Segment Information - Segment Revenues and Earnings (Detail) - USD ($) $ in Millions	3 Months Ended								9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ 347.6	$ 177.9	$ 159.1	$ 247.2	$ 236.3	$ 88.2	$ 65.3	$ 38.5	$ 78.3	$ 83.5	$ 931.8	$ 428.3
Cost of sales									49.5	90.1	813.7	362.7
Selling, general and administrative expense									19.4	51.2	45.7	28.9
Equity in earnings of unconsolidated ventures									(4.8)	(10.9)	(0.7)	(0.5)
Gain on sale of assets									(27.8)	(113.4)	(0.1)	(3.0)
Interest expense									3.7	6.4	0.0	0.0
Interest and other income									(6.4)	(2.4)	(4.9)	(5.5)
Income before income taxes	32.0	10.3	13.7	22.2	16.0	8.4	16.4	4.9	44.7	62.5	78.1	45.7
Net income attributable to noncontrolling interests	$ 0.3	$ 0.0	$ 0.8	$ (0.1)	$ (0.1)	$ 0.0	$ 2.7	$ 0.6	1.2	$ 0.1	0.9	3.3
Income before income taxes attributable to Forestar Group Inc.									43.5
Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Segment Reporting Information [Line Items]
Revenues									22.2		$ 3.5	$ 1.9
FMF Littleton [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Segment Reporting Information [Line Items]
Gain on sale of assets									(14.6)
Operating Segments [Member] | Real Estate
Segment Reporting Information [Line Items]
Revenues									78.3
Cost of sales									48.9
Selling, general and administrative expense									7.1
Equity in earnings of unconsolidated ventures									(4.8)
Gain on sale of assets									(18.6)
Interest expense									0.0
Interest and other income									(1.8)
Income before income taxes									47.5
Net income attributable to noncontrolling interests									1.2
Income before income taxes attributable to Forestar Group Inc.									46.3
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues									0.0
Cost of sales									0.6
Selling, general and administrative expense									0.3
Equity in earnings of unconsolidated ventures									0.0
Gain on sale of assets									(9.2)
Interest expense									0.0
Interest and other income									0.0
Income before income taxes									8.3
Net income attributable to noncontrolling interests									0.0
Income before income taxes attributable to Forestar Group Inc.									8.3
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues									0.0
Cost of sales									0.0
Selling, general and administrative expense									12.0
Equity in earnings of unconsolidated ventures									0.0
Gain on sale of assets									0.0
Interest expense									3.7
Interest and other income									(4.6)
Income before income taxes									(11.1)
Net income attributable to noncontrolling interests									0.0
Income before income taxes attributable to Forestar Group Inc.									$ (11.1)