Capitalized Interest (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2023	Sep. 30, 2022	Mar. 31, 2022	Sep. 30, 2021
Statement [Line Items]
Real Estate Inventory, Capitalized Interest Costs	$ 60.5	$ 51.7	$ 60.5	$ 51.7	$ 58.8	$ 52.5	$ 50.6	$ 53.7
Interest incurred	8.2	8.3	24.6	24.7
Interest charged to cost of sales	$ (6.5)	$ (7.2)	$ (16.6)	$ (26.7)

Capitalized Interest

No definition available.

Name:	for_CapitalizedInterestLineItems
Namespace Prefix:	for_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Total interest costs incurred during the period and either capitalized or charged against earnings.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483013/835-20-50-1

+ Details

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

- Definition

Carrying amount as of the balance sheet date of interest costs that were capitalized to properties under development during the land development and construction period of a project, and which are included in inventory.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483013/835-20-50-1

+ Details

Name:	us-gaap_RealEstateInventoryCapitalizedInterestCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of interest cost charged to cost of sales that had been previously capitalized to inventory.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-SubTopic 20
-Topic 835
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483013/835-20-50-1

+ Details

Name:	us-gaap_RealEstateInventoryCapitalizedInterestCostsCostOfSales1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration