Summary of Quarterly Results of Operations (Unaudited) (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total 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
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	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
Net income (loss) per share -- basic
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
Net income (loss) per share -- diluted
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