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Investment in Unconsolidated Real Estate Ventures - Investment in Real Estate Ventures and Share of Real Estate Ventures' Income (Loss) (Parenthetical) (Details) $ in Thousands |  |  |  |  | 12 Months Ended |  |  |  |  |  |
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Dec. 28, 2018 USD ($) property | Aug. 01, 2018 USD ($) | Jun. 26, 2018 USD ($) | Jan. 31, 2017 USD ($) property | Dec. 31, 2018 USD ($) ft² property | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 20, 2018 USD ($) ft² property | Dec. 11, 2018 ft² property | Sep. 30, 2018 property | Jan. 05, 2018 | Oct. 21, 2014 | Jan. 20, 2011
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayment of mortgage debt |  |  |  |  |  | $ 122,180 |  | $ 4,931 | $ 357,151 |  |  |  |  |  |
Number of Properties Sold | property |  |  |  |  |  | 10 |  |  |  |  |  |  |  |  |
Area of Real Estate Property | ft² |  |  |  |  |  | 16,777,249 |  |  |  |  |  |  |  |  |
Gross sales price |  |  |  |  |  | $ 359,300 |  |  |  |  |  |  |  |  |
Equity Method Investment Summarized Financial Information Debt |  |  |  |  |  | $ 370,319 |  | 768,912 |  |  |  |  |  |  |
Number of Properties acquired | property |  |  |  |  |  | 97 |  |  |  |  |  |  |  |  |
Debt, net |  |  |  |  |  | $ 365,707 |  | 766,420 |  |  |  |  |  |  |
Mortgage notes payable, net |  |  |  |  |  | $ 320,869 |  | $ 317,216 |  |  |  |  |  |  |
DRA Austin Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of Real Estate Property | ft² |  |  |  |  |  |  |  |  |  |  | 1,570,123 |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |
Percentage of ownership interests |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |
Number of Properties acquired | property |  |  |  |  |  |  |  |  |  |  | 12 |  |  |  |
Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties contributed to a joint venture | property |  |  |  |  |  |  |  |  |  |  |  | 8 |  |  |
TB-BDN Plymouth Apartments [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties Sold | property |  |  |  |  | 398 |  |  |  |  |  |  |  |  |  |
Gross sales price |  |  |  |  | $ 100,500 |  |  |  |  |  |  |  |  |  |
Percentage of ownership interests |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
Sale of ownership interest percentage |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
Brandywine - AI Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  | 50.00% | [1],[2] | 50.00% | 50.00% |  |  |  |  |  |
Equity Method Investment Summarized Financial Information Debt | [2] |  |  |  |  | $ 26,111 |  | $ 93,117 |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage | [2] |  |  |  |  | 4.65% |  |  |  |  |  |  |  |  |
Debt, net |  |  |  |  |  | $ 26,020 |  | 92,917 |  |  |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of properties contributed to a joint venture | property |  |  |  |  |  |  |  |  |  | 8 |  |  |  |  |
Area of Real Estate Property | ft² |  |  |  |  |  |  |  |  |  | 1,293,197 |  |  |  |  |
Gross sales price |  |  |  |  |  |  |  |  |  | $ 312,000 |  |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  | 15.00% |  |  |  |  |
Herndon Innovation Center Metro Portfolio Venture, LLC [Member] | Metropolitan Washington, D.C. - Office [Member] | Rockpoint [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  | 85.00% |  |  |  |  |
MAP Ground Lease Venture LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument Maturity Date |  |  | Feb. 09, 2019 |  |  |  |  |  |  |  |  |  |  |  |
Equity Method Investment Summarized Financial Information Debt |  |  | $ 180,800 |  |  |  |  |  |  |  |  |  |  |  |
Seven Tower Bridge [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage | [1],[3] |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  |
Equity Method Investment Summarized Financial Information Debt | [3] |  |  |  |  | $ 0 |  | 14,629 |  |  |  |  |  |  |
Sale of ownership interest percentage |  |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |
Four Tower Bridge [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  | 65.00% | [1],[3] |  |  |  |  |  | 35.00% |  |
Equity Method Investment Summarized Financial Information Debt | [3] |  |  |  |  | $ 0 |  | 9,749 |  |  |  |  |  |  |
1919 Ventures [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  | 50.00% |  | 50.00% | [1],[4] |  |  |  |  |  |  |  | 50.00%
Equity Method Investment Summarized Financial Information Debt | [4] |  |  |  |  | $ 88,860 |  | 88,860 |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage | [4] |  |  |  |  | 4.00% |  |  |  |  |  |  |  |  |
Debt, net |  |  |  | $ 44,400 |  |  |  |  |  |  |  |  |  |  |
Note receivable |  |  |  | $ 44,400 |  |  |  |  |  |  |  |  |  |  |
Loan interest rate |  |  |  | 4.00% |  |  |  |  |  |  |  |  |  |  |
Mortgage loans scheduled maturity date |  |  |  | Jun. 25, 2023 |  |  |  |  |  |  |  |  |  |  |
Repayment of construction loan |  |  |  | $ 88,800 |  |  |  |  |  |  |  |  |  |  |
Construction loan, principal amount |  |  |  | 88,600 |  |  |  |  |  |  |  |  |  |  |
Accrued interest on construction loan |  |  |  | 200 |  |  |  |  |  |  |  |  |  |  |
1919 Ventures [Member] | LCOR/CalSTRS [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |
Debt, net |  |  |  | 44,400 |  |  |  |  |  |  |  |  |  |  |
1919 Ventures [Member] | Other Assets [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Note receivable |  |  |  | $ 44,400 |  |  |  |  |  |  |  |  |  |  |
4040 Wilson Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  | 50.00% | [1],[5] |  | 50.00% |  |  |  |  |  |
Equity Method Investment Summarized Financial Information Debt | [5] |  |  |  |  | $ 57,288 |  | 6,664 |  |  |  |  |  |  |
Mortgage notes payable, net |  |  |  |  |  |  |  | $ 150,000 |  |  |  |  |  |  |
Secured JV Debt [Member] | Brandywine - AI Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 3.22% |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Number of Fixed Rate Mortgages |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |
Repayment of mortgage debt |  | $ 66,500 |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties Sold | property |  | 3 |  |  |  |  |  |  |  |  |  |  |  |  |
Secured JV Debt [Member] | Brandywine - AI Venture [Member] | Fixed rate loan maturing January 1, 2022 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  |  |  |  | $ 26,100 |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  | 4.65% |  |  |  |  |  |  |  |  |
Debt Instrument Maturity Date |  |  |  |  |  | Jan. 01, 2022 |  |  |  |  |  |  |  |  |
Secured Debt [Member] | MAP Ground Lease Venture LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  |  | $ 185,000 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument Maturity Date |  |  | Aug. 01, 2023 |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  |  | 2.45% |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt [Member] | MAP Ground Lease Venture LLC [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  | 6.00% |  |  |  |  |  |  |  |  |  |  |  |
[1] | Ownership percentage represents the Company’s entitlement to residual distributions after payments of priority returns, where applicable.
[2] | See “Brandywine - AI Venture: Station Square and 7101 Wisconsin Avenue” and “Brandywine - AI Venture: Other Than Temporary Impairment” sections below for information discussing activity that occurred during 2018 and 2017 relating to this venture.
[3] | On January 5, 2018, the Company exchanged its 20% interest in Seven Tower Bridge to acquire the remaining 35% interest in Four Tower Bridge. For further information regarding the accounting of the transaction, see “Four Tower Bridge Acquisition” section below.
[4] | The basis difference associated with this venture is allocated between cost and the underlying equity in the net assets of the investee and is accounted for as if the entity were consolidated (i.e., allocated to the Company’s relative share of assets and liabilities with an adjustment to recognize equity in earnings for the appropriate depreciation/amortization).
[5] | During the fourth quarter of 2017, 4040 Wilson obtained a secured construction loan with a total borrowing capacity of $150.0 million.

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- Definition Maturity date of mortgage loan within investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 12 -Paragraph Column C -Subsection 29 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 310 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-29(Column C)) -URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(19)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
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- Definition Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10(3)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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