Net Income (Loss) per Share - Earnings Attributable to Common Shareholders and Weighted Average Common Shares Outstanding (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018
Continuing operations
Net income (loss) from continuing operations	$ 6.9			$ 10.3			$ 23.6	$ (7.9)
Net income	6.9	$ 12.8	$ 3.9	10.3	$ 4.6	$ (15.6)	23.6	(0.7)
Less: Net income attributable to noncontrolling interests	0.0			0.9			3.3	2.9
Net income (loss) attributable to Forestar Group Inc.	6.9			9.4			20.3	(10.8)
Discontinued operations
Income from discontinued operations, net of taxes	$ 0.0			$ 0.0			$ 0.0	$ 7.2
Denominator:
Weighted average common shares outstanding — basic	41,959,866			41,938,936			41,957,408	41,945,099
Dilutive effect of share based compensation	50,355			27,377			20,931	0
Total weighted average shares outstanding — diluted	42,010,221			41,966,313			41,978,339	41,945,099
Anti-dilutive awards excluded from diluted weighted average shares (in shares)	0			0			0	28,558