Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Continuing operations
Net income (loss) attributable to Forestar Group Inc.	$ 47.8	$ 28.4	$ 88.3	$ 50.4
Denominator:
Weighted average common shares outstanding — basic	49,820,937	48,412,213	49,752,153	48,270,718
Dilutive effect of stock-based compensation	57,229	97,711	53,364	80,692
Total weighted average shares outstanding — diluted	49,878,166	48,509,924	49,805,517	48,351,410
Basic net income per common share attributable to Forestar Group Inc.	$ 0.96	$ 0.59	$ 1.77	$ 1.04
Diluted net income per common share attributable to Forestar Group Inc.	$ 0.96	$ 0.59	$ 1.77	$ 1.04