Investment in Unconsolidated Real Estate Ventures (Textual) (Details)											3 Months Ended			9 Months Ended
		Oct. 24, 2016 USD ($)	Sep. 22, 2016 USD ($)	Aug. 31, 2016 USD ($)	Aug. 19, 2016 USD ($)	Jun. 30, 2016 USD ($) ft²	Jun. 10, 2016 USD ($)	Mar. 02, 2016 USD ($)	Feb. 04, 2016 USD ($) ft² property	Jan. 29, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) ft²	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) property	Sep. 30, 2015 USD ($)	Sep. 30, 2016	Sep. 30, 2016 property	Sep. 30, 2016 ft²	Sep. 30, 2016 Real_Estate_Investment	Sep. 30, 2016 a	Sep. 30, 2016 Apartment_unit	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) a Storey Bed	Sep. 30, 2014 a Storey Bed	Jan. 25, 2013 USD ($) a	Jul. 10, 2012 USD ($) ft² property	Dec. 20, 2011 property
Schedule Of Equity Method Investments [Line Items]
Management fees											$ 6,248,000		$ 4,274,000	$ 17,691,000	$ 12,805,000
Accounts receivable											9,925,000			9,925,000								$ 17,126,000
Deconsolidated mortgage loan														20,582,000	0
Deconsolidated financing liability														12,384,000	0
Investment in Real Estate Ventures, equity method											282,162,000			282,162,000								241,004,000
Capital funded to venture for mortgage debt														27,174,000	62,868,000
Repayment of mortgage debt														302,514,000	10,598,000
Number of Properties | property																	117
Proceeds from sale of real estate venture														20,551,000	0
Net gain on Real Estate Venture transactions											10,472,000		0	19,529,000	0
Advance received for refinancing														195,000,000	0
Cash distribution from unconsolidated real estate ventures														12,565,000	7,401,000
Proceeds from the sale of properties														758,931,000	152,243,000
Gain (Loss) on Sale											(104,000)		6,083,000	114,625,000	16,673,000
TB-BDN Plymouth Apartments [Member]
Schedule Of Equity Method Investments [Line Items]
Construction Loan											56,000,000			56,000,000
Guarantor obligations, current carrying value											3,200,000			3,200,000
1919 Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Guarantor obligations, current carrying value											88,900,000			88,900,000
Office Properties [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²																		5,082,963
Number of Properties | property																	99
Sales Price											807,200,000			$ 807,200,000
Number of Properties Sold | property														63
Proceeds from the sale of properties														$ 757,785,000
Gain (Loss) on Sale	[1]													114,601,000
Office Properties [Member] | 3141 Fairview Park Drive [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²																		587,317
Real Estate Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Management fees											6,200,000		$ 4,300,000	17,700,000	$ 12,800,000
Accounts receivable											1,100,000			1,100,000								1,700,000
Unconsolidated Real Estate Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment																			14
Equity Method Investments including net liabilities											282,200,000			282,200,000
Investment in Real Estate Ventures, equity method	[2],[3]										282,162,000			282,162,000								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%
Unconsolidated Real Estate Ventures [Member] | Seven 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																			7
Rentable Square Feet | ft²																		8,000,000
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] | Two Other 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																			2
Acreage of land | a																				1.4
Unconsolidated Real Estate Ventures [Member] | Two Other Real Estate Ventures [Member] | Residential Tower One [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] | Two Other Real Estate Ventures [Member] | Residential Tower Two [Member]
Schedule Of Equity Method Investments [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment																			1
Number of Property units																	321				321
Unconsolidated Real Estate Ventures [Member] | One Real Estate Venture [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
Brandywine - AI Venture LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																											50.00%
Provision for impairment of investment in real estate ventures														10,400,000
Brandywine - AI Venture LLC [Member] | Current Creek Investments, LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage																											50.00%
Brandywine - AI Venture LLC [Member] | 3141 Fairview Park Drive [Member]
Schedule Of Equity Method Investments [Line Items]
Deconsolidated net assets				$ 45,600,000
Deconsolidated mortgage loan				20,582,000
Deconsolidated financing liability				12,384,000
Investment in Real Estate Ventures, equity method				$ 12,600,000
Recognizing gain or loss on sale												$ 0
Capital funded to venture for mortgage debt														10,300,000
Brandywine - AI Venture LLC [Member] | 3141 Fairview Park Drive [Member] | Subsequent Event [Member]
Schedule Of Equity Method Investments [Line Items]
Repayment of mortgage debt		$ 20,600,000
Brandywine - AI Venture LLC [Member] | Station Square [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²																										497,896
Equity method investment percentage																										50.00%
Provision for impairment of investment in real estate ventures														10,400,000
Number of Properties | property																										3
Cost of equity method investment																										$ 120,600,000
Equity method investment other than temporary impairment charge														5,200,000
Brandywine - AI Venture LLC [Member] | Remaining Properties in Real Estate Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment other than temporary impairment charge														0
Brandywine - AI Venture LLC [Member] | Office Properties [Member] | 3141 Fairview Park Drive [Member]
Schedule Of Equity Method Investments [Line Items]
Number of office properties owned | property																											3
PJP V [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage			25.00%
Proceeds from sale of real estate venture			$ 3,400,000
Equity in income (loss) of Real Estate Ventures			200,000
Net gain on Real Estate Venture transactions			$ 3,200,000
PJP V [Member] | Office Properties [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²																		73,997
Invesco Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity in income (loss) of Real Estate Ventures					$ 0
Net gain on Real Estate Venture transactions					7,000,000
Amount of proceeds, net of closing costs					$ 7,000,000
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%						50.00%
Proceeds from sale of real estate venture						$ 9,800,000
Net gain on Real Estate Venture transactions						3,200,000
Sales Price						32,100,000						$ 32,100,000
Debt assumed by buyer						$ 23,200,000
Working capital contribution											200,000			$ 200,000
1000 Chesterbrook [Member] | One Real Estate Venture [Member] | Office Properties [Member]
Schedule Of Equity Method Investments [Line Items]
Net Rentable Square Feet | ft²						172,286						172,286
evo at Cira Centre South Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Equity method investment percentage								50.00%																	30.00%
Investment in Real Estate Ventures, equity method								$ 28,300,000
Equity Method Investment, Underlying Equity in Net Assets																							$ 0		$ 8,500,000
Area of developed parcels of land (in acres) owned by a Real Estate Venture | a																							0	1	1
Number of floors of a building | Storey																							0	33
Student housing bed capacity | Bed																							0	850
Percent of share of contribution made on real estate venture																							100.00%		0.00%
Construction Loan							$ 97,800,000																$ 97,800,000		$ 0
Cash paid to acquire real estate ventures								$ 12,800,000
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							$ 6,300,000
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							$ 117,000,000
Debt instrument, description of variable rate basis														LIBOR + 2.25
Debt instrument optional extended maturity date							Jun. 30, 2021
Advance received for refinancing							$ 105,000,000
Additional capacity under the term loan may be funded if certain criterion relating to the operating performance of the student housing tower are met							$ 12,000,000
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								$ 6,600,000
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 [Member]
Schedule Of Equity Method Investments [Line Items]
Rentable Square Feet | ft²									3,924,783
Equity method investment percentage									50.00%
Sales Price									$ 398,100,000
Number of Properties Sold | property									58
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%
Investment in Real Estate Ventures, equity method										$ 1,100,000	0			$ 0								$ 1,100,000
PJP VII [Member]
Schedule Of Equity Method Investments [Line Items]
Guarantor obligations, current carrying value											$ 500,000			$ 500,000

[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 September 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.