Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Parenthetical) (Details) - property	Sep. 30, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Number of Properties	117
Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties	99
Held for Sale Properties Included in Continuing Operations [Member]
Segment Reporting Information [Line Items]
Number of Properties	4
Held for Sale Properties Included in Continuing Operations [Member] | Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties	3
Cira Square [Member] | Held for Sale Properties Included in Continuing Operations [Member]
Segment Reporting Information [Line Items]
Number of Properties		58
620, 640 and 660 Allendale Road [Member] | Held for Sale Properties Included in Continuing Operations [Member] | Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties	1
Oakland Lot B [Member] | Held for Sale Properties Included in Continuing Operations [Member] | Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties	1
Marine Center Piers [Member] | Held for Sale Properties Included in Continuing Operations [Member] | Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties	1