Investment in Unconsolidated Real Estate Ventures (Textual) (Details)					3 Months Ended
		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.