Debt Obligations (Details Textuals) (USD $)						12 Months Ended					0 Months Ended													12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Secured Debt [Member]	Dec. 31, 2010 Secured Debt [Member]	Dec. 31, 2009 Secured Debt [Member]	Dec. 31, 2011 Unsecured Debt [Member]	Dec. 31, 2010 Unsecured Debt [Member]	Dec. 15, 2011 Unsecured Debt [Member]	Dec. 31, 2011 Letter of Credit [Member] Unsecured Debt [Member]	Dec. 15, 2011 Credit Facility [Member]	Jun. 29, 2011 Bank Term Loan and the Credit Facility [Member]	Dec. 31, 2011 Bank Term Loan and the Credit Facility [Member]	Dec. 31, 2011 $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member]	Dec. 31, 2011 $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member] Unsecured Debt [Member]	Oct. 20, 2011 $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member] Unsecured Debt [Member]	Sep. 30, 2011 $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $300.0M 5.750% Guaranteed Notes due 2012 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $250.0M 5.400% Guaranteed Notes due 2014 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $250.0M 7.500% Guaranteed Notes due 2015 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $250.0M 6.000% Guaranteed Notes due 2016 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $300.0M 5.700% Guaranteed Notes due 2017 [Member] Unsecured Debt [Member]	Dec. 31, 2011 $325.0M 4.950% Guaranteed Notes due 2018 [Member] Unsecured Debt [Member]	Mar. 31, 2011 $325.0M 4.950% Guaranteed Notes due 2018 [Member] Unsecured Debt [Member]	Dec. 31, 2011 One Logan Square [Member] Secured Debt [Member]
Debt obligations (Textuals)
Debt instrument, face amount								$ 600,000,000						$ 345,000,000			$ 300,000,000	$ 250,000,000	$ 250,000,000	$ 250,000,000	$ 300,000,000	$ 325,000,000
Interest rate floor on a monthly basis																								4.50%
Initial Maturity Date																Oct. 20, 2011
Extended Maturity date												Jun. 29, 2012
Guaranteed Exchangeable Notes tendered and accepted for purchase															59,500,000
Outstanding balance remaining after redeem notes at par plus accrued and unpaid interest															400,000
Debt Instrument, Extension Fee Amount											1,200,000
Basis points of the outstanding principal balance related to debt extension fee											1.50%
Maximum amount by which the Company may increase its credit facility										600,000,000
Option for additional borrowings subject to customary conditions and limitations										200,000,000
Borrowings under the credit facility						275,500,000			2,500,000
Unused availability under the Credit Facility						322,000,000
Weighted average effective interest rate			6.72%	6.59%	6.45%	0.99%	1.03%
Properties encumbered mortgage indebtedness carrying value, net	678,000,000	989,800,000
Weighted average interest rate on the Credit Facility						1.01%	1.02%
Debt instrument stated interest rate													3.875%	3.875%			5.75%	5.40%	7.50%	6.00%	5.70%	4.95%	4.95%
Carrying amount of the equity component													$ 24,400,000