Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Real estate investments at cost:
Notes receivable	$ 11,917	$ 11,960
Liabilities:
Notes payable	1,946,063	1,779,697
Unsecured credit facilities	85,000	75,000
Parent Company [Member]
Real estate investments at cost:
Land	1,318,518	1,249,779
Buildings and improvements	2,711,144	2,590,302
Properties in development	225,793	186,450
Gross real estate investments at cost	4,255,455	4,026,531
Less: accumulated depreciation	892,695	844,873
Total Cost Net of Accumulated Depreciation	3,362,760	3,181,658
Investments in real estate partnerships	339,922	358,849
Net real estate investments	3,702,682	3,540,507
Cash and cash equivalents	36,736	80,684
Restricted cash	8,912	9,520
Accounts receivable, net of allowance for doubtful accounts of $4,286 and $3,922 at June 30, 2014 and December 31, 2013, respectively	33,510	26,319
Straight-line rent receivable, net of reserve of $716 and $547 at June 30, 2014 and December 31, 2013, respectively	53,673	50,612
Notes receivable	11,917	11,960
Deferred costs, less accumulated amortization of $76,838 and $73,231 at June 30, 2014 and December 31, 2013, respectively	73,659	69,963
Acquired lease intangible assets, less accumulated amortization of $30,930 and $25,591 at June 30, 2014 and December 31, 2013, respectively	53,543	44,805
Trading securities held in trust, at fair value	27,604	26,681
Other assets (note 4)	41,535	52,465
Total assets	4,043,771	3,913,516
Liabilities:
Notes payable	1,946,063	1,779,697
Unsecured credit facilities	85,000	75,000
Accounts payable and other liabilities	141,063	147,045
Acquired lease intangible liabilities, less accumulated accretion of $12,130 and $10,102 at June 30, 2014 and December 31, 2013, respectively	29,703	26,729
Tenants’ security and escrow deposits and prepaid rent	23,540	23,911
Total liabilities	2,225,369	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 June 30, 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; 92,357,585 and 92,333,161 shares issued at June 30, 2014 and December 31, 2013, respectively	923	923
Treasury stock at cost, 418,001 and 373,042 shares held at June 30, 2014 and December 31, 2013, respectively	(18,952)	(16,726)
Additional paid in capital	2,431,928	2,426,477
Accumulated other comprehensive loss	(36,412)	(17,404)
Distributions in excess of net income	(916,576)	(874,916)
Total stockholders’ equity	1,785,911	1,843,354
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $8,872 and $7,676 at June 30, 2014 and December 31, 2013, respectively	(1,817)	(1,426)
Limited partners’ interests in consolidated partnerships	34,308	19,206
Total noncontrolling interests	32,491	17,780
Total equity/capital	1,818,402	1,861,134
Total liabilities and equity/capital	4,043,771	3,913,516
Partnership Interest [Member]
Real estate investments at cost:
Land	1,318,518	1,249,779
Buildings and improvements	2,711,144	2,590,302
Properties in development	225,793	186,450
Gross real estate investments at cost	4,255,455	4,026,531
Less: accumulated depreciation	892,695	844,873
Total Cost Net of Accumulated Depreciation	3,362,760	3,181,658
Investments in real estate partnerships	339,922	358,849
Net real estate investments	3,702,682	3,540,507
Cash and cash equivalents	36,736	80,684
Restricted cash	8,912	9,520
Accounts receivable, net of allowance for doubtful accounts of $4,286 and $3,922 at June 30, 2014 and December 31, 2013, respectively	33,510	26,319
Straight-line rent receivable, net of reserve of $716 and $547 at June 30, 2014 and December 31, 2013, respectively	53,673	50,612
Notes receivable	11,917	11,960
Deferred costs, less accumulated amortization of $76,838 and $73,231 at June 30, 2014 and December 31, 2013, respectively	73,659	69,963
Acquired lease intangible assets, less accumulated amortization of $30,930 and $25,591 at June 30, 2014 and December 31, 2013, respectively	53,543	44,805
Trading securities held in trust, at fair value	27,604	26,681
Other assets (note 4)	41,535	52,465
Total assets	4,043,771	3,913,516
Liabilities:
Notes payable	1,946,063	1,779,697
Unsecured credit facilities	85,000	75,000
Accounts payable and other liabilities	141,063	147,045
Acquired lease intangible liabilities, less accumulated accretion of $12,130 and $10,102 at June 30, 2014 and December 31, 2013, respectively	29,703	26,729
Tenants’ security and escrow deposits and prepaid rent	23,540	23,911
Total liabilities	2,225,369	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 June 30, 2014 and December 31, 2013, liquidation preference of $25 per unit	325,000	325,000
General partner; 92,357,585 and 92,333,161 units outstanding at June 30, 2014 and December 31, 2013, respectively	1,497,323	1,535,758
Limited partners; 159,338 and 165,796 units outstanding at June 30, 2014 and December 31, 2013	(1,817)	(1,426)
Accumulated other comprehensive loss	(36,412)	(17,404)
Total partners’ capital	1,784,094	1,841,928
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	34,308	19,206
Total noncontrolling interests	34,308	19,206
Total equity/capital	1,818,402	1,861,134
Total liabilities and equity/capital	$ 4,043,771	$ 3,913,516