Summary of Quarterly Results of Operations (Unaudited) (Detail) - USD ($)	3 Months Ended							9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenues	$ 32,158,000	$ 23,565,000	$ 22,599,000	$ 30,866,000	$ 33,136,000	$ 28,015,000	$ 22,305,000	$ 78,322,000	$ 83,456,000	$ 114,322,000	$ 197,314,000
Gross profit (loss)	8,926,000	13,581,000	6,340,000	8,278,000	10,763,000	10,911,000	(28,324,000)
Operating income (loss)	25,197,000	8,413,000	3,601,000	(15,816,000)	12,381,000	6,965,000	36,235,000	37,211,000	55,581,000	39,765,000	140,354,000
Equity in earnings of unconsolidated ventures	2,266,000	1,000,000	1,529,000	7,026,000	1,764,000	2,747,000	6,362,000	4,795,000	10,873,000	17,899,000	6,123,000
Income (loss) before taxes	29,272,000	9,570,000	4,600,000	(12,298,000)	13,223,000	8,120,000	40,998,000	44,660,000	62,453,000	52,121,000	92,346,000
Income (loss) discontinued operations, net of taxes				7,191,000	37,193,000	1,229,000	418,000	0	38,840,000	46,031,000	(16,865,000)
Net income (loss) attributable to Forestar Group Inc.	$ 54,859,000	$ 9,421,000	$ 4,534,000	$ (17,574,000)	$ 45,202,000	$ (2,579,000)	$ 25,205,000	$ 68,814,000	$ 67,828,000	$ 50,254,000	$ 58,648,000
Net income (loss) per share -- basic
Continuing operations, basic (usd per share)				$ (0.59)	$ 0.19	$ (0.09)	$ 0.59	$ 1.64	$ 0.69	$ 0.10	$ 1.80
Discontinued operations, basic (usd per share)				0.17	0.88	0.03	0.01	0.00	0.92	1.09	(0.40)
Basic (usd per share)	$ 1.31	$ 0.22	$ 0.11	(0.42)	1.07	(0.06)	0.60	1.64	1.61	1.19	1.40
Net income (loss) per share -- diluted
Continuing operations, diluted (usd per share)				(0.58)	0.19	(0.09)	0.58	1.64	0.68	0.10	1.78
Discontinued operations, diluted (usd per share)				0.18	0.87	0.03	0.01
Diluted (usd per share)	$ 1.31	$ 0.22	$ 0.11	$ (0.40)	$ 1.06	$ (0.06)	$ 0.59	$ 1.64	$ 1.59	$ 1.19	$ 1.38
Oil and Gas Properties [Member] | Continuing Operations | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Net income (loss) per share -- diluted
Goodwill, Impairment Loss							$ 37,900,000