Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income attributable to Forestar Group Inc.	$ 178.8	$ 110.2	$ 60.8
Weighted average number of common shares	49,818,132	48,901,987	48,037,018
Dilutive effect of stock-based compensation	31,762	73,674	57,093
Adjusted weighted average number of common shares	49,849,894	48,975,661	48,094,111
Basic net income per common share attributable to Forestar Group Inc.	$ 3.59	$ 2.25	$ 1.26
Diluted net income per common share attributable to Forestar Group Inc.	$ 3.59	$ 2.25	$ 1.26