Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation, net of capitalization	$ 9,824	$ 6,615	$ 3,932
Equity in income (loss) of investments in real estate partnerships	(9,643)	12,884	26,373
Cash flows from investing activities:
Payments to Acquire Notes Receivable			(20,400)
Proceeds from Collection of Notes Receivable			8,900
Parent Company [Member]
Cash flows from operating activities:
Net income	55,788	13,001	(29,036)
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	133,756	123,933	117,979
Amortization of deferred loan cost and debt premium	12,327	8,533	5,822
Amortization and (accretion) of above and below market lease intangibles, net	(931)	(1,161)	(1,867)
Stock-based compensation, net of capitalization	9,824	6,615	4,668
Equity in income (loss) of investments in real estate partnerships	(9,643)	12,884	26,373
Net gain on sale of properties	(8,346)	(8,648)	(25,192)
Provision for doubtful accounts	3,166	3,954	9,078
Provision for impairment	15,883	26,615	104,402
Gains (Losses) on Extinguishment of Debt	0	4,243	2,784
Proceeds from Distributions Received from Real Estate Partnerships	188,514	90,092	23,548
Settlement of derivative instruments	0	(63,435)	(19,953)
Gain on derivative instruments	54	(1,419)	3,294
Deferred compensation expense (income)	(2,136)	5,068	(247)
Realized gain on trading securities in held in trust	(383)	(667)	1,447
Unrealized (gain) loss on trading securities in held in trust	567	(1,342)	4,226
Changes in assets and liabilities:
Increase (Decrease) in Restricted Cash	(651)	(1,778)	5,126
Accounts receivable	(6,274)	(1,297)	(2,995)
Straight-line rent receivables, net	(4,642)	(6,202)	(3,959)
Increase (Decrease) in Other Receivables	0	0	19,700
Deferred leasing costs	(15,013)	(15,563)	(9,799)
Other assets	(971)	(4,681)	(16,493)
Accounts payable and other liabilities	(17,892)	(1,281)	(30,352)
Tenants' security and escrow deposits	9,789	33	(454)
Net cash provided by operating activities	217,633	138,459	195,804
Cash flows from investing activities:
Acquisition of operating real estate	(70,629)	(24,569)	0
Development of real estate including acquisition of land	(82,069)	(65,889)	(142,989)
Proceeds from sale of real estate investments	86,233	47,333	180,307
Payments to Acquire Notes Receivable	(78)
Proceeds from Collection of Notes Receivable		883	13,572
Investments in real estate partnerships	(198,688)	(231,847)	(28,709)
Equity Method Investment, Dividends or Distributions	43,361	41,054	31,252
Dividends on trading securities held in trust	225	297	247
Acquisition of trading securities held in trust	(19,377)	(10,312)	(12,220)
Proceeds from sale of trading securities held in trust	18,146	9,555	17,789
Net cash used in investing activities	(77,723)	(184,457)	51,545
Cash flows from financing activities:
Proceeds from Issuance of Common Stock	215,369	0	345,800
Proceeds from sale of treasury stock	2,128	1,431	(2,632)
Payments for Repurchase of Common Stock	(14)	0	0
Repayments of Unsecured Debt	181,691	209,879	116,053
Distributions to exchangeable operating partnership unit holders	(160,154)	(148,649)	(158,690)
Distributions to preferred unit holders/partners	19,675	19,675	19,675
Dividends paid to preferred stockholders	(19,675)	(19,675)	(19,675)
Repayment of fixed rate unsecured notes	(181,691)	(209,879)	(116,053)
Proceeds from Issuance of Unsecured Debt	0	398,599	0
Proceeds from unsecured credit facilities	455,000	250,000	135,000
Repayment of unsecured credit facilities	(425,000)	(240,000)	(432,667)
Proceeds from notes payable	1,940	6,068	106,992
Repayment of notes payable	(16,919)	(51,687)	(8,056)
Scheduled principal payments	(5,699)	(5,024)	(5,214)
Net cash (used in) provided by financing activities	(6,070)	(4,361)	(1,195)
Net Cash Provided by (Used in) Financing Activities	(145,569)	(32,797)	(164,279)
Net (decrease) increase in cash and cash equivalents	0	(4,000)	(2,312)
Cash and Cash Equivalents, Period Increase (Decrease)	(5,659)	(78,795)	83,070
Cash and cash equivalents at beginning of the period	17,061	95,856	12,786
Cash and cash equivalents at end of the period	11,402	17,061	95,856
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $1,480, $5,099, and $19,062 in 2011, 2010 and 2009, respectively)	128,649	127,591	112,730
Supplemental disclosure of non-cash transactions:
Common stock issued for partnership units exchanged	0	7,630	0
Real estate received through distribution in kind	47,512	0	100,717
Mortgage loans assumed through distribution in kind	28,760	0	70,541
Mortgage loans assumed for aquistion of real estate	31,292	58,981	0
Real estate contributed for investments in real estate partnerships	0	0	26,410
Notes receivable taken in connection with sales of properties in development	0	0	11,413
Real estate received through foreclosure on notes receivable	0	990	0
Change in fair value of derivative instruments	18	28,363	55,328
Common stock issued for dividend reinvestment plan	1,081	1,847	3,219
Stock-based compensation capitalized	1,104	852	1,574
Contributions from limited partners in consolidated partnerships, net	2,411	132	4,188
Common stock issued for dividend reinvestment in trust	631	640	808
Contribution of stock awards into trust	1,132	1,142	1,823
Distribution of stock held in trust	0	51	3,025
Payments of Dividends, Common Stock	160,154	148,649	158,690
Partnership Interest [Member]
Cash flows from operating activities:
Net income	55,788	13,001	(29,036)
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	133,756	123,933	117,979
Amortization of deferred loan cost and debt premium	12,327	8,533	5,822
Amortization and (accretion) of above and below market lease intangibles, net	(931)	(1,161)	(1,867)
Stock-based compensation, net of capitalization	9,824	6,615	4,668
Equity in income (loss) of investments in real estate partnerships	(9,643)	12,884	26,373
Net gain on sale of properties	(8,346)	(8,648)	(25,192)
Provision for doubtful accounts	3,166	3,954	9,078
Provision for impairment	15,883	26,615	104,402
Gains (Losses) on Extinguishment of Debt	0	4,243	2,784
Proceeds from Distributions Received from Real Estate Partnerships	188,514	90,092	23,548
Settlement of derivative instruments	0	(63,435)	(19,953)
Gain on derivative instruments	54	(1,419)	3,294
Deferred compensation expense (income)	(2,136)	5,068	(247)
Realized gain on trading securities in held in trust	(383)	(667)	1,447
Unrealized (gain) loss on trading securities in held in trust	567	(1,342)	4,226
Changes in assets and liabilities:
Increase (Decrease) in Restricted Cash	(651)	(1,778)	5,126
Accounts receivable	(6,274)	(1,297)	(2,995)
Straight-line rent receivables, net	(4,642)	(6,202)	(3,959)
Increase (Decrease) in Other Receivables	0	0	19,700
Deferred leasing costs	(15,013)	(15,563)	(9,799)
Other assets	(971)	(4,681)	(16,493)
Accounts payable and other liabilities	(17,892)	(1,281)	(30,352)
Tenants' security and escrow deposits	9,789	33	(454)
Net cash provided by operating activities	217,633	138,459	195,804
Cash flows from investing activities:
Acquisition of operating real estate	(70,629)	(24,569)	0
Development of real estate including acquisition of land	(82,069)	(65,889)	(142,989)
Proceeds from sale of real estate investments	86,233	47,333	180,307
Payments to Acquire Notes Receivable	(78)
Proceeds from Collection of Notes Receivable		883	13,572
Investments in real estate partnerships	(198,688)	(231,847)	(28,709)
Equity Method Investment, Dividends or Distributions	43,361	41,054	31,252
Dividends on trading securities held in trust	225	297	247
Acquisition of trading securities held in trust	(19,377)	(10,312)	(12,220)
Proceeds from sale of trading securities held in trust	18,146	9,555	17,789
Net cash used in investing activities	(77,723)	(184,457)	51,545
Cash flows from financing activities:
Net proceeds from common stock issuance	215,369	0	345,800
Proceeds from sale of treasury stock	2,128	1,431	(2,632)
Payments for Repurchase of Common Stock	(14)	0	0
Repayments of Unsecured Debt	181,691	209,879	116,053
Distributions to exchangeable operating partnership unit holders	(160,478)	(149,117)	(159,670)
Distributions to preferred unit holders/partners	23,400	23,400	23,400
Dividends paid to preferred stockholders	(23,400)	(23,400)	(23,400)
Repayment of fixed rate unsecured notes	(181,691)	(209,879)	(116,053)
Proceeds from Issuance of Unsecured Debt	0	398,599	0
Proceeds from unsecured credit facilities	455,000	250,000	135,000
Repayment of unsecured credit facilities	(425,000)	(240,000)	(432,667)
Proceeds from notes payable	1,940	6,068	106,992
Repayment of notes payable	(16,919)	(51,687)	(8,056)
Scheduled principal payments	(5,699)	(5,024)	(5,214)
Net cash (used in) provided by financing activities	(6,070)	(4,361)	(1,195)
Net Cash Provided by (Used in) Financing Activities	(145,569)	(32,797)	(164,279)
Net (decrease) increase in cash and cash equivalents	0	(4,000)	(2,312)
Cash and Cash Equivalents, Period Increase (Decrease)	(5,659)	(78,795)	83,070
Cash and cash equivalents at beginning of the period	17,061	95,856	12,786
Cash and cash equivalents at end of the period	11,402	17,061	95,856
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $1,480, $5,099, and $19,062 in 2011, 2010 and 2009, respectively)	128,649	127,591	112,730
Supplemental disclosure of non-cash transactions:
Common stock issued for partnership units exchanged	0	7,630	0
Real estate received through distribution in kind	47,512	0	100,717
Mortgage loans assumed through distribution in kind	28,760	0	70,541
Mortgage loans assumed for aquistion of real estate	31,292	58,981	0
Real estate contributed for investments in real estate partnerships	0	0	26,410
Notes receivable taken in connection with sales of properties in development	0	0	11,413
Real estate received through foreclosure on notes receivable	0	990	0
Change in fair value of derivative instruments	18	28,363	55,328
Common stock issued for dividend reinvestment plan	1,081	1,847	3,219
Stock-based compensation capitalized	1,104	852	1,574
Contributions from limited partners in consolidated partnerships, net	2,411	132	4,188
Common stock issued for dividend reinvestment in trust	631	640	808
Contribution of stock awards into trust	1,132	1,142	1,823
Distribution of stock held in trust	0	51	3,025
Payments of Dividends, Common Stock	160,478	149,117	159,670
Payments of Dividends, Noncontrolling Interest	735	1,427	872
Limited Partner [Member] | Parent Company [Member]
Supplemental disclosure of non-cash transactions:
Payments of Dividends, Noncontrolling Interest	735	1,427	872
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Supplemental disclosure of non-cash transactions:
Payments of Dividends, Noncontrolling Interest	324	468	980
Noncontrolling Interests Preferred Units [Member] | Parent Company [Member]
Supplemental disclosure of non-cash transactions:
Payments of Dividends, Noncontrolling Interest	$ (3,725)	$ (3,725)	$ (3,725)