Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014 Parent Company [Member]	Dec. 31, 2013 Parent Company [Member]	Sep. 30, 2014 Partnership Interest [Member]	Dec. 31, 2013 Partnership Interest [Member]
Real estate investments at cost:
Land	$ 1,323,452	$ 1,249,779	$ 1,323,452	$ 1,249,779
Buildings and improvements	2,703,109	2,590,302	2,703,109	2,590,302
Properties in development	273,710	186,450	273,710	186,450
Gross real estate investments at cost	4,300,271	4,026,531	4,300,271	4,026,531
Less: accumulated depreciation	909,572	844,873	909,572	844,873
Total Cost Net of Accumulated Depreciation	3,390,699	3,181,658	3,390,699	3,181,658
Real Estate Held-for-sale	12,203	0	12,203	0
Investments in real estate partnerships	328,398	358,849	328,398	358,849
Net real estate investments	3,731,300	3,540,507	3,731,300	3,540,507
Cash and cash equivalents	100,708	80,684	100,708	80,684
Restricted cash	9,339	9,520	9,339	9,520
Accounts receivable, net of allowance for doubtful accounts of $4,543 and $3,922 at September 30, 2014 and December 31, 2013, respectively	25,719	26,319	25,719	26,319
Straight-line rent receivable, net of reserve of $509 and $547 at September 30, 2014 and December 31, 2013, respectively	54,947	50,612	54,947	50,612
Notes receivable	12,132	11,960	12,132	11,960
Deferred costs, less accumulated amortization of $79,504 and $73,231 at September 30, 2014 and December 31, 2013, respectively	72,559	69,963	72,559	69,963
Acquired lease intangible assets, less accumulated amortization of $35,505 and $25,591 at September 30, 2014 and December 31, 2013, respectively	52,240	44,805	52,240	44,805
Trading securities held in trust, at fair value	27,365	26,681	27,365	26,681
Other assets (note 5)	41,779	52,465	41,779	52,465
Total assets	4,128,088	3,913,516	4,128,088	3,913,516
Liabilities:
Notes payable	1,948,243	1,779,697	1,948,243	1,779,697
Unsecured credit facilities	75,000	75,000	75,000	75,000
Accounts payable and other liabilities	173,997	147,045	173,997	147,045
Acquired lease intangible liabilities, less accumulated accretion of $13,013 and $10,102 at September 30, 2014 and December 31, 2013, respectively	31,831	26,729	31,831	26,729
Tenants’ security and escrow deposits and prepaid rent	24,888	23,911	24,888	23,911
Total liabilities	2,253,959	2,052,382	2,253,959	2,052,382
Commitments and contingencies (note 13)
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, 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; 93,243,593 and 92,333,161 shares issued at September 30, 2014 and December 31, 2013, respectively	932	923
Treasury stock at cost, 421,758 and 373,042 shares held at September 30, 2014 and December 31, 2013, respectively	(19,167)	(16,726)
Additional paid in capital	2,484,460	2,426,477
Preferred units of general partner, $0.01 par value per unit, 13,000,000 units issued and outstanding at September 30, 2014 and December 31, 2013, liquidation preference of $25 per unit			325,000	325,000
General partner; 93,243,593 and 92,333,161 units outstanding at September 30, 2014 and December 31, 2013, respectively			1,554,184	1,535,758
Limited partners; 154,170 and 165,796 units outstanding at September 30, 2014 and December 31, 2013			(1,935)	(1,426)
Accumulated other comprehensive loss	(33,963)	(17,404)	(33,963)	(17,404)
Distributions in excess of net income	(912,041)	(874,916)
Total stockholders’ equity	1,845,221	1,843,354
Total partners’ capital			1,843,286	1,841,928
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $8,299 and $7,676 at September 30, 2014 and December 31, 2013, respectively	(1,935)	(1,426)
Limited partners’ interests in consolidated partnerships	30,843	19,206	30,843	19,206
Total noncontrolling interests	28,908	17,780	30,843	19,206
Total equity/capital	1,874,129	1,861,134	1,874,129	1,861,134
Total liabilities and equity/capital	$ 4,128,088	$ 3,913,516	$ 4,128,088	$ 3,913,516