DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ / shares in Units, $ in Thousands, $ in Thousands		12 Months Ended
		Sep. 30, 2019 USD ($) segment $ / shares	Sep. 30, 2019 CAD ($) segment	Sep. 30, 2018 AUD ($)	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2018 CAD ($)	Sep. 30, 2017 USD ($)	Oct. 01, 2019 USD ($)	Oct. 01, 2018 USD ($)	Nov. 16, 2017 USD ($)	Nov. 15, 2017 USD ($)
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Number of Reportable Segments | segment		3	3
Number of operating segments | segment		3	3
Environmental exit costs, costs accrued to date					$ 5,700,000
Cash in non U.S. bank accounts		$ 34,200,000			24,900,000
Fair value of insurance contracts		3,408,000
Proceeds from sale of trading securities		4,126,000
Contracts revenue		$ 2,209,289,000	$ 3,500		$ 1,977,918,000	$ 700	$ 1,524,997,000
Contracts weighted average rate price (in Dollars per share) | $ / shares		$ 1.32			$ 1.29
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax		$ (8,460,000)			$ 9,403,000
Foreign currency transaction gain (loss), realized		(14,000)			(7,000)
Defined benefit plan, fair value of plan assets		145,319,000			150,634,000
Income (loss) from operations		135,265,000			92,801,000		67,034,000
Customer program reserves and cash discounts netted against accounts receivable		17,322,000			15,530,000
Costs in excess of billings, noncurrent		13,100,000			29,500,000
Contract with Customer, Asset, Allowance for Credit Loss		0			400,000
Depreciation, depletion and amortization, nonproduction		51,926,000			46,733,000		41,220,000
Accumulated capitalized interest costs		2,925,000			2,896,000		4,891,000
Original cost of fully depreciated property plant and equipment		$ 229,456,000
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.
Fair value projections		5 years	5 years
Fair value terminal value		3.00%	3.00%
Goodwill, impairment loss		$ 0			0		0
Unrecognized tax benefits		4,061,000			4,519,000		4,825,000
Research and development arrangement, contract to perform for others, costs incurred, gross		15,400,000			15,400,000		17,700,000
Assets		2,074,939,000			2,084,890,000		$ 1,873,541,000		$ 2,085,933,000
Accumulated other comprehensive loss		$ (65,916,000)			$ (34,112,000)
U.S. Federal income tax provision (benefit) rate		21.00%	21.00%	24.50%	24.50%	24.50%	35.00%
Selling, general and administrative expenses		$ 460,004,000			$ 436,380,000		$ 341,092,000
Provisional income tax (expense) benefit		$ 13,100,000			(20,587,000)
Revenue, Performance Obligation Satisfied At A Point In Time, Percentage		80.00%	80.00%
Revenue, Performance Obligation Satisfied Over In Time, Percentage		20.00%	20.00%
Liabilities		$ 1,597,176,000			1,610,499,000				1,617,160,000
Tax Cuts and Jobs Act, Transition Tax for Accumulated Foreign Earnings, Income Tax Expense		12,699,000
Other Income [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Trading securities, realized loss		1,251,000
Selling, general and administrative expenses [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Depreciation		$ 19,026,000			$ 16,306,000		12,995,000
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%
Home Depot [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Percentage of consolidated accounts receivable		12.00%	12.00%
Designated as hedging instrument [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Contracts revenue		$ 14,000,000		$ 12,000
Contracts weighted average rate price (in Dollars per share) | $ / shares		$ 1.48			$ 1.38
Accumulated other comprehensive income (loss), foreign currency translation adjustment, before tax		$ 327,000			$ 443,000
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax		213,000			288,000
Gain (loss) on hedging activity		1,932,000			657,000
Level 2 [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Defined benefit plan, fair value of plan assets		32,297,000			68,114,000
Level 2 [Member] | Estimate of fair value measurement [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Trading securities		2,754,000			2,644,000
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		2,233,000			2,086,000
Senior notes [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Long-term debt, fair value		$ 1,010,000,000
Maximum [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Maturity period of highly liquid investments		3 months	3 months
Contracts expiration days		90 days	90 days
Fair value discount rates		9.50%	9.50%
Finite-lived intangible asset, useful life		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
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
Minimum [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Contracts expiration days		1 day	1 day
Fair value discount rates		7.00%	7.00%
Finite-lived intangible asset, useful life		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
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
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		$ 68,000			(161,000)
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		360 days	360 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
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		$ 304,000			1,299,000		4,440,000
Accounting Standards Update 2014-09 [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Cumulative catch-up adjustment related to adoption of ASC 606	[1]								5,618,000
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										$ 465,000,000	$ 465,000,000
Foreign currency translation adjustments [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Accumulated other comprehensive loss		(31,284,000)			(22,824,000)
Retained Earnings [Member] | Accounting Standards Update 2014-09 [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Cumulative catch-up adjustment related to adoption of ASC 606	[1]								$ 5,618,000
Shipping and Handling [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Selling, general and administrative expenses		66,400,000			59,600,000		32,500,000
Advertising [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Selling, general and administrative expenses		$ 20,000,000			$ 21,000,000		$ 22,000,000
Forecast [Member] | Accounting Standards Update 2016-02 [Member] | Maximum [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Assets								$ 170,000,000
Liabilities								170,000
Forecast [Member] | Accounting Standards Update 2016-02 [Member] | Minimum [Member]
DESCRIPTION OF BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) [Line Items]
Assets								160,000,000
Liabilities								$ 160,000

[1] See Note 1 - Recently adopted accounting pronouncements and Note 2 - Revenue for additional information.