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Summary of Significant Accounting Policies - Text (Detail) $ in Millions				9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) Lot	Feb. 08, 2018 USD ($) a Lot Project	Oct. 05, 2017	Sep. 30, 2018 USD ($) Lot	Sep. 30, 2017 USD ($)	Sep. 30, 2019 USD ($) Lot	Dec. 31, 2017 USD ($)	Oct. 01, 2018 USD ($)
Significant Accounting Policies [Line Items]
Real estate	$ 1,028.9			$ 498.0		$ 1,028.9
Contract with Customer, Liability	2.5			0.0		2.5
Income (loss) discontinued operations, net of taxes				0.0	$ 38.8	0.0	$ 46.0
Depreciation expense of property and equipment				0.2		0.3	0.4
Inventory Write-down				0.3		0.8	3.4
Deferred Revenue	$ 9.3			$ 11.6		$ 9.3
Number of Units in Real Estate Property \| Lot	38,300			20,100		38,300
Related Party Transaction, Purchase Obligation from Parent	$ 953.8			$ 522.2		$ 953.8
Minimum \| Building and Building Improvements
Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life						10 years
Minimum \| Property and Equipment
Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life						2 years
Maximum \| Building and Building Improvements
Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life						40 years
Maximum \| Property and Equipment
Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life						10 years
Land and Land Improvements [Member]
Significant Accounting Policies [Line Items]
Real estate	1,011.8			463.1		$ 1,011.8
Land [Member]
Significant Accounting Policies [Line Items]
Real estate	$ 17.1			34.9		17.1
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Starwood Land, L.P.. [Member]
Significant Accounting Policies [Line Items]
Number of Projects Sold \| Project		24
Purchase price of properties sold		$ 232.0
Proceeds from Sale of Real Estate		$ 217.5
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Starwood Land, L.P.. [Member] \| Residential Real Estate [Member]
Significant Accounting Policies [Line Items]
Number of Lots \| Lot		750
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Starwood Land, L.P.. [Member] \| Land and Land Improvements [Member]
Significant Accounting Policies [Line Items]
Number of Lots \| Lot		4,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Starwood Land, L.P.. [Member] \| Land [Member]
Significant Accounting Policies [Line Items]
Area of Land \| a		730
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Starwood Land, L.P.. [Member] \| Multifamily [Member]
Significant Accounting Policies [Line Items]
Number of Projects Sold \| Project		1
D.R. Horton, Inc. [Member] \| D.R. Horton Merger Agreement [Member]
Significant Accounting Policies [Line Items]
Sale of Stock, Percentage of Ownership after Transaction	66.00%		75.00%
D.R. Horton, Inc. [Member] \| Under Contract [Member]
Significant Accounting Policies [Line Items]
Related Party Deposit Liabilities	$ 88.7			$ 45.3		$ 88.7
Number of Units in Real Estate Property \| Lot	12,800			5,500		12,800
Accounting Standards Update 2016-18 [Member]
Significant Accounting Policies [Line Items]
Increase (Decrease) in Restricted Cash				$ (23.8)			$ 39.8
Accounting Standards Update 2014-09 [Member]
Significant Accounting Policies [Line Items]
Real estate	$ 2.1					$ 2.1
Contract with Customer, Liability	2.5					2.5		$ 6.4
Deferred Revenue	(0.4)					(0.4)
Pro Forma [Member] \| Accounting Standards Update 2014-09 [Member]
Significant Accounting Policies [Line Items]
Real estate	1,026.8					1,026.8
Contract with Customer, Liability	0.0					0.0
Deferred Revenue	$ 9.7					$ 9.7