Consolidated Balance Sheets - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Document Fiscal Year Focus	2015
Real estate investments at cost:
Notes receivable	$ 11,314	$ 12,132
Liabilities:
Notes payable	1,808,652	1,946,357
Unsecured credit facilities	215,000	75,000
Parent Company [Member]
Real estate investments at cost:
Land	1,431,612	1,380,211
Buildings and improvements	2,880,141	2,790,137
Properties in development	187,240	239,538
Gross real estate investments at cost	4,498,993	4,409,886
Less: accumulated depreciation	1,014,788	933,708
Total Cost Net of Accumulated Depreciation	3,484,205	3,476,178
Investments in real estate partnerships	321,164	333,167
Net real estate investments	3,805,369	3,809,345
Cash and cash equivalents	23,880	113,776
Restricted cash	5,142	8,013
Accounts receivable, net of allowance for doubtful accounts of $5,377 and $4,523 at September 30, 2015 and December 31, 2014, respectively	24,642	30,999
Straight-line rent receivable, net of reserve of $1,176 and $652 at September 30, 2015 and December 31, 2014, respectively	61,435	55,768
Notes receivable	11,314	12,132
Deferred costs, less accumulated amortization of $85,591 and $81,822 at September 30, 2015 and December 31, 2014, respectively	77,599	71,502
Acquired lease intangible assets, less accumulated amortization of $43,350 and $36,112 at September 30, 2015 and December 31, 2014, respectively	108,400	52,365
Trading securities held in trust, at fair value	28,291	28,134
Other assets	24,815	15,136
Total assets	4,170,887	4,197,170
Liabilities:
Notes payable	1,808,652	1,946,357
Unsecured credit facilities	215,000	75,000
Accounts payable and other liabilities	171,304	181,197
Acquired lease intangible liabilities, less accumulated accretion of $16,689 and $13,993 at September 30, 2015 and December 31, 2014, respectively	43,161	32,143
Tenants’ security, escrow deposits and prepaid rent	24,726	25,991
Total liabilities	$ 2,262,843	$ 2,260,688
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 September 30, 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; 94,161,761 and 94,108,061 shares issued at September 30, 2015 and December 31, 2014, respectively	942	941
Treasury stock at cost, 414,541 and 425,246 shares held at September 30, 2015 and December 31, 2014, respectively	(19,438)	(19,382)
Additional paid in capital	2,542,791	2,540,153
Accumulated other comprehensive loss	(62,307)	(57,748)
Distributions in excess of net income	(907,962)	(882,372)
Total stockholders’ equity	1,879,026	1,906,592
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $9,582 and $9,833 at September 30, 2015 and December 31, 2014, respectively	(1,940)	(1,914)
Limited partners’ interests in consolidated partnerships	30,958	31,804
Total noncontrolling interests	29,018	29,890
Total equity	1,908,044	1,936,482
Total liabilities and equity	4,170,887	4,197,170
Partnership Interest [Member]
Real estate investments at cost:
Land	1,431,612	1,380,211
Buildings and improvements	2,880,141	2,790,137
Properties in development	187,240	239,538
Gross real estate investments at cost	4,498,993	4,409,886
Less: accumulated depreciation	1,014,788	933,708
Total Cost Net of Accumulated Depreciation	3,484,205	3,476,178
Investments in real estate partnerships	321,164	333,167
Net real estate investments	3,805,369	3,809,345
Cash and cash equivalents	23,880	113,776
Restricted cash	5,142	8,013
Accounts receivable, net of allowance for doubtful accounts of $5,377 and $4,523 at September 30, 2015 and December 31, 2014, respectively	24,642	30,999
Straight-line rent receivable, net of reserve of $1,176 and $652 at September 30, 2015 and December 31, 2014, respectively	61,435	55,768
Notes receivable	11,314	12,132
Deferred costs, less accumulated amortization of $85,591 and $81,822 at September 30, 2015 and December 31, 2014, respectively	77,599	71,502
Acquired lease intangible assets, less accumulated amortization of $43,350 and $36,112 at September 30, 2015 and December 31, 2014, respectively	108,400	52,365
Trading securities held in trust, at fair value	28,291	28,134
Other assets	24,815	15,136
Total assets	4,170,887	4,197,170
Liabilities:
Notes payable	1,808,652	1,946,357
Unsecured credit facilities	215,000	75,000
Accounts payable and other liabilities	171,304	181,197
Acquired lease intangible liabilities, less accumulated accretion of $16,689 and $13,993 at September 30, 2015 and December 31, 2014, respectively	43,161	32,143
Tenants’ security, escrow deposits and prepaid rent	24,726	25,991
Total liabilities	$ 2,262,843	$ 2,260,688
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, 2015 and December 31, 2014, liquidation preference of $25 per unit	$ 325,000	$ 325,000
General partner; 94,161,761 and 94,108,061 units outstanding at September 30, 2015 and December 31, 2014, respectively	1,616,333	1,639,340
Limited partners; 154,170 units outstanding at September 30, 2015 and December 31, 2014	(1,940)	(1,914)
Accumulated other comprehensive loss	(62,307)	(57,748)
Total partners’ capital	1,877,086	1,904,678
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	30,958	31,804
Total noncontrolling interests	30,958	31,804
Total equity	1,908,044	1,936,482
Total liabilities and equity	$ 4,170,887	$ 4,197,170