Segment Information - Segment Revenues and Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 38,500	$ 30,900
Cost of sales	30,700	22,600
Selling, general and administrative expense	5,700	24,100
Equity in earnings of unconsolidated ventures	(600)	(7,000)
Interest expense	0	2,100
Interest and other income	(1,300)	(600)
Income (loss) from continuing operations before taxes	4,900	(10,300)
Noncontrolling interest	$ 600	2,000
Income (loss) from continuing operations before taxes attributable to Forestar Group Inc.		(12,300)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues		0
Cost of sales		0
Selling, general and administrative expense		18,200
Equity in earnings of unconsolidated ventures		0
Interest expense		2,100
Interest and other income		0
Income (loss) from continuing operations before taxes		(20,300)
Noncontrolling interest		0
Income (loss) from continuing operations before taxes attributable to Forestar Group Inc.		(20,300)
Real Estate [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues		30,900
Cost of sales		16,600
Selling, general and administrative expense		5,900
Equity in earnings of unconsolidated ventures		(7,000)
Interest expense		0
Interest and other income		(600)
Income (loss) from continuing operations before taxes		16,000
Noncontrolling interest		(2,000)
Income (loss) from continuing operations before taxes attributable to Forestar Group Inc.		14,000
Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues		0
Cost of sales		6,000
Selling, general and administrative expense		0
Equity in earnings of unconsolidated ventures		0
Interest expense		0
Interest and other income		0
Income (loss) from continuing operations before taxes		(6,000)
Noncontrolling interest		0
Income (loss) from continuing operations before taxes attributable to Forestar Group Inc.		$ (6,000)