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Commitments and Contingencies - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2017
Sep. 30, 2016
Commitments and Contingencies [Abstract]        
Liabilities for various claims, complaints and other legal actions $ 419.7 $ 435.6 $ 420.6 $ 423.5
Construction defect portion of loss contingency accrual, percentage 98.00%   98.00%  
Expenses related to legal claims $ 8.8 27.0    
Estimated insurance recoveries related to legal claims 73.1 $ 84.7 $ 74.4  
Earnest money deposits 269.7      
Remaining purchase price of land under option contracts 5,000.0      
Surety bonds 1,200.0      
Outstanding letters of credit 114.9      
Variable Interest Entity, Not Primary Beneficiary [Member]        
Commitments and Contingencies [Abstract]        
Earnest money deposits 256.6   222.9  
Option Contracts Subject to Specific Performance Clauses [Member]        
Commitments and Contingencies [Abstract]        
Remaining purchase price of land under option contracts 35.4      
Cash [Member]        
Commitments and Contingencies [Abstract]        
Earnest money deposits 263.8      
Notes Payable, Other Payables [Member]        
Commitments and Contingencies [Abstract]        
Earnest money deposits 5.9      
Homebuilding [Member] | Line of Credit [Member]        
Commitments and Contingencies [Abstract]        
Outstanding letters of credit 98.0      
Homebuilding [Member] | Standby Letters of Credit [Member]        
Commitments and Contingencies [Abstract]        
Outstanding letters of credit 2.5      
Forestar Group [Member]        
Commitments and Contingencies [Abstract]        
Outstanding letters of credit $ 14.4   $ 14.3