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Investment in Unconsolidated Real Estate Ventures (Textual) (Details) $ in Thousands |  |  |  |  | 3 Months Ended | 6 Months Ended |  |  |  |  |
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Jun. 10, 2016 USD ($) | Mar. 02, 2016 USD ($) | Feb. 04, 2016 USD ($) ft² property | Jan. 29, 2016 USD ($) | Jun. 30, 2016 USD ($) | Jun. 30, 2015 USD ($) | Jun. 30, 2016 USD ($) property | Jun. 30, 2015 USD ($) | Jun. 30, 2016 | Jun. 30, 2016 property | Jun. 30, 2016 ft² | Jun. 30, 2016 Real_Estate_Investment | Jun. 30, 2016 a | Jun. 30, 2016 Building | Jun. 30, 2016 Apartment_unit | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Sep. 30, 2014 a Storey Bed | Jan. 25, 2013 USD ($) a
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties | property |  |  |  |  |  |  |  |  |  |  | 120 |  |  |  |  |  |  |  |  |
Management fees |  |  |  |  |  | $ 6,208 | $ 4,659 | $ 11,443 | $ 8,531 |  |  |  |  |  |  |  |  |  |  |
Accounts receivable |  |  |  |  |  | 14,804 |  | 14,804 |  |  |  |  |  |  |  |  | $ 17,126 |  |  |
Proceeds from sale of real estate venture |  |  |  |  |  |  |  | 4,812 | 0 |  |  |  |  |  |  |  |  |  |  |
Net gain on Real Estate Venture transactions |  |  |  |  |  | 3,128 | 0 | 9,057 | 0 |  |  |  |  |  |  |  |  |  |  |
Company's share of equity (Company's basis) |  |  |  |  |  | 272,148 |  | 272,148 |  |  |  |  |  |  |  |  | 241,004 |  |  |
Advance received for refinancing |  |  |  |  |  |  |  | 195,000 | 0 |  |  |  |  |  |  |  |  |  |  |
Cash distribution from unconsolidated real estate ventures |  |  |  |  |  |  |  | 10,298 | 5,774 |  |  |  |  |  |  |  |  |  |  |
Proceeds from the sale of properties |  |  |  |  |  |  |  | 748,395 | 80,268 |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale |  |  |  |  |  | (727) | 1,571 | 114,729 | 10,590 |  |  |  |  |  |  |  |  |  |  |
TB-BDN Plymouth Apartments [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Construction Loan |  |  |  |  |  | 56,000 |  | 56,000 |  |  |  |  |  |  |  |  |  |  |  |
Guarantor obligations, current carrying value |  |  |  |  |  | 3,200 |  | 3,200 |  |  |  |  |  |  |  |  |  |  |  |
1919 Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor obligations, current carrying value |  |  |  |  |  | 88,900 |  | 88,900 |  |  |  |  |  |  |  |  |  |  |  |
Office Properties [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties | property |  |  |  |  |  |  |  |  |  |  | 102 |  |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  |  |  |  |  |  |  |  |  |  | 4,984,700 |  |  |  |  |  |  |  |
Sales Price |  |  |  |  |  | 796,600 |  | $ 796,600 |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties Sold | property |  |  |  |  |  |  |  | 61 |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from the sale of properties |  |  |  |  |  |  |  | $ 747,533 |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale | [1] |  |  |  |  |  |  | 114,704 |  |  |  |  |  |  |  |  |  |  |  |
One Real Estate Ventures [Member] | Office Properties [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |
Area of office property | ft² |  |  |  |  |  |  |  |  |  |  |  | 172,286 |  |  |  |  |  |  |  |
Real Estate Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management fees |  |  |  |  |  | 3,800 | $ 3,400 | 7,200 | $ 6,000 |  |  |  |  |  |  |  |  |  |  |
Accounts receivable |  |  |  |  |  | 1,500 |  | 1,500 |  |  |  |  |  |  |  |  | 1,700 |  |  |
Unconsolidated Real Estate Ventures [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  |  |  |  |  |  |  |  |  | 16 |  |  |  |  |  |  |
Equity Method Investments including net liabilities |  |  |  |  |  | 272,100 |  | 272,100 |  |  |  |  |  |  |  |  |  |  |  |
Company's share of equity (Company's basis) | [2],[3] |  |  |  |  | 272,148 |  | 272,148 |  |  |  |  |  |  |  |  | $ 241,004 |  |  |
Unconsolidated Real Estate Ventures [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  |  |
Unconsolidated Real Estate Ventures [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  | 70.00% |  |  |  |  |  |  |  |  |  |
Unconsolidated Real Estate Ventures [Member] | Eight Real Estate Ventures [Member] | Office Properties [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties | Building |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 73 |  |  |  |  |
Unconsolidated Real Estate Ventures [Member] | Eight Real Estate Ventures [Member] | Office And Industrial Buildings [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  |  |  |  |  |  |  |  |  | 8 |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  |  |  |  |  |  |  |  |  |  | 7,900,000 |  |  |  |  |  |  |  |
Unconsolidated Real Estate Ventures [Member] | Eight Real Estate Ventures [Member] | Industrial Building [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties | Building |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 14 |  |  |  |  |
Unconsolidated Real Estate Ventures [Member] | Two Real Estate Ventures [Member] | Undeveloped Land [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |
Acreage of land | a |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.3 |  |  |  |  |  |
Unconsolidated Real Estate Ventures [Member] | Three Real Estate Ventures [Member] | Under Active Development Land [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |
Acreage of land | a |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.2 |  |  |  |  |  |
Unconsolidated Real Estate Ventures [Member] | One Real Estate Ventures [Member] | Residential Tower [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |
Number of Property units |  |  |  |  |  |  |  |  |  |  | 345 |  |  |  |  | 345 |  |  |  |
Unconsolidated Real Estate Ventures [Member] | One Other Real Estate Ventures [Member] | Apartment Complex [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |
Number of Property units |  |  |  |  |  |  |  |  |  |  | 398 |  |  |  |  | 398 |  |  |  |
1000 Chesterbrook [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
Sales Price |  |  |  |  |  | 32,100 |  | 32,100 |  |  |  |  |  |  |  |  |  |  |  |
Debt assumed by buyer |  |  |  |  |  |  |  | 23,200 |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of real estate venture |  |  |  |  |  |  |  | 9,800 |  |  |  |  |  |  |  |  |  |  |  |
Net gain on Real Estate Venture transactions |  |  |  |  |  |  |  | 3,200 |  |  |  |  |  |  |  |  |  |  |  |
Working capital contribution |  |  |  |  |  | 200 |  | 200 |  |  |  |  |  |  |  |  |  |  |  |
Company's share of equity (Company's basis) |  |  |  |  |  | 5,100 |  | $ 5,100 |  |  |  |  |  |  |  |  |  |  |  |
1000 Chesterbrook [Member] | One Real Estate Ventures [Member] | Office Properties [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
Area of office property | ft² |  |  |  |  |  |  |  |  |  |  |  | 172,286 |  |  |  |  |  |  |  |
evo at Cira Centre South Venture [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30.00%
Company's share of equity (Company's basis) |  |  | $ 28,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity Method Investment, Underlying Equity in Net Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,500
Area of developed parcels of land (in acres) owned by a Real Estate Venture | a |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1 | 1
Number of floors of a building | Storey |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 33 |
Student housing bed capacity | Bed |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 850 |
Percent of share of contribution made on real estate venture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |
Construction Loan |  | $ 97,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 97,800 |  |
Cash paid to acquire real estate ventures |  |  | $ 12,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage of interests acquired by the Company and HSRE |  |  | 30.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity date |  | Jul. 25, 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash distribution from unconsolidated real estate ventures |  | $ 6,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
evo at Cira Centre South Venture [Member] | Secured Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument maturity date |  | Oct. 31, 2019 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term loans |  | $ 117,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, description of variable rate basis |  |  |  |  |  |  |  | LIBOR + 2.25 |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument optional extended maturity date |  | Jun. 30, 2021 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Advance received for refinancing |  | $ 105,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional capacity under the term loan may be funded if certain criterion relating to the operating performance of the student housing tower are met |  | $ 12,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
evo at Cira Centre South Venture [Member] | Secured Debt [Member] | LIBOR [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 2.25% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
evo at Cira Centre South Venture [Member] | Campus Crest Properties, LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30.00%
evo at Cira Centre South Venture [Member] | HSRE, LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40.00%
Cash paid to acquire real estate ventures |  |  | $ 6,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
evo at Cira Centre South Venture [Member] | Maximum [Member] | Secured Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effective interest rate |  | 5.25% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
MAP Venture [Member] | Variable Interest Entity, Not Primary Beneficiary |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  |  | 3,924,783 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sales Price |  |  |  | $ 398,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Properties Sold | property |  |  |  | 58 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of fee interests in land sold |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ground lease annual payments |  |  |  |  |  |  |  | $ 11,900 |  |  |  |  |  |  |  |  |  |  |  |
Ground lease annual rent expense increase percentage |  |  |  |  |  |  |  |  |  | 2.50% |  |  |  |  |  |  |  |  |  |
Reassessment period for change in ground lease annual payments |  |  |  |  |  |  |  | 10 years |  |  |  |  |  |  |  |  |  |  |  |
Ground Lease Terms |  |  |  |  |  |  |  | Annual payments by the MAP Venture, as tenant under the Ground Leases, initially total $11.9 million and increase 2.5% annually through November 2025 |  |  |  |  |  |  |  |  |  |  |  |
Non-Recourse Debt |  |  |  | $ 180,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from the sale of properties |  |  |  | 354,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage value |  |  |  | $ 25,200 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Coppell Associates [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of real estate venture |  |  |  |  | $ 4,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale |  |  |  |  | 5,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Coppell Associates [Member] | Other Liabilities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
Company's share of equity (Company's basis) |  |  |  |  | $ 1,100 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
PJP VII [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Guarantor obligations, current carrying value |  |  |  |  |  | $ 500 |  | $ 500 |  |  |  |  |  |  |  |  |  |  |  |
[1] | Gain/(Loss) on Sale is net of closing and other transaction related costs.
[2] | Does not include the negative investment balance in one real estate venture totaling $1.1 million as of December 31, 2015, which is included in other liabilities. There is no negative investment balance as of June 30, 2016 because the Company sold its entire remaining 50% interest in the applicable real estate venture (See “Coppell Associates” section below) during the three month period ended March 31, 2016.
[3] | This amount includes the effect of the basis difference between the Company's historical cost basis and the basis recorded at the Real Estate Venture level, which is typically amortized over the life of the related assets and liabilities. Basis differentials occur from the impairment of investments, purchases of third party interests in existing Real Estate Ventures and upon the transfer of assets that were previously owned by the Company into a Real Estate Venture. In addition, certain acquisition, transaction and other costs may not be reflected in the net assets at the Real Estate Venture level.

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