Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Continuing operations
Net income (loss) attributable to Forestar Group Inc.	$ 39.7	$ 15.8	$ 128.0	$ 66.2
Denominator:
Weighted average common shares outstanding — basic	49,879,394	49,419,594	49,794,566	48,653,676
Dilutive effect of stock-based compensation	14,246	89,485	40,325	83,623
Total weighted average shares outstanding — diluted	49,893,640	49,509,079	49,834,891	48,737,299
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