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Investment in Unconsolidated Real Estate Ventures (Textual) (Details) |  |  |  | 3 Months Ended |  |  |  |
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Mar. 02, 2016 USD ($) | Feb. 04, 2016 USD ($) ft² property | Jan. 29, 2016 USD ($) | Mar. 31, 2016 USD ($) ft² a property real_estate_venture Building Apartment_unit | Mar. 31, 2015 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Sep. 30, 2014 a Storey Bed | Jan. 25, 2013 USD ($) a
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | real_estate_venture |  |  |  |  | 16 |  |  |  |  |
Equity Method Investments including net liabilities |  |  |  |  | $ 278,700,000 |  |  |  |  |
Number of Properties | property |  |  |  |  | 121 |  |  |  |  |
Management fees |  |  |  |  | $ 5,235,000 | $ 3,872,000 |  |  |  |
Accounts receivable |  |  |  |  | 14,027,000 |  | $ 17,126,000 |  |  |
Company's share of equity (Company's basis) |  |  |  |  | 278,659,000 |  | 241,004,000 |  |  |
Proceeds from the sale of properties |  |  |  |  | 704,911,000 | 26,778,000 |  |  |  |
Proceeds from sale of real estate venture |  |  |  |  | 4,812,000 | 0 |  |  |  |
Gain (Loss) on Sale |  |  |  |  | 115,456,000 | 9,019,000 |  |  |  |
TB-BDN Plymouth Apartments [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Construction Loan |  |  |  |  | 56,000,000 |  |  |  |  |
Guarantor obligations, current carrying value |  |  |  |  | 3,200,000 |  |  |  |  |
1919 Venture [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Guarantor obligations, current carrying value |  |  |  |  | 88,900,000 |  |  |  |  |
Unconsolidated Real Estate Ventures [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Company's share of equity (Company's basis) | [1],[2] |  |  |  | $ 278,659,000 |  | 241,004,000 |  |  |
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% |  |  |  |  |
evo at Cira Centre South Venture [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Construction Loan |  |  |  |  | $ 97,800,000 |  |  |  |  |
Guarantor obligations, current carrying value |  |  |  |  | 26,500,000 |  |  |  |  |
evo at Cira Centre South Venture [Member] | Construction Financing [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Guarantor obligations, maximum payment |  |  |  |  |  |  | 24,700,000 |  |  |
evo at Cira Centre South Venture [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  | 50.00% |  |  |  |  |  |  |  | 30.00%
Equity Method Investment, Underlying Equity in Net Assets |  |  |  |  |  |  |  |  |  | $ 8,500,000
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 |  |  |  |  |  |  |  | $ 97,800,000 |  |
Cash paid to acquire real estate ventures |  | $ 12,800,000 |  |  |  |  |  |  |  |
Equity method investment percentage of interests acquired by the Company and HSRE |  | 30.00% |  |  |  |  |  |  |  |
Company's share of equity (Company's basis) |  | $ 28,300,000 |  |  |  |  |  |  |  |
evo at Cira Centre South Venture [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] | Construction Financing [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Guarantor obligations, maximum payment |  | $ 26,500,000 |  |  |  |  |  |  |  |
evo at Cira Centre South Venture [Member] | Variable Interest Entity, Not Primary Beneficiary [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] | Variable Interest Entity, Not Primary Beneficiary [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 |  | $ 6,600,000 |  |  |  |  |  |  |  |
MAP Venture [Member] | Variable Interest Entity, Not Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Rentable Square Feet | ft² |  |  | 3,924,783 |  |  |  |  |  |  |
Equity method investment percentage |  |  | 50.00% |  |  |  |  |  |  |
Number of Properties Sold | property |  |  | 58 |  |  |  |  |  |  |
Sales Price |  |  | $ 398,100,000 |  |  |  |  |  |  |
Percentage of fee interests in land sold |  |  | 100.00% |  |  |  |  |  |  |
Ground lease annual payments |  |  |  |  | $ 11,900,000 |  |  |  |  |
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 |  |  | $ 180,800,000 |  |  |  |  |  |  |
Proceeds from the sale of properties |  |  | 354,000,000 |  |  |  |  |  |  |
Ownership percentage value |  |  | $ 25,200,000 |  |  |  |  |  |  |
Coppell Associates [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Equity method investment percentage |  |  |  | 50.00% |  |  |  |  |  |
Proceeds from sale of real estate venture |  |  |  | $ 4,600,000 |  |  |  |  |  |
Gain (Loss) on Sale |  |  |  | 5,700,000 |  |  |  |  |  |
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) |  |  |  | $ 1,100,000 |  |  |  |  |  |
PJP VII [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Guarantor obligations, current carrying value |  |  |  |  | $ 500,000 |  |  |  |  |
Office Properties [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Number of Properties | property |  |  |  |  | 106 |  |  |  |  |
Rentable Square Feet | ft² |  |  |  |  | 4,787,475 |  |  |  |  |
Number of Properties Sold | property |  |  |  |  | 59 |  |  |  |  |
Sales Price |  |  |  |  | $ 752,100,000 |  |  |  |  |
Proceeds from the sale of properties |  |  |  |  | 704,121,000 |  |  |  |  |
Gain (Loss) on Sale | [3] |  |  |  | $ 115,456,000 |  |  |  |  |
Nine Real Estate Ventures [Member] | Office Properties [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Number of unconsolidated investments in Real Estate Ventures | real_estate_venture |  |  |  |  | 9 |  |  |  |  |
Number of Properties | Building |  |  |  |  | 88 |  |  |  |  |
Rentable Square Feet | ft² |  |  |  |  | 8,100,000 |  |  |  |  |
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_venture |  |  |  |  | 2 |  |  |  |  |
Acreage of land | a |  |  |  |  | 4.3 |  |  |  |  |
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_venture |  |  |  |  | 3 |  |  |  |  |
Acreage of land | a |  |  |  |  | 2.2 |  |  |  |  |
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_venture |  |  |  |  | 1 |  |  |  |  |
Number of Property units | Apartment_unit |  |  |  |  | 345 |  |  |  |  |
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_venture |  |  |  |  | 1 |  |  |  |  |
Number of Property units | Apartment_unit |  |  |  |  | 398 |  |  |  |  |
Real Estate Venture [Member] |  |  |  |  |  |  |  |  |  |
Schedule Of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |
Management fees |  |  |  |  | $ 1,500,000 | $ 1,200,000 |  |  |  |
Accounts receivable |  |  |  |  | $ 1,300,000 |  | $ 1,700,000 |  |  |
[1] | 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 March 31, 2016 because the Company sold its entire remaining 50% interest in the applicable real estate venture (See “Coppell Associates” section below) during the three months ended March 31, 2016.
[2] | 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.
[3] | Gain/(Loss) on Sale is net of closing and other transaction related costs.

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