Segment Information - Segment Revenues and Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 88.2	$ 23.6	$ 192.0	$ 77.0
Cost of sales	75.3	10.0	149.6	48.9
Selling, general and administrative expense	7.9	6.5	19.8	36.1
Equity in earnings of unconsolidated ventures	0.0	(1.0)	(0.5)	(9.6)
Gain (Loss) on Disposition of Assets	1.5	1.3	2.4	4.1
Interest expense	0.0	1.6	0.0	5.8
Interest and other income	(1.9)	(2.6)	(4.1)	(4.8)
Income (loss) from continuing operations before taxes	8.4	10.4	29.6	4.7
Net income attributable to noncontrolling interests	$ 0.0	0.9	$ 3.3	2.9
Income (loss) from continuing operations before taxes attributable to Forestar Group Inc.		9.5		1.8
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues		0.0		0.0
Cost of sales		0.0		0.0
Selling, general and administrative expense		3.6		25.6
Equity in earnings of unconsolidated ventures		0.0		0.0
Gain (Loss) on Disposition of Assets		0.0		0.0
Interest expense		1.6		5.8
Interest and other income		(1.6)		(3.0)
Income (loss) from continuing operations before taxes		(3.6)		(28.4)
Net income attributable to noncontrolling interests		0.0		0.0
Income (loss) from continuing operations before taxes attributable to Forestar Group Inc.		(3.6)		(28.4)
Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues		23.6		77.0
Cost of sales		9.9		42.4
Selling, general and administrative expense		2.9		10.3
Equity in earnings of unconsolidated ventures		(1.0)		(9.6)
Gain (Loss) on Disposition of Assets		1.3		4.1
Interest expense		0.0		0.0
Interest and other income		(1.0)		(1.8)
Income (loss) from continuing operations before taxes		14.1		39.8
Net income attributable to noncontrolling interests		0.9		2.9
Income (loss) from continuing operations before taxes attributable to Forestar Group Inc.		13.2		36.9
Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues		0.0		0.0
Cost of sales		0.1		6.5
Selling, general and administrative expense		0.0		0.2
Equity in earnings of unconsolidated ventures		0.0		0.0
Gain (Loss) on Disposition of Assets		0.0		0.0
Interest expense		0.0		0.0
Interest and other income		0.0		0.0
Income (loss) from continuing operations before taxes		(0.1)		(6.7)
Net income attributable to noncontrolling interests		0.0		0.0
Income (loss) from continuing operations before taxes attributable to Forestar Group Inc.		$ (0.1)		$ (6.7)