Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation, net of capitalization	$ 9,806	$ 9,824	$ 6,615
Equity in (income) loss of investments in real estate partnerships	(23,807)	(9,643)	12,884
Deferred income tax expense (benefit) of taxable REIT subsidiary	13,727	2,422	(860)
Cash flows from financing activities:
Net proceeds from issuance of preferred stock	313,900	0	0
Redemption of preferred stock and partnership units	(323,125)	0	0
Parent Company [Member]
Cash flows from operating activities:
Net income	26,209	55,788	13,001
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	127,839	133,756	123,933
Amortization of deferred loan cost and debt premium	12,759	12,327	8,533
Amortization and (accretion) of above and below market lease intangibles, net	(1,043)	(931)	(1,161)
Stock-based compensation, net of capitalization	9,806	9,824	6,615
Equity in (income) loss of investments in real estate partnerships	(23,807)	(9,643)	12,884
Net gain on sale of properties	(24,013)	(8,346)	(8,648)
Provision for impairment	74,816	15,883	26,615
Early extinguishment of debt	852	0	4,243
Deferred income tax expense (benefit) of taxable REIT subsidiary	13,727	2,422	(860)
Distribution of earnings from operations of investments in real estate partnerships	44,809	43,361	41,054
Settlement of derivative instruments	0	0	(63,435)
(Gain) loss on derivative instruments	(22)	54	(1,419)
Deferred compensation expense (income)	2,069	(2,136)	5,068
Realized and unrealized (gain) loss on trading securities held in trust	(2,095)	184	(2,009)
Changes in assets and liabilities:
Restricted cash	(423)	(651)	(1,778)
Accounts receivable	6,157	(3,108)	2,657
Straight-line rent receivables, net	(6,059)	(4,642)	(6,202)
Deferred leasing costs	(12,642)	(15,013)	(15,563)
Other assets	(1,079)	(3,393)	(3,821)
Accounts payable and other liabilities	10,994	(17,892)	(1,281)
Tenants’ security and escrow deposits and prepaid rent	(1,639)	9,789	33
Net cash provided by operating activities	257,215	217,633	138,459
Cash flows from investing activities:
Acquisition of operating real estate	(156,026)	(70,629)	(24,569)
Real estate development and capital improvements	(164,588)	(82,069)	(65,889)
Proceeds from sale of real estate investments	352,707	86,233	47,333
(Issuance) collection of notes receivable	(552)	(78)	883
Investments in real estate partnerships	(66,663)	(198,688)	(231,847)
Distributions received from investments in real estate partnerships	38,353	188,514	90,092
Dividends on trading securities held in trust	245	225	297
Acquisition of trading securities held in trust	(17,930)	(19,377)	(10,312)
Proceeds from sale of trading securities held in trust	18,077	18,146	9,555
Net cash provided by (used in) investing activities	3,623	(77,723)	(184,457)
Cash flows from financing activities:
Net proceeds from common stock issuance	21,542	215,369	0
Proceeds from sale of treasury stock	338	2,128	1,431
Acquisition of treasury stock	(4)	(14)	0
Dividends paid to common stockholders	(164,423)	(160,154)	(148,649)
Dividends paid to preferred stockholders	(23,254)	(19,675)	(19,675)
Proceeds from notes payable	0	1,940	6,068
Repayment of notes payable	(1,332)	(16,919)	(51,687)
Scheduled principal payments	(7,259)	(5,699)	(5,024)
Payment of loan costs	(4,544)	(6,070)	(4,361)
Payment of premium on tender offer	0	0	(4,000)
Net cash used in financing activities	(249,891)	(145,569)	(32,797)
Net increase (decrease) in cash and cash equivalents	10,947	(5,659)	(78,795)
Cash and cash equivalents at beginning of the period	11,402	17,061	95,856
Cash and cash equivalents at end of the period	22,349	11,402	17,061
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $3,686, $1,480, and $5,099 in 2012, 2011, and 2010, respectively)	115,879	128,649	127,591
Supplemental disclosure of non-cash transactions:
Common stock issued for partnership units exchanged	0	0	7,630
Real estate received through distribution in kind	0	47,512	0
Mortgage loans assumed through distribution in kind	0	28,760	0
Mortgage loans assumed for the acquisition of real estate	30,467	31,292	58,981
Real estate contributed for investments in real estate partnerships	47,500	0	0
Real estate received through foreclosure on notes receivable	12,585	0	990
Change in fair value of derivative instruments	(4,285)	18	28,363
Common stock issued for dividend reinvestment plan	988	1,081	1,847
Stock-based compensation capitalized	1,979	1,104	852
Contributions from limited partners in consolidated partnerships, net	986	2,411	132
Common stock issued for dividend reinvestment in trust	440	631	640
Contribution of stock awards into trust	819	1,132	1,142
Distribution of stock held in trust	1,191	0	51
Partnership Interest [Member]
Cash flows from operating activities:
Net income	26,209	55,788	13,001
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	127,839	133,756	123,933
Amortization of deferred loan cost and debt premium	12,759	12,327	8,533
Amortization and (accretion) of above and below market lease intangibles, net	(1,043)	(931)	(1,161)
Stock-based compensation, net of capitalization	9,806	9,824	6,615
Equity in (income) loss of investments in real estate partnerships	(23,807)	(9,643)	12,884
Net gain on sale of properties	(24,013)	(8,346)	(8,648)
Provision for impairment	74,816	15,883	26,615
Early extinguishment of debt	852	0	4,243
Deferred income tax expense (benefit) of taxable REIT subsidiary	13,727	2,422	(860)
Distribution of earnings from operations of investments in real estate partnerships	44,809	43,361	41,054
Settlement of derivative instruments	0	0	(63,435)
(Gain) loss on derivative instruments	(22)	54	(1,419)
Deferred compensation expense (income)	2,069	(2,136)	5,068
Realized and unrealized (gain) loss on trading securities held in trust	(2,095)	184	(2,009)
Changes in assets and liabilities:
Restricted cash	(423)	(651)	(1,778)
Accounts receivable	6,157	(3,108)	2,657
Straight-line rent receivables, net	(6,059)	(4,642)	(6,202)
Deferred leasing costs	(12,642)	(15,013)	(15,563)
Other assets	(1,079)	(3,393)	(3,821)
Accounts payable and other liabilities	10,994	(17,892)	(1,281)
Tenants’ security and escrow deposits and prepaid rent	(1,639)	9,789	33
Net cash provided by operating activities	257,215	217,633	138,459
Cash flows from investing activities:
Acquisition of operating real estate	(156,026)	(70,629)	(24,569)
Real estate development and capital improvements	(164,588)	(82,069)	(65,889)
Proceeds from sale of real estate investments	352,707	86,233	47,333
(Issuance) collection of notes receivable	(552)	(78)	883
Investments in real estate partnerships	(66,663)	(198,688)	(231,847)
Distributions received from investments in real estate partnerships	38,353	188,514	90,092
Dividends on trading securities held in trust	245	225	297
Acquisition of trading securities held in trust	(17,930)	(19,377)	(10,312)
Proceeds from sale of trading securities held in trust	18,077	18,146	9,555
Net cash provided by (used in) investing activities	3,623	(77,723)	(184,457)
Cash flows from financing activities:
Net proceeds from common units issued as a result of common stock issued by Parent Company	21,542	215,369	0
Proceeds from sale of treasury stock	338	2,128	1,431
Acquisition of treasury stock	(4)	(14)	0
Distributions to limited partners in consolidated partnerships, net	1,375	(735)	(1,427)
Dividends paid to common stockholders	(164,747)	(160,478)	(149,117)
Dividends paid to preferred stockholders	(23,658)	(23,400)	(23,400)
Proceeds from notes payable	0	1,940	6,068
Repayment of notes payable	(1,332)	(16,919)	(51,687)
Scheduled principal payments	(7,259)	(5,699)	(5,024)
Payment of loan costs	(4,544)	(6,070)	(4,361)
Payment of premium on tender offer	0	0	(4,000)
Net cash used in financing activities	(249,891)	(145,569)	(32,797)
Net increase (decrease) in cash and cash equivalents	10,947	(5,659)	(78,795)
Cash and cash equivalents at beginning of the period	11,402	17,061	95,856
Cash and cash equivalents at end of the period	22,349	11,402	17,061
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $3,686, $1,480, and $5,099 in 2012, 2011, and 2010, respectively)	115,879	128,649	127,591
Supplemental disclosure of non-cash transactions:
Common stock issued for partnership units exchanged	0	0	7,630
Real estate received through distribution in kind	0	47,512	0
Mortgage loans assumed through distribution in kind	0	28,760	0
Mortgage loans assumed for the acquisition of real estate	30,467	31,292	58,981
Real estate contributed for investments in real estate partnerships	47,500	0	0
Real estate received through foreclosure on notes receivable	12,585	0	990
Change in fair value of derivative instruments	(4,285)	18	28,363
Common stock issued for dividend reinvestment plan	988	1,081	1,847
Stock-based compensation capitalized	1,979	1,104	852
Contributions from limited partners in consolidated partnerships, net	986	2,411	132
Common stock issued for dividend reinvestment in trust	440	631	640
Contribution of stock awards into trust	819	1,132	1,142
Distribution of stock held in trust	1,191	0	51
Limited Partner [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions to limited partners in consolidated partnerships, net	1,375	(735)	(1,427)
Noncontrolling Interest Exchangeable Operating Partnership Units | Parent Company [Member]
Cash flows from financing activities:
Distributions to limited partners in consolidated partnerships, net	(324)	(324)	(468)
Noncontrolling Interests Preferred Units | Parent Company [Member]
Cash flows from financing activities:
Distributions to limited partners in consolidated partnerships, net	(404)	(3,725)	(3,725)
Fixed Rate Unsecured Loans [Member] | Parent Company [Member]
Cash flows from financing activities:
Repayment of fixed rate unsecured notes	(192,377)	(181,691)	(209,879)
Proceeds from issuance of fixed rate unsecured notes, net	0	0	398,599
Fixed Rate Unsecured Loans [Member] | Partnership Interest [Member]
Cash flows from financing activities:
Repayment of fixed rate unsecured notes	(192,377)	(181,691)	(209,879)
Proceeds from issuance of fixed rate unsecured notes, net	0	0	398,599
Unsecured Credit Facilities [Member] | Parent Company [Member]
Cash flows from financing activities:
Repayment of fixed rate unsecured notes	(620,000)	(425,000)	(240,000)
Proceeds from issuance of fixed rate unsecured notes, net	750,000	455,000	250,000
Unsecured Credit Facilities [Member] | Partnership Interest [Member]
Cash flows from financing activities:
Repayment of fixed rate unsecured notes	(620,000)	(425,000)	(240,000)
Proceeds from issuance of fixed rate unsecured notes, net	$ 750,000	$ 455,000	$ 250,000