Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Parenthetical) (Details) - property	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Number of Properties	113
Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties	93
Held for Sale Properties Included in Continuing Operations [Member]
Segment Reporting Information [Line Items]
Number of Properties	5
Cira Square [Member] | Held for Sale Properties Included in Continuing Operations [Member]
Segment Reporting Information [Line Items]
Number of Properties			58
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Number of Properties		2	3
Metropolitan DC [Member] | Held for Sale Properties Included in Continuing Operations [Member] | Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties	3
Other [Member]
Segment Reporting Information [Line Items]
Number of Properties	3
Other [Member] | Held for Sale Properties Included in Continuing Operations [Member] | Office Properties [Member]
Segment Reporting Information [Line Items]
Number of Properties	2