Subsequent Events (Textual) (Details) $ in Thousands				2 Months Ended	12 Months Ended
	Feb. 05, 2016 USD ($) ft²	Feb. 04, 2016 USD ($) ft² property	Jan. 14, 2016 USD ($)	Feb. 26, 2016 segment	Dec. 31, 2015 USD ($) ft² property segment	Dec. 31, 2014 USD ($) ft² property	Dec. 31, 2013 USD ($) ft² property
Subsequent Event [Line Items]
Rentable Square Feet | ft²					4,300,000
Proceeds from the sale of properties					$ 247,228	$ 118,855	$ 423,480
Gain on sale of properties					$ 23,515	6,085	29,166
Write off of Deferred Debt Issuance Cost						152	40
Number of Properties | property					179
Number of Reportable Segments | segment					7
Cira Square [Member] | Secured Debt [Member]
Subsequent Event [Line Items]
Debt Instrument Maturity Date					Sep. 10, 2030
Cira South Garage [Member] | Secured Debt [Member]
Subsequent Event [Line Items]
Debt Instrument Maturity Date					Sep. 10, 2030
Och Ziff Properties [Member]
Subsequent Event [Line Items]
Percentage of property management fee		2.50%
Office Building [Member]
Subsequent Event [Line Items]
Sales Price					$ 374,600	$ 101,150	$ 491,825
Rentable Square Feet | ft²					2,649,926	645,273	2,472,036
Proceeds from the sale of properties					$ 340,098	$ 113,024	$ 418,547
Number of Properties | property					27	8	21
Subsequent Event [Member]
Subsequent Event [Line Items]
Number of Reportable Segments | segment				4
Subsequent Event [Member] | Cira Square [Member] | Secured Debt [Member]
Subsequent Event [Line Items]
Repayments of debt			$ 221,400
Principal repayment of debt			176,800
Repayment of prepayment charges			44,500
Repayment of accrued interest			100
Subsequent Event [Member] | Cira South Garage [Member] | Secured Debt [Member]
Subsequent Event [Line Items]
Repayments of debt			44,500
Principal repayment of debt			35,400
Repayment of prepayment charges			8,900
Subsequent Event [Member] | Cira Square And Cira South Garage | Secured Debt [Member]
Subsequent Event [Line Items]
Write off of Deferred Debt Issuance Cost			$ 13,200
Subsequent Event [Member] | Och Ziff Properties [Member]
Subsequent Event [Line Items]
Rentable Square Feet | ft²		3,924,783
Cash proceeds from series of related transactions		$ 353,400
Number of Properties | property		58
Percentage of fee interests in land sold		100.00%
Subsequent Event [Member] | Office Building [Member] | Cira Square [Member]
Subsequent Event [Line Items]
Sales Price	$ 354,000
Rentable Square Feet | ft²	862,692
Sale of properties, closing cost	$ 3,700
Proceeds from the sale of properties	350,300
Gain on sale of properties	$ 113,400
Property management fee, description			The transaction agreements provide for the purchaser to engage the Company’s management company subsidiary to provide customary property management services for the property with an annual management fee (exclusive of expense reimbursement) not to exceed 1.5% of annual gross revenues of the property.
Property management fee, agreement term			10 years
Property management fee, maximum years required to entitle management fee			7 years
Subsequent Event [Member] | Office Building [Member] | Cira Square [Member] | Maximum [Member]
Subsequent Event [Line Items]
Percentage of property management fee			1.50%