Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Document Fiscal Year Focus	2014
Real estate investments at cost (notes 2 and 3):
Properties in development	$ 239,538	$ 186,450
Investments in real estate partnerships (note 4)	333,167	358,849
Accounts receivable, net of allowance for doubtful accounts of $4,523 and $3,922 at December 31, 2014 and 2013, respectively	30,999	26,319
Deferred costs, less accumulated amortization of $81,822 and $73,231 at December 31, 2014 and 2013, respectively	71,502	69,963
Acquired lease intangible assets, less accumulated amortization of $36,112 and $25,591 at December 31, 2014 and 2013, respectively (note 6)	52,365	44,805
Parent Company [Member]
Real estate investments at cost (notes 2 and 3):
Land	1,380,211	1,249,779
Buildings and improvements	2,790,137	2,590,302
Properties in development	239,538	186,450
Gross real estate investments at cost	4,409,886	4,026,531
Less: accumulated depreciation	933,708	844,873
Total Cost Net of Accumulated Depreciation	3,476,178	3,181,658
Investments in real estate partnerships (note 4)	333,167	358,849
Net real estate investments	3,809,345	3,540,507
Cash and cash equivalents	113,776	80,684
Restricted cash	8,013	9,520
Accounts receivable, net of allowance for doubtful accounts of $4,523 and $3,922 at December 31, 2014 and 2013, respectively	30,999	26,319
Straight-line rent receivable, net of reserve of $652 and $547 at December 31, 2014 and 2013, respectively	55,768	50,612
Notes receivable (note 5)	12,132	11,960
Deferred costs, less accumulated amortization of $81,822 and $73,231 at December 31, 2014 and 2013, respectively	71,502	69,963
Acquired lease intangible assets, less accumulated amortization of $36,112 and $25,591 at December 31, 2014 and 2013, respectively (note 6)	52,365	44,805
Trading securities held in trust, at fair value (note 14)	28,134	26,681
Other assets	15,136	52,465
Total assets	4,197,170	3,913,516
Liabilities:
Notes payable (note 9)	1,946,357	1,779,697
Unsecured Debt	75,000	75,000
Accounts payable and other liabilities	181,197	147,045
Acquired lease intangible liabilities, less accumulated accretion of $13,993 and $10,102 at December 31, 2014 and 2013, respectively (note 6)	32,143	26,729
Tenants’ security and escrow deposits and prepaid rent	25,991	23,911
Total liabilities	2,260,688	2,052,382
Commitments and contingencies (notes 16 and 17)	0	0
Stockholders’ equity (notes 12 and 13):
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 December 31, 2013 and December 31, 2012, with liquidation preferences of $25 per share	325,000	325,000
Common stock $0.01 par value per share,150,000,000 shares authorized; 94,108,061 and 92,333,161 shares issued at December 31, 2014 and 2013, respectively	941	923
Treasury stock at cost, 425,246 and 373,042 shares held at December 31, 2014 and 2013, respectively	(19,382)	(16,726)
Additional paid in capital	2,540,153	2,426,477
Accumulated other comprehensive loss	(57,748)	(17,404)
Distributions in excess of net income	(882,372)	(874,916)
Total stockholders’ equity	1,906,592	1,843,354
Noncontrolling interests (note 12):
Exchangeable operating partnership units, aggregate redemption value of $7,676 and $8,348 at December 31, 2013 and 2012, respectively	(1,914)	(1,426)
Limited partners’ interests in consolidated partnerships	31,804	19,206
Total noncontrolling interests	29,890	17,780
Total equity	1,936,482	1,861,134
Total liabilities and equity	4,197,170	3,913,516
Partnership Interest [Member]
Real estate investments at cost (notes 2 and 3):
Land	1,380,211	1,249,779
Buildings and improvements	2,790,137	2,590,302
Properties in development	239,538	186,450
Gross real estate investments at cost	4,409,886	4,026,531
Less: accumulated depreciation	933,708	844,873
Total Cost Net of Accumulated Depreciation	3,476,178	3,181,658
Investments in real estate partnerships (note 4)	333,167	358,849
Net real estate investments	3,809,345	3,540,507
Cash and cash equivalents	113,776	80,684
Restricted cash	8,013	9,520
Accounts receivable, net of allowance for doubtful accounts of $4,523 and $3,922 at December 31, 2014 and 2013, respectively	30,999	26,319
Straight-line rent receivable, net of reserve of $652 and $547 at December 31, 2014 and 2013, respectively	55,768	50,612
Notes receivable (note 5)	12,132	11,960
Deferred costs, less accumulated amortization of $81,822 and $73,231 at December 31, 2014 and 2013, respectively	71,502	69,963
Acquired lease intangible assets, less accumulated amortization of $36,112 and $25,591 at December 31, 2014 and 2013, respectively (note 6)	52,365	44,805
Trading securities held in trust, at fair value (note 14)	28,134	26,681
Other assets	15,136	52,465
Total assets	4,197,170	3,913,516
Liabilities:
Notes payable (note 9)	1,946,357	1,779,697
Unsecured Debt	75,000	75,000
Accounts payable and other liabilities	181,197	147,045
Acquired lease intangible liabilities, less accumulated accretion of $13,993 and $10,102 at December 31, 2014 and 2013, respectively (note 6)	32,143	26,729
Tenants’ security and escrow deposits and prepaid rent	25,991	23,911
Total liabilities	2,260,688	2,052,382
Commitments and contingencies (notes 16 and 17)	0	0
Stockholders’ equity (notes 12 and 13):
Preferred Units General Partner	325,000	325,000
General partner; 94,108,061 and 92,333,161 units outstanding at December 31, 2014 and 2013, respectively	1,639,340	1,535,758
Limited partners; 154,170 and 165,796 units outstanding at December 31, 2014 and 2013, respectively	(1,914)	(1,426)
Accumulated other comprehensive loss	(57,748)	(17,404)
Total partners' capital	1,904,678	1,841,928
Noncontrolling interests (note 12):
Limited partners’ interests in consolidated partnerships	31,804	19,206
Total noncontrolling interests	31,804	19,206
Total equity	1,936,482	1,861,134
Total liabilities and equity	$ 4,197,170	$ 3,913,516