Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Real estate investments at cost:
Notes receivable	$ 10,480	$ 10,480
Liabilities:
Notes payable	1,674,631	1,699,771
Unsecured credit facilities	309,585	164,514
Parent Company [Member]
Real estate investments at cost:
Land, including amounts held for future development	1,639,958	1,479,814
Buildings and improvements	3,044,793	2,896,396
Properties in development	137,562	169,690
Gross real estate investments at cost	4,822,313	4,545,900
Less: accumulated depreciation	1,079,448	1,043,787
Total Cost Net of Accumulated Depreciation	3,742,865	3,502,113
Properties held for sale	29,365	0
Investments in real estate partnerships	279,270	306,206
Net real estate investments	4,051,500	3,808,319
Cash and cash equivalents	24,939	36,856
Restricted cash	3,505	3,767
Accounts receivable, net of allowance for doubtful accounts of $5,534 and $5,295 at June 30, 2016 and December 31, 2015, respectively	27,765	32,292
Straight-line rent receivable, net of reserve of $2,961 and $1,365 at June 30, 2016 and December 31, 2015, respectively	66,291	63,392
Notes receivable	10,480	10,480
Deferred Costs, Leasing, Net	68,297	66,367
Acquired lease intangible assets, less accumulated amortization of $50,553 and $45,639 at June 30, 2016 and December 31, 2015, respectively	124,325	105,380
Trading securities held in trust, at fair value	29,939	29,093
Other assets	29,561	26,935
Total assets	4,436,602	4,182,881
Liabilities:
Notes payable	1,674,631	1,699,771
Unsecured credit facilities	309,585	164,514
Accounts payable and other liabilities	165,611	164,515
Acquired lease intangible liabilities, less accumulated accretion of $20,240 and $17,555 at June 30, 2016 and December 31, 2015, respectively	57,776	42,034
Tenants’ security, escrow deposits and prepaid rent	26,974	29,427
Total liabilities	2,234,577	2,100,261
Commitments and contingencies (note 11)
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, 2016 and December 31, 2015, with liquidation preferences of $25 per share	325,000
Common stock, $0.01 par value per share,150,000,000 shares authorized; 99,486,537 and 97,212,638 shares issued at June 30, 2016 and December 31, 2015, respectively	995
Treasury stock at cost, 343,951 and 417,862 shares held at June 30, 2016 and December 31, 2015, respectively	(16,843)
Additional paid in capital	2,888,583
Accumulated other comprehensive loss	(80,038)
Distributions in excess of net income	(950,941)
Total stockholders’ equity	2,166,756
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $12,909 and $10,502 at June 30, 2016 and December 31, 2015, respectively	(2,013)
Limited partners’ interests in consolidated partnerships	37,282
Stockholders' Equity Attributable to Noncontrolling Interest	35,269
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,202,025	2,082,620
Total liabilities and equity	4,436,602	4,182,881
Partnership Interest [Member]
Real estate investments at cost:
Land, including amounts held for future development	1,639,958	1,479,814
Buildings and improvements	3,044,793	2,896,396
Properties in development	137,562	169,690
Gross real estate investments at cost	4,822,313	4,545,900
Less: accumulated depreciation	1,079,448	1,043,787
Total Cost Net of Accumulated Depreciation	3,742,865	3,502,113
Properties held for sale	29,365	0
Investments in real estate partnerships	279,270	306,206
Net real estate investments	4,051,500	3,808,319
Cash and cash equivalents	24,939	36,856
Restricted cash	3,505	3,767
Accounts receivable, net of allowance for doubtful accounts of $5,534 and $5,295 at June 30, 2016 and December 31, 2015, respectively	27,765	32,292
Straight-line rent receivable, net of reserve of $2,961 and $1,365 at June 30, 2016 and December 31, 2015, respectively	66,291	63,392
Notes receivable	10,480	10,480
Deferred Costs, Leasing, Net	68,297	66,367
Acquired lease intangible assets, less accumulated amortization of $50,553 and $45,639 at June 30, 2016 and December 31, 2015, respectively	124,325	105,380
Trading securities held in trust, at fair value	29,939	29,093
Other assets	29,561	26,935
Total assets	4,436,602	4,182,881
Liabilities:
Notes payable	1,674,631	1,699,771
Unsecured credit facilities	309,585	164,514
Accounts payable and other liabilities	165,611	164,515
Acquired lease intangible liabilities, less accumulated accretion of $20,240 and $17,555 at June 30, 2016 and December 31, 2015, respectively	57,776	42,034
Tenants’ security, escrow deposits and prepaid rent	26,974	29,427
Total liabilities	2,234,577	2,100,261
Commitments and contingencies (note 11)
Stockholders’ equity/Partners' capital:
Preferred units of general partner, $0.01 par value per unit, 13,000,000 units issued and outstanding at March 31, 2016 and December 31, 2015, liquidation preference of $25 per unit	325,000	325,000
General partner; 99,486,537 and 97,212,638 units outstanding at March 31, 2016 and December 31, 2015, respectively	1,921,794	1,787,802
Limited partners; 154,170 units outstanding at March 31, 2016 and December 31, 2015	(2,013)	(1,975)
Accumulated other comprehensive loss	(80,038)	(58,693)
Total partners’ capital	2,164,743	2,052,134
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	37,282	30,486
Total noncontrolling interests	37,282	30,486
Total equity	2,202,025	2,082,620
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,202,025	2,082,620
Total liabilities and equity	4,436,602	4,182,881
Preferred Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	325,000	325,000
Common Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	995	972
Treasury Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(16,843)	(19,658)
Additional Paid-in Capital [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,888,583	2,742,508
AOCI Attributable to Parent [Member]
Stockholders’ equity/Partners' capital:
Accumulated other comprehensive loss	(80,038)	(58,693)
AOCI Attributable to Parent [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(80,038)	(58,693)
AOCI Attributable to Parent [Member] | Partnership Interest [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(80,038)	(58,693)
Accumulated Distributions in Excess of Net Income [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(950,941)	(936,020)
Total Stockholders' Equity [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,166,756	2,054,109
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(2,013)	(1,975)
Noncontrolling Interests in Limited Partners Interest in Consolidated Partnerships [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	37,282	30,486
Noncontrolling Interests in Limited Partners Interest in Consolidated Partnerships [Member] | Partnership Interest [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	37,282	30,486
Total Noncontrolling Interests [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 35,269	$ 28,511