Investments in Unconsolidated Joint Ventures Narrative (Details) $ in Thousands							3 Months Ended		9 Months Ended
	Sep. 20, 2019 USD ($) ft² a	Sep. 16, 2019 USD ($)	Sep. 05, 2019 USD ($) ft² yr	Jun. 27, 2019 USD ($) ft²	Apr. 26, 2019 USD ($) ft² yr	Jan. 24, 2019 USD ($) ft²	Sep. 30, 2019 USD ($)		Sep. 30, 2019 USD ($)		Sep. 30, 2018 USD ($)	Jun. 15, 2019 ft²	May 28, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Mortgage notes payable, net							$ 2,952,006		$ 2,952,006					$ 2,964,572
Payments to Acquire Equity Method Investments									$ 65,499		$ 314,075
Annapolis Junction Building Six [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage						50.00%
Mortgage notes payable, net						$ 13,000
Debt Instrument, Basis Spread on Variable Rate						2.00%
Extension Option (in years)						1
Debt instrument, borrowing capacity						$ 14,300
Net Rentable Area (in sf) | ft²						119,000
7750 Wisconsin Avenue LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					50.00%		50.00%		50.00%
Construction Loan Facility Borrowing Capacity					$ 255,000
Debt Instrument, Basis Spread on Variable Rate					1.25%
Number of extensions					2
Extension Option (in years) | yr					1
Proceeds from Construction Loans Payable							$ 39,600
Net Rentable Area (in sf) | ft²					734,000
The Hub on Causeway - Podium and 100 Causeway [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage													50.00%
Maximum proceeds from a state infrastructure development program													$ 30,000
Net proceeds from a state infrastructure development program		$ 28,800
Hub on Causeway - Podium [Member]
Schedule of Equity Method Investments [Line Items]
Repayments of Construction Loans Payable		$ 28,800
Net Rentable Area (in sf) | ft²												375,000
540 Madison Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				60.00%			60.00%	[1]	60.00%	[1]
Contractual Sales Price				$ 310,300
Loans Assumed				$ 120,000
Debt Instrument, Basis Spread on Variable Rate				1.10%
Proceeds from Sale of Equity Method Investments				$ 178,700
Equity Method Investment, Realized Gain (Loss) on Disposal				$ 47,300			$ (500)
Net Rentable Area (in sf) | ft²				284,000
100 Causeway Street [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage			50.00%				50.00%		50.00%
Construction Loan Facility Borrowing Capacity			$ 400,000
Debt Instrument, Basis Spread on Variable Rate			1.50%
Number of extensions			2
Extension Option (in years) | yr			1
Proceeds from Construction Loans Payable							$ 47,700
Net Rentable Area (in sf) | ft²			632,000
Platform 16 [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	55.00%						55.00%	[2]	55.00%	[2]
Net Rentable Area (in sf) | ft²	1,100,000
Payments to Acquire Equity Method Investments	$ 28,200
Lessee, Finance Lease, Term of Contract	65 years
Area of Land | a	5.6
Company's Share [Member] | 540 Madison Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from Sale of Equity Method Investments				$ 107,100
Joint Venture Partner [Member] | Platform 16 [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	45.00%
Payments to Acquire Equity Method Investments	$ 23,100
Stabilization as defined in the agreement [Member] | 100 Causeway Street [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.375%

[1] The property was sold on June 27, 2019. As of September 30, 2019, the investment is comprised of undistributed cash. See note below for additional details.

[2] This entity is a VIE (See Note 2).