Acquisition of Real Estate (Properties Acquired) (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2017
USD ($)
ft²
Business Acquisition [Line Items]  
GLA | ft² 409,296
Aggregate Purchase Price $ 104,811
Outparcel building adjacent to Annex of Arlington  
Business Acquisition [Line Items]  
GLA | ft² 5,760
Aggregate Purchase Price $ 1,006
Outparcel adjacent to Northeast Plaza  
Business Acquisition [Line Items]  
Aggregate Purchase Price $ 1,537
Arborland Center  
Business Acquisition [Line Items]  
GLA | ft² 403,536
Aggregate Purchase Price $ 102,268