Document And Entity Information - USD ($)	12 Months Ended
	Sep. 30, 2018	Oct. 31, 2018	Mar. 31, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	GRIFFON CORP
Document Type	10-K
Current Fiscal Year End Date	--09-30
Entity Common Stock, Shares Outstanding		45,674,821
Entity Public Float			$ 656,000,000
Amendment Flag	false
Entity Central Index Key	0000050725
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Well-known Seasoned Issuer	Yes
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY