Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Parent Company [Member]
Real estate investments at cost:
Land	$ 1,228,920	$ 1,215,659
Buildings and improvements	2,515,644	2,502,186
Properties in development	208,432	192,067
Gross real estate investments at cost	3,952,996	3,909,912
Less: accumulated depreciation	808,699	782,749
Total Cost Net of Accumulated Depreciation	3,144,297	3,127,163
Investments in real estate partnerships	432,287	442,927
Net real estate investments	3,576,584	3,570,090
Cash and cash equivalents	22,402	22,349
Restricted cash	6,090	6,472
Accounts receivable, net of allowance for doubtful accounts of $3,697 and $3,915 at March 31, 2013 and December 31, 2012, respectively	24,589	26,601
Straight-line rent receivable, net of reserve of $568 and $870 at March 31, 2013 and December 31, 2012, respectively	51,403	49,990
Notes receivable	19,727	23,751
Deferred costs, less accumulated amortization of $71,640 and $69,224 at March 31, 2013 and December 31, 2012, respectively	68,206	69,506
Acquired lease intangible assets, less accumulated amortization of $21,104 and $19,148 at March 31, 2013 and December 31, 2012, respectively	40,391	42,459
Trading securities held in trust, at fair value	24,495	23,429
Other assets	27,500	18,811
Total assets	3,861,387	3,853,458
Liabilities:
Notes payable	1,785,443	1,771,891
Unsecured credit facilities	145,000	170,000
Accounts payable and other liabilities	121,120	127,185
Acquired lease intangible liabilities, less accumulated accretion of $7,393 and $6,636 at March 31, 2013 and December 31, 2012, respectively	19,510	20,325
Tenants’ security and escrow deposits and prepaid rent	14,618	18,146
Total liabilities	2,085,691	2,107,547
Commitments and contingencies (note 11)
Stockholders’ equity/Partners' capital:
Preferred stock, $0.01 par value per share, 30,000,000 shares authorized; 13,000,000 Series 6 and 7 shares issued and outstanding at March 31, 2013 and December 31, 2012, respectively, with liquidation preferences of $25 per share	325,000	325,000
Common stock $0.01 par value per share,150,000,000 shares authorized; 91,397,030 and 90,394,486 shares issued at March 31, 2013 and December 31, 2012, respectively	914	904
Treasury stock at cost, 333,446 and 335,347 shares held at March 31, 2013 and December 31, 2012, respectively	(15,912)	(14,924)
Additional paid in capital	2,365,607	2,312,310
Accumulated other comprehensive loss	(51,983)	(57,715)
Distributions in excess of net income	(860,832)	(834,810)
Total stockholders’ equity	1,762,794	1,730,765
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $9,374 and $8,348 at March 31, 2013 and December 31, 2012, respectively	(1,193)	(1,153)
Limited partners’ interests in consolidated partnerships	14,095	16,299
Total noncontrolling interests	12,902	15,146
Total equity/capital	1,775,696	1,745,911
Total liabilities and equity/capital	3,861,387	3,853,458
Partnership Interest [Member]
Real estate investments at cost:
Land	1,228,920	1,215,659
Buildings and improvements	2,515,644	2,502,186
Properties in development	208,432	192,067
Gross real estate investments at cost	3,952,996	3,909,912
Less: accumulated depreciation	808,699	782,749
Total Cost Net of Accumulated Depreciation	3,144,297	3,127,163
Investments in real estate partnerships	432,287	442,927
Net real estate investments	3,576,584	3,570,090
Cash and cash equivalents	22,402	22,349
Restricted cash	6,090	6,472
Accounts receivable, net of allowance for doubtful accounts of $3,697 and $3,915 at March 31, 2013 and December 31, 2012, respectively	24,589	26,601
Straight-line rent receivable, net of reserve of $568 and $870 at March 31, 2013 and December 31, 2012, respectively	51,403	49,990
Notes receivable	19,727	23,751
Deferred costs, less accumulated amortization of $71,640 and $69,224 at March 31, 2013 and December 31, 2012, respectively	68,206	69,506
Acquired lease intangible assets, less accumulated amortization of $21,104 and $19,148 at March 31, 2013 and December 31, 2012, respectively	40,391	42,459
Trading securities held in trust, at fair value	24,495	23,429
Other assets	27,500	18,811
Total assets	3,861,387	3,853,458
Liabilities:
Notes payable	1,785,443	1,771,891
Unsecured credit facilities	145,000	170,000
Accounts payable and other liabilities	121,120	127,185
Acquired lease intangible liabilities, less accumulated accretion of $7,393 and $6,636 at March 31, 2013 and December 31, 2012, respectively	19,510	20,325
Tenants’ security and escrow deposits and prepaid rent	14,618	18,146
Total liabilities	2,085,691	2,107,547
Commitments and contingencies (note 11)
Stockholders’ equity/Partners' capital:
Preferred units of general partner, $0.01 par value per unit, 13,000,000 units issued and outstanding at March 31, 2013 and December 31, 2012, respectively, liquidation preference of $25 per unit	325,000	325,000
General partner; 91,397,030 and 90,394,486 units outstanding at March 31, 2013 and December 31, 2012, respectively	1,489,777	1,463,480
Limited partners; 177,164 units outstanding at March 31, 2013 and December 31, 2012	(1,193)	(1,153)
Accumulated other comprehensive loss	(51,983)	(57,715)
Total partners’ capital	1,761,601	1,729,612
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	14,095	16,299
Total noncontrolling interests	14,095	16,299
Total equity/capital	1,775,696	1,745,911
Total liabilities and equity/capital	$ 3,861,387	$ 3,853,458