Consolidated Statements of Operations (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Revenues	$ 307.1	$ 247.2
Cost of sales	262.9	216.6
Selling, general and administrative expense	15.5	10.5
Equity in earnings of unconsolidated ventures	(0.2)	(0.5)
Loss on sale of assets	0.0	0.1
Interest and other income	(0.3)	(1.7)
Income before income taxes	29.2	22.2
Income tax expense	7.1	5.4
Net income	22.1	16.8
Net income (loss) attributable to noncontrolling interests	0.1	(0.1)
Net income attributable to Forestar Group Inc.	$ 22.0	$ 16.9
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Basic (in shares)	48,129,222	48,011,132
Diluted (in shares)	48,192,895	48,066,418
Earnings Per Share [Abstract]
Earnings Per Share, Basic	$ 0.46	$ 0.35
NET INCOME (LOSS) PER DILUTED SHARE
Earnings Per Share, Diluted	$ 0.46	$ 0.35