Fair Value of Financial Instruments - Financial Instruments for which Estimates of Fair Value Differ from Carrying Amounts (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Carrying Amount [Member] | Mortgages Receivable [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Mortgage note receivable	[1]	$ 0	$ 88,000
Carrying Amount [Member] | Unsecured Debt [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Notes payable		1,512,554	1,510,181
Carrying Amount [Member] | Variable Rate Debt [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Variable rate debt		326,410	277,533
Carrying Amount [Member] | Mortgages Payable [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Notes payable	[2]	545,753	639,631
Fair Value [Member] | Mortgages Receivable [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Mortgage note receivable	[1]	0	87,692
Fair Value [Member] | Unsecured Debt [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Notes payable		1,529,346	1,593,212
Fair Value [Member] | Variable Rate Debt [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Variable rate debt		305,522	257,188
Fair Value [Member] | Mortgages Payable [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Notes payable	[2]	$ 597,377	$ 707,241

[1] See Note 2, “Summary of Significant Accounting Policies - Notes Receivable,” for further discussion.

[2] The IRS Philadelphia Campus mortgage debt and Cira South Garage mortgage debt were prepaid prior to their scheduled maturity. See Note 21, “Subsequent Events,” for additional information regarding the prepayment. As of December 31, 2015 the fair value of the IRS Philadelphia Campus mortgage debt and Cira South Garage mortgage debt were $177.4 million and $35.5 million, respectively.