Debt Obligations (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Consolidated debt obligations
Long-term Debt, Gross		$ 59,800
Plus: unamortized fixed-rate debt premiums (discounts), net	7,427
Less: unamortized exchangeable debt discount	400	900
Total mortgage indebtedness	588,823	711,789
Total unsecured indebtedness	2,466,021
Arboretum I, II, III & V [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0	20,386
Effective interest rate	7.59%
Debt Instrument, Maturity Date	Jul-11
Midlantic Drive/Lenox Drive/DCC I [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0	56,514
Effective interest rate	8.05%
Debt Instrument, Maturity Date	Oct-11
Research Office Center [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0	39,145
Effective interest rate	5.30%
Debt Instrument, Maturity Date	Oct-11
Concord Airport Plaza [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	33,914	34,494
Effective interest rate	5.55%
Debt Instrument, Maturity Date	Jan-12
Newtown Square/Berwyn Park/Libertyview [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	57,334	58,102
Effective interest rate	7.25%
Debt Instrument, Maturity Date	May-13
Southpoint III [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	2,249	2,597
Effective interest rate	7.75%
Debt Instrument, Maturity Date	Apr-14
Tysons Corner [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	95,698	96,507
Effective interest rate	5.36%
Debt Instrument, Maturity Date	Aug-15
Two Logan Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	89,800	89,800
Effective interest rate	7.57%
Debt Instrument, Maturity Date	Apr-16
One Logan Square [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	60,000	60,000
Effective interest rate	4.50%
Debt Instrument, Maturity Date	Jul-16
IRS Philadelphia Campus [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	205,676	208,366
Effective interest rate	6.95%
Debt Instrument, Maturity Date	Sep-30
Cira South Garage [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	45,372	46,335
Effective interest rate	7.11%
Debt Instrument, Maturity Date	Sep-30
$345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	59,835	59,835
Effective interest rate	5.50%
Debt Instrument, Maturity Date	Oct-11
Bank Term Loan [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	183,000	183,000
Effective interest rate	LIBOR + 0.800%
Debt Instrument, Maturity Date	Jun-12
Credit Facility [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	42,000	183,000
Effective interest rate	LIBOR + 0.725%
Debt Instrument, Maturity Date	Jun-12
$300.0M 5.750% Guaranteed Notes due 2012 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	153,694	175,200
Effective interest rate	5.73%
Debt Instrument, Maturity Date	Apr-12
$250.0M 5.400% Guaranteed Notes due 2014 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	242,681	242,681
Effective interest rate	5.53%
Debt Instrument, Maturity Date	Nov-14
$250.0M 7.500% Guaranteed Notes due 2015 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	248,585	250,000
Effective interest rate	7.77%
Debt Instrument, Maturity Date	May-15
$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 Date	Apr-16
$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	5.68%
Debt Instrument, Maturity Date	May-17
$325.0M 4.950% Guaranteed Notes due 2018 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	325,000	0
Effective interest rate	5.14%
Debt Instrument, Maturity Date	Apr-18
Indenture IA (Preferred Trust I) [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	27,062	27,062
Effective interest rate	LIBOR + 1.25%
Debt Instrument, Maturity Date	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	LIBOR + 1.25%
Debt Instrument, Maturity Date	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	LIBOR + 1.25%
Debt Instrument, Maturity Date	Jul-35
Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	590,043	712,246
Plus: unamortized fixed-rate debt premiums (discounts), net	(1,220)	(457)
Total mortgage indebtedness	588,823	711,789
Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	1,883,405	1,722,326
Less: unamortized exchangeable debt discount	(362)	(906)
Less: unamortized fixed-rate debt discounts, net	(5,845)	(2,763)
Total unsecured indebtedness	1,877,198	1,718,657
Total unsecured indebtedness	$ 2,466,021	$ 2,430,446