchase Program	(10,002)						$ (10,002)
Repurchase of common stock - other (in shares)							47,000
Repurchases of common stock - other	(4,850)						$ (4,850)
Exercise of stock options (in shares)				313,000
Exercise of stock options	13,195			$ 1				13,194
Issuance of common stock in connection with acquisition of KMG Chemicals, Inc (in shares)				3,237,000
Issuance of restricted stock under Deposit Share Program (in shares)					131,000
Issuance of restricted stock under Deposit Share Program		75							75
Issuance of stock under Employee Stock Purchase Program (in shares)						49,000
Issuance of restricted stock under Deposit Share Program			3,941							3,941
Issuance of stock under Employee Stock Purchase Plan			3,941							3,941
Net income	39,215										39,215
Dividends paid	(48,454)										(48,454)
Foreign currency translation adjustments	(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				39,592,000
Ending Balance, Treasure Stock (in shares) at Sep. 30, 2019	10,491,000						10,491,000
Ending Balance at Sep. 30, 2019	$ 980,377			$ 40			$ (446,906)	988,980			461,501	(23,238)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense	16,396							16,396
Repurchase of common stock under Share Repurchase Program (in shares)							318,000
Repurchases of common stock under Share Repurchase Program	(35,009)						$ (35,009)
Repurchase of common stock - other (in shares)							25,000
Repurchases of common stock - other	(3,229)						$ (3,229)
Exercise of stock options (in shares)				181,000
Exercise of stock options	9,491							9,491
Issuance of common stock in connection with acquisition of KMG Chemicals, Inc.	331,048			$ 3				331,045
Issuance of restricted stock under Deposit Share Program (in shares)					95,000
Issuance of restricted stock under Deposit Share Program		$ 150							$ 150
Issuance of stock under Employee Stock Purchase Program (in shares)						46,000
Issuance of restricted stock under Deposit Share Program			4,786							4,786
Issuance of stock under Employee Stock Purchase Plan			$ 4,786							$ 4,786
Net income	142,828										142,828
Dividends paid	(51,099)										(51,099)
Foreign currency translation adjustments	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				39,914,000
Ending Balance, Treasure Stock (in shares) at Sep. 30, 2020	10,834,000						10,834,000
Ending Balance at Sep. 30, 2020	$ 1,074,313			$ 40			$ (485,144)	$ 1,019,803			$ 553,718	$ (14,104)