Real Estate Investments - Summary of Properties Sold (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2018 USD ($) ft² property	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) ft² property
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property		10
Rentable Square Feet | ft²		1,414,457
Sales Price		$ 359,300
Net Proceeds on Sale		309,169
Gain (Loss) on Sale		$ 2,932	$ 31,657	$ 116,983
Mixed-use Properties [Member] | Subaru National Training Center [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property	[1]	1
Rentable Square Feet | ft²	[1]	83,000
Sales Price	[1]	$ 45,300
Net Proceeds on Sale	[1]	44,877
Gain (Loss) on Sale	[1]	$ 2,570
Office Properties [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property				66
Rentable Square Feet | ft²				5,239,632
Sales Price				$ 820,000
Gain (Loss) on Sale	[2]			$ 116,983
Office Properties [Member] | Rockpoint Portfolio [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property	[3]	8
Rentable Square Feet | ft²	[3]	1,293,197
Sales Price	[3]	$ 312,000
Net Proceeds on Sale	[3]	262,442
Gain (Loss) on Sale	[3]	$ 397
Office Properties [Member] | 20 East Clementon Road [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of Properties Sold | property		1
Rentable Square Feet | ft²		38,260
Sales Price		$ 2,000
Net Proceeds on Sale		1,850
Gain (Loss) on Sale		$ (35)

[1] During the second quarter of 2018, Subaru exercised its purchase option under the lease agreement for the Subaru NSTC and the sale occurred during the fourth quarter of 2018. See Note 2, “Summary of Significant Accounting Policies,” for further discussion of the lease agreement and related revenue recognition.

[2] Gain/(Loss) on Sale is net of closing and other transaction related costs.

[3] On December 20, 2018, the Company contributed a portfolio of eight properties containing an aggregate of 1,293,197 square feet, located in its Metropolitan Washington, D.C. segment (the “Rockpoint Portfolio”) to a newly-formed joint venture (the “Herndon Innovation Center Metro Portfolio Venture, LLC”) for a gross sales price of $312.0 million. The Company and its partner own 15% and 85% interests in the Herndon Innovation Center Metro Portfolio Venture, LLC, respectively. The Herndon Innovation Center Metro Portfolio Venture, LLC funded the acquisition with $265.2 million of cash, which was distributed to the Company at closing. After funding its share of closing costs and working capital contributions of $2.2 million and $0.6 million, respectively, the Company received $262.4 million of cash proceeds at settlement. The Company recorded an impairment charge of $56.9 million for the Rockpoint Portfolio during the third quarter of 2018. The Company recorded a $0.4 million gain on sale, which represents an adjustment to estimated closing costs used to determine the impairment charge in the third quarter of 2018. For further information related to this transaction, see the “Herndon Innovation Center Metro Portfolio Venture, LLC” section in Note 4, “Investment in Unconsolidated Real Estate Ventures.”