Consolidated Statements of Operations (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenues	$ 177.9	$ 88.2	$ 584.3	$ 192.0
Cost of sales	157.1	75.3	510.3	149.6
Selling, general and administrative expense	11.2	7.9	32.8	19.8
Equity in loss (earnings) of unconsolidated ventures	0.1	0.0	(0.6)	(0.5)
Gain on sale of assets	0.0	(1.5)	(0.1)	(2.4)
Interest and other income	(0.8)	(1.9)	(4.2)	(4.1)
Income before income taxes	10.3	8.4	46.1	29.6
Income tax expense	0.2	1.5	8.9	6.0
Net income	10.1	6.9	37.2	23.6
Net income attributable to noncontrolling interests	0.0	0.0	0.7	3.3
Net income attributable to Forestar Group Inc.	$ 10.1	$ 6.9	$ 36.5	$ 20.3
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Basic (in shares)	48,050,379	41,959,866	48,028,957	41,957,408
Diluted (in shares)	48,075,210	42,010,221	48,074,860	41,978,339
Earnings Per Share [Abstract]
Earnings Per Share, Basic	$ 0.21	$ 0.16	$ 0.76	$ 0.48
NET INCOME (LOSS) PER DILUTED SHARE
Earnings Per Share, Diluted	$ 0.21	$ 0.16	$ 0.76	$ 0.48