Document and Entity Information - shares	6 Months Ended
	Mar. 31, 2020	Apr. 22, 2020
Entity Information [Line Items]
Entity Current Reporting Status	Yes
Document Type	10-Q
Document Period End Date	Mar. 31, 2020
Amendment Flag	false
Document Fiscal Period Focus	Q2
Document Fiscal Year Focus	2020
Trading Symbol	FOR
Title of 12(b) Security	Common Stock, par value $1.00 per share
Entity Registrant Name	FORESTAR GROUP INC.
Entity Central Index Key	0001406587
Current Fiscal Year End Date	--09-30
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		48,036,414
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Small Business	false
Document Quarterly Report	true
Document Transition Report	false
Entity Address, Address Line One	2221 E. Lamar Blvd., 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
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1336998
Entity File Number	001-33662
Security Exchange Name	NYSE
Entity Interactive Data Current	Yes