Investment in Unconsolidated Real Estate Ventures (Textual) (Details) $ in Thousands						3 Months Ended		6 Months Ended
		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.