Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Parent Company [Member]
Real estate investments at cost:
Land	$ 1,264,492	$ 1,273,606
Buildings and improvements	2,608,756	2,604,229
Properties in development	250,342	224,077
Gross real estate investments at cost	4,123,590	4,101,912
Less: accumulated depreciation	813,187	791,619
Total Cost Net of Accumulated Depreciation	3,310,403	3,310,293
Investments in real estate partnerships	395,933	386,882
Net real estate investments	3,706,336	3,697,175
Cash and cash equivalents	27,115	11,402
Restricted cash	6,406	6,050
Accounts receivable, net of allowance for doubtful accounts of $3,124 and $3,442 at March 31, 2012 and and December 31, 2011, respectively	42,338	37,733
Straight-line rent receivable, net of allowance of $1,871 and $2,075 at March 31, 2012 and December 31, 2011, respectively	49,625	48,132
Notes receivable	23,883	35,784
Deferred costs, less accumulated amortization of $71,385 and $71,265 at March 31, 2012 and December 31, 2011, respectively	71,094	70,204
Acquired lease intangible assets, less accumulated amortization of $16,705 and $15,588 at March 31, 2012 and December 31, 2011, respectively	25,937	27,054
Trading securities held in trust, at fair value	23,411	21,713
Other assets	28,731	31,824
Total assets	4,004,876	3,987,071
Liabilities:
Notes payable	1,748,358	1,942,440
Unsecured credit facilities	275,000	40,000
Accounts payable and other liabilities	104,819	101,862
Derivative instruments, at fair value	59	37
Acquired lease intangible liabilities, less accumulated accretion of $5,143 and $4,750 at March 31, 2012 and December 31, 2011, respectively	12,269	12,662
Tenants' security and escrow deposits and prepaid rent	19,757	20,416
Total liabilities	2,160,262	2,117,417
Commitments and contingencies (note 10)
Stockholders’ equity/Partners' capital:
Preferred stock, $.01 par value per share, 32,000,000 shares authorized; 13,000,000 and 11,000,000 Series 3-6 shares issued and outstanding at March 31, 2012 and December 31, 2011, respectively, with liquidation preferences of $25 per share	325,000	275,000
Common stock $.01 par value per share, 150,000,000 shares authorized; 89,929,528 and 89,921,858 shares issued at March 31, 2012 and December 31, 2011, respectively	899	899
Treasury stock at cost, 315,867 and 338,714 shares held at March 31, 2012 and December 31, 2011, respectively	(14,222)	(15,197)
Additional paid in capital	2,280,781	2,281,817
Accumulated other comprehensive loss	(69,083)	(71,429)
Distributions in excess of net income	(690,845)	(662,735)
Total stockholders' equity	1,832,530	1,808,355
Noncontrolling interests:
Series D preferred units, aggregate redemption value of $50,000 at December 31, 2011	0	49,158
Exchangeable operating partnership units, aggregate redemption value of $7,880 and $6,665 at March 31, 2012 and December 31, 2011, respectively	(990)	(963)
Limited partners' interests in consolidated partnerships	13,074	13,104
Total noncontrolling interests	12,084	61,299
Total equity/capital	1,844,614	1,869,654
Total liabilities and equity/capital	4,004,876	3,987,071
Partnership Interest [Member]
Real estate investments at cost:
Land	1,264,492	1,273,606
Buildings and improvements	2,608,756	2,604,229
Properties in development	250,342	224,077
Gross real estate investments at cost	4,123,590	4,101,912
Less: accumulated depreciation	813,187	791,619
Total Cost Net of Accumulated Depreciation	3,310,403	3,310,293
Investments in real estate partnerships	395,933	386,882
Net real estate investments	3,706,336	3,697,175
Cash and cash equivalents	27,115	11,402
Restricted cash	6,406	6,050
Accounts receivable, net of allowance for doubtful accounts of $3,124 and $3,442 at March 31, 2012 and and December 31, 2011, respectively	42,338	37,733
Straight-line rent receivable, net of allowance of $1,871 and $2,075 at March 31, 2012 and December 31, 2011, respectively	49,625	48,132
Notes receivable	23,883	35,784
Deferred costs, less accumulated amortization of $71,385 and $71,265 at March 31, 2012 and December 31, 2011, respectively	71,094	70,204
Acquired lease intangible assets, less accumulated amortization of $16,705 and $15,588 at March 31, 2012 and December 31, 2011, respectively	25,937	27,054
Trading securities held in trust, at fair value	23,411	21,713
Other assets	28,731	31,824
Total assets	4,004,876	3,987,071
Liabilities:
Notes payable	1,748,358	1,942,440
Unsecured credit facilities	275,000	40,000
Accounts payable and other liabilities	104,819	101,862
Derivative instruments, at fair value	59	37
Acquired lease intangible liabilities, less accumulated accretion of $5,143 and $4,750 at March 31, 2012 and December 31, 2011, respectively	12,269	12,662
Tenants' security and escrow deposits and prepaid rent	19,757	20,416
Total liabilities	2,160,262	2,117,417
Stockholders’ equity/Partners' capital:
Series D preferred units, par value $100: 500,000 units issued and outstanding at December 31, 2011	0	49,158
Preferred units of general partner, $.01 par value per unit, 13,000,000 and 11,000,000 units issued and outstanding at March 31, 2012 and December 31, 2011, respectively, liquidation preference of $25 per unit	325,000	275,000
General partner; 89,929,528 and 89,921,858 units outstanding at March 31, 2012 and December 31, 2011, respectively	1,576,613	1,604,784
Limited partners; 177,164 units outstanding at March 31, 2012 and December 31, 2011	(990)	(963)
Accumulated other comprehensive loss	(69,083)	(71,429)
Total partners' capital	1,831,540	1,856,550
Noncontrolling interests:
Limited partners' interests in consolidated partnerships	13,074	13,104
Total noncontrolling interests	13,074	13,104
Total equity/capital	1,844,614	1,869,654
Total liabilities and equity/capital	$ 4,004,876	$ 3,987,071