CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ 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
Income Statement [Abstract]
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 ON SALE OF ASSETS								(27.8)	(113.4)	(3.0)	(113.4)
Interest expense								(3.7)	(6.4)	0.0	(8.5)
Loss on extinguishment of debt								0.0		0.0	0.6
Interest and other income								(6.4)	(2.4)	(5.5)	(3.6)
INCOME FROM CONTINUING OPERATIONS 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)
NET INCOME FROM CONTINUING OPERATIONS								70.0	29.1	36.3	6.3
Income (loss) discontinued operations, net of taxes								0.0	38.8	0.0	46.0
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 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
Weighted Average Common Shares Outstanding:
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
Net Income per Basic Share:
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
Net Income per Diluted Share:
Continuing operations, diluted (usd per share)								1.64	0.68	0.79	0.10
Discontinued operations								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