Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Nov. 12, 2020	Mar. 31, 2020
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Sep. 30, 2020
Current Fiscal Year End Date	--09-30
Document Transition Report	false
Entity File Number	001-33662
Entity Registrant Name	Forestar Group Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1336998
Entity Address, Address Line One	2221 E. Lamar Blvd.
Entity Address, Address Line Two	Suite 790
Entity Address, City or Town	Arlington
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	76006
City Area Code	817
Local Phone Number	769-1860
Title of 12(b) Security	Common Stock, par value $1.00 per share
Trading Symbol	FOR
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 190
Entity Common Stock, Shares Outstanding		48,070,347
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001406587