Summary of Quarterly Results of Operations (Unaudited) - Summary of Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 30,866	$ 33,136	$ 28,015	$ 22,305	$ 64,497	$ 47,207	$ 47,992	$ 37,618	$ 114,322	$ 197,314	$ 218,576
Gross profit	10,065	11,251	11,559	(28,332)	17,352	17,403	(24,953)	18,579
Operating income (loss)	(15,816)	12,381	6,965	36,235	50,980	6,256	69,528	13,590	39,765	140,354	23,942
Equity in earnings of unconsolidated ventures	7,026	1,764	2,747	6,362	2,251	3,637	188	47	17,899	6,123	16,008
Income (loss) before taxes	(12,298)	13,223	8,120	40,998	51,069	7,163	26,591	5,992	52,121	92,346	8,890
Income (loss) discontinued operations, net of taxes	7,191	37,193	1,229	418	563	(7,164)	(2,048)	(8,216)	46,031	(16,865)	(186,130)
Net income (loss) attributable to Forestar Group Inc.	$ (17,574)	$ 45,202	$ (2,579)	$ 25,205	$ 43,745	$ 9,665	$ 9,614	$ (4,376)	$ 50,254	$ 58,648	$ (213,047)
Net income (loss) per share -- basic
Continuing operations, basic (usd per share)	$ (0.59)	$ 0.19	$ (0.09)	$ 0.59	$ 1.03	$ 0.40	$ 0.28	$ 0.11	$ 0.10	$ 1.80	$ (0.79)
Discontinued operations, basic (usd per share)	0.17	0.88	0.03	0.01	0.01	(0.17)	(0.05)	(0.24)	1.09	(0.40)	(5.43)
Basic (usd per share)	(0.42)	1.07	(0.06)	0.60	1.04	0.23	0.23	(0.13)	1.19	1.40	(6.22)
Net income (loss) per share -- diluted
Continuing operations, diluted (usd per share)	(0.58)	0.19	(0.09)	0.58	1.02	0.40	0.28	0.09	0.10	1.78	(0.79)
Discontinued operations, diluted (usd per share)	0.18	0.87	0.03	0.01	0.01	(0.17)	(0.05)	(0.19)	1.09	(0.40)	(5.43)
Diluted (usd per share)	$ (0.40)	$ 1.06	$ (0.06)	$ 0.59	$ 1.03	$ 0.23	$ 0.23	$ (0.10)	$ 1.19	$ 1.38	$ (6.22)
Continuing Operations
Supplemental Oil And Gas Reserve Information [Line Items]
Impairment of oil and gas properties	$ 9,272	$ 0	$ 0	$ 37,900	$ 3,874	$ 7,627	$ 48,826	$ 0
Discontinued Operations
Supplemental Oil And Gas Reserve Information [Line Items]
Impairment of oil and gas properties	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 612	$ 0