Consolidated Statements of Operations (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Income Statement [Abstract]
Revenues	$ 407.6	$ 307.1
Cost of sales	334.2	262.9
Selling, general and administrative expense	21.5	15.5
Equity in earnings of unconsolidated ventures	(1.1)	(0.2)
Gain on sale of assets	(0.5)	0.0
Interest and other income	0.0	(0.3)
Income before income taxes	53.5	29.2
Income tax expense	13.0	7.1
Net income	40.5	22.1
Net income attributable to noncontrolling interests	0.0	0.1
Net income attributable to Forestar Group Inc.	$ 40.5	$ 22.0
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted average number of common shares	49,683,369	48,129,222
Adjusted weighted average number of common shares	49,732,869	48,192,895
Earnings Per Share [Abstract]
Basic net income per common share attributable to Forestar Group Inc.	$ 0.81	$ 0.46
Diluted net income per common share attributable to Forestar Group Inc.	$ 0.81	$ 0.46