Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income attributable to Forestar Group Inc.	$ 26.9	$ 47.8	$ 47.7	$ 88.3
Weighted average number of common shares	49,943,373	49,820,937	49,916,927	49,752,153
Dilutive effect of stock-based compensation	67,166	57,229	37,854	53,364
Adjusted weighted average number of common shares	50,010,539	49,878,166	49,954,781	49,805,517
Basic net income per common share attributable to Forestar Group Inc.	$ 0.54	$ 0.96	$ 0.95	$ 1.77
Diluted net income per common share attributable to Forestar Group Inc.	$ 0.54	$ 0.96	$ 0.95	$ 1.77