Transition Period Comparative Data (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Transition Period Comparative Data [Abstract]
Total revenues	$ 78.3	$ 83.5
Cost of sales	49.5	90.1
Selling, general and administrative expense	19.4	51.2
Equity in earnings of unconsolidated ventures	(4.8)	(10.9)
Gain on sale of assets	(27.8)	(113.4)
Interest expense	3.7	6.4
Interest and other income	6.4	2.4
Income before income taxes	44.7	62.5
Income tax expense (benefit)	25.3	(33.4)
Net income from continuing operations	70.0	29.1
Income from discontinued operations, net of taxes	0.0	38.8
Net income	70.0	67.9
Net income attributable to noncontrolling interests	1.2	0.1
Net income attributable to Forestar Group Inc.	$ 68.8	$ 67.8
Weighted average number of common shares	41,938,987	42,200,000
Adjusted weighted average number of common shares	41,969,056	42,500,000
Continuing operations, basic (usd per share)	$ 1.64	$ 0.69
Discontinued operations, basic (usd per share)	0	0.92
Basic net income per common share attributable to Forestar Group Inc.	1.64	1.61
Continuing operations, diluted (usd per share)	1.64	0.68
Discontinued operations, diluted (usd per share)	0	0.91
Diluted net income per common share attributable to Forestar Group Inc.	$ 1.64	$ 1.59