Transition Period Comparative Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended							9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2019	Dec. 31, 2017
Transition Period Comparative Data [Abstract]
Total revenues	$ 236.3	$ 88.2	$ 65.3	$ 38.5	$ 32.2	$ 23.6	$ 22.6	$ 78.3	$ 83.5	$ 428.3	$ 114.3
Cost of Goods and Services Sold								49.5	90.1	362.7	112.6
Selling, General and Administrative Expense								19.4	51.2	28.9	75.3
Equity in earnings of unconsolidated ventures								(4.8)	(10.9)	(0.5)	(17.8)
Gain of sale of assets								(27.8)	(113.4)	(3.0)	(113.4)
Interest expense								3.7	6.4	0.0	8.5
Other Nonoperating Income								6.4	2.4	5.5	3.6
Income (loss) before taxes	16.0	8.4	16.4	4.9	29.6	10.4	4.7	44.7	62.5	45.7	52.1
Income tax expense (benefit)	(3.4)	(1.5)	(3.6)	(1.0)	25.6	(0.1)	(0.1)	25.3	(33.4)	(9.4)	(45.8)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest								70.0	29.1	36.3	6.3
Income (loss) discontinued operations, net of taxes								0.0	38.8	0.0	46.0
Consolidated net income	12.6	6.9	12.8	3.9	55.2	10.3	4.6	70.0	67.9	36.3	52.3
Net Income (Loss) Attributable to Noncontrolling Interest	(0.1)	0.0	2.7	0.6	0.3	0.9	0.1	1.2	0.1	3.3	2.0
Net income (loss) attributable to Forestar Group Inc.	$ 12.7	$ 6.9	$ 10.1	$ 3.3	$ 54.9	$ 9.4	$ 4.5	$ 68.8	$ 67.8	$ 33.0	$ 50.3
Basic (shares)								41,938,987	42,200,000	41,974,429	42,142,690
Diluted (shares)								41,969,056	42,500,000	42,005,141	42,381,023
Continuing operations, basic (usd per share)								$ 1.64	$ 0.69	$ 0.79	$ 0.10
Discontinued operations, basic (usd per share)								0.00	0.92	0.00	1.09
Basic (usd per share)	$ 0.30	$ 0.16	$ 0.24	$ 0.08	$ 1.31	$ 0.22	$ 0.11	1.64	1.61	0.79	1.19
Continuing operations, diluted (usd per share)								1.64	0.68	0.79	0.10
Discontinued operations, diluted (usd per share)								0.00	0.91	0.00	1.09
Diluted (usd per share)	$ 0.30	$ 0.16	$ 0.24	$ 0.08	$ 1.31	$ 0.22	$ 0.11	$ 1.64	$ 1.59	$ 0.79	$ 1.19