Debt Obligations (Details Textuals 2) (USD $)	0 Months Ended	12 Months Ended				0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended					12 Months Ended
	Aug. 26, 2010	Dec. 31, 2011 Years	Dec. 31, 2010	Dec. 31, 2009	Jun. 29, 2009	Dec. 15, 2011 Credit Facility [Member]	Dec. 31, 2011 Credit Facility [Member]	Dec. 15, 2011 Three-year Term Loan [Member]	Dec. 31, 2011 Three-year Term Loan [Member]	Dec. 15, 2011 Four-year Term Loan [Member]	Dec. 31, 2011 Four-year Term Loan [Member]	Dec. 15, 2011 Seven-year Term Loan [Member]	Dec. 31, 2011 Seven-year Term Loan [Member]	Dec. 31, 2011 Three and Four-year Term Loan [Member]	Dec. 31, 2011 $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member]	Jun. 29, 2009 IRS Philadelphia Campus [Member]	Jun. 29, 2009 Cira South Garage [Member]	Dec. 31, 2011 IRS Philadelphia Campus and Cira South Garage [Member]	Sep. 10, 2010 IRS Philadelphia Campus and Cira South Garage [Member]	Jun. 29, 2009 IRS Philadelphia Campus and Cira South Garage [Member]	Dec. 31, 2011 Credit Facility Decemeber 2011 [Member] Federal Funds Rate [Member]	Dec. 31, 2011 Unsecured Debt [Member]	Dec. 15, 2011 Unsecured Debt [Member]	Dec. 15, 2011 Unsecured Debt [Member] Three-year Term Loan [Member]	Dec. 15, 2011 Unsecured Debt [Member] Four-year Term Loan [Member]	Dec. 15, 2011 Unsecured Debt [Member] Seven-year Term Loan [Member]	Dec. 31, 2011 Unsecured Debt [Member] $300.0M 5.750% Guaranteed Notes due 2012 [Member]	Dec. 31, 2011 Unsecured Debt [Member] $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity						$ 600,000,000
Debt instrument, face amount																							600,000,000	150,000,000	250,000,000	200,000,000	300,000,000	345,000,000
Debt Instrument, Term						4 years		3 years		4 years		7 years
Repayments of Credit Facility borrowings																							600,000,000	150,000,000	250,000,000	200,000,000	300,000,000	345,000,000
Repayments of unsecured notes																						275,500,000
Debt instrument stated interest rate															3.875%				5.93%								5.75%	3.875%
Declining prepayment penalty in year one													3.00%
Declining prepayment penalty in year two													2.00%
Declining prepayment penalty in year three													1.00%
Debt instrument, basis spread on variable rate							1.50%		1.75%		1.75%		1.90%								0.50%
Option to extend Term Loan							1 year		1 year		1 year
Derivative, Average Fixed Interest Rate													3.623%
Notional amount of interest rate cash flow hedge derivatives														300,000,000
Derivative, lower fixed interest rate range														2.47%
Derivative, higher fixed interest rate range														2.91%
Derivative, lower remaining maturity range														3 years
Derivative, higher remaining maturity range														5 years
Gross proceeds from Issuance of First Mortgage Bond	254,000,000
Forward financing commitment					256,500,000											209,700,000	46,800,000
Loans proceeds withheld pending completion																				2,500,000
Proceeds from mortgage notes payable retained by lender and included in other receivables																		2,100,000
Debt Related Commitment Fees and Debt Issuance Costs	17,700,000
Amortization of deferred financing costs		4,991,000	3,770,000	5,864,000														19,900,000
Amortization of life of the loan using effective interest rate method in years		20
Debt related commitment fees reduced by refund resulting from overpayment		16,000,000
Refund of deferred financing costs related to forward commitment		0	1,659,000	0
Other related transaction costs included in total financing costs		$ 3,800,000