CONSOLIDATED STATEMENTS OF OPERATIONS - 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
Income Statement [Abstract]
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
Income tax expense (benefit)	(7.5)	(0.2)	(3.3)	(5.4)	(3.4)	(1.5)	(3.6)	(1.0)	25.3	(33.4)	(16.4)	(9.4)
Net income	24.5	10.1	10.4	16.8	12.6	6.9	12.8	3.9	70.0	67.9	61.7	36.3
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
Net income attributable to Forestar Group Inc.	$ 24.2	$ 10.1	$ 9.6	$ 16.9	$ 12.7	$ 6.9	$ 10.1	$ 3.3	$ 68.8	$ 67.8	$ 60.8	$ 33.0
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted average number of common shares									41,938,987	42,200,000	48,037,018	41,974,429
Adjusted weighted average number of common shares									41,969,056	42,500,000	48,094,111	42,005,141
Earnings Per Share, Basic [Abstract]
Basic net income per common share attributable to Forestar Group Inc.	$ 0.50	$ 0.21	$ 0.20	$ 0.35	$ 0.30	$ 0.16	$ 0.24	$ 0.08	$ 1.64	$ 1.61	$ 1.26	$ 0.79
Earnings Per Share, Diluted [Abstract]
Diluted net income per common share attributable to Forestar Group Inc.	$ 0.50	$ 0.21	$ 0.20	$ 0.35	$ 0.30	$ 0.16	$ 0.24	$ 0.08	$ 1.64	$ 1.59	$ 1.26	$ 0.79