Property and Equipment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	$ 895.0		$ 895.0		$ 683.7
Depreciation	17.0	$ 17.3	34.8	$ 34.5
Intersegment Eliminations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	(6.1)		(6.1)		(3.0)
Other Segments [Member]
Property, Plant and Equipment [Line Items]
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(30.6)		(30.6)		(28.2)
Property, Plant and Equipment, Net	432.4		432.4		308.9
Other Segments [Member] | Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	16.8		16.8		15.6
Other Segments [Member] | Other Assets [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	71.1		71.1		69.7
Other Segments [Member] | LandMember
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	19.7		19.7		19.7
Forestar Group [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	2.0		2.0		1.1
Financial Services [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	3.5		3.5		3.9
HomeBuildingMember
Property, Plant and Equipment [Line Items]
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(271.4)		(271.4)		(256.2)
Property, Plant and Equipment, Net	463.2		463.2		372.8
HomeBuildingMember | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	275.0		275.0		267.4
HomeBuildingMember | 2510 Household Furniture [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	133.1		133.1		134.0
HomeBuildingMember | Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	110.8		110.8		108.1
HomeBuildingMember | LandMember
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	33.1		33.1		32.3
MultifamilyMember
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	355.4		355.4		232.1
MultifamilyMember | LandMember
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	106.5		106.5		58.3
MultifamilyMember | Property, Plant and Equipment, Other Types
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	248.9		248.9		173.8
SingleFamilyMember
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	182.6		182.6		87.2
SingleFamilyMember | LandMember
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	23.9		23.9		19.1
SingleFamilyMember | Property, Plant and Equipment, Other Types
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 158.7		$ 158.7		$ 68.1