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Real Estate - Text (Detail)
$ in Millions
9 Months Ended 12 Months Ended
Feb. 08, 2018
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2019
USD ($)
a
Real Estate Properties [Line Items]      
Document Period End Date     Sep. 30, 2019
Payments to Acquire Residential Real Estate     $ 559.4
Payments to Develop Real Estate Assets     290.3
Real estate   $ 498.0 1,028.9
Land and Land Improvements [Member]      
Real Estate Properties [Line Items]      
Real estate   463.1 $ 1,011.8
Starwood Land, L.P.. [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations      
Real Estate Properties [Line Items]      
Disposal Group, Including Discontinued Operation, Consideration $ 232.0    
Proceeds from Sale of Real Estate $ 217.5    
Gain (Loss) on Sale of Project   $ 0.7  
Equity Method Investments [Member] | Other Customer [Member]      
Real Estate Properties [Line Items]      
Area of Land | a     63
Proceeds from Sale of Real Estate     $ 44.2
Maximum | D.R. Horton, Inc. [Member]      
Real Estate Properties [Line Items]      
Related Party Transaction, Rate     16.00%
Minimum | D.R. Horton, Inc. [Member]      
Real Estate Properties [Line Items]      
Related Party Transaction, Rate     12.00%
Contract Termination [Member] | D.R. Horton, Inc. [Member]      
Real Estate Properties [Line Items]      
Related Party Transaction, Amounts of Transaction     $ 2.1