Debt Obligations (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Mar. 31, 2011	Dec. 31, 2010
Consolidated debt obligations
Net unamortized premiums/(discounts)	$ (6,507)
Less: unamortized exchangeable debt discount				(900)
Total mortgage indebtedness	511,061			711,789
Total Debt Obligations	2,393,995
Effective interest rate	5.50%			5.50%
Effective interest rate	LIBOR plus 0.725			LIBOR plus 0.725
Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	512,391			712,246
Net unamortized premiums/(discounts)	(1,330)			(457)
Total mortgage indebtedness	511,061			711,789
Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	1,888,111			1,722,326
Less: unamortized exchangeable debt discount	0			(906)
Less: unamortized fixed-rate debt discounts, net	(5,177)			(2,763)
Total unsecured indebtedness	1,882,934			1,718,657
Total Debt Obligations	2,393,995			2,430,446
$345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member]
Consolidated debt obligations
Long-term Debt, Gross	59,800
$345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0			59,835
Effective interest rate	5.50%	[1]
Debt Instrument Maturity Dates	Oct-11
Bank Term Loan [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	37,500			183,000
Effective interest rate	LIBOR+0.80%	[2]
Debt Instrument Maturity Dates	Jun-12
Credit Facility [Member] | Unsecured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	275,500			183,000
Effective interest rate	LIBOR+0.725%	[2]
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	151,491			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	227,329			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		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	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	LIBOR+1.25%
Debt Instrument Maturity Dates	Jul-35
Arboretum I, II, III & V [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	0			20,386
Effective interest rate	7.59%	[3]
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%	[4]
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%	[5],[6]
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%	[5],[7]
Debt Instrument Maturity Dates	Jan-12
Newtown Square/Berwyn Park/Libertyview [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	56,538			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	1,887			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	94,882			96,507
Effective interest rate	5.36%	[5]
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%	[8]
Debt Instrument Maturity Dates	Jul-16
Fairview Eleven Tower [Member] | Secured Debt [Member]
Consolidated debt obligations
Long-term Debt, Gross	22,000			0
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	202,905			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	$ 44,379			$ 46,335
Effective interest rate	7.12%
Debt Instrument Maturity Dates	Sep-30

[1] On October 20, 2011, holders representing $59.5 million of the outstanding Exchangeable Notes exercised their right to cause the Company to redeem their notes at par plus accrued and unpaid interest leaving an outstanding balance of $0.4 million. On December 8, 2011, the Company redeemed the remaining balance of the Exchangeable Notes pursuant to its right under the indenture agreement (see related discussion below).

[2] On March 31, 2011, the maturity dates of the Bank Term Loan and the Credit Facility were extended to June 29, 2012 from June 29, 2011. On June 29, 2011, the Company paid a total extension fee amounting to $1.2 million which is equal to 15 basis points of the outstanding principal balance of the Bank Term Loan and of the committed amount under the Credit Facility. The extension of the maturity dates was at the Company's option under the Bank Term Loan and the Credit Facility agreements. There were no changes in the terms and conditions of the loan agreements as a result of the maturity date extensions. On December 15, 2011, the Company entered into binding agreements for lender commitments related a new $600.0 million four -year unsecured credit facility (the "New Credit Facility") and three unsecured term loans in the same aggregate amount of $600.0 million (collectively, the “New Term Loans”), consisting of a $150.0 million three-year loan, a $250.0 million four-year loan and a $200.0 million seven-year loan. Please see below for a related discussion regarding the New Credit Facility and Term Loans

[3] On April 1, 2011, the Company prepaid the remaining balance of the loan without penalty.

[4] On June 3, 2011, the Company prepaid the remaining balance of the loan without penalty.

[5] These loans were assumed upon acquisition of the related properties. The interest rates reflect the market rate at the time of acquisition.

[6] On June 30, 2011, the Company prepaid the remaining balance of the loan without penalty. The unamortized fixed-rate debt premium of $0.3 million related to this loan was included as part of the gain (loss) on early extinguishment of debt in the Company's consolidated statement of operations during the current year.

[7] On September 1, 2011, the Company prepaid the remaining balance of the loan without penalty. The unamortized fixed-rate debt premium of $0.2 million related to this loan was included as part of the gain (loss) on early extinguishment of debt in the Company's consolidated statement of operations during the year.

[8] This mortgage was subject to an interest rate floor of 4.50% on a monthly basis. On July 11, 2011, the Company prepaid the balance of the loan without penalty.