CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Revenues	$ 1,519.1	$ 1,325.8	$ 931.8
Cost of sales	1,195.1	1,096.6	813.7
Selling, general and administrative expense	93.6	68.4	45.7
Equity in earnings of unconsolidated ventures	(1.2)	(0.2)	(0.7)
Gain on sale of assets	(3.2)	(2.5)	(0.1)
Interest and other income	1.0	1.2	4.9
Loss on extinguishment of debt	0.0	18.1	0.0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest, Total	235.8	146.6	78.1
Income tax expense	(57.0)	(36.1)	(16.4)
Net income	178.8	110.5	61.7
Net income attributable to noncontrolling interests	0.0	0.3	0.9
Net income attributable to Forestar Group Inc.	$ 178.8	$ 110.2	$ 60.8
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted average number of common shares	49,818,132	48,901,987	48,037,018
Adjusted weighted average number of common shares	49,849,894	48,975,661	48,094,111
Earnings Per Share, Basic [Abstract]
Basic net income per common share attributable to Forestar Group Inc.	$ 3.59	$ 2.25	$ 1.26
Earnings Per Share, Diluted [Abstract]
Diluted net income per common share attributable to Forestar Group Inc.	$ 3.59	$ 2.25	$ 1.26