BUSINESS SEGMENTS - Depreciation, Amortization and Capital Expenditures (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
DEPRECIATION and AMORTIZATION	$ 15,523	$ 15,595	$ 47,067	$ 46,172
CAPITAL EXPENDITURES	12,232	10,376	34,751	27,794
Operating Segments
Segment Reporting Information [Line Items]
DEPRECIATION and AMORTIZATION	15,370	15,453	46,611	45,757
CAPITAL EXPENDITURES	12,207	10,339	34,444	27,547
Corporate
Segment Reporting Information [Line Items]
DEPRECIATION and AMORTIZATION	153	142	456	415
CAPITAL EXPENDITURES	25	37	307	247
Consumer and Professional Products \| Operating Segments
Segment Reporting Information [Line Items]
DEPRECIATION and AMORTIZATION	8,197	8,158	24,650	24,148
CAPITAL EXPENDITURES	7,029	3,187	14,561	11,327
Home and Building Products \| Operating Segments
Segment Reporting Information [Line Items]
DEPRECIATION and AMORTIZATION	4,507	4,626	13,975	13,683
CAPITAL EXPENDITURES	3,640	5,088	15,135	10,423
Defense Electronics \| Operating Segments
Segment Reporting Information [Line Items]
DEPRECIATION and AMORTIZATION	2,666	2,669	7,986	7,926
CAPITAL EXPENDITURES	$ 1,538	$ 2,064	$ 4,748	$ 5,797

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

+ Details

Name:	us-gaap_DepreciationDepletionAndAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3213-108585

+ Details

Name:	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_SegmentReportingInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_CorporateNonSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=gff_ConsumerAndProfessionalProductsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=gff_HomeAndBuildingProductsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=gff_DefenseElectronicsSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: