| Cover | Apr. 20, 2021 |
| Document Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Apr. 20, 2021 |
| Entity Registrant Name | Forestar Group Inc. |
| Entity Incorporation, State or Country Code | DE |
| 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 File Number | 001-33662 |
| Entity Tax Identification Number | 26-1336998 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $1.00 per share |
| Trading Symbol | FOR |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001406587 |
| Amendment Flag | false |
