BUSINESS COMBINATION (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Nov. 15, 2018	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Business combination disclosure [Abstract]
Acquisition of business, net of cash acquired		$ 1,182,186	$ 0
Face amount of debt	$ 1,265,000
Total consideration [Abstract]
Fair value of Cabot Microelectronics common stock issued for KMG outstanding common stock and equity awards		331,048	0
Fair values of assets acquired and liabilities assumed [Abstract]
Goodwill		706,108		$ 101,083
Net income (loss)		13,443	(3,083)
Non-recurring costs [Abstract]
Share-based compensation expense		8,170	5,881
Retention and severance expense		27,294	0
Revolving Credit Facility [Member]
Business combination disclosure [Abstract]
Face amount of debt	200,000
Line of Credit Facility, Maximum Borrowing Capacity	200,000
Term Loan [Member]
Business combination disclosure [Abstract]
Face amount of debt	$ 1,065,000
Selling, general, and administrative [Member]
Fair values of assets acquired and liabilities assumed [Abstract]
Transaction and acquisition-related expenses		27,294
Cost of Sales [Member]
Fair values of assets acquired and liabilities assumed [Abstract]
Transaction and acquisition-related expenses		10,261
Electronic Materials [Member]
Fair values of assets acquired and liabilities assumed [Abstract]
Goodwill		359,187
Performance Materials [Member]
Fair values of assets acquired and liabilities assumed [Abstract]
Goodwill		346,921
Acquisition-related Costs [Member]
Non-recurring costs [Abstract]
Transaction costs			29,326
Share-based compensation expense			10,316
Retention and severance expense			25,461
KMG [Member]
Business combination disclosure [Abstract]
Business acquisition, percentage of voting interests acquired	100.00%
Acquisition of business, net of cash acquired	$ 1,513,235
Cash acquired from acquisition	$ 23,217
Amount received for each share of KMG common stock (in dollars per share)	$ 55.65
Business Acquisition Share Percentage	0.2000
Shares issued to acquire entity	3,237,005
Business Acquisition, Share Price	$ 102.27
Face amount of debt	$ 1,065,000
Line of Credit Facility, Maximum Borrowing Capacity	200,000
Debt Issuance Costs, Net	21,408
Total consideration [Abstract]
Cash provided to payoff KMG debt	304,648
Total cash consideration paid	1,205,404
Total consideration transferred	1,536,452
Fair values of assets acquired and liabilities assumed [Abstract]
Cash	23,217
Accounts receivable	74,649
Inventories	68,963
Prepaid expenses and other current assets	4,860
Property, plant and equipment	149,504
Intangible assets	844,800
Other long-term assets	6,208
Accounts payable	(31,761)
Accrued expenses and other current liabilities	(40,584)
Deferred tax liabilities	(164,778)
Other long-term liabilities	(3,754)
Total identifiable net assets acquired	931,324
Goodwill	605,128
Total	1,536,452
Net sales		62,001
Net income (loss)		14,174
Transaction and acquisition-related expenses	25,661
Business acquisition, pro forma information [Abstract]
Revenue		283,756	252,734
Net income		$ 30,604	$ (56,109)
Earnings per share - basic (in dollars per share)		$ 1.06	$ (1.96)
Earnings per share - diluted (in dollars per share)		$ 1.04	$ (1.96)
Intangible assets acquired [Abstract]
Fair value	$ 844,800
Weighted average useful life	17 years 10 months 24 days
KMG [Member] | Revolving Credit Facility [Member]
Business combination disclosure [Abstract]
Debt Issuance Costs, Net	$ 859
KMG [Member] | Term Loan [Member]
Business combination disclosure [Abstract]
Debt Issuance Costs, Net	20,549
KMG [Member] | Electronic Materials [Member]
Fair values of assets acquired and liabilities assumed [Abstract]
Goodwill	263,208
KMG [Member] | Performance Materials [Member]
Fair values of assets acquired and liabilities assumed [Abstract]
Goodwill	341,920
KMG [Member] | Customer Relationships [Member]
Intangible assets acquired [Abstract]
Fair value	704,000
KMG [Member] | Customer Relationships [Member] | Minimum [Member]
Intangible assets acquired [Abstract]
Estimated useful lives		15 years
KMG [Member] | Customer Relationships [Member] | Maximum [Member]
Intangible assets acquired [Abstract]
Estimated useful lives		20 years
KMG [Member] | Technology and Know-how [Member]
Intangible assets acquired [Abstract]
Fair value	85,500
KMG [Member] | Technology and Know-how [Member] | Minimum [Member]
Intangible assets acquired [Abstract]
Estimated useful lives		9 years
KMG [Member] | Technology and Know-how [Member] | Maximum [Member]
Intangible assets acquired [Abstract]
Estimated useful lives		11 years
KMG [Member] | Trade Name - Flowchem [Member]
Intangible assets acquired [Abstract]
Fair value	46,000
KMG [Member] | Trade Name - All Other [Member]
Intangible assets acquired [Abstract]
Fair value	7,000
KMG [Member] | Trade Name - All Other [Member] | Minimum [Member]
Intangible assets acquired [Abstract]
Estimated useful lives		1 year
KMG [Member] | Trade Name - All Other [Member] | Maximum [Member]
Intangible assets acquired [Abstract]
Estimated useful lives		15 years
KMG [Member] | EPA Product Registration Rights [Member]
Intangible assets acquired [Abstract]
Fair value	2,300
Estimated useful lives		15 years
KMG [Member] | Common Stock and Equity Awards [Member]
Total consideration [Abstract]
Total cash consideration paid	900,756
Fair value of Cabot Microelectronics common stock issued for KMG outstanding common stock and equity awards	$ 331,048