Annual report pursuant to Section 13 and 15(d)

Related Party Transactions (Details)

v3.8.0.1
Related Party Transactions (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
ft²
property
item
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Related Party Transaction [Line Items]      
Lease revenue $ 501,334,000 $ 506,877,000 $ 487,041,000
Mack [Member]      
Related Party Transaction [Line Items]      
Due from related parties 0 0  
RG [Member]      
Related Party Transaction [Line Items]      
Related party revenue $ 1,873,000 2,464,000 2,542,000
Leased Property [Member]      
Related Party Transaction [Line Items]      
Number of real estate properties | property 1    
Lease expiration date May 2018    
Number of lease options | item 2    
Lease extension period 3 years    
Leased Property [Member] | Mack [Member]      
Related Party Transaction [Line Items]      
Area Of Real Estate Property | ft² 7,034    
Lease revenue $ 187,000 $ 193,000 $ 204,000