Net Income (Loss) per Share - Earnings Attributable to Common Shareholders and Weighted Average Common Shares Outstanding (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Continuing operations
Net income (loss) from continuing operations	$ 3,900	$ (22,800)
Less: Net income attributable to noncontrolling interests	600	2,000
Net income (loss) attributable to Forestar Group Inc.	3,300	(24,800)
Discontinued operations
Income from discontinued operations, net of taxes	$ 0	$ 7,200
Denominator:
Weighted average common shares outstanding — basic	41,952,493	41,957,424
Dilutive effect of share based compensation	6,206	0
Total weighted average shares outstanding — diluted	41,958,699	41,957,424
Anti-dilutive awards excluded from diluted weighted average shares (in shares)	0	30,868