Consolidated Statements of Operations (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Revenues	$ 159.1	$ 65.3	$ 406.4	$ 103.8
Cost of sales	136.6	43.6	353.2	74.3
Selling, general and administrative expense	11.2	6.2	21.7	11.9
Equity in earnings of unconsolidated ventures	(0.3)	0.0	(0.8)	(0.6)
Gain on sale of assets	(0.3)	0.0	(0.1)	(0.9)
Interest and other income	(1.8)	(0.9)	(3.4)	(2.2)
Income before income taxes	13.7	16.4	35.8	21.3
Income tax expense	3.3	3.6	8.7	4.6
Net income	10.4	12.8	27.1	16.7
Net income attributable to noncontrolling interests	0.8	2.7	0.7	3.3
Net income attributable to Forestar Group Inc.	$ 9.6	$ 10.1	$ 26.4	$ 13.4
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Basic (in shares)	48,025,359	41,959,866	48,018,246	41,956,179
Diluted (in shares)	48,082,951	41,966,099	48,074,685	41,962,399
Earnings Per Share [Abstract]
Earnings Per Share, Basic	$ 0.20	$ 0.24	$ 0.55	$ 0.32
NET INCOME (LOSS) PER DILUTED SHARE
Earnings Per Share, Diluted	$ 0.20	$ 0.24	$ 0.55	$ 0.32