Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets
Land	$ 949,244	$ 949,777
Buildings and improvements	5,404,616	5,389,674
Real estate assets, at cost, total	6,353,860	6,339,451
Accumulated depreciation	(1,552,499)	(1,518,896)
Net operating real estate assets	4,801,361	4,820,555
Properties under development, including land	339,848	334,463
Investments in joint ventures	45,260	45,092
Properties held for sale	14,986	30,517
Total real estate assets	5,201,455	5,230,627
Accounts receivable - affiliates	26,948	33,625
Other assets, net	89,233	88,260
Cash and cash equivalents	59,642	26,669
Restricted cash	5,578	5,991
Total assets	5,382,856	5,385,172
Liabilities
Unsecured notes payable	1,538,471	1,538,212
Secured notes payable	945,134	972,256
Accounts payable and accrued expenses	102,307	101,896
Accrued real estate taxes	20,683	28,452
Distributions payable	56,559	49,969
Other liabilities	69,679	67,679
Total liabilities	2,732,833	2,758,464
Commitments and contingencies
Equity
Common shares of beneficial interest; $0.01 par value per share; 175,000 shares authorized; 99,294 and 99,106 issued; 96,249 and 96,201 outstanding at March 31, 2013 and December 31, 2012, respectively	962	962
Additional paid-in capital	3,590,261	3,587,505
Distributions in excess of net income attributable to common shareholders	(590,831)	(598,951)
Treasury shares, at cost (11,419 and 11,771 common shares at March 31, 2013 and December 31, 2012, respectively)	(412,643)	(425,355)
Accumulated other comprehensive loss	(1,048)	(1,062)
Total common equity	2,586,701	2,563,099
Non-controlling interests	63,322	63,609
Total equity	2,650,023	2,626,708
Total liabilities and equity	$ 5,382,856	$ 5,385,172