Segment Information (Textual) (Details)		12 Months Ended
	Feb. 02, 2017 property	Dec. 31, 2017 property segment	Dec. 31, 2016 property	Dec. 31, 2015 property
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment		5
Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties Sold			66	27
Office Properties [Member] | 1200 & 1220 Concord Avenue (Concord Airport Plaza) [Member]
Segment Reporting Information [Line Items]
Number of Properties Sold	2	2