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Investment in unconsolidated Joint Ventures (Narrative) (Details) |  |  |  |  |  |  |  | 2 Months Ended | 6 Months Ended | 12 Months Ended |  |  |  |
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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

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- Definition This element represents the carrying value of a short-term real estate loan to finance building costs. The funds are disbursed as needed or in accordance with a prearranged plan; generally, a portion of the funds is disbursed at inception and the remainder as construction progresses. The money is repaid on completion of the project, usually from the proceeds of a mortgage loan. The rate is normally higher than the prime rate, and there is usually an origination fee. The effective yield on these loans tends to be high, and the lender has a security interest in the real property. Note that there are separate concepts for the current and noncurrent portions of long-term construction loans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20 -Article 5 + Details
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