Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Real estate investments at cost:
Notes receivable	$ 11,938	$ 11,960
Liabilities:
Notes payable	1,855,690	1,779,697
Unsecured credit facilities	145,000	75,000
Parent Company [Member]
Real estate investments at cost:
Land	1,315,900	1,249,779
Buildings and improvements	2,683,333	2,590,302
Properties in development	212,783	186,450
Gross real estate investments at cost	4,212,016	4,026,531
Less: accumulated depreciation	864,975	844,873
Total Cost Net of Accumulated Depreciation	3,347,041	3,181,658
Investments in real estate partnerships	353,772	358,849
Net real estate investments	3,700,813	3,540,507
Cash and cash equivalents	35,159	80,684
Restricted cash	9,192	9,520
Accounts receivable, net of allowance for doubtful accounts of $4,009 and $3,922 at March 31, 2014 and December 31, 2013, respectively	32,781	26,319
Straight-line rent receivable, net of reserve of $528 and $547 at March 31, 2014 and December 31, 2013, respectively	52,205	50,612
Notes receivable	11,938	11,960
Deferred costs, less accumulated amortization of $75,413 and $73,231 at March 31, 2014 and December 31, 2013, respectively	70,543	69,963
Acquired lease intangible assets, less accumulated amortization of $27,842 and $25,591 at March 31, 2014 and December 31, 2013, respectively	57,028	44,805
Trading securities held in trust, at fair value	26,976	26,681
Other assets	38,916	52,465
Total assets	4,035,551	3,913,516
Liabilities:
Notes payable	1,855,690	1,779,697
Unsecured credit facilities	145,000	75,000
Accounts payable and other liabilities	136,800	147,045
Acquired lease intangible liabilities, less accumulated accretion of $11,158 and $10,102 at March 31, 2014 and December 31, 2013, respectively	31,128	26,729
Tenants’ security and escrow deposits and prepaid rent	25,847	23,911
Total liabilities	2,194,465	2,052,382
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 March 31, 2014 and December 31, 2013, with liquidation preferences of $25 per share	325,000	325,000
Common stock, $0.01 par value per share,150,000,000 shares authorized; 92,340,435 and 92,333,161 shares issued at March 31, 2014 and December 31, 2013, respectively	923	923
Treasury stock at cost, 406,685 and 373,042 shares held at March 31, 2014 and December 31, 2013, respectively	(18,351)	(16,726)
Additional paid in capital	2,428,166	2,426,477
Accumulated other comprehensive loss	(28,599)	(17,404)
Distributions in excess of net income	(898,787)	(874,916)
Total stockholders’ equity	1,808,352	1,843,354
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $8,136 and $7,676 at March 31, 2014 and December 31, 2013, respectively	(1,782)	(1,426)
Limited partners’ interests in consolidated partnerships	34,516	19,206
Total noncontrolling interests	32,734	17,780
Total equity/capital	1,841,086	1,861,134
Total liabilities and equity/capital	4,035,551	3,913,516
Partnership Interest [Member]
Real estate investments at cost:
Land	1,315,900	1,249,779
Buildings and improvements	2,683,333	2,590,302
Properties in development	212,783	186,450
Gross real estate investments at cost	4,212,016	4,026,531
Less: accumulated depreciation	864,975	844,873
Total Cost Net of Accumulated Depreciation	3,347,041	3,181,658
Investments in real estate partnerships	353,772	358,849
Net real estate investments	3,700,813	3,540,507
Cash and cash equivalents	35,159	80,684
Restricted cash	9,192	9,520
Accounts receivable, net of allowance for doubtful accounts of $4,009 and $3,922 at March 31, 2014 and December 31, 2013, respectively	32,781	26,319
Straight-line rent receivable, net of reserve of $528 and $547 at March 31, 2014 and December 31, 2013, respectively	52,205	50,612
Notes receivable	11,938	11,960
Deferred costs, less accumulated amortization of $75,413 and $73,231 at March 31, 2014 and December 31, 2013, respectively	70,543	69,963
Acquired lease intangible assets, less accumulated amortization of $27,842 and $25,591 at March 31, 2014 and December 31, 2013, respectively	57,028	44,805
Trading securities held in trust, at fair value	26,976	26,681
Other assets	38,916	52,465
Total assets	4,035,551	3,913,516
Liabilities:
Notes payable	1,855,690	1,779,697
Unsecured credit facilities	145,000	75,000
Accounts payable and other liabilities	136,800	147,045
Acquired lease intangible liabilities, less accumulated accretion of $11,158 and $10,102 at March 31, 2014 and December 31, 2013, respectively	31,128	26,729
Tenants’ security and escrow deposits and prepaid rent	25,847	23,911
Total liabilities	2,194,465	2,052,382
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 March 31, 2014 and December 31, 2013, liquidation preference of $25 per unit	325,000	325,000
General partner; 92,340,435 and 92,333,161 units outstanding at March 31, 2014 and December 31, 2013, respectively	1,511,951	1,535,758
Limited partners; 159,338 and 165,796 units outstanding at March 31, 2014 and December 31, 2013	(1,782)	(1,426)
Accumulated other comprehensive loss	(28,599)	(17,404)
Total partners’ capital	1,806,570	1,841,928
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	34,516	19,206
Total noncontrolling interests	34,516	19,206
Total equity/capital	1,841,086	1,861,134
Total liabilities and equity/capital	$ 4,035,551	$ 3,913,516