CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY (USD $) In Thousands	Total	Common Stock [Member]	Capital In Excess Of Par [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Comprehensive Income, Net Of Tax [Member]	Comprehensive Treasury Stock [Member]
Balance at Sep. 30, 2009	$ 470,729	$ 26	$ 213,031	$ 334,309	$ 13,690		$ (90,327)
Share-based compensation expense	11,643		11,643
Repurchases of common stock under share repurchase plans, at cost	(24,998)						(24,998)
Repurchases of common stock - other, at cost	(766)						(766)
Exercise of stock options	2,283		2,283
Issuance of Cabot Microelectronics restricted stock under deposit share plan	45		45
Issuance of Cabot Microelectronics stock under Employee Stock Purchase Plan	1,101		1,101
Net income	49,458			49,458		49,458
Foreign currency translation adjustment					4,580	4,580
Minimum pension liability adjustment					268	268
Total comprehensive income	54,306					54,306
Balance at Sep. 30, 2010	514,343	26	228,103	383,767	18,538		(116,091)
Share-based compensation expense	12,646		12,646
Repurchases of common stock under share repurchase plans, at cost	(54,106)						(54,106)
Repurchases of common stock - other, at cost	(1,393)						(1,393)
Exercise of stock options	35,955	2	35,953
Issuance of Cabot Microelectronics restricted stock under deposit share plan	145		145
Issuance of Cabot Microelectronics stock under Employee Stock Purchase Plan	1,951		1,951
Tax deduction for the exercise of stock options granted prior to adoption of ASC 718	262		262
Deferred tax effect of long-term incentives	(700)		(700)
Net income	51,662			51,662		51,662
Foreign currency translation adjustment					5,490	5,490
Minimum pension liability adjustment					99	99
Total comprehensive income	57,251					57,251
Balance at Sep. 30, 2011	566,354	28	278,360	435,429	24,127		(171,590)
Share based compensation expense, net of compensation related to dividends on unvested restricted stock	12,980		12,980
Repurchases of common stock under share repurchase plans, at cost	(33,026)						(33,026)
Repurchases of common stock - other, at cost	(1,511)						(1,511)
Exercise of stock options	34,107	1	34,106
Issuance of Cabot Microelectronics restricted stock under deposit share plan	155		155
Issuance of Cabot Microelectronics stock under Employee Stock Purchase Plan	2,228		2,228
Tax deduction for the exercise of stock options granted prior to adoption of ASC 718	498		498
Net income	40,826			40,826		40,826
Foreign currency translation adjustment					6,876	6,876
Minimum pension liability adjustment					(537)	(537)
Total comprehensive income	47,165					47,165
Dividends paid, net of expected forfeitures of unvested restricted stock	(346,814)			(346,814)
Tax deduction for the dividend paid on unvested restricted stock, net of expected forfeitures	1,455		1,455
Balance at Sep. 30, 2012	$ 283,591	$ 29	$ 329,782	$ 129,441	$ 30,466		$ (206,127)