Investment in unconsolidated Joint Ventures (Narrative) (Details)									2 Months Ended	6 Months Ended	12 Months Ended
		Dec. 19, 2016 USD ($) ft² yr	Dec. 07, 2016 USD ($) ft² yr	Nov. 28, 2016 USD ($) yr	Nov. 15, 2016 USD ($) ft²	Oct. 20, 2016 USD ($) ft²	Jul. 01, 2016 USD ($) ft² a Vehicles Buildings	Apr. 11, 2016 USD ($) ft² yr	Dec. 31, 2016 USD ($)	Oct. 16, 2016	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 08, 2016 USD ($)	Nov. 28, 2016 ft²	Nov. 28, 2016 apartments	Nov. 28, 2016 floor	Oct. 21, 2016	Oct. 01, 2016 ft²
Schedule of Equity Method Investments [Line Items]
Mortgage and notes payable, net									$ 2,063,087,000		$ 2,063,087,000	$ 3,435,242,000
Payments to Acquire Equity Method Investments											575,795,000	38,207,000	$ 52,052,000
Proceeds from sale of investment in unconsolidated joint venture											55,707,000	0	0
Gain on sales of real estate											59,370,000	0	0
Gains (losses) from early extinguishments of debt											(371,000)	(22,040,000)	(10,633,000)
Repayments of Secured Debt											$ 1,326,865,000	$ 54,801,000	$ 87,758,000
Metropolitian Square [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage						51.00%												20.00%
Net Rentable Area | ft²						607,000
Contractual Sales Price						$ 282,400,000
Carrying Value of Debt Assumed in Sale						133,400,000
Payments for Leasing Costs						14,200,000
Proceeds from sale of investment in unconsolidated joint venture						58,200,000
Gain on sales of real estate						$ 59,400,000
1265 Main Street [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage																			50.00%
Net Rentable Area | ft²																			115,000
1265 Main Office JV LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage									50.00%		50.00%
Annapolis Junction Building Eight [Member]
Schedule of Equity Method Investments [Line Items]
Net Rentable Area | ft²			126,000
Debt Instrument, Basis Spread on Variable Rate			1.50%
Construction Loan			$ 15,100,000
Number of extensions			2
Extension Option (in years) | yr			1
Annapolis Junction Building Seven and Eight [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage			50.00%
Mortgage and notes payable, net			$ 36,700,000
Debt Instrument, Basis Spread on Variable Rate			2.35%
Number of extensions			3
Extension Option (in years) | yr			1
Mortgage note borrowing capacity, maximum			$ 42,000,000
Annapolis Junction Building Seven [Member]
Schedule of Equity Method Investments [Line Items]
Net Rentable Area | ft²			127,000
Debt Instrument, Basis Spread on Variable Rate			1.65%
Construction Loan			$ 21,600,000
Number of extensions			1
Extension Option (in years) | yr			1
Annapolis Junction Building Six [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					50.00%
Net Rentable Area | ft²					119,000
Construction Loan Facility Borrowing Capacity					$ 15,400,000
Debt Instrument, Basis Spread on Variable Rate					2.25%
Construction Loan					$ 12,900,000
The Hub on causeway - Hotel [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				50.00%
Contribution of Property				$ 700,000
Air rights lease (in years) | yr				99
Payments to Acquire Equity Method Investments				$ 6,700,000
The Hub on Causeway - Residential [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				50.00%
Payments and improvements for equity method investments				$ 17,700,000
Net Rentable Area | ft²															320,000
Payments to Acquire Equity Method Investments				$ 6,500,000
Number of apartment units																440	40
Annapolis Junction NFM, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage	[1]								50.00%		50.00%
Dock 72 at Brooklyn Navy Yard [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		50.00%
Net Rentable Area | ft²		670,000
Construction Loan Facility Borrowing Capacity		$ 250,000,000
Debt Instrument, Basis Spread on Variable Rate		2.25%
Number of extensions		2
Extension Option (in years) | yr		1
Construction facilty, amount outstanding									$ 0		$ 0
joint venture partner [Member] | Metropolitian Square [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage						49.00%												80.00%
joint venture partner [Member] | The Hub on causeway - Hotel [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				50.00%
Payments to Acquire Equity Method Investments				$ 7,400,000
joint venture partner [Member] | The Hub on Causeway - Residential [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage				50.00%
Payments to Acquire Equity Method Investments				$ 24,200,000
Secured Debt [Member] | Annapolis Junction Buidling One [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage								50.00%
Mortgage and notes payable, net								$ 39,600,000
Net Rentable Area | ft²								118,000
Debt Instrument, Basis Spread on Variable Rate									5.75%	1.75%
Number of extensions								1
Extension Option (in years) | yr								3
Secured Debt [Member] | 1265 Main Street [Member]
Schedule of Equity Method Investments [Line Items]
Coupon/Stated Rate														3.77%
Mortgage Loans on Real Estate, Face Amount of Mortgages														$ 40,400,000
Colorado Center [Member] | Colorado Center [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage							49.80%
Aggregate purchase price							$ 511,100,000
Net Rentable Area | ft²							1,184,000
Net assets							$ 503,600,000
number of buildings | Buildings							6
Land acquired (in acres) | a							15
Vehicles In Structured Parking | Vehicles							3,100

[1] The joint venture owns four in-service buildings and two undeveloped land parcels