Segment Information - Segment Revenues and Earnings (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]
Gain of sale of assets											$ 27,756,000		$ 113,411,000	$ 113,411,000		$ 166,747,000
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
Gain (Loss) on Sale of Properties											27,756,000			113,411,000		166,747,000
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 from continuing operations attributable to Forestar Group Inc.											43,442,000			50,043,000		90,815,000
Total assets		893,146,000	[1]			761,912,000	[1]				893,146,000	[1]		761,912,000	[1]	733,194,000
Investment in unconsolidated ventures		11,721,000				64,579,000					11,721,000			64,579,000		77,611,000
Assets of discontinued operations																14,000
Operating Segments | Real Estate
Segment Reporting Information [Line Items]
Revenues											78,298,000			112,746,000		190,273,000
Gain (Loss) on Sale of Properties											18,638,000			1,915,000		117,856,000
Equity in earnings of unconsolidated ventures											4,795,000			16,500,000		5,778,000
Income (loss) before taxes from continuing operations attributable to Forestar Group Inc.											46,252,000			47,281,000		121,420,000
Total assets		522,083,000	[1]			386,222,000	[1]				522,083,000	[1]		386,222,000	[1]	403,062,000
Investment in unconsolidated ventures		11,721,000				64,579,000					11,721,000			64,579,000		77,611,000
Operating Segments | Oil and Gas Properties [Member]
Segment Reporting Information [Line Items]
Revenues											0			1,502,000		5,076,000
Gain (Loss) on Sale of Properties											0			82,422,000		0
Equity in earnings of unconsolidated ventures											0			1,395,000		173,000
Income (loss) before taxes from continuing operations attributable to Forestar Group Inc.											0			45,552,000		3,327,000
Total assets		0	[1]			0	[1]				0	[1]		0	[1]	38,907,000
Investment in unconsolidated ventures		0				0					0			0		0
Operating Segments | Other Segments [Member]
Segment Reporting Information [Line Items]
Revenues											24,000			74,000		1,965,000
Gain (Loss) on Sale of Properties											9,118,000			400,000		0
Equity in earnings of unconsolidated ventures											0			4,000		172,000
Income (loss) before taxes from continuing operations attributable to Forestar Group Inc.											8,262,000			(6,393,000)		(4,625,000)
Total assets		1,943,000	[1]			3,346,000	[1]				1,943,000	[1]		3,346,000	[1]	11,531,000
Investment in unconsolidated ventures		0				0					0			0		0
Items Not Allocated To Segments
Segment Reporting Information [Line Items]
Revenues											0			0		0
Gain (Loss) on Sale of Properties											0			28,674,000		48,891,000
Equity in earnings of unconsolidated ventures											0			0		0
Income (loss) before taxes from continuing operations attributable to Forestar Group Inc.	[2]										(11,072,000)			(36,397,000)		(29,307,000)
Total assets		369,120,000	[1]			372,344,000	[1]				369,120,000	[1]		372,344,000	[1]	279,694,000
Investment in unconsolidated ventures		$ 0				$ 0					$ 0			$ 0		$ 0
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Oil and Gas Properties [Member]
Segment Reporting Information [Line Items]
Purchase price of properties sold										85,700,000
Gain of sale of assets										82,422,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Oil and Gas Properties [Member] | Continuing Operations
Segment Reporting Information [Line Items]
Goodwill, Impairment Loss										$ 37,900,000

[1] Total assets excludes assets of discontinued operations of $14,000 in 2016.

[2] Items not allocated to segments consist of: For the Nine Months Ended September 30, 2018 For the Twelve Months Ended December 31, 2017 2016 (In thousands)General and administrative expense$(11,443) $(50,354) $(18,274)Share-based and long-term incentive compensation expense(512) (7,201) (4,425)Gain on sale of assets— 28,674 48,891Interest expense(3,741) (8,532) (19,985)Loss on extinguishment of debt, net— (611) (35,864)Interest and other income4,624 1,627 350 $(11,072) $(36,397) $(29,307)