Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Document Fiscal Year Focus	2015
Real estate investments at cost:
Notes receivable	$ 12,169	$ 12,132
Liabilities:
Notes payable	1,889,727	1,946,357
Unsecured credit facilities	105,000	75,000
Parent Company [Member]
Real estate investments at cost:
Land	1,386,126	1,380,211
Buildings and improvements	2,793,939	2,790,137
Properties in development	247,503	239,538
Gross real estate investments at cost	4,427,568	4,409,886
Less: accumulated depreciation	961,830	933,708
Total Cost Net of Accumulated Depreciation	3,465,738	3,476,178
Operating properties held for sale	18,219	0
Investments in real estate partnerships	325,243	333,167
Net real estate investments	3,809,200	3,809,345
Cash and cash equivalents	36,489	113,776
Restricted cash	6,590	8,013
Accounts receivable, net of allowance for doubtful accounts of $5,016 and $4,523 at March 31, 2015 and December 31, 2014, respectively	27,293	30,999
Straight-line rent receivable, net of reserve of $685 and $652 at March 31, 2015 and December 31, 2014, respectively	57,554	55,768
Notes receivable	12,169	12,132
Deferred costs, less accumulated amortization of $84,728 and $81,822 at March 31, 2015 and December 31, 2014, respectively	70,018	71,502
Acquired lease intangible assets, less accumulated amortization of $38,670 and $36,112 at March 31, 2015 and December 31, 2014, respectively	49,363	52,365
Trading securities held in trust, at fair value	29,595	28,134
Other assets	20,126	15,136
Total assets	4,118,397	4,197,170
Liabilities:
Notes payable	1,889,727	1,946,357
Unsecured credit facilities	105,000	75,000
Accounts payable and other liabilities	166,659	181,197
Acquired lease intangible liabilities, less accumulated accretion of $14,930 and $13,993 at March 31, 2015 and December 31, 2014, respectively	31,087	32,143
Tenants’ security and escrow deposits and prepaid rent	27,045	25,991
Total liabilities	2,219,518	2,260,688
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, 2015 and December 31, 2014, with liquidation preferences of $25 per share	325,000	325,000
Common stock, $0.01 par value per share,150,000,000 shares authorized; 94,323,823 and 94,108,061 shares issued at March 31, 2015 and December 31, 2014, respectively	943	941
Treasury stock at cost, 443,514 and 425,246 shares held at March 31, 2015 and December 31, 2014, respectively	(20,638)	(19,382)
Additional paid in capital	2,536,471	2,540,153
Accumulated other comprehensive loss	(69,277)	(57,748)
Distributions in excess of net income	(902,845)	(882,372)
Total stockholders’ equity	1,869,654	1,906,592
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $10,490 and $9,833 at March 31, 2015 and December 31, 2014, respectively	(1,959)	(1,914)
Limited partners’ interests in consolidated partnerships	31,184	31,804
Total noncontrolling interests	29,225	29,890
Total equity	1,898,879	1,936,482
Total liabilities and equity	4,118,397	4,197,170
Partnership Interest [Member]
Real estate investments at cost:
Land	1,386,126	1,380,211
Buildings and improvements	2,793,939	2,790,137
Properties in development	247,503	239,538
Gross real estate investments at cost	4,427,568	4,409,886
Less: accumulated depreciation	961,830	933,708
Total Cost Net of Accumulated Depreciation	3,465,738	3,476,178
Operating properties held for sale	18,219	0
Investments in real estate partnerships	325,243	333,167
Net real estate investments	3,809,200	3,809,345
Cash and cash equivalents	36,489	113,776
Restricted cash	6,590	8,013
Accounts receivable, net of allowance for doubtful accounts of $5,016 and $4,523 at March 31, 2015 and December 31, 2014, respectively	27,293	30,999
Straight-line rent receivable, net of reserve of $685 and $652 at March 31, 2015 and December 31, 2014, respectively	57,554	55,768
Notes receivable	12,169	12,132
Deferred costs, less accumulated amortization of $84,728 and $81,822 at March 31, 2015 and December 31, 2014, respectively	70,018	71,502
Acquired lease intangible assets, less accumulated amortization of $38,670 and $36,112 at March 31, 2015 and December 31, 2014, respectively	49,363	52,365
Trading securities held in trust, at fair value	29,595	28,134
Other assets	20,126	15,136
Total assets	4,118,397	4,197,170
Liabilities:
Notes payable	1,889,727	1,946,357
Unsecured credit facilities	105,000	75,000
Accounts payable and other liabilities	166,659	181,197
Acquired lease intangible liabilities, less accumulated accretion of $14,930 and $13,993 at March 31, 2015 and December 31, 2014, respectively	31,087	32,143
Tenants’ security and escrow deposits and prepaid rent	27,045	25,991
Total liabilities	2,219,518	2,260,688
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, 2015 and December 31, 2014, liquidation preference of $25 per unit	325,000	325,000
General partner; 94,323,823 and 94,108,061 units outstanding at March 31, 2015 and December 31, 2014, respectively	1,613,931	1,639,340
Limited partners; 154,170 units outstanding at March 31, 2015 and December 31, 2014	(1,959)	(1,914)
Accumulated other comprehensive loss	(69,277)	(57,748)
Total partners’ capital	1,867,695	1,904,678
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	31,184	31,804
Total noncontrolling interests	31,184	31,804
Total equity	1,898,879	1,936,482
Total liabilities and equity	$ 4,118,397	$ 4,197,170