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Commitments (Details) - USD ($)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Commitments      
Rent expense $ 411,000 $ 408,000 $ 407,000
Maximum amount of loan 90,300,000    
Amount drawn and outstanding 81,780,000 $ 118,641,000  
Office buildings and industrial properties      
Future minimum rental income:      
2017 188,381,000    
2018 174,557,000    
2019 146,073,000    
2020 121,368,000    
2021 101,861,000    
Thereafter (2021-2029) 236,020,000    
Total $ 968,260,000    
Office buildings and industrial properties | Minimum      
Commitments      
Lease term 1 year    
Corporate office space      
Commitments      
Extension period 5 years    
Future minimum lease payments:      
2017 $ 424,000    
2018 404,000    
2019 412,000    
2020 421,000    
2021 429,000    
Thereafter 1,224,000    
Total $ 3,314,000