Debt Obligations (Details Textuals) (USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended	12 Months Ended		9 Months Ended				9 Months Ended									9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Mar. 31, 2011	Sep. 30, 2011 One Logan Square [Member] Secured Debt [Member]	Sep. 30, 2011 Letter of Credit [Member] Unsecured Debt [Member]	Jun. 29, 2011 Bank Term Loan and the Credit Facility [Member]	Sep. 30, 2011 $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member]	Sep. 30, 2011 $345.0M 3.875% Guaranteed Exchangeable Notes due 2026 [Member] Unsecured Debt [Member]	Sep. 30, 2011 $300.0M 5.750% Guaranteed Notes due 2012 [Member]	Sep. 30, 2011 $250.0M 5.400% Guaranteed Notes due 2014 [Member]	Sep. 30, 2011 $250.0M 7.500% Guaranteed Notes due 2015 [Member]	Sep. 30, 2011 $250.0M 6.000% Guaranteed Notes due 2016 [Member]	Sep. 30, 2011 $300.0M 5.700% Guaranteed Notes due 2017 [Member]	Sep. 30, 2011 $325.0M 4.950% Guaranteed Notes due 2018 [Member]	Sep. 30, 2011 Secured Debt [Member]	Sep. 30, 2010 Secured Debt [Member]	Sep. 30, 2011 Unsecured Debt [Member]	Sep. 30, 2010 Unsecured Debt [Member]
Debt obligations (Textuals)
Debt instrument, face amount							$ 345.0		$ 300.0	$ 250.0	$ 250.0	$ 250.0	$ 300.0	$ 325.0
Interest rate floor on a monthly basis	5.50%	5.50%		4.50%
Initial Maturity Date						Jun. 29, 2011		Oct. 20, 2011
Extended Maturity date						Jun. 29, 2012
Redemption price under right to redeem guaranteed exchangeable notes								100% of the principal amount of the Exchangeable Notes plus accrued and unpaid interest
Guaranteed Exchangeable Notes tendered and accepted for purchase								59.5
Percentage of principal amount of the notes to be redeemed								100.00%
Weighted average effective interest rate on mortgage notes payable															6.83%	6.62%	0.98%	1.14%
Extension fee						1.2
Payment of extension fee under the Credit Facility						15 basis points of the committed amount under the Credit Facility
Maximum amount by which the Company may increase its credit facility																	800.0
Borrowings under the credit facility					10.6												166.0
Unused availability under the Credit Facility																	423.4
Weighted-average interest rate on credit facility borrowings															6.83%	6.62%	0.98%	1.14%
Weighted average interest rate on the Credit Facility																	0.95%	0.98%
Interest rate on Loans			4.95%				3.875%		5.75%	5.40%	7.50%	6.00%	5.70%	4.95%
Initial exchange rate for Guaranteed Exchangeable Notes								25.4065 shares per $1,000 principal amount of notes
Initial exchange price per share								$ 39.36
Carrying amount of the equity component							$ 24.4