Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Document Fiscal Year Focus	2015
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in income of investments in real estate partnerships (note 4)	$ (22,508)	$ (31,270)	$ (31,718)
Cash flows from investing activities:
Payments for Deposits on Real Estate Acquisitions	2,300
Proceeds from sale of real estate investments	108,822	118,787	212,632
Cash flows from financing activities:
Proceeds from Issuance of Common Stock	12,843	103,821
Parent Company [Member]
Cash flows from operating activities:
Net income	152,543	188,847	151,285
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	146,829	147,791	134,454
Amortization of deferred loan cost and debt premium	9,677	10,521	12,339
Amortization and (accretion) of above and below market lease intangibles, net	(1,598)	(3,101)	(2,488)
Stock-based compensation, net of capitalization	11,081	9,662	12,191
Equity in income of investments in real estate partnerships (note 4)	(22,508)	(31,270)	(31,718)
Business Combination, Step Acquisition, Equity Interest in Acquiree, Remeasurement Gain	0	18,271	0
Gain Loss On Sale Of Properties Including Discontinued Operations	(35,606)	(55,077)	(59,656)
Provision for impairment	0	1,257	6,000
Early extinguishment of debt	8,239	18	32
Proceeds from Equity Method Investment, Dividends or Distributions	46,646	42,767	45,377
Loss on settlement of derivative instruments	(7,267)	4,648	0
Gain on derivative instruments	0	(13)	(19)
Deferred compensation expense	207	1,386	3,294
Realized and unrealized gain on investments (note 8 and 14)	(626)	(9,158)	(3,293)
Changes in assets and liabilities:
Increase (Decrease) in Restricted Cash for Operating Activities	1,926	848	(62)
Accounts receivable	(11,965)	(6,225)	(5,042)
Straight-line rent receivable, net	(8,231)	(6,544)	(5,459)
Deferred leasing costs	(12,949)	(8,252)	(10,086)
Other assets	(496)	89	(1,866)
Accounts payable and other liabilities	(3,810)	6,201	(672)
Tenants’ security and escrow deposits and prepaid rent	3,545	1,618	6,120
Net cash provided by operating activities	275,637	277,742	250,731
Cash flows from investing activities:
Acquisition of operating real estate	(42,983)	(112,120)	(107,790)
Payments for Deposits on Real Estate Acquisitions	(2,250)	0	0
Real estate development and capital improvements	(205,103)	(238,237)	(213,282)
Proceeds from sale of real estate investments	108,822	118,787	212,632
Collection of notes receivable	1,719	0	27,354
Investments in real estate partnerships (note 4)	(20,054)	(23,577)	(10,883)
Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital	23,801	37,152	87,111
Payments for (Proceeds from) Other Investing Activities	243	243	194
Acquisition of securities	(31,941)	(23,760)	(19,144)
Proceeds from sale of securities	28,400	31,222	13,991
Net cash used in investing activities	(139,346)	(210,290)	(9,817)
Cash flows from financing activities:
Proceeds from Issuance of Common Stock	198,494	102,453	99,753
Proceeds from sale of treasury stock	0	0	34
Redemption of preferred stock and partnership units	0	(300)	0
Proceeds from (Payments to) Noncontrolling Interests	(5,341)	(5,303)	1,514
Payments of Ordinary Dividends, Noncontrolling Interest	(299)	(300)	(322)
Dividends paid to common stockholders	(181,392)	(172,600)	(167,773)
Dividends paid to preferred stockholders	(21,062)	(21,062)	(21,062)
Dividends, Preferred Stock, Cash	21,062	21,062	21,062
Repayment of fixed rate unsecured notes	(450,000)	(150,000)	0
Proceeds from Issuance of Unsecured Debt	248,160	248,705	0
Proceeds from unsecured credit facilities	445,000	255,000	82,000
Repayment of unsecured credit facilities	(355,000)	(255,000)	(177,000)
Proceeds from notes payable	4,316	12,739	36,350
Repayment of notes payable	(76,168)	(38,717)	(27,960)
Scheduled principal payments	(5,878)	(6,909)	(7,530)
Payment of loan costs	(5,998)	(3,066)	(583)
Payments of Debt Extinguishment Costs	(8,043)	0	0
Net cash used in financing activities	(213,211)	(34,360)	(182,579)
Net (decrease) increase in cash and cash equivalents	(76,920)	33,092	58,335
Cash and cash equivalents at beginning of the period	113,776	80,684	22,349
Cash and cash equivalents at end of the period	36,856	113,776	80,684
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $000, $6,078, and $3,686 in 2014, 2013, and 2012, respectively)	101,527	109,425	107,312
Income Taxes Paid	1,015	2,169	0
Supplemental disclosure of non-cash transactions:
Mortgage loans assumed through distribution in kind	0	0	7,500
Stock Issued During Period, Value, Conversion of Units	0	137	302
Real estate received through distribution in kind	0	0	7,576
Mortgage loans assumed for the acquisition of real estate	42,799	103,187	0
Unrealized Gain (Loss) on Securities	(43)	0	0
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value	0	15,385	0
Acquisition of previously unconsolidated real estate	0	16,182	0
Change in fair value of derivative instruments	(9,012)	(49,968)	30,952
Contributions from limited partners in consolidated partnerships, net	13	1,579	156
Common stock issued for dividend reinvestment in trust	833	779	660
Common stock issued for dividend reinvestment plan	1,250	1,184	1,075
Stock-based compensation capitalized	2,988	2,707	2,188
Contribution of stock awards into trust	1,651	1,881	1,537
Distribution of stock held in trust	1,898	4	201
Partnership Interest [Member]
Cash flows from operating activities:
Net income	152,543	188,847	151,285
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	146,829	147,791	134,454
Amortization of deferred loan cost and debt premium	9,677	10,521	12,339
Amortization and (accretion) of above and below market lease intangibles, net	(1,598)	(3,101)	(2,488)
Stock-based compensation, net of capitalization	11,081	9,662	12,191
Equity in income of investments in real estate partnerships (note 4)	(22,508)	(31,270)	(31,718)
Business Combination, Step Acquisition, Equity Interest in Acquiree, Remeasurement Gain	0	18,271	0
Gain Loss On Sale Of Properties Including Discontinued Operations	(35,606)	(55,077)	(59,656)
Provision for impairment	0	1,257	6,000
Early extinguishment of debt	8,239	18	32
Proceeds from Equity Method Investment, Dividends or Distributions	46,646	42,767	45,377
Loss on settlement of derivative instruments	(7,267)	4,648	0
Gain on derivative instruments	0	(13)	(19)
Deferred compensation expense	207	1,386	3,294
Realized and unrealized gain on investments (note 8 and 14)	(626)	(9,158)	(3,293)
Changes in assets and liabilities:
Increase (Decrease) in Restricted Cash for Operating Activities	1,926	848	(62)
Accounts receivable	(11,965)	(6,225)	(5,042)
Straight-line rent receivable, net	(8,231)	(6,544)	(5,459)
Deferred leasing costs	(12,949)	(8,252)	(10,086)
Other assets	(496)	89	(1,866)
Accounts payable and other liabilities	(3,810)	6,201	(672)
Tenants’ security and escrow deposits and prepaid rent	3,545	1,618	6,120
Net cash provided by operating activities	275,637	277,742	250,731
Cash flows from investing activities:
Acquisition of operating real estate	(42,983)	(112,120)	(107,790)
Payments for Deposits on Real Estate Acquisitions	(2,250)	0	0
Real estate development and capital improvements	(205,103)	(238,237)	(213,282)
Proceeds from sale of real estate investments	108,822	118,787	212,632
Collection of notes receivable	1,719	0	27,354
Investments in real estate partnerships (note 4)	(20,054)	(23,577)	(10,883)
Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital	23,801	37,152	87,111
Payments for (Proceeds from) Other Investing Activities	243	243	194
Acquisition of securities	(31,941)	(23,760)	(19,144)
Proceeds from sale of securities	28,400	31,222	13,991
Net cash used in investing activities	(139,346)	(210,290)	(9,817)
Cash flows from financing activities:
Net proceeds from common units issued as a result of common stock issued by Parent Company	198,494	102,453	99,753
Proceeds from sale of treasury stock	0	0	34
Redemption of preferred stock and partnership units	0	(300)	0
Proceeds from (Payments to) Noncontrolling Interests	(5,341)	(5,303)	1,514
Dividends paid to common stockholders	(181,691)	(172,900)	(168,095)
Dividends paid to preferred stockholders	(21,062)	(21,062)	(21,062)
Dividends, Preferred Stock, Cash	21,062	21,062	21,062
Repayment of fixed rate unsecured notes	(450,000)	(150,000)	0
Proceeds from Issuance of Unsecured Debt	248,160	248,705	0
Proceeds from unsecured credit facilities	445,000	255,000	82,000
Repayment of unsecured credit facilities	(355,000)	(255,000)	(177,000)
Proceeds from notes payable	4,316	12,739	36,350
Repayment of notes payable	(76,168)	(38,717)	(27,960)
Scheduled principal payments	(5,878)	(6,909)	(7,530)
Payment of loan costs	(5,998)	(3,066)	(583)
Payments of Debt Extinguishment Costs	(8,043)	0	0
Net cash used in financing activities	(213,211)	(34,360)	(182,579)
Net (decrease) increase in cash and cash equivalents	(76,920)	33,092	58,335
Cash and cash equivalents at beginning of the period	113,776	80,684	22,349
Cash and cash equivalents at end of the period	36,856	113,776	80,684
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $000, $6,078, and $3,686 in 2014, 2013, and 2012, respectively)	101,527	109,425	107,312
Supplemental disclosure of non-cash transactions:
Mortgage loans assumed through distribution in kind	0	0	7,500
Stock Issued During Period, Value, Conversion of Units	0	137	302
Real estate received through distribution in kind	0	0	7,576
Mortgage loans assumed for the acquisition of real estate	42,799	103,187	0
Unrealized Gain (Loss) on Securities	(43)	0	0
Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value	0	15,385	0
Acquisition of previously unconsolidated real estate	0	16,182	0
Change in fair value of derivative instruments	(9,012)	(49,968)	30,952
Contributions from limited partners in consolidated partnerships, net	13	1,579	156
Common stock issued for dividend reinvestment in trust	833	779	660
Common stock issued for dividend reinvestment plan	1,250	1,184	1,075
Stock-based compensation capitalized	2,988	2,707	2,188
Contribution of stock awards into trust	1,651	1,881	1,537
Distribution of stock held in trust	$ 1,898	4	201
Common Stock | Parent Company [Member]
Cash flows from financing activities:
Proceeds from Issuance of Common Stock			99,753
Supplemental disclosure of non-cash transactions:
Stock Issued During Period, Value, Conversion of Units		$ 0	$ 0