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Investment in Unconsolidated Entities - Narrative (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Sep. 30, 2017
USD ($)
Schedule of Equity Method Investments [Line Items]          
Gain (Loss) on Sale of Properties $ 0.0 $ 0.0 $ 14.5 $ 0.0  
Investment in unconsolidated entities 32.2   32.2   $ 0.0
Outstanding debt, unconsolidated entities 45.6   45.6    
Equity in earnings of unconsolidated entities 0.4 $ 0.0 3.1 $ 0.0  
Nonrecourse Debt [Member]          
Schedule of Equity Method Investments [Line Items]          
Outstanding debt, unconsolidated entities 41.1   41.1    
Segment Reconciling Items [Member]          
Schedule of Equity Method Investments [Line Items]          
Gain (Loss) on Sale of Properties (1.3)   (2.9)    
Investment in unconsolidated entities 14.2   14.2    
Equity in earnings of unconsolidated entities (0.6)   (7.0)    
Forestar Group [Member]          
Schedule of Equity Method Investments [Line Items]          
Gain (Loss) on Sale of Properties $ 1.3   $ 4.0    
Number of Equity Method Investments 6   6    
Investment in unconsolidated entities $ 18.0   $ 18.0    
Equity in earnings of unconsolidated entities $ 1.0   10.1    
Multifamily [Member] | Forestar Group [Member]          
Schedule of Equity Method Investments [Line Items]          
Gain (Loss) on Sale of Properties     $ 19.0