Annual report pursuant to Section 13 and 15(d)

Concentration of Revenue (Details)

v3.8.0.1
Concentration of Revenue (Details) - Tenants [Member] - United States Government [Member]
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Rental revenue [Member]      
Concentration Risk      
Percentage of revenue 22.00% 21.00% 20.00%
Construction contract revenue [Member] | Minimum [Member]      
Concentration Risk      
Percentage of revenue 80.00% 80.00% 80.00%