Annual report pursuant to Section 13 and 15(d)

Concentration of Revenue (Details)

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Concentration of Revenue (Details) - Tenants [Member] - United States Government [Member]
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Rental revenue [Member]      
Concentration Risk      
Percentage of revenue 24.00% 22.00% 21.00%
Construction contract revenue [Member]      
Concentration Risk      
Percentage of revenue 95.00% 98.00% 87.00%