Real Estate Investments - Summary of Properties Classified as Held for Sale Included in Continuing Operations (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
ASSETS HELD FOR SALE
Total assets held for sale, net		$ 297,194	$ 392
LIABILITIES HELD FOR SALE
Total liabilities held for sale		826	$ 0
Held for Sale Properties Included in Continuing Operations [Member]
ASSETS HELD FOR SALE
Operating properties		396,072
Accumulated depreciation		(112,600)
Operating real estate investments, net		283,472
Construction-in-progress		1,748
Land inventory		11,575
Total real estate investments		296,795
Other assets		399
Total assets held for sale, net		297,194
LIABILITIES HELD FOR SALE
Other liabilities		826
Total liabilities held for sale		826
Held for Sale Properties Included in Continuing Operations [Member] | Metropolitan Washington, D.C. - Office [Member]
ASSETS HELD FOR SALE
Operating properties	[1]	396,072
Accumulated depreciation	[1]	(112,600)
Operating real estate investments, net	[1]	283,472
Construction-in-progress	[1]	1,748
Land inventory	[1]	0
Total real estate investments	[1]	285,220
Other assets	[1]	399
Total assets held for sale, net	[1]	285,619
LIABILITIES HELD FOR SALE
Other liabilities	[1]	826
Total liabilities held for sale	[1]	826
Held for Sale Properties Included in Continuing Operations [Member] | Pennsylvania Suburbs - Land [Member]
ASSETS HELD FOR SALE
Operating properties	[2]	0
Accumulated depreciation	[2]	0
Operating real estate investments, net	[2]	0
Construction-in-progress	[2]	0
Land inventory	[2]	4,254
Total real estate investments	[2]	4,254
Other assets	[2]	0
Total assets held for sale, net	[2]	4,254
LIABILITIES HELD FOR SALE
Other liabilities	[2]	0
Total liabilities held for sale	[2]	0
Held for Sale Properties Included in Continuing Operations [Member] | Other - Land [Member]
ASSETS HELD FOR SALE
Operating properties	[2]	0
Accumulated depreciation	[2]	0
Operating real estate investments, net	[2]	0
Construction-in-progress	[2]	0
Land inventory	[2]	7,321
Total real estate investments	[2]	7,321
Other assets	[2]	0
Total assets held for sale, net	[2]	7,321
LIABILITIES HELD FOR SALE
Other liabilities	[2]	0
Total liabilities held for sale	[2]	$ 0

[1] As of September 30, 2018, the Company determined that the sale of eight office properties, containing 1,293,197 rentable square feet, in the Metropolitan Washington, D.C. segment, was probable and classified these properties as held for sale in accordance with applicable accounting standards for long-lived assets. At such date, the $366.0 million carrying value of the properties exceeded the estimated $309.1 million fair value less the anticipated costs of sale. As a result, the Company recognized an impairment loss totaling approximately $56.9 million during the three-month period ended September 30, 2018. The Company measured this impairment based on a discounted cash flow analysis, using a hold period of ten years and residual capitalization rates and discount rates of 7.47% and 8.60%, respectively. The results were comparable to indicative pricing in the market. As significant inputs to the model are unobservable, the Company determined that the value determined for this property falls within Level 3 fair value reporting.

[2] As of September 30, 2018, the Company determined that the sale of one land parcel in the Pennsylvania Suburbs segment and two parcels of land in the Other segment was probable and classified these properties as held for sale in accordance with applicable accounting standards for long-lived assets. At such date, the fair value less the anticipated costs of sale of the properties exceeded the carrying values. As a result, the Company expects to record gains on sale. The fair value measurement will be based on the pricing in the purchase and sale agreements.