Consolidated Statements of Operations (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Income Statement [Abstract]
Revenues	$ 308.5	$ 312.9	$ 1,137.7	$ 907.1
Cost of sales	234.6	257.1	902.9	753.8
Selling, general and administrative expense	24.1	16.9	69.9	48.7
Equity in earnings of unconsolidated ventures	0.0	0.0	(1.1)	(0.2)
Gain on sale of assets	(2.7)	0.0	(3.2)	0.0
Interest and other income	(0.2)	(0.3)	(0.2)	(1.2)
Loss on extinguishment of debt	0.0	18.1	0.0	18.1
Income before income taxes	52.7	21.1	169.4	87.9
Income tax expense	13.0	5.2	41.4	21.5
Net income	39.7	15.9	128.0	66.4
Net income attributable to noncontrolling interests	0.0	0.1	0.0	0.2
Net income attributable to Forestar Group Inc.	$ 39.7	$ 15.8	$ 128.0	$ 66.2
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted average number of common shares	49,879,394	49,419,594	49,794,566	48,653,676
Adjusted weighted average number of common shares	49,893,640	49,509,079	49,834,891	48,737,299
Earnings Per Share [Abstract]
Basic net income per common share attributable to Forestar Group Inc.	$ 0.80	$ 0.32	$ 2.57	$ 1.36
Diluted net income per common share attributable to Forestar Group Inc.	$ 0.80	$ 0.32	$ 2.57	$ 1.36