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Notes Payable - General Narrative (Details) - Notes Payable - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Debt      
Weighted average interest rate (as a percent) 4.04%    
Weighted average remaining years until maturity 8 years 8 months 12 days    
Interest incurred on notes and bonds $ 213.8 $ 197.1 $ 171.5
Maximum      
Debt      
Debt to total adjusted assets ratio (as a percent) 60.00%    
Secured debt to total adjusted assets ratio (as a percent) 40.00%    
Minimum      
Debt      
Debt service coverage ratio 1.5    
Total unencumbered assets as a percentage of outstanding unsecured debt 150.00%