Debt Obligations (Consolidated Debt Obligations Outstanding) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Consolidated debt obligations
Plus: premiums/(discounts), net	$ (4,084,000)
Total mortgage indebtedness	670,151,000		442,974,000
Total Debt Obligations	2,595,381,000		2,465,330,000
Repayments of Unsecured Debt	0		190,485,000	145,500,000
Long Term Debt Maturities Repayments Of Principal	2,599,465,000
Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	669,762,000		443,942,000
Plus: premiums/(discounts), net	389,000		(968,000)
Total mortgage indebtedness	670,151,000		442,974,000
Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	1,929,703,000		2,027,953,000
Plus: premiums/(discounts), net	(4,473,000)		(5,597,000)
Total unsecured indebtedness	1,925,230,000		2,022,356,000
New Revolving Credit Facility [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0		69,000,000
Debt Instrument, Interest Rate Terms	LIBOR + 1.50%
Debt Instrument Maturity Dates	Feb-16
Debt instrument, basis spread on variable rate	1.50%
Debt Instrument, Term	4 years
Option to extend Term Loan	1 year
Three-Year Term Loan - Swapped to fixed [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	150,000,000		150,000,000
Effective interest rate	2.60%
Debt Instrument Maturity Dates	Feb-15
Debt instrument, basis spread on variable rate	1.75%
Debt Instrument, Term	3 years
Option to extend Term Loan	1 year
Debt Instrument, Face Amount	150,000,000
Four-Year Term Loan - Variable [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	100,000,000		100,000,000
Debt Instrument, Interest Rate Terms	LIBOR + 1.75%
Debt Instrument Maturity Dates	Feb-16
Debt instrument, basis spread on variable rate	1.75%
Debt Instrument, Term	4 years
Option to extend Term Loan	1 year
Debt Instrument, Face Amount	100,000,000
Seven Year Term Loan - Swapped to fixed [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	200,000,000		200,000,000
Effective interest rate	3.62%
Debt Instrument Maturity Dates	Feb-19
Debt instrument, basis spread on variable rate	1.90%
Debt Instrument, Term	7 years
Debt Instrument, Face Amount	200,000,000
$300.0M 5.750% Guaranteed Notes due 2012 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Debt instrument stated interest rate	5.75%
Debt Instrument, Face Amount	300,000,000
$250.0M 5.400% Guaranteed Notes due 2014 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	218,549,000		238,379,000
Effective interest rate	5.53%
Debt Instrument Maturity Dates	Nov-14
Debt instrument stated interest rate	5.40%
Debt Instrument, Face Amount	250,000,000
$250.0M 7.500% Guaranteed Notes due 2015 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	157,625,000		166,535,000
Effective interest rate	7.76%
Debt Instrument Maturity Dates	May-15
Debt instrument stated interest rate	7.50%
Debt Instrument, Face Amount	250,000,000
$250.0M 6.000% Guaranteed Notes due 2016 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	149,919,000		150,429,000
Effective interest rate	5.95%
Debt Instrument Maturity Dates	Apr-16
Debt instrument stated interest rate	6.00%
Debt Instrument, Face Amount	250,000,000
$300.0M 5.700% Guaranteed Notes due 2017 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	300,000,000		300,000,000
Effective interest rate	5.68%
Debt Instrument Maturity Dates	May-17
Debt instrument stated interest rate	5.70%
Debt Instrument, Face Amount	300,000,000
$325.0M 4.950% Guaranteed Notes due 2018 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	325,000,000		325,000,000
Effective interest rate	5.13%
Debt Instrument Maturity Dates	Apr-18
Debt instrument stated interest rate	4.95%
Debt Instrument, Face Amount	325,000,000
Note 2023 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	250,000,000		250,000,000
Effective interest rate	4.02%
Debt Instrument Maturity Dates	Feb-23
Indenture IA (Preferred Trust I) [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	27,062,000		27,062,000
Effective interest rate	2.75%
Debt Instrument Maturity Dates	Mar-35
Indenture IB (Preferred Trust I) [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	25,774,000		25,774,000
Effective interest rate	3.30%
Debt Instrument Maturity Dates	Apr-35
Indenture II (Preferred Trust II) [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	25,774,000		25,774,000
Effective interest rate	3.09%
Debt Instrument Maturity Dates	Jul-35
Tysons Corner [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	91,395,000		93,188,000
Effective interest rate	5.36%
Debt Instrument Maturity Dates	Aug-15
One Commerce Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	125,089,000		0
Effective interest rate	3.68%	[1]
Debt Instrument Maturity Dates	Jan-16
Two Logan Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	88,583,000		89,340,000
Effective interest rate	7.57%
Debt Instrument Maturity Dates	Apr-16
Fairview Eleven Tower [Member]
Participating Mortgage Loans [Line Items]
Equity Method Investment, Ownership Percentage	50.00%	[2]
Fairview Eleven Tower [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	21,630,000		22,000,000
Effective interest rate	4.25%	[2]
Debt Instrument Maturity Dates	Jan-17
Two Commerce Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	112,000,000		0
Effective interest rate	4.51%	[1]
Debt Instrument Maturity Dates	Apr-23
IRS Philadelphia Campus [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	190,964,000		197,111,000
Effective interest rate	7.00%
Debt Instrument Maturity Dates	Sep-30
Cira South Garage [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	$ 40,101,000		$ 42,303,000
Effective interest rate	7.12%
Debt Instrument Maturity Dates	Sep-30

[1] This loan was assumed upon acquisition of the related properties on December 19, 2013. The interest rate reflects the market rate at the time of acquisition.

[2] (b)Represents the full debt amount of a property in a consolidated joint venture for which the Company maintains a 50% interest.