Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
Real estate investments at cost:
Notes receivable	$ 26,128,000	$ 23,751,000
Liabilities:
Notes payable	1,764,437,000	1,771,891,000
Unsecured credit facilities	100,000,000	170,000,000
Parent Company [Member]
Real estate investments at cost:
Land	1,175,815,000	1,215,659,000
Buildings and improvements	2,479,326,000	2,502,186,000
Properties in development	305,249,000	192,067,000
Gross real estate investments at cost	3,960,390,000	3,909,912,000
Less: accumulated depreciation	835,447,000	782,749,000
Total Cost Net of Accumulated Depreciation	3,124,943,000	3,127,163,000
Real Estate Held-for-sale	10,607,000	0
Investments in real estate partnerships	414,509,000	442,927,000
Net real estate investments	3,550,059,000	3,570,090,000
Cash and cash equivalents	62,702,000	22,349,000
Restricted cash	5,288,000	6,472,000
Accounts receivable, net of allowance for doubtful accounts of $3,626 and $3,915 at September 30, 2013 and December 31, 2012, respectively	23,379,000	26,601,000
Straight-line rent receivable, net of reserve of $451 and $870 at September 30, 2013 and December 31, 2012, respectively	50,422,000	49,990,000
Notes receivable	26,128,000	23,751,000
Deferred costs, less accumulated amortization of $72,808 and $69,224 at September 30, 2013 and December 31, 2012, respectively	69,553,000	69,506,000
Acquired lease intangible assets, less accumulated amortization of $23,750 and $19,148 at September 30, 2013 and December 31, 2012, respectively	39,134,000	42,459,000
Trading securities held in trust, at fair value	25,421,000	23,429,000
Other assets	44,467,000	18,811,000
Total assets	3,896,553,000	3,853,458,000
Liabilities:
Notes payable	1,764,437,000	1,771,891,000
Unsecured credit facilities	100,000,000	170,000,000
Accounts payable and other liabilities	147,087,000	127,185,000
Acquired lease intangible liabilities, less accumulated accretion of $9,123 and $6,636 at September 30, 2013 and December 31, 2012, respectively	25,480,000	20,325,000
Tenants’ security and escrow deposits and prepaid rent	22,454,000	18,146,000
Total liabilities	2,059,458,000	2,107,547,000
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 September 30, 2013 and December 31, 2012, with liquidation preferences of $25 per share	325,000,000	325,000,000
Common stock, $0.01 par value per share,150,000,000 shares authorized; 92,327,151 and 90,394,486 shares issued at September 30, 2013 and December 31, 2012, respectively	923,000	904,000
Treasury stock at cost, 369,144 and 335,347 shares held at September 30, 2013 and December 31, 2012, respectively	(16,540,000)	(14,924,000)
Additional paid in capital	2,422,466,000	2,312,310,000
Accumulated other comprehensive loss	(28,430,000)	(57,715,000)
Distributions in excess of net income	(878,753,000)	(834,810,000)
Total stockholders’ equity	1,824,666,000	1,730,765,000
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $8,016 and $8,348 at September 30, 2013 and December 31, 2012, respectively	(1,462,000)	(1,153,000)
Limited partners’ interests in consolidated partnerships	13,891,000	16,299,000
Total noncontrolling interests	12,429,000	15,146,000
Total equity/capital	1,837,095,000	1,745,911,000
Total liabilities and equity/capital	3,896,553,000	3,853,458,000
Partnership Interest [Member]
Real estate investments at cost:
Land	1,175,815,000	1,215,659,000
Buildings and improvements	2,479,326,000	2,502,186,000
Properties in development	305,249,000	192,067,000
Gross real estate investments at cost	3,960,390,000	3,909,912,000
Less: accumulated depreciation	835,447,000	782,749,000
Total Cost Net of Accumulated Depreciation	3,124,943,000	3,127,163,000
Real Estate Held-for-sale	10,607,000	0
Investments in real estate partnerships	414,509,000	442,927,000
Net real estate investments	3,550,059,000	3,570,090,000
Cash and cash equivalents	62,702,000	22,349,000
Restricted cash	5,288,000	6,472,000
Accounts receivable, net of allowance for doubtful accounts of $3,626 and $3,915 at September 30, 2013 and December 31, 2012, respectively	23,379,000	26,601,000
Straight-line rent receivable, net of reserve of $451 and $870 at September 30, 2013 and December 31, 2012, respectively	50,422,000	49,990,000
Notes receivable	26,128,000	23,751,000
Deferred costs, less accumulated amortization of $72,808 and $69,224 at September 30, 2013 and December 31, 2012, respectively	69,553,000	69,506,000
Acquired lease intangible assets, less accumulated amortization of $23,750 and $19,148 at September 30, 2013 and December 31, 2012, respectively	39,134,000	42,459,000
Trading securities held in trust, at fair value	25,421,000	23,429,000
Other assets	44,467,000	18,811,000
Total assets	3,896,553,000	3,853,458,000
Liabilities:
Notes payable	1,764,437,000	1,771,891,000
Unsecured credit facilities	100,000,000	170,000,000
Accounts payable and other liabilities	147,087,000	127,185,000
Acquired lease intangible liabilities, less accumulated accretion of $9,123 and $6,636 at September 30, 2013 and December 31, 2012, respectively	25,480,000	20,325,000
Tenants’ security and escrow deposits and prepaid rent	22,454,000	18,146,000
Total liabilities	2,059,458,000	2,107,547,000
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 September 30, 2013 and December 31, 2012, liquidation preference of $25 per unit	325,000,000	325,000,000
General partner; 92,279,321 and 90,394,486 units outstanding at September 30, 2013 and December 31, 2012, respectively	1,528,096,000	1,463,480,000
Limited partners; 165,796 and 177,164 units outstanding at September 30, 2013 and December 31, 2012	(1,462,000)	(1,153,000)
Accumulated other comprehensive loss	(28,430,000)	(57,715,000)
Total partners’ capital	1,823,204,000	1,729,612,000
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	13,891,000	16,299,000
Total noncontrolling interests	13,891,000	16,299,000
Total equity/capital	1,837,095,000	1,745,911,000
Total liabilities and equity/capital	3,896,553,000	3,853,458,000
Preferred Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	325,000,000
Common Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	923,000
Treasury Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	(16,540,000)
Additional Paid-in Capital [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	2,422,466,000
Accumulated Other Comprehensive Income (Loss) [Member]
Stockholders’ equity/Partners' capital:
Accumulated other comprehensive loss	(28,430,000)	(57,715,000)
Accumulated Other Comprehensive Income (Loss) [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	(28,430,000)
Accumulated Other Comprehensive Income (Loss) [Member] | Partnership Interest [Member]
Noncontrolling interests:
Total equity/capital	(28,430,000)	(57,715,000)
Accumulated Distributions in Excess of Net Income [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	(878,753,000)
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	(1,462,000)
Noncontrolling Interests in Limited Partners' Interest in Consolidated Partnerships [Member] | Parent Company [Member]
Noncontrolling interests:
Total equity/capital	13,891,000
Noncontrolling Interests in Limited Partners' Interest in Consolidated Partnerships [Member] | Partnership Interest [Member]
Noncontrolling interests:
Total equity/capital	13,891,000	16,299,000
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Real estate investments at cost:
Trading securities held in trust, at fair value	$ 25,421,000	$ 23,429,000