Summary of Quarterly Results of Operations (Unaudited) (Detail) - 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
Quarterly Financial Information Disclosure [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
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
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
Net income (loss) per share -- basic
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 (loss) per share -- diluted
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