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Summary of Significant Accounting Policies (Intangible Assets) (Details) - Favorable Lease Rates
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]    
Favorable lease rates, original useful lives 38 years 31 years 4 months 24 days
Minimum    
Finite-Lived Intangible Assets [Line Items]    
Favorable lease rates, original useful lives 41 years  
Maximum    
Finite-Lived Intangible Assets [Line Items]    
Favorable lease rates, original useful lives 52 years