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Basis of Presentation (Details)
$ in Millions
3 Months Ended
Dec. 31, 2018
USD ($)
Lot
Home
Dec. 31, 2017
USD ($)
Oct. 01, 2018
USD ($)
Sep. 30, 2018
USD ($)
Oct. 05, 2017
Entity Information [Line Items]          
Payments to Acquire Businesses, Net of Cash Acquired $ 293.0 $ 156.4      
Cumulative Effect on Retained Earnings, Net of Tax 27.1        
Business Acquisition, Percentage of Voting Interests Acquired         75.00%
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners         25.00%
Goodwill 158.4     $ 109.2  
Contract Asset Insurance Renewals 33.4     0.0  
Accounting Standards Update 2016-18 [Member]          
Entity Information [Line Items]          
Increase (Decrease) in Restricted Cash   $ (37.2)      
Accounting Standards Update 2014-09 [Member]          
Entity Information [Line Items]          
Contract Asset Insurance Renewals     $ 32.4    
Series of Individually Immaterial Business Acquisitions [Member]          
Entity Information [Line Items]          
Payments to Acquire Businesses, Net of Cash Acquired 320.7        
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory 271.3        
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets 14.8        
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other (25.9)        
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest 320.7        
Goodwill 49.2        
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill $ 11.3        
Business Acquisition, Number of Homes Acquired | Home 700        
Business Acquisition, Number of Finished Lots Acquired | Lot 4,500        
Business Acquisition, Number of Lots Under Option Contracts | Lot 4,300        
Business Acquisition, Sales Order Backlog Acquired (Homes) | Home 700        
Homebuilding [Member]          
Entity Information [Line Items]          
Goodwill $ 129.2     $ 80.0  
Homebuilding [Member] | Midwest [Member]          
Entity Information [Line Items]          
Goodwill 43.2        
Homebuilding [Member] | East [Member]          
Entity Information [Line Items]          
Goodwill $ 6.0        
Minimum [Member] | Series of Individually Immaterial Business Acquisitions [Member]          
Entity Information [Line Items]          
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life 1 year        
Maximum [Member] | Series of Individually Immaterial Business Acquisitions [Member]          
Entity Information [Line Items]          
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life 3 years        
Retained Earnings [Member]          
Entity Information [Line Items]          
Cumulative Effect on Retained Earnings, Net of Tax $ 27.1        
Retained Earnings [Member] | Accounting Standards Update 2014-09 [Member]          
Entity Information [Line Items]          
Cumulative Effect on Retained Earnings, Net of Tax $ 27.1