Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Continuing operations
Net income (loss) attributable to Forestar Group Inc.	$ 22.0	$ 16.9
Denominator:
Weighted average common shares outstanding — basic	48,129,222	48,011,132
Dilutive effect of stock-based compensation	63,673	55,286
Total weighted average shares outstanding — diluted	48,192,895	48,066,418
Earnings Per Share, Basic	$ 0.46	$ 0.35
Earnings Per Share, Diluted	$ 0.46	$ 0.35