Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Assets
Land	$ 868,964	$ 768,016
Buildings and improvements	5,068,560	4,751,654
Real estate assets, at cost, total	5,937,524	5,519,670
Accumulated depreciation	(1,458,451)	(1,432,799)
Net operating real estate assets	4,479,073	4,086,871
Properties under development, including land	301,282	299,870
Investments in joint ventures	49,436	44,844
Properties held for sale	0	11,131
Total real estate assets	4,829,791	4,442,716
Accounts receivable - affiliates	29,742	31,035
Other assets, net	89,706	88,089
Cash and cash equivalents	49,702	55,159
Restricted cash	5,074	5,076
Total assets	5,004,015	4,622,075
Liabilities
Unsecured notes payable	1,380,952	1,380,755
Secured notes payable	1,050,154	1,051,357
Accounts payable and accrued expenses	105,370	93,747
Accrued real estate taxes	17,991	21,883
Distributions payable	47,594	39,364
Other liabilities	90,423	109,276
Total liabilities	2,692,484	2,696,382
Commitments and contingencies
Perpetual preferred units	0	97,925
Equity
Common shares of beneficial interest; $0.01 par value per share; 100,000 shares authorized; 94,927 and 87,377 issued; 91,957 and 84,517 outstanding at March 31, 2012 and December 31, 2011, respectively	919	845
Additional paid-in capital	3,327,961	2,901,024
Distributions in excess of net income attributable to common shareholders	(648,074)	(690,466)
Treasury shares, at cost (12,100 and 12,509 common shares at March 31, 2012 and December 31, 2011, respectively)	(437,215)	(452,003)
Accumulated other comprehensive loss	(675)	(683)
Total common equity	2,242,916	1,758,717
Noncontrolling interests	68,615	69,051
Total equity	2,311,531	1,827,768
Total liabilities and equity	$ 5,004,015	$ 4,622,075