Debt Obligations (Consolidated Debt Obligations Outstanding) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Debt Instrument [Line Items]
Net unamortized premiums/(discounts)	$ (4,247)
Total mortgage indebtedness	666,305		670,151
Total Debt Obligations	2,591,806		2,595,381
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	666,350		669,762
Net unamortized premiums/(discounts)	(45)		389
Total mortgage indebtedness	666,305		670,151
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	1,929,703		1,929,703
Net unamortized premiums/(discounts)	(4,202)		(4,473)
Total unsecured indebtedness	1,925,501		1,925,230
Unsecured Debt [Member] | Three Year Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	150,000		150,000
Debt Instrument, Interest Rate, Effective Percentage	2.60%
Debt Instrument Maturity Dates	Feb-15
Unsecured Debt [Member] | Four-Year Term Loan - Variable [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	100,000		100,000
Debt Instrument, Interest Rate Terms	LIBOR + 1.75%	[1]
Debt Instrument Maturity Dates	Feb-16
Unsecured Debt [Member] | Seven Year Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	200,000		200,000
Debt Instrument, Interest Rate, Effective Percentage	3.62%
Debt Instrument Maturity Dates	Feb-19
Unsecured Debt [Member] | 2014 5.400% Notes
Debt Instrument [Line Items]
Long-term Debt, Gross	218,549		218,549
Debt Instrument, Interest Rate, Effective Percentage	5.53%
Debt Instrument Maturity Dates	Nov-14
Unsecured Debt [Member] | $250.0M 7.500% Guaranteed Notes due 2015
Debt Instrument [Line Items]
Long-term Debt, Gross	157,625		157,625
Debt Instrument, Interest Rate, Effective Percentage	7.76%
Debt Instrument Maturity Dates	May-15
Unsecured Debt [Member] | $250.0M 6.000% Guranteed Notes due 2016 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	149,919		149,919
Debt Instrument, Interest Rate, Effective Percentage	5.95%
Debt Instrument Maturity Dates	Apr-16
Unsecured Debt [Member] | $300.0M 5.700% Guaranteed Notes due 2017
Debt Instrument [Line Items]
Long-term Debt, Gross	300,000		300,000
Debt Instrument, Interest Rate, Effective Percentage	5.68%
Debt Instrument Maturity Dates	May-17
Unsecured Debt [Member] | $325.0M 4.950% Guaranteed Notes due 2018
Debt Instrument [Line Items]
Long-term Debt, Gross	325,000		325,000
Debt Instrument, Interest Rate, Effective Percentage	5.13%
Debt Instrument Maturity Dates	Apr-18
Unsecured Debt [Member] | $250.0M 3.950% Guaranteed Notes due 2023
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000		250,000
Debt Instrument, Interest Rate, Effective Percentage	4.02%
Debt Instrument Maturity Dates	Feb-23
Unsecured Debt [Member] | Indenture IA (Preferred Trust I) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	27,062		27,062
Debt Instrument, Interest Rate, Effective Percentage	2.75%
Debt Instrument Maturity Dates	Mar-35
Unsecured Debt [Member] | Indenture IB (Preferred Trust I) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	25,774		25,774
Debt Instrument, Interest Rate, Effective Percentage	3.30%
Debt Instrument Maturity Dates	Apr-35
Unsecured Debt [Member] | Indenture II (Preferred Trust II) [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	25,774		25,774
Debt Instrument, Interest Rate, Effective Percentage	3.09%
Debt Instrument Maturity Dates	Jul-35
Tysons Corner [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	90,918		91,395
Debt Instrument, Interest Rate, Effective Percentage	5.36%	[2]
Debt Instrument Maturity Dates	Aug-15
One Commerce Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	124,604		125,089
Debt Instrument, Interest Rate, Effective Percentage	3.68%	[2]
Debt Instrument Maturity Dates	Jan-16
Two Logan Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	88,362		88,583
Debt Instrument, Interest Rate, Effective Percentage	7.57%
Debt Instrument Maturity Dates	Apr-16
Fairview Eleven Tower [Member]
Debt Instrument [Line Items]
Equity Method Investment, Ownership Percentage	50.00%
Fairview Eleven Tower [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	21,566		21,630
Debt Instrument, Interest Rate, Effective Percentage	4.25%	[3]
Debt Instrument Maturity Dates	Jan-17
Two Commerce Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	112,000		112,000
Debt Instrument, Interest Rate, Effective Percentage	4.51%	[2]
Debt Instrument Maturity Dates	Apr-23
IRS Philadelphia Campus [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	189,370		190,964
Debt Instrument, Interest Rate, Effective Percentage	7.00%
Debt Instrument Maturity Dates	Sep-30
Cira South Garage [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 39,530		$ 40,101
Debt Instrument, Interest Rate, Effective Percentage	7.12%
Debt Instrument Maturity Dates	Sep-30

[1] (c)London Interbank Offered Rate (“LIBOR”).

[2] (a)These loans were assumed upon acquisition of the related properties. The interest rate reflects the market rate at the time of acquisition.

[3] (b)Represents the full debt amount secured by a property owned by a consolidated joint venture in which the Company holds a 50% interest.