Debt Obligations (Consolidated Debt Obligations Outstanding) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended								12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended							12 Months Ended			12 Months Ended					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Secured Debt [Member]		Dec. 31, 2011 Secured Debt [Member]	Dec. 31, 2012 Unsecured Debt [Member]	Feb. 01, 2012 Unsecured Debt [Member]	Dec. 31, 2011 Unsecured Debt [Member]	Dec. 31, 2012 Bank Term Loan [Member] Unsecured Debt [Member]		Dec. 31, 2011 Bank Term Loan [Member] Unsecured Debt [Member]	Dec. 31, 2012 Former Revolving Credit Facility [Member] Unsecured Debt [Member]		Dec. 31, 2011 Former Revolving Credit Facility [Member] Unsecured Debt [Member]	Dec. 31, 2012 New Revolving Credit Facility [Member] Unsecured Debt [Member]		Dec. 31, 2011 New Revolving Credit Facility [Member] Unsecured Debt [Member]	Dec. 31, 2012 Three-Year Term Loan - Swapped to fixed [Member] Unsecured Debt [Member]		Feb. 01, 2012 Three-Year Term Loan - Swapped to fixed [Member] Unsecured Debt [Member]	Dec. 31, 2011 Three-Year Term Loan - Swapped to fixed [Member] Unsecured Debt [Member]	Dec. 31, 2012 Four Year Term Loan [Member] Unsecured Debt [Member]	Feb. 01, 2012 Four Year Term Loan [Member] Unsecured Debt [Member]	Dec. 31, 2012 Four-Year Term Loan - Swapped to fixed [Member] Unsecured Debt [Member]	Dec. 31, 2012 Four-Year Term Loan - Variable [Member] Unsecured Debt [Member]		Dec. 31, 2011 Four-Year Term Loan - Variable [Member] Unsecured Debt [Member]	Dec. 31, 2012 Seven Year Term Loan - Swapped to fixed [Member] Unsecured Debt [Member]		Feb. 01, 2012 Seven Year Term Loan - Swapped to fixed [Member] Unsecured Debt [Member]		Dec. 31, 2011 Seven Year Term Loan - Swapped to fixed [Member] Unsecured Debt [Member]	Dec. 31, 2012 Bank Term Loan And The Credit Facility [Member] Unsecured Debt [Member]		Dec. 31, 2012 $300.0M 5.750% Guaranteed Notes due 2012 [Member] Unsecured Debt [Member]		Dec. 31, 2011 $300.0M 5.750% Guaranteed Notes due 2012 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $250.0M 5.400% Guaranteed Notes due 2014 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $250.0M 5.400% Guaranteed Notes due 2014 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $250.0M 7.500% Guaranteed Notes due 2015 [Member] Unsecured Debt [Member]		Dec. 31, 2011 $250.0M 7.500% Guaranteed Notes due 2015 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $250.0M 6.000% Guaranteed Notes due 2016 [Member] Unsecured Debt [Member]		Dec. 31, 2011 $250.0M 6.000% Guaranteed Notes due 2016 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $300.0M 5.700% Guaranteed Notes due 2017 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $300.0M 5.700% Guaranteed Notes due 2017 [Member] Unsecured Debt [Member]	Dec. 31, 2012 $325.0M 4.950% Guaranteed Notes due 2018 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $325.0M 4.950% Guaranteed Notes due 2018 [Member] Unsecured Debt [Member]	Dec. 31, 2012 Note 2023 [Member] Unsecured Debt [Member]		Dec. 31, 2011 Note 2023 [Member] Unsecured Debt [Member]	Dec. 31, 2012 Indenture IA (Preferred Trust I) [Member] Unsecured Debt [Member]	Dec. 31, 2011 Indenture IA (Preferred Trust I) [Member] Unsecured Debt [Member]	Dec. 31, 2012 Indenture IB (Preferred Trust I) [Member] Unsecured Debt [Member]	Dec. 31, 2011 Indenture IB (Preferred Trust I) [Member] Unsecured Debt [Member]	Dec. 31, 2012 Indenture II (Preferred Trust II) [Member] Unsecured Debt [Member]	Dec. 31, 2011 Indenture II (Preferred Trust II) [Member] Unsecured Debt [Member]	Dec. 31, 2012 Newtown Square/Berwyn Park/Libertyview [Member] Secured Debt [Member]		Dec. 31, 2011 Newtown Square/Berwyn Park/Libertyview [Member] Secured Debt [Member]	Dec. 31, 2012 Southpoint III [Member] Secured Debt [Member]		Dec. 31, 2011 Southpoint III [Member] Secured Debt [Member]	Dec. 31, 2012 Tysons Corner [Member] Secured Debt [Member]		Dec. 31, 2011 Tysons Corner [Member] Secured Debt [Member]	Dec. 31, 2012 Two Logan Square [Member] Secured Debt [Member]	Dec. 31, 2011 Two Logan Square [Member] Secured Debt [Member]	Dec. 31, 2012 Fairview Eleven Tower [Member] Secured Debt [Member]	Dec. 31, 2011 Fairview Eleven Tower [Member] Secured Debt [Member]	Dec. 31, 2012 IRS Philadelphia Campus [Member] Secured Debt [Member]	Dec. 31, 2011 IRS Philadelphia Campus [Member] Secured Debt [Member]	Dec. 31, 2012 Cira South Garage [Member] Secured Debt [Member]	Dec. 31, 2011 Cira South Garage [Member] Secured Debt [Member]
Participating Mortgage Loans [Line Items]
Secured debt prepayment penalty			$ 100	[1]
Consolidated debt obligations
Long-term Debt, Gross			443,942		512,391	2,027,953		1,888,111	0		37,500	0		275,500	69,000		0	150,000			0				100,000		0	200,000				0			0		151,491	238,379	242,681	166,535		227,329	150,429		250,000	300,000	300,000	325,000	325,000	250,000		0	27,062	27,062	25,774	25,774	25,774	25,774	0		56,538	0		1,887	93,188		94,882	89,340	89,800	22,000	22,000	197,111	202,905	42,303	44,379
Plus: premiums/(discounts), net	(6,565)		(968)		(1,330)	(5,597)		(5,177)
Total mortgage indebtedness	442,974	511,061	442,974		511,061
Total unsecured indebtedness						2,022,356		1,882,934
Total Debt Obligations	2,465,330	2,393,995
Effective interest rate									LIBOR + 0.80%	[2]		LIBOR + 0.725%	[2]		LIBOR + 1.50%	[2]									LIBOR + 1.75%	[2]
Effective interest rate																		2.60%	[2]									3.62%	[2]						5.73%	[3]		5.53%		7.76%	[4]		5.95%	[4]		5.68%		5.13%		4.02%	[4]		2.75%		3.30%		3.09%		7.25%	[1]		7.75%	[1]		5.36%	[5]		7.57%		4.25%		7.00%		7.12%
Debt Instrument Maturity Dates									Feb-12			Feb-12			Feb-16			Feb-15							Feb-16			Feb-19							Apr-12			Nov-14		May-15			Apr-16			May-17		Apr-18		Feb-23			Mar-35		Apr-35		Jul-35		May-13			Apr-14			Aug-15			Apr-16		Jan-17		Sep-30		Sep-30
Debt instrument stated interest rate																																			5.75%			5.40%		7.50%			6.00%			5.70%		4.95%		3.95%	[4]
Debt instrument, basis spread on variable rate									0.80%			0.725%			1.50%			1.75%							1.75%			1.90%
Debt Instrument, Term															4 years			3 years							4 years			7 years
Option to extend Term Loan															1 year			1 year							1 year
Line of Credit Facility, Maximum Borrowing Capacity							600,000
Debt Instrument, Face Amount																		150,000		150,000		100,000	250,000					200,000		200,000	[2]				300,000			250,000		250,000			250,000			300,000		325,000
Repayments of Unsecured Debt	190,485	145,500																															183,000	[2]
Long Term Debt Maturities Repayments Of Principal	$ 2,471,895																							$ 150,000

[1] On December 28, 2012, the Company prepaid the remaining balances of the loans, incurring associated prepayment penalties of $0.1 million.

[2] On February 1, 2012, the Company closed on a new $600.0 million four-year unsecured credit facility and three unsecured term loans (the "New Term Loans") totaling $600.0 which consist of a $150.0 million three-year loan, a $250.0 million four-year loan (with $150.0 million swapped to fixed) and a $200.0 million seven-year loan. The Company used a portion of the net proceeds from the New Term Loans to repay all balances outstanding under its prior revolving credit facility and its prior $183.0 million bank term loan which were then retired prior to their scheduled June 29, 2012 maturity. On December 31, 2012, the Company repaid the entire portion of the four-year loan that was swapped to fixed (see related discussion below).

[3] Notes matured on April 1, 2012, and were repaid using a portion of the proceeds from the term loans.

[4] On December 18, 2012, the Company issued new 3.95% $250.0 million Guaranteed Notes due 2023. The Company used the net proceeds from this debt offering to redeem portions of its 7.500% Guaranteed Notes due May 15, 2015 and 6.000% Guaranteed Notes due April 1, 2016 (see related discussion below).

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