Debt Obligations (Consolidated Debt Obligations Outstanding) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014		Dec. 31, 2013
Debt Instrument [Line Items]
Net unamortized premiums/(discounts)	$ (4,409)
Total mortgage indebtedness	662,478		670,151
Total Debt Obligations	2,588,250		2,595,381
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	662,956		669,762
Net unamortized premiums/(discounts)	(478)		389
Total mortgage indebtedness	662,478		670,151
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	1,929,703		1,929,703
Net unamortized premiums/(discounts)	(3,931)		(4,473)
Total unsecured indebtedness	1,925,772		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 Date	Feb. 01, 2015
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 Date	Feb. 01, 2016
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 Date	Feb. 01, 2019
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 Date	Nov. 01, 2014
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 Date	May 15, 2015
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 Date	Apr. 01, 2016
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 Date	May 01, 2017
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 Date	Apr. 15, 2018
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 Date	Feb. 15, 2023
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 Date	Mar. 30, 2035
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 Date	Apr. 30, 2035
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 Date	Jul. 30, 2035
Tysons Corner [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	90,460		91,395
Debt Instrument, Interest Rate, Effective Percentage	5.36%	[2]
Debt Instrument, Maturity Date	Aug. 01, 2015
One Commerce Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	124,151		125,089
Debt Instrument, Interest Rate, Effective Percentage	3.68%	[2]
Debt Instrument, Maturity Date	Jan. 06, 2016
Two Logan Square [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	88,174		88,583
Debt Instrument, Interest Rate, Effective Percentage	7.57%
Debt Instrument, Maturity Date	Apr. 01, 2016
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	21,470		21,630
Debt Instrument, Interest Rate, Effective Percentage	4.25%	[3]
Debt Instrument, Maturity Date	Jan. 01, 2017
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 Date	Apr. 05, 2023
IRS Philadelphia Campus [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	187,751		190,964
Debt Instrument, Interest Rate, Effective Percentage	7.00%
Debt Instrument, Maturity Date	Sep. 10, 2030
Cira South Garage [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 38,950		$ 40,101
Debt Instrument, Interest Rate, Effective Percentage	7.12%
Debt Instrument, Maturity Date	Sep. 10, 2030

[1] 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 real estate venture in which the Company holds a 50% interest.