Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Real estate investments at cost:
Notes receivable	$ 18,502	$ 23,751
Trading securities held in trust, at fair value	24,457	23,429
Liabilities:
Notes payable	1,767,124	1,771,891
Unsecured credit facilities	125,000	170,000
Parent Company [Member]
Real estate investments at cost:
Land	1,193,500	1,215,659
Buildings and improvements	2,502,979	2,502,186
Properties in development	246,990	192,067
Gross real estate investments at cost	3,943,469	3,909,912
Less: accumulated depreciation	823,601	782,749
Total Cost Net of Accumulated Depreciation	3,119,868	3,127,163
Real Estate Held-for-sale	15,961	0
Investments in real estate partnerships	428,606	442,927
Net real estate investments	3,564,435	3,570,090
Cash and cash equivalents	59,143	22,349
Restricted cash	5,354	6,472
Accounts receivable, net of allowance for doubtful accounts of $3,766 and $3,915 at June 30, 2013 and December 31, 2012, respectively	21,824	26,601
Straight-line rent receivable, net of reserve of $566 and $870 at June 30, 2013 and December 31, 2012, respectively	50,258	49,990
Notes receivable	18,502	23,751
Deferred costs, less accumulated amortization of $72,758 and $69,224 at June 30, 2013 and December 31, 2012, respectively	68,141	69,506
Acquired lease intangible assets, less accumulated amortization of $21,860 and $19,148 at June 30, 2013 and December 31, 2012, respectively	41,331	42,459
Trading securities held in trust, at fair value	24,457	23,429
Other assets	46,458	18,811
Total assets	3,899,903	3,853,458
Liabilities:
Notes payable	1,767,124	1,771,891
Unsecured credit facilities	125,000	170,000
Accounts payable and other liabilities	131,181	127,185
Acquired lease intangible liabilities, less accumulated accretion of $8,245 and $6,636 at June 30, 2013 and December 31, 2012, respectively	26,337	20,325
Tenants’ security and escrow deposits and prepaid rent	14,229	18,146
Total liabilities	2,063,871	2,107,547
Commitments and contingencies (note 12)
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 June 30, 2013 and December 31, 2012, with liquidation preferences of $25 per share	325,000	325,000
Common stock $0.01 par value per share,150,000,000 shares authorized; 92,279,321 and 90,394,486 shares issued at June 30, 2013 and December 31, 2012, respectively	923	904
Treasury stock at cost, 365,303 and 335,347 shares held at June 30, 2013 and December 31, 2012, respectively	(16,352)	(14,924)
Additional paid in capital	2,416,632	2,312,310
Accumulated other comprehensive loss	(31,319)	(57,715)
Distributions in excess of net income	(871,266)	(834,810)
Total stockholders’ equity	1,823,618	1,730,765
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $9,002 and $8,348 at June 30, 2013 and December 31, 2012, respectively	(1,165)	(1,153)
Limited partners’ interests in consolidated partnerships	13,579	16,299
Total noncontrolling interests	12,414	15,146
Total equity/capital	1,836,032	1,745,911
Total liabilities and equity/capital	3,899,903	3,853,458
Partnership Interest [Member]
Real estate investments at cost:
Land	1,193,500	1,215,659
Buildings and improvements	2,502,979	2,502,186
Properties in development	246,990	192,067
Gross real estate investments at cost	3,943,469	3,909,912
Less: accumulated depreciation	823,601	782,749
Total Cost Net of Accumulated Depreciation	3,119,868	3,127,163
Real Estate Held-for-sale	15,961	0
Investments in real estate partnerships	428,606	442,927
Net real estate investments	3,564,435	3,570,090
Cash and cash equivalents	59,143	22,349
Restricted cash	5,354	6,472
Accounts receivable, net of allowance for doubtful accounts of $3,766 and $3,915 at June 30, 2013 and December 31, 2012, respectively	21,824	26,601
Straight-line rent receivable, net of reserve of $566 and $870 at June 30, 2013 and December 31, 2012, respectively	50,258	49,990
Notes receivable	18,502	23,751
Deferred costs, less accumulated amortization of $72,758 and $69,224 at June 30, 2013 and December 31, 2012, respectively	68,141	69,506
Acquired lease intangible assets, less accumulated amortization of $21,860 and $19,148 at June 30, 2013 and December 31, 2012, respectively	41,331	42,459
Trading securities held in trust, at fair value	24,457	23,429
Other assets	46,458	18,811
Total assets	3,899,903	3,853,458
Liabilities:
Notes payable	1,767,124	1,771,891
Unsecured credit facilities	125,000	170,000
Accounts payable and other liabilities	131,181	127,185
Acquired lease intangible liabilities, less accumulated accretion of $8,245 and $6,636 at June 30, 2013 and December 31, 2012, respectively	26,337	20,325
Tenants’ security and escrow deposits and prepaid rent	14,229	18,146
Total liabilities	2,063,871	2,107,547
Commitments and contingencies (note 12)
Stockholders’ equity/Partners' capital:
Preferred units of general partner, $0.01 par value per unit, 13,000,000 units issued and outstanding at June 30, 2013 and December 31, 2012, liquidation preference of $25 per unit	325,000	325,000
General partner; 92,279,321 and 90,394,486 units outstanding at June 30, 2013 and December 31, 2012, respectively	1,529,937	1,463,480
Limited partners; 177,164 units outstanding at June 30, 2013 and December 31, 2012	(1,165)	(1,153)
Accumulated other comprehensive loss	(31,319)	(57,715)
Total partners’ capital	1,822,453	1,729,612
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	13,579	16,299
Total noncontrolling interests	13,579	16,299
Total equity/capital	1,836,032	1,745,911
Total liabilities and equity/capital	3,899,903	3,853,458
Preferred Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	325,000
Common Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	923
Treasury Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	(16,352)
Additional Paid-in Capital [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	2,416,632
Accumulated Other Comprehensive Income (Loss) [Member]
Stockholders’ equity/Partners' capital:
Accumulated other comprehensive loss	(31,319)	(57,715)
Accumulated Other Comprehensive Income (Loss) [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	(31,319)
Accumulated Other Comprehensive Income (Loss) [Member] | Partnership Interest [Member]
Noncontrolling interests:
Total equity/capital	(31,319)	(57,715)
Accumulated Distributions in Excess of Net Income [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	(871,266)
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	(1,165)
Noncontrolling Interests in Limited Partners' Interest in Consolidated Partnerships [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	13,579
Noncontrolling Interests in Limited Partners' Interest in Consolidated Partnerships [Member] | Partnership Interest [Member]
Noncontrolling interests:
Total equity/capital	$ 13,579	$ 16,299