NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details)											1 Months Ended							12 Months Ended
	Sep. 30, 2018 USD ($)	Apr. 16, 2018 USD ($) $ / shares	Oct. 02, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 AUD ($)	Jan. 17, 2017 USD ($) shares	Jul. 31, 2016 AUD ($) loan	Jul. 14, 2016 USD ($)	Mar. 13, 2015	Feb. 27, 2014 USD ($)	Mar. 31, 2019 AUD ($)	Jul. 31, 2018 GBP (£)	Sep. 30, 2017 AUD ($)	Aug. 31, 2016 USD ($)	Jul. 31, 2016 AUD ($) loan	Nov. 30, 2012 CAD ($)	Oct. 31, 2006 USD ($)	Sep. 30, 2019 USD ($) option $ / shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2019 GBP (£)	Sep. 30, 2019 AUD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 CAD ($)	Feb. 05, 2018 USD ($)	Dec. 18, 2017 USD ($)	Dec. 31, 2016 AUD ($)	Nov. 30, 2016 AUD ($)	Sep. 30, 2016 USD ($)	Jul. 20, 2016 USD ($)	May 18, 2016 USD ($)	Mar. 31, 2016 USD ($) property	Mar. 22, 2016 USD ($)	Mar. 21, 2016 USD ($)	Jun. 18, 2014 USD ($)	Dec. 21, 2009 USD ($)
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Minimum payments under capital leases for 2018																							$ 3,950,000
Minimum payments under capital leases for 2019																							2,153,000
Minimum payments under capital leases for 2020																							668,000
Minimum payments under capital leases for 2021																							157,000
Minimum payments under capital leases for 2022																							0
Minimum payments under capital leases thereafter																							0
Capital leased assets, gross	$ 41,742,000																		$ 41,742,000				41,742,000
Capital leases, lessee balance sheet, assets by major class, accumulated depreciation	31,969,000																		31,969,000				35,196,000
Deferred interest charges	80,000																		80,000				55,000
Amortization																		$ 3,967,000	3,514,000	$ 1,683,000
Proceeds from long-term debt																		$ 201,748,000	443,058,000	233,443,000
Payments to acquire buildings																	$ 10,000,000
Long-term debt, percentage bearing fixed interest, percentage rate																	5.00%
Minimum payments under debt agreements for 2018																							10,525,000
Minimum payments under debt agreements for 2019																							86,108,000
Minimum payment under debt engagement for 2020																							1,016,109,000
Minimum payments under debt agreements for 2021																							441,000
Minimum payments under debt agreements for 2022																							217,000
Minimum payments under debt agreements thereafter																							731,000
Debt instrument, face amount														$ 35,092,000
Capitalized fees & expenses	14,830,000																		14,830,000				10,724,000
Underwriting fees and other expense capitalized																				$ 13,329,000
Maximum percentage of equity interest of subsidiaries borrowings guaranteed									65.00%
Line of credit facility, amount outstanding																								$ 0
Debt instrument, basis spread on variable rate																1.30%
Outstanding debt	1,121,082,000																		1,121,082,000				1,104,274,000
Long-term debt, gross	$ 1,134,692,000																		$ 1,134,692,000				1,114,131,000
Amount of line note available to purchase common stock in open market														$ 10,908,000
Common stock, dividends, per share, cash paid (in Dollars per share) | $ / shares																		$ 0.29	$ 0.28	$ 0.24
Stock issued during period, shares, ESOP | shares																			621,875
Debt instrument, interest rate during period																			5.66%
Issuance of treasury stock in settlement of convertible debt																				$ 48,858,000
Term Loan [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid | £												£ 7,000,000
Term Loan [Member] | Northcote Holdings Pty. Ltd [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt Instrument, Number of Loans | loan							2								2
Term Loan [Member] | Ames UK [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount | £												14,000,000
Debt instrument, periodic payment, principal | £												£ 350,000
Revolver due 2019 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Line of credit facility, remaining borrowing capacity																							278,719,000
Revolving Credit Facility [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, basis spread on variable rate	2.25%											1.50%
Revolving Credit Facility [Member] | Ames UK [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount | £												£ 5,000,000
Term And Mortgage Loan July 2018 [Member] | Ames UK [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Outstanding debt																					£ 15,831,000		19,485,000
Letter Of Credit Subfacility [Member] | Revolver due 2019 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Line of credit facility, current borrowing capacity																																	$ 50,000,000
Line of credit facility, amount outstanding																							21,281,000
Multicurrency Subfacility [Member] | Revolver due 2019 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Line of credit facility, current borrowing capacity																																	100,000,000
Margin Rate [Member] | Revolver due 2019 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Line of credit facility, interest rate during period																		1.75%
LIBOR Rate [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Line of credit facility, interest rate during period																		2.75%
Convert. debt due 2017 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount																																				$ 100,000,000
Stated percentage																																				4.00%
ESOP Loans [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Stock issued during period, value, ESOP																			$ 10,908,000
ESOP, weighted average purchase price of shares purchased (in Dollars per share) | $ / shares																			$ 17.54
Revolving Facility, June 2017 [Member] | Revolving Credit Facility [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Line of credit facility, amount outstanding																						$ 10,000,000	$ 6,760,000
Maximum borrowing capacity							$ 10,000,000								$ 10,000,000												$ 20,000	$ 10,000,000
Receivables Purchase Facility [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, basis spread on variable rate															1.00%
Proceeds from long-term lines of credit (in Euro)											$ 10,000,000
Debt instrument, interest rate at period end																					2.01%	2.01%	2.01%	2.01%
Internal Loan March 2019 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Outstanding debt																							$ 32,418,000
Debt instrument, periodic payment, principal																		$ 569,000
Secured Debt [Member] | Term Loan 1 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Repayments of Debt		$ 5,705,000
Capital lease - real estate [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Proceeds from long-term debt																	$ 14,290,000
Capitalized fees & expenses	$ 80,000																		$ 80,000				55,000
Outstanding debt	7,423,000																		7,423,000				4,333,000
Long-term debt, gross	$ 7,503,000																		$ 7,503,000				$ 4,388,000
Debt instrument, interest rate during period																				5.50%
Debt instrument, interest rate at period end	5.00%																		5.00%		5.00%	5.00%	5.00%	5.00%
Senior notes [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount																									$ 275,000,000					$ 125,000,000
Senior notes [Member] | Level 1 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Notes payable, fair value disclosure																							$ 1,010,000,000
Senior notes [Member] | Reissued Senior Notes [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount																									$ 725,000,000
Senior notes [Member] | Senior Notes due 2022, May 2016 Add-On Issuance [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount			$ 275,000,000																												$ 125,000,000
Issuance price, percentage			101.00%																												98.76%
Senior notes [Member] | Senior note due 2022 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount										$ 600,000,000
Unamortized debt issuance expense																							9,175,000
Stated percentage										5.25%
Proceeds from issuance of debt			$ 275,000,000
Outstanding debt																							1,000,000,000
Senior notes [Member] | Senior note due 2022 [Member] | Senior notes due 2018 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount										$ 550,000,000
Stated percentage										7.125%
Payment of tender offer premium										$ 31,530,000
Debt instrument, periodic payment, interest										$ 16,716,000
Senior note due 2022 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Capitalized fees & expenses																							9,175,000			$ 8,472,000
Outstanding debt																							991,692,000
Long-term debt, gross																							$ 1,000,000,000
Debt instrument, interest rate during period																		5.66%		5.55%
Debt instrument, interest rate at period end																					5.25%	5.25%	5.25%	5.25%
Senior Notes Two Thousand Twenty Two [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount																																			$ 600,000,000
Stated percentage			5.25%							5.25%																					5.25%
Original debt, amount								$ 125,000,000
Convertible Notes 2017 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt Conversion, Converted Instrument, Amount						$ 173,855,000
Debt conversion, converted instrument, cash received						125,000,000
Issuance of treasury stock in settlement of convertible debt						$ 48,858,000
Converted instrument, shares issued | shares						1,954,993,000
Senior notes due 2018 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Capitalized fees & expenses	$ 12,968,000																		$ 12,968,000
Outstanding debt	988,252,000																		988,252,000
Long-term debt, gross	$ 1,000,000,000																		$ 1,000,000,000
Debt instrument, interest rate at period end	5.25%																		5.25%
Revolver due 2019 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Capitalized fees & expenses	$ 1,413,000																		$ 1,413,000				$ 1,243,000
Line of credit facility, current borrowing capacity																																	$ 350,000	$ 250,000,000
Outstanding debt	23,587,000																		23,587,000				48,757,000
Long-term debt, gross	25,000,000																		$ 25,000,000				50,000,000
Real estate mortgages [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount																													$ 32,280,000			$ 8,000,000
Number of secured properties | property																																4
Debt instrument, description of variable rate basis																		LIBOR plus 2.75%
Debt instrument, basis spread on variable rate																		1.50%
Debt instrument, interest rate during period																			3.30%	2.60%
ESOP Loans [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Capitalized fees & expenses	186,000																		$ 186,000
Debt instrument, description of variable rate basis																		The loan bears interest at a) LIBOR plus 2.38% or b) the lender’s prime rate, at Griffon’s option.
Outstanding debt	34,508,000																		34,508,000
Long-term debt, gross	34,694,000																		$ 34,694,000
Debt instrument, periodic payment, principal				$ 569,000
Debt instrument, interest rate during period																		6.30%	6.30%	4.20%
Revolver due 2013 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, description of variable rate basis																		The revolving credit facility accrues interest at EURIBOR plus 2.20% per annum.
Non U.S. term loans [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Capitalized fees & expenses	148,000																		$ 148,000				188,000
Outstanding debt	53,385,000																		53,385,000				36,789,000
Long-term debt, gross	53,533,000																		53,533,000				36,977,000
Non U.S. lines of credit [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Capitalized fees & expenses	16,000																		16,000				45,000
Line of credit facility, remaining borrowing capacity																							11,315,000	$ 15,000,000
Outstanding debt	7,935,000																		7,935,000				17,531,000
Long-term debt, gross	$ 7,951,000																		$ 7,951,000				$ 17,576,000
Proceeds from long-term lines of credit (in Euro)																$ 15,000,000		$ 11,315,000
Line of credit facility, interest rate description																		The facility accrues interest at LIBOR (USD) or the Bankers Acceptance Rate (CDN) plus 1.3% per annum.
Non U.S. lines of credit [Member] | LIBOR Rate [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Line of credit facility, interest rate at period end																					3.38%	3.38%	3.38%	3.38%
Non U.S. lines of credit [Member] | Bankers Acceptance Rate [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Line of credit facility, interest rate at period end																					3.13%	3.13%	3.13%	3.13%
Non U.S. lines of credit [Member] | Line of Credit One [Member] | Northcote Holdings Pty. Ltd [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Outstanding debt																						$ 16,000,000	$ 10,816,000
Medium-term Notes [Member] | Term Loan Due 2019 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount							$ 30,000,000								$ 30,000,000
Number of loans refinanced with new debt instrument | loan							2								2
Line of credit facility, amount outstanding																						$ 25,875,000	$ 17,492,000
Debt instrument, periodic payment, principal															$ 1,250,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid							$ 13,375,000								$ 13,375,000
Term Loan May 2014 [Member] | Northcote Holdings Pty. Ltd [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Maximum borrowing capacity increase					$ 5,000,000						$ 10,000,000		$ 15,000,000
Mortgages [Member] | Ames UK [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, face amount | £												4,000,000
Debt instrument, periodic payment, principal | £												83,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid | £												£ 2,333,000
London Interbank Offered Rate (LIBOR) [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, basis spread on variable rate	2.56%											1.80%
London Interbank Offered Rate (LIBOR) [Member] | Term Loan [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, basis spread on variable rate	3.01%											2.25%
London Interbank Offered Rate (LIBOR) [Member] | Internal Loan March 2019 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, basis spread on variable rate																		2.91%
London Interbank Offered Rate (LIBOR) [Member] | Secured Debt [Member] | Term Loan 1 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, basis spread on variable rate				2.91%
Bank Bill Swap Bid Rate [Member] | Revolving Facility, June 2017 [Member] | Revolving Credit Facility [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, basis spread on variable rate															1.80%
Line of credit facility, interest rate at period end																					2.81%	2.81%	2.81%	2.81%
Bank Bill Swap Bid Rate [Member] | Medium-term Notes [Member] | Term Loan Due 2019 [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Debt instrument, basis spread on variable rate							1.90%
Debt instrument, interest rate at period end																					2.85%	2.85%	2.85%	2.85%
Special Dividends [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Common stock, dividends, per share, cash paid (in Dollars per share) | $ / shares		$ 1.00
Troy, Ohio [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Long-term debt, percentage bearing fixed interest, percentage rate																					5.00%	5.00%	5.00%	5.00%
Ocala, Florida [Member]
NOTES PAYABLE, CAPITALIZED LEASES AND LONG-TERM DEBT (Details) [Line Items]
Long-term debt, percentage bearing fixed interest, percentage rate																					8.00%	8.00%	8.00%	8.00%
Lessee, Finance Lease, Number Of Option to Extend | option																		2
Lessee, Finance Lease, Renewal Term																					5 years	5 years	5 years	5 years