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Notes Payable (Unsecured Senior Notes) (Details) - Unsecured Senior Notes
1 Months Ended 12 Months Ended
Jul. 31, 2017
USD ($)
Apr. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
tranche
Debt Instrument [Line Items]      
Debt amount     $ 350,000,000
Number of tranches | tranche     2
Minimum      
Debt Instrument [Line Items]      
Unencumbered interest coverage ratio     1.75
Fixed charge coverage ratio     1.50
Maximum      
Debt Instrument [Line Items]      
Leverage ratio (percent)     60.00%
Secured leverage ratio (percent)     40.00%
10-year Note with Interest Rate of 4.09%      
Debt Instrument [Line Items]      
Debt amount   $ 100,000,000  
Debt term   10 years  
Interest rate (percent)   4.09%  
8-year Note with Interest Rate of 3.91%      
Debt Instrument [Line Items]      
Debt amount $ 250,000,000    
Debt term 8 years    
Interest rate (percent) 3.91%