Debt Obligations (Consolidated Debt Obligations Outstanding) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012		Dec. 31, 2011
Consolidated debt obligations
Plus: premiums/(discounts), net	$ (5,750)
Total mortgage indebtedness	505,214		511,061
Total Debt Obligations	2,509,841		2,393,995
Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	506,363		512,391
Plus: premiums/(discounts), net	(1,149)		(1,330)
Total mortgage indebtedness	505,214		511,061
Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	2,009,228		1,888,111
Plus: premiums/(discounts), net	(4,601)		(5,177)
Total unsecured indebtedness	2,004,627		1,882,934
Bank Term Loan [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0		37,500
Effective interest rate	LIBOR + 0.80%	[1]
Debt Instrument Maturity Dates	Feb-12
Debt instrument, basis spread on variable rate	0.80%
Former Revolving Credit Facility [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0		275,500
Effective interest rate	LIBOR + 0.725%	[1]
Debt Instrument Maturity Dates	Feb-12
Debt instrument, basis spread on variable rate	0.725%
New Revolving Credit Facility [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0		0
Effective interest rate	LIBOR + 1.50%	[1]
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		0
Effective interest rate	2.60%	[1]
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
Four Year Term Loan [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Debt instrument, basis spread on variable rate	1.75%
Debt Instrument, Term	4 years
Option to extend Term Loan	1 year
Four-Year Term Loan - Swapped to fixed [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	150,000		0
Effective interest rate	2.88%	[1]
Debt Instrument Maturity Dates	Feb-16
Four-Year Term Loan - Variable [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	100,000		0
Effective interest rate	LIBOR + 1.75%	[1]
Debt Instrument Maturity Dates	Feb-16
Seven Year Term Loan - Swapped to fixed [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	200,000		0
Effective interest rate	3.62%	[1]
Debt Instrument Maturity Dates	Feb-19
Debt instrument, basis spread on variable rate	1.90%
Debt Instrument, Term	7 years
$300.0M 5.750% Guaranteed Notes due 2012 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0		151,491
Effective interest rate	5.73%
Debt Instrument Maturity Dates	Apr-12
Debt instrument stated interest rate	5.75%
$250.0M 5.400% Guaranteed Notes due 2014 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	238,379		242,681
Effective interest rate	5.53%
Debt Instrument Maturity Dates	Nov-14
Debt instrument stated interest rate	5.40%
$250.0M 7.500% Guaranteed Notes due 2015 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	217,239		227,329
Effective interest rate	7.77%
Debt Instrument Maturity Dates	May-15
Debt instrument stated interest rate	7.50%
$250.0M 6.000% Guaranteed Notes due 2016 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	250,000		250,000
Effective interest rate	5.95%
Debt Instrument Maturity Dates	Apr-16
Debt instrument stated interest rate	6.00%
$300.0M 5.700% Guaranteed Notes due 2017 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	300,000		300,000
Effective interest rate	5.75%
Debt Instrument Maturity Dates	May-17
Debt instrument stated interest rate	5.70%
$325.0M 4.950% Guaranteed Notes due 2018 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	325,000		325,000
Effective interest rate	5.14%
Debt Instrument Maturity Dates	Apr-18
Debt instrument stated interest rate	4.95%
Indenture IA (Preferred Trust I) [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	27,062		27,062
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		25,774
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		25,774
Effective interest rate	3.09%
Debt Instrument Maturity Dates	Jul-35
Newtown Square/Berwyn Park/Libertyview [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	55,713		56,538
Effective interest rate	7.25%
Debt Instrument Maturity Dates	May-13
Southpoint III [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	1,511		1,887
Effective interest rate	7.75%
Debt Instrument Maturity Dates	Apr-14
Tysons Corner [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	94,045		94,882
Effective interest rate	5.36%	[2]
Debt Instrument Maturity Dates	Aug-15
Two Logan Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	89,687		89,800
Effective interest rate	7.57%
Debt Instrument Maturity Dates	Apr-16
Fairview Eleven Tower [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	22,000		22,000
Effective interest rate	4.25%
Debt Instrument Maturity Dates	Jan-17
IRS Philadelphia Campus [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	200,051		202,905
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	$ 43,356		$ 44,379
Effective interest rate	7.12%
Debt Instrument Maturity Dates	Sep-30

[1] On February 1, 2012, the Company closed on a new $600.0 million four-year unsecured credit facility and three unsecured term loans totaling $600.0 million which consist of a $150.0 million three-year loan, a $250.0 million four-year loan and a $200.0 million seven-year loan. The Company used a portion of the net proceeds from the term loans to repay all balances outstanding under its Former Revolving Credit Facility and its former $183.0 million Bank Term Loan which were then retired prior to their scheduled June 29, 2012 maturity.

[2] This loan was assumed upon acquisition of the related property. The interest rate reflects the market rate at the time of acquisition.