CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenues	$ 1,325.8	$ 931.8	$ 428.3
Cost of sales	1,096.6	813.7	362.7
Selling, general and administrative expense	68.4	45.7	28.9
Equity in earnings of unconsolidated ventures	(0.2)	(0.7)	(0.5)
Gain on sale of assets	(2.5)	(0.1)	(3.0)
Interest and other income	1.2	4.9	5.5
Loss on extinguishment of debt	18.1	0.0	0.0
Income before income taxes	146.6	78.1	45.7
Income tax expense	(36.1)	(16.4)	(9.4)
Net income	110.5	61.7	36.3
Net income attributable to noncontrolling interests	0.3	0.9	3.3
Net income attributable to Forestar Group Inc.	$ 110.2	$ 60.8	$ 33.0
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted average number of common shares	48,901,987	48,037,018	41,974,429
Adjusted weighted average number of common shares	48,975,661	48,094,111	42,005,141
Earnings Per Share, Basic [Abstract]
Basic net income per common share attributable to Forestar Group Inc.	$ 2.25	$ 1.26	$ 0.79
Earnings Per Share, Diluted [Abstract]
Diluted net income per common share attributable to Forestar Group Inc.	$ 2.25	$ 1.26	$ 0.79