Debt Obligations (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010
Consolidated debt obligations
Long-term Debt, Gross	$ 59,800	$ 59,800
Net unamortized premiums/(discounts)	(7,068)
Less: unamortized exchangeable debt discount	100	900
Total mortgage indebtedness	491,867	711,789
Total Debt Obligations	2,492,227
Effective interest rate	LIBOR plus 0.725%
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 Dates	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 Dates	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 Dates	Oct-11
Concord Airport Plaza [Member] \| Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0	34,494
Effective interest rate	5.55%
Debt Instrument Maturity Dates	Jan-12
Newtown Square/Berwyn Park/Libertyview [Member] \| Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	56,940	58,102
Effective interest rate	7.25%
Debt Instrument Maturity Dates	May-13
Southpoint III [Member] \| Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	2,070	2,597
Effective interest rate	7.75%
Debt Instrument Maturity Dates	Apr-14
Tysons Corner [Member] \| Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	95,299	96,507
Effective interest rate	5.36%
Debt Instrument Maturity Dates	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 Dates	Apr-16
One Logan Square [Member] \| Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0	60,000
Effective interest rate	4.50%
Debt Instrument Maturity Dates	Jul-16
IRS Philadelphia Campus [Member] \| Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	204,301	208,366
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	44,879	46,335
Effective interest rate	7.12%
Debt Instrument Maturity Dates	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 Dates	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 Dates	Jun-12
Credit Facility [Member] \| Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	166,000	183,000
Effective interest rate	LIBOR + 0.725%
Debt Instrument Maturity Dates	Jun-12
$300.0M 5.750% Guaranteed Notes due 2012 [Member] \| Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	152,296	175,200
Effective interest rate	5.73%
Debt Instrument Maturity Dates	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 Dates	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 Dates	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 Dates	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 Dates	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 Dates	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 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	LIBOR + 1.25%
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	LIBOR + 1.25%
Debt Instrument Maturity Dates	May-35
Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	493,288	712,246
Net unamortized premiums/(discounts)	(1,421)	(457)
Total mortgage indebtedness	491,867	711,789
Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	2,006,007	1,722,326
Less: unamortized exchangeable debt discount	(91)	(906)
Less: unamortized fixed-rate debt discounts, net	(5,556)	(2,763)
Total unsecured indebtedness	2,000,360	1,718,657
Total Debt Obligations	$ 2,492,227	$ 2,430,446

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Debt Instrument, Interest Rates Effective Percentage.

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Unamortized exchangeable debt discount.

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Total unsecured indebtedness.

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Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date before deducting unamortized discount or premiums (if any). May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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