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Segment Information - Narrative (Details) $ in Millions	3 Months Ended
Segment Information - Narrative (Details) $ in Millions	Dec. 31, 2020 USD ($) Segments Home Market State Projects OperatingDivisions	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) Home
Segment Reporting Information [Line Items]
Gain (Loss) on Sale of Properties	$ 14.0	$ 31.1
Total assets	19,780.4		$ 18,912.3
Other Segments [Member]
Segment Reporting Information [Line Items]
Gain (Loss) on Sale of Properties	0.9	31.2
Total assets	$ 438.7		379.4
Home Building [Member]
Segment Reporting Information [Line Items]
Number of housing construction markets \| Market	90
Number of housing construction states \| State	29
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	$ 16,065.5		15,221.4
Forestar Group [Member]
Segment Reporting Information [Line Items]
Number of housing construction markets \| Market	51
Number of housing construction states \| State	21
Gain (Loss) on Sale of Properties	$ 0.0	$ (0.1)
Total assets	1,809.1		1,739.9
Single Family [Member]
Segment Reporting Information [Line Items]
Property, Plant and Equipment, Gross	106.6		87.2
Multifamily [Member] \| Other Segments [Member]
Segment Reporting Information [Line Items]
Total assets	$ 294.3		246.2
Multi-Family Units, Total \| Home	2,325
Multi-Family Units, Under Active Construction \| Home	1,015
Multi-Family Units, Completed \| Home	1,310
Single Family [Member] \| Home Building [Member]
Segment Reporting Information [Line Items]
Unit Sales, Single-Family \| Home	124
Home sales	$ 31.8
Property, Plant and Equipment, Gross	$ 106.6		$ 87.2
Single Family Rental Units, Completed Homes \| Home	440		440
Single Family Rental Units, Homes and Lots, Total \| Home	890		740
Asset under Construction [Member] \| Multifamily [Member] \| Other Segments [Member]
Segment Reporting Information [Line Items]
Number of Projects \| Projects	4
Assets Leased to Others [Member] \| Single Family [Member]
Segment Reporting Information [Line Items]
Property, Plant and Equipment, Gross	$ 89.3		$ 68.1
Assets Leased to Others [Member] \| Multifamily [Member] \| Other Segments [Member]
Segment Reporting Information [Line Items]
Number of Projects \| Projects	4