Consolidated Statements of Operations (Unaudited) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Revenues	$ 287.1	$ 159.1	$ 594.2	$ 406.4
Cost of sales	233.8	136.6	496.7	353.2
Selling, general and administrative expense	16.3	11.2	31.8	21.7
Equity in earnings of unconsolidated ventures	(0.1)	(0.3)	(0.3)	(0.8)
Gain on sale of assets	0.0	(0.3)	0.0	(0.1)
Interest and other income	(0.5)	(1.8)	(0.8)	(3.4)
Income before income taxes	37.6	13.7	66.8	35.8
Income tax expense	9.2	3.3	16.3	8.7
Net income	28.4	10.4	50.5	27.1
Net income attributable to noncontrolling interests	0.0	0.8	0.1	0.7
Net income attributable to Forestar Group Inc.	$ 28.4	$ 9.6	$ 50.4	$ 26.4
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Basic (in shares)	48,412,213	48,025,359	48,270,718	48,018,246
Diluted (in shares)	48,509,924	48,082,951	48,351,410	48,074,685
Earnings Per Share [Abstract]
Earnings Per Share, Basic	$ 0.59	$ 0.20	$ 1.04	$ 0.55
NET INCOME (LOSS) PER DILUTED SHARE
Earnings Per Share, Diluted	$ 0.59	$ 0.20	$ 1.04	$ 0.55

The aggregate costs related to goods produced and sold and services rendered by an entity during the reporting period. This excludes costs incurred during the reporting period related to financial services rendered and other revenue generating activities.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 924
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-URI http://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(a),(d))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227