DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2018 AUD ($) component segment	Sep. 30, 2018 USD ($) component segment $ / shares	Sep. 30, 2018 CAD ($) component segment	Sep. 30, 2017 AUD ($)	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2017 CAD ($)	Sep. 30, 2016 USD ($)	Nov. 16, 2017 USD ($)	Nov. 15, 2017 USD ($)
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Number of operating segments | segment	2	2	2
Environmental exit costs, costs accrued to date					$ 5,700,000
Cash in non U.S. bank accounts		$ 24,900,000			26,500,000
Fair value of insurance contracts		3,032,000
Proceeds from sale of trading securities		4,126,000					$ 715,000
Contracts revenue		$ 1,977,918,000	$ 700		$ 1,524,997,000	$ 4,690	1,477,035,000
Contracts weighted average rate price (in Dollars per share) | $ / shares		$ 1.29			$ 1.25
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax		$ 9,403,000			$ 10,667,000
Foreign currency transaction gain (loss), realized		(7,000)			378,000
Share-based compensation, excess tax benefit, amount		2,193,000
Defined benefit plan, fair value of plan assets		$ 150,634,000			150,822,000
Contract period	1 year	1 year	1 year
Income (loss) from operations		$ 96,450,000			69,027,000		82,340,000
Provision for loss on contracts		12,200,000
Customer program reserves and cash discounts netted against accounts receivable		15,530,000			11,249,000
Costs in excess of billings, noncurrent		29,500,000			20,100,000
Unbilled contracts receivable		400,000			2,850,000
Depreciation, depletion and amortization, nonproduction		46,733,000			41,220,000		39,734,000
Accumulated capitalized interest costs		2,896,000			4,891,000		3,956,000
Original cost of fully depreciated property plant and equipment		$ 213,501,000
Description of fair value calculation	Griffon used five year projections and a 3.0% terminal value to which discount rates between 9% and 10% were applied to calculate each unit’s fair value.	Griffon used five year projections and a 3.0% terminal value to which discount rates between 9% and 10% were applied to calculate each unit’s fair value.	Griffon used five year projections and a 3.0% terminal value to which discount rates between 9% and 10% were applied to calculate each unit’s fair value.
Fair value projections	5 years	5 years	5 years
Fair value terminal value	3.00%	3.00%	3.00%
Goodwill, impairment loss		$ 0			0		0
Unrecognized tax benefits		4,519,000			4,825,000		4,709,000
Research and development arrangement, contract to perform for others, costs incurred, gross		15,400,000			17,700,000		18,000,000
Net cash used in financing activities		39,065,000			(700,000)		$ 15,414,000
Accumulated other comprehensive loss		$ (34,112,000)			$ (60,481,000)
U.S. Federal income tax provision (benefit) rate	24.50%	24.50%	24.50%	35.00%	35.00%	35.00%	35.00%
Selling, general and administrative expenses		$ 433,110,000			$ 339,089,000		$ 318,353,000
Home and Building Products (HBP) [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Number of operating segments | segment	2	2	2
Home & Building Products [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Number of components | component	2	2	2
Other Income [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Trading securities, realized loss		$ 1,251,000					13,000
Selling, general and administrative expenses [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Depreciation		$ 16,306,000			12,995,000		11,721,000
U.S. Government [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Percentage of consolidated accounts receivable	8.00%	8.00%	8.00%
Home Depot [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Percentage of consolidated accounts receivable	20.00%	20.00%	20.00%
Contracts accounted for under percentage of completion [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Income (loss) from operations		$ 1,400,000			$ 600,000		(700,000)
Designated as hedging instrument [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Contracts revenue	$ 12,000			$ 14,500
Contracts weighted average rate price (in Dollars per share) | $ / shares		$ 1.38			$ 1.28
Accumulated other comprehensive income (loss), foreign currency translation adjustment, before tax		$ 443,000			$ 175,000
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax		288,000			114,000
Gain (loss) on hedging activity		657,000			(1,458,000)
Level 2 [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Defined benefit plan, fair value of plan assets		68,114,000			68,723,000
Level 2 [Member] | Estimate of fair value measurement [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Trading securities		2,644,000			3,352,000
Level 2 [Member] | Portion at other than fair value measurement [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Trading securities		2,086,000			1,000,000
Senior notes [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Long-term debt, fair value		$ 990,000,000
Maximum [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Maturity period of highly liquid investments	3 months	3 months	3 months
Contracts expiration days	92 days	92 days	92 days
Fair value discount rates	9.50%	9.50%	9.50%
Finite-lived intangible asset, useful life	25 years	25 years	25 years
Maximum [Member] | Building and building improvements [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Property, plant and equipment, useful life	40 years	40 years	40 years
Maximum [Member] | Machinery and equipment [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Property, plant and equipment, useful life	15 years	15 years	15 years
Minimum [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Contracts expiration days	31 days	31 days	31 days
Fair value discount rates	7.00%	7.00%	7.00%
Finite-lived intangible asset, useful life	8 years	8 years	8 years
Minimum [Member] | Building and building improvements [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Property, plant and equipment, useful life	25 years	25 years	25 years
Minimum [Member] | Machinery and equipment [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Property, plant and equipment, useful life	2 years	2 years	2 years
Canadian Dollar Forward Contracts [Member] | Level 2 [Member] | Not Designated as Hedging Instrument [Member] | Other Income [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Gain (loss) on sale of derivatives		$ (161,000)			200,000
Canadian Dollar Forward Contracts [Member] | Maximum [Member] | Not Designated as Hedging Instrument [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Foreign currency contracts duration	358 days	358 days	358 days
Canadian Dollar Forward Contracts [Member] | Minimum [Member] | Not Designated as Hedging Instrument [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Foreign currency contracts duration	30 days	30 days	30 days
New Accounting Pronouncement, Early Adoption, Effect [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Share-based compensation, excess tax benefit, amount		$ 1,299,000			4,440,000
Accounting Standards Update 2016-09, Statutory Tax Withholding Component [Member] | New Accounting Pronouncement, Early Adoption, Effect [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Net cash provided by operating activities		2,291,000
Net cash used in financing activities		2,291,000
Discontinued Operations, Held-for-sale [Member] | Plastics [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Disposal group, including discontinued operation, consideration								$ 475,000,000	$ 475,000,000
Foreign currency translation adjustments [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Accumulated other comprehensive loss		(22,824,000)			(32,227,000)
Shipping and Handling [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Selling, general and administrative expenses		59,600,000			32,500,000		30,600,000
Advertising [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Selling, general and administrative expenses		$ 21,000,000			$ 22,000,000		$ 23,000,000