Consolidated Balance Sheets - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Document Fiscal Year Focus	2015
Real estate investments at cost:
Notes receivable	$ 12,281	$ 12,132
Liabilities:
Notes payable	1,868,700	1,946,357
Unsecured credit facilities	120,000	75,000
Parent Company [Member]
Real estate investments at cost:
Land	1,405,931	1,380,211
Buildings and improvements	2,817,680	2,790,137
Properties in development	216,408	239,538
Gross real estate investments at cost	4,440,019	4,409,886
Less: accumulated depreciation	990,744	933,708
Total Cost Net of Accumulated Depreciation	3,449,275	3,476,178
Operating properties held for sale	29,017	0
Investments in real estate partnerships	309,544	333,167
Net real estate investments	3,787,836	3,809,345
Cash and cash equivalents	27,114	113,776
Restricted cash	6,726	8,013
Accounts receivable, net of allowance for doubtful accounts of $5,066 and $4,523 at June 30, 2015 and December 31, 2014, respectively	25,506	30,999
Straight-line rent receivable, net of reserve of $767 and $652 at June 30, 2015 and December 31, 2014, respectively	59,168	55,768
Notes receivable	12,281	12,132
Deferred costs, less accumulated amortization of $82,561 and $81,822 at June 30, 2015 and December 31, 2014, respectively	74,978	71,502
Acquired lease intangible assets, less accumulated amortization of $41,058 and $36,112 at June 30, 2015 and December 31, 2014, respectively	46,737	52,365
Trading securities held in trust, at fair value	29,530	28,134
Other assets	28,392	15,136
Total assets	4,098,268	4,197,170
Liabilities:
Notes payable	1,868,700	1,946,357
Unsecured credit facilities	120,000	75,000
Accounts payable and other liabilities	142,415	181,197
Acquired lease intangible liabilities, less accumulated accretion of $15,718 and $13,993 at June 30, 2015 and December 31, 2014, respectively	30,125	32,143
Tenants’ security and escrow deposits and prepaid rent	27,161	25,991
Total liabilities	$ 2,188,401	$ 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 June 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; 94,347,602 and 94,108,061 shares issued at June 30, 2015 and December 31, 2014, respectively	943	941
Treasury stock at cost, 412,745 and 425,246 shares held at June 30, 2015 and December 31, 2014, respectively	(19,390)	(19,382)
Additional paid in capital	2,539,008	2,540,153
Accumulated other comprehensive loss	(48,799)	(57,748)
Distributions in excess of net income	(916,027)	(882,372)
Total stockholders’ equity	1,880,735	1,906,592
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $9,093 and $9,833 at June 30, 2015 and December 31, 2014, respectively	(1,940)	(1,914)
Limited partners’ interests in consolidated partnerships	31,072	31,804
Total noncontrolling interests	29,132	29,890
Total equity	1,909,867	1,936,482
Total liabilities and equity	4,098,268	4,197,170
Partnership Interest [Member]
Real estate investments at cost:
Land	1,405,931	1,380,211
Buildings and improvements	2,817,680	2,790,137
Properties in development	216,408	239,538
Gross real estate investments at cost	4,440,019	4,409,886
Less: accumulated depreciation	990,744	933,708
Total Cost Net of Accumulated Depreciation	3,449,275	3,476,178
Operating properties held for sale	29,017	0
Investments in real estate partnerships	309,544	333,167
Net real estate investments	3,787,836	3,809,345
Cash and cash equivalents	27,114	113,776
Restricted cash	6,726	8,013
Accounts receivable, net of allowance for doubtful accounts of $5,066 and $4,523 at June 30, 2015 and December 31, 2014, respectively	25,506	30,999
Straight-line rent receivable, net of reserve of $767 and $652 at June 30, 2015 and December 31, 2014, respectively	59,168	55,768
Notes receivable	12,281	12,132
Deferred costs, less accumulated amortization of $82,561 and $81,822 at June 30, 2015 and December 31, 2014, respectively	74,978	71,502
Acquired lease intangible assets, less accumulated amortization of $41,058 and $36,112 at June 30, 2015 and December 31, 2014, respectively	46,737	52,365
Trading securities held in trust, at fair value	29,530	28,134
Other assets	28,392	15,136
Total assets	4,098,268	4,197,170
Liabilities:
Notes payable	1,868,700	1,946,357
Unsecured credit facilities	120,000	75,000
Accounts payable and other liabilities	142,415	181,197
Acquired lease intangible liabilities, less accumulated accretion of $15,718 and $13,993 at June 30, 2015 and December 31, 2014, respectively	30,125	32,143
Tenants’ security and escrow deposits and prepaid rent	27,161	25,991
Total liabilities	$ 2,188,401	$ 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; 94,347,602 and 94,108,061 units outstanding at June 30, 2015 and December 31, 2014, respectively	1,604,534	1,639,340
Limited partners; 154,170 units outstanding at June 30, 2015 and December 31, 2014	(1,940)	(1,914)
Accumulated other comprehensive loss	(48,799)	(57,748)
Total partners’ capital	1,878,795	1,904,678
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	31,072	31,804
Total noncontrolling interests	31,072	31,804
Total equity	1,909,867	1,936,482
Total liabilities and equity	$ 4,098,268	$ 4,197,170