Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income attributable to Forestar Group Inc.	$ 24.2	$ 10.1	$ 9.6	$ 16.9	$ 12.7	$ 6.9	$ 10.1	$ 3.3	$ 68.8	$ 67.8	$ 60.8	$ 33.0
Weighted average number of common shares									41,938,987	42,200,000	48,037,018	41,974,429
Dilutive effect of stock-based compensation									30,069		57,093	30,712
Adjusted weighted average number of common shares									41,969,056	42,500,000	48,094,111	42,005,141
Basic net income per common share attributable to Forestar Group Inc.	$ 0.50	$ 0.21	$ 0.20	$ 0.35	$ 0.30	$ 0.16	$ 0.24	$ 0.08	$ 1.64	$ 1.61	$ 1.26	$ 0.79
Diluted net income per common share attributable to Forestar Group Inc.	$ 0.50	$ 0.21	$ 0.20	$ 0.35	$ 0.30	$ 0.16	$ 0.24	$ 0.08	$ 1.64	$ 1.59	$ 1.26	$ 0.79
Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.75%										3.75%
Maturities of Senior Debt			$ 118.9