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Segment Information - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
Segment Information - Narrative (Details) $ in Millions	Mar. 31, 2021 USD ($) Segments Market Home OperatingDivisions Projects State	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) Market Home Projects State	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($) Home
Segment Reporting Information [Line Items]
Gain (Loss) on Sale of Properties	$ 0.0	$ 28.5	$ 14.0	$ 59.5
Total assets	21,083.8		21,083.8		$ 18,912.3
Other Segments [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Sale of Properties		28.2	0.9	59.4
Total assets	$ 547.5		$ 547.5		379.4
Forestar Group [Member]
Segment Reporting Information [Line Items]
Number of housing construction markets \| Market	54		54
Number of housing construction states \| State	22		22
Gain (Loss) on Sale of Properties		0.3	$ 0.0	0.1
Total assets	$ 1,898.9		$ 1,898.9		1,739.9
HomeBuildingMember
Segment Reporting Information [Line Items]
Number of housing construction markets \| Market	91		91
Number of housing construction states \| State	29		29
Number of home building operating divisions \| OperatingDivisions	55
Number of homebuilding reporting segments \| Segments	6
Gain (Loss) on Sale of Properties		$ 0.0	$ 13.1	$ 0.0
Total assets	$ 16,783.9		16,783.9		15,221.4
Multifamily [Member] \| Other Segments [Member]
Segment Reporting Information [Line Items]
Total assets	$ 394.4		$ 394.4		246.2
Multi-Family Units, Total \| Home	3,760		3,760
Multi-Family Units, Under Active Construction \| Home	2,450		2,450
Multi-Family Units, Completed \| Home	1,310		1,310
Single Family [Member] \| HomeBuildingMember
Segment Reporting Information [Line Items]
Unit Sales, Single-Family \| Home			124
Home sales			$ 31.8
Property, Plant and Equipment, Gross	$ 182.6		$ 182.6		$ 87.2
Single Family Rental Units, Completed Homes \| Home	570		570		440
Single Family Rental Units, Homes and Lots, Total \| Home	1,650		1,650		740
Asset under Construction [Member] \| Multifamily [Member] \| Other Segments [Member]
Segment Reporting Information [Line Items]
Number of Projects \| Projects	8		8
Property, Plant and Equipment, Other Types \| Multifamily [Member] \| Other Segments [Member]
Segment Reporting Information [Line Items]
Number of Projects \| Projects	4		4