Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Liabilities:
Unsecured credit facilities	$ 305,000	$ 40,000
Parent Company [Member]
Real estate investments at cost:
Land	1,276,966	1,273,606
Buildings and improvements	2,614,724	2,604,229
Properties in development	249,509	224,077
Gross real estate investments at cost	4,141,199	4,101,912
Less: accumulated depreciation	834,681	791,619
Total Cost Net of Accumulated Depreciation	3,306,518	3,310,293
Investments in real estate partnerships	424,726	386,882
Net real estate investments	3,731,244	3,697,175
Cash and cash equivalents	23,030	11,402
Restricted cash	6,075	6,050
Accounts receivable, net of allowance for doubtful accounts of $3,322 and $3,442 at June 30, 2012 and and December 31, 2011, respectively	37,681	37,733
Straight-line rent receivable, net of allowance of $1,648 and $2,075 at June 30, 2012 and December 31, 2011, respectively	50,934	48,132
Notes receivable	23,865	35,784
Deferred costs, less accumulated amortization of $73,609 and $71,265 at June 30, 2012 and December 31, 2011, respectively	71,597	70,204
Acquired lease intangible assets, less accumulated amortization of $17,339 and $15,588 at June30, 2012 and December 31, 2011, respectively	26,835	27,054
Trading securities held in trust, at fair value	22,455	21,713
Other assets	32,150	31,824
Total assets	4,025,866	3,987,071
Liabilities:
Notes payable	1,758,299	1,942,440
Unsecured credit facilities	305,000	40,000
Accounts payable and other liabilities	111,995	101,862
Derivative instruments, at fair value	73	37
Acquired lease intangible liabilities, less accumulated accretion of $5,452 and $4,750 at June 30, 2012 and December 31, 2011, respectively	11,971	12,662
Tenants' security and escrow deposits and prepaid rent	20,929	20,416
Total liabilities	2,208,267	2,117,417
Commitments and contingencies (note 10)
Stockholders’ equity/Partners' capital:
Preferred stock, $.01 par value per share, 30,000,000 shares authorized; 13,000,000 and 11,000,000 Series 3-6 shares issued and outstanding at June 30, 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; 89,938,760 and 89,921,858 shares issued at June 30, 2012 and December 31, 2011, respectively	899	899
Treasury stock at cost, 328,330 and 338,714 shares held at June 30, 2012 and December 31, 2011, respectively	(14,792)	(15,197)
Additional paid in capital	2,284,464	2,281,817
Accumulated other comprehensive loss	(66,731)	(71,429)
Distributions in excess of net income	(726,444)	(662,735)
Total stockholders' equity	1,802,396	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 $8,428 and $6,665 at June 30, 2012 and December 31, 2011, respectively	(1,041)	(963)
Limited partners' interests in consolidated partnerships	16,244	13,104
Total noncontrolling interests	15,203	61,299
Total equity/capital	1,817,599	1,869,654
Total liabilities and equity/capital	4,025,866	3,987,071
Partnership Interest [Member]
Real estate investments at cost:
Land	1,276,966	1,273,606
Buildings and improvements	2,614,724	2,604,229
Properties in development	249,509	224,077
Gross real estate investments at cost	4,141,199	4,101,912
Less: accumulated depreciation	834,681	791,619
Total Cost Net of Accumulated Depreciation	3,306,518	3,310,293
Investments in real estate partnerships	424,726	386,882
Net real estate investments	3,731,244	3,697,175
Cash and cash equivalents	23,030	11,402
Restricted cash	6,075	6,050
Accounts receivable, net of allowance for doubtful accounts of $3,322 and $3,442 at June 30, 2012 and and December 31, 2011, respectively	37,681	37,733
Straight-line rent receivable, net of allowance of $1,648 and $2,075 at June 30, 2012 and December 31, 2011, respectively	50,934	48,132
Notes receivable	23,865	35,784
Deferred costs, less accumulated amortization of $73,609 and $71,265 at June 30, 2012 and December 31, 2011, respectively	71,597	70,204
Acquired lease intangible assets, less accumulated amortization of $17,339 and $15,588 at June30, 2012 and December 31, 2011, respectively	26,835	27,054
Trading securities held in trust, at fair value	22,455	21,713
Other assets	32,150	31,824
Total assets	4,025,866	3,987,071
Liabilities:
Notes payable	1,758,299	1,942,440
Unsecured credit facilities	305,000	40,000
Accounts payable and other liabilities	111,995	101,862
Derivative instruments, at fair value	73	37
Acquired lease intangible liabilities, less accumulated accretion of $5,452 and $4,750 at June 30, 2012 and December 31, 2011, respectively	11,971	12,662
Tenants' security and escrow deposits and prepaid rent	20,929	20,416
Total liabilities	2,208,267	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 June 30, 2012 and December 31, 2011, respectively, liquidation preference of $25 per unit	325,000	275,000
General partner; 89,938,760 and 89,921,858 units outstanding at June 30, 2012 and December 31, 2011, respectively	1,544,127	1,604,784
Limited partners; 177,164 units outstanding at June 30, 2012 and December 31, 2011	(1,041)	(963)
Accumulated other comprehensive loss	(66,731)	(71,429)
Total partners' capital	1,801,355	1,856,550
Noncontrolling interests:
Limited partners' interests in consolidated partnerships	16,244	13,104
Total noncontrolling interests	16,244	13,104
Total equity/capital	1,817,599	1,869,654
Total liabilities and equity/capital	$ 4,025,866	$ 3,987,071