Segment Information - Narrative (Details) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2021 USD ($) Home Market OperatingDivisions Projects Segments State	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) Home Market Projects State	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) Home
Segment Reporting Information [Line Items]
Gain (Loss) on Sale of Properties	$ 0.0		$ 0.0	$ 14.0	$ 59.5
Total assets	21,793.5			21,793.5		$ 18,912.3
Rental properties	722.3			722.3		0.0
Other Segments [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Sale of Properties				0.9	59.4
Total assets	$ 632.2			$ 632.2		379.4
Forestar Group [Member]
Segment Reporting Information [Line Items]
Number of housing construction markets | Market	55			55
Number of housing construction states | State	22			22
Gain (Loss) on Sale of Properties				$ 0.0	0.1
Total assets	$ 2,015.0			2,015.0		1,739.9
Rental properties	$ 0.0			$ 0.0
HomeBuildingMember
Segment Reporting Information [Line Items]
Number of housing construction markets | Market	96			96
Number of housing construction states | State	30			30
Number of home building operating divisions | OperatingDivisions	55
Number of homebuilding reporting segments | Segments	6
Gain (Loss) on Sale of Properties				$ 13.1	$ 0.0
Total assets	$ 17,504.1			17,504.1		15,221.4
MultifamilyMember
Segment Reporting Information [Line Items]
Property, Plant and Equipment, Gross	0.0			0.0		232.1
Multifamily [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Property, Plant and Equipment, Gross						232.1
Total assets	$ 458.3			$ 458.3		246.2
Multi-Family Units, Total | Home	4,540			4,540
Multi-Family Units, Under Active Construction | Home	3,230			3,230
Multi-Family Units, Completed | Home	1,310			1,310
Rental properties	$ 436.3			$ 436.3
Single Family [Member] | HomeBuildingMember
Segment Reporting Information [Line Items]
Home sales	$ 23.1	$ 31.8
Gain (Loss) on Sale of Properties		$ 14.0
Property, Plant and Equipment, Gross						$ 87.2
Single Family Rental Units, Completed Homes | Home	680			680		440
Single Family Rental Units, Homes and Lots, Total | Home	2,340			2,340		740
Gross Profit	$ 11.4
Rental properties	$ 303.1			$ 303.1
Asset under Construction [Member] | Multifamily [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Number of Projects | Projects	11			11
Inventories [Member] | Multifamily [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Number of Projects | Projects	4			4