Summary of Significant Accounting Policies (Textual) (Details)			3 Months Ended	12 Months Ended
		May 18, 2017 shares	Dec. 31, 2016 USD ($) Customer	Dec. 31, 2017 USD ($) Customer shares	Dec. 31, 2016 USD ($) Customer	Dec. 31, 2015 USD ($) Customer
Summary Of Significant Accounting Policies (Textual) [Abstract]
Shares used for employee taxes upon vesting of share awards				$ 674,000	$ 879,000	$ 2,055,000
Real estate ventures aggregate indebtedness to third parties			$ 997,466,000	768,912,000	997,466,000
Variable interest entity, assets			417,100,000	412,900,000	417,100,000
Variable interest entity, liabilities			254,600,000	250,400,000	254,600,000
Revenue increased by straight-line rent adjustment				24,900,000	26,300,000	21,600,000
Increased revenue due to amortization of deferred rent				2,200,000	2,100,000	2,000,000
Decrease in revenue due to amortization of lease incentives				1,800,000	2,000,000	1,800,000
Share-based compensation expense				6,300,000	5,600,000	7,300,000
Share-based compensation expense, capitalized				$ 1,200,000	1,000,000	$ 1,900,000
Common shares for future awards | shares				2,661,992
Additional shares authorized for issuance | shares		2,663,886
Total number of shares authorized for issuance | shares		6,500,000
Number of awards granted | shares				3,838,008
Restricted cash			$ 700,000	$ 1,300,000	$ 700,000
Employee Stock Option And Share Appreciation Rights
Summary Of Significant Accounting Policies (Textual) [Abstract]
Common shares for future awards | shares				2,238,590
Customer Concentration Risk [Member] | Rental Revenue [Member]
Summary Of Significant Accounting Policies (Textual) [Abstract]
Number of tenant accounted for 10% or more of the Company's rents | Customer			0	0	0	0
Maximum [Member]
Summary Of Significant Accounting Policies (Textual) [Abstract]
Property, Plant and Equipment, Useful Life				55 years
Tenant Improvements Useful Lives				16 years
Minimum [Member]
Summary Of Significant Accounting Policies (Textual) [Abstract]
Property, Plant and Equipment, Useful Life				5 years
Tenant Improvements Useful Lives				1 year
Unconsolidated Real Estate Ventures [Member]
Summary Of Significant Accounting Policies (Textual) [Abstract]
Real estate ventures aggregate indebtedness to third parties				$ 768,912,000
Capitalized interest				400,000	$ 1,900,000	$ 2,000,000
evo at Cira Centre South Venture [Member]
Summary Of Significant Accounting Policies (Textual) [Abstract]
Guarantees, maximum exposure amount				55,400,000
PJP VII [Member]
Summary Of Significant Accounting Policies (Textual) [Abstract]
Real estate ventures aggregate indebtedness to third parties			$ 4,956,000	4,792,000	4,956,000
Guarantees, maximum exposure amount				400,000
4040 Wilson Venture [Member]
Summary Of Significant Accounting Policies (Textual) [Abstract]
Real estate ventures aggregate indebtedness to third parties	[1]		1,004,000	6,664,000	1,004,000
Guarantees, maximum exposure amount				$ 41,300,000
ASU 2016-09 [Member] | Reclassified from Operating Activities to Financing Activities [Member]
Summary Of Significant Accounting Policies (Textual) [Abstract]
Shares used for employee taxes upon vesting of share awards					$ 900,000	$ 2,100,000
Impairment Charges [Member]
Summary Of Significant Accounting Policies (Textual) [Abstract]
Out-of-period error immaterial in current year			$ 1,200,000

[1] During the fourth quarter of 2017, 4040 Wilson obtained a secured construction loan with a total borrowing capacity of $150.0 million.