Net Income (Loss) per Share - Earnings Attributable to Common Shareholders and Weighted Average Common Shares Outstanding (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Continuing operations
Net income (loss) attributable to Forestar Group Inc.	$ 16.9	$ 3.3
Denominator:
Weighted average common shares outstanding — basic	48,011,132	41,952,493
Dilutive effect of share based compensation	55,286	6,206
Total weighted average shares outstanding — diluted	48,066,418	41,958,699
Anti-dilutive awards excluded from diluted weighted average shares (in shares)	0	0
Earnings Per Share, Basic	$ 0.35	$ 0.08
Earnings Per Share, Diluted	$ 0.35	$ 0.08