Fair Value of Financial Instruments - Financial Instruments for which Estimates of Fair Value Differ from Carrying Amounts (Parenthetical) (Details) - Mortgages Payable [Member] - Fair Value [Member] - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Notes payable	[1]	$ 597,377	$ 707,241
IRS Philadelphia Campus [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Notes payable		177,400
Cira South Garage [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Notes payable		$ 35,500

[1] 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.