Consolidated Statements of Cash Flows (USD $) In Thousands	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation, net of capitalization	$ 5,010	$ 3,199
Parent Company [Member]
Cash flows from operating activities:
Net income	27,067	31,143
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	67,247	62,832
Amortization of deferred loan cost and debt premium	6,140	3,185
Accretion of above and below market lease intangibles, net	(390)	(777)
Stock-based compensation, net of capitalization	5,010	3,199
Equity in loss of investments in real estate partnerships	37	2,110
Net gain on sale of properties	0	(7,350)
Provision for doubtful accounts	2,212	2,361
Distribution of earnings from operations of investments in real estate partnerships	22,872	21,694
Settlement of derivative instruments	0	(26,761)
Loss on derivative instruments	0	922
Deferred compensation expense (income)	1,724	(240)
Realized gain on trading securities in held in trust	(172)	(157)
Unrealized (gain) loss on trading securities in held in trust	(731)	517
Changes in assets and liabilities:
Accounts receivable	5,163	5,418
Straight-line rent receivables, net	(2,526)	(2,365)
Deferred leasing costs	(7,003)	(6,278)
Other assets	(6,833)	4,168
Accounts payable and other liabilities	(17,220)	(8,630)
Tenants' security and escrow deposits	310	(126)
Net cash provided by operating activities	102,907	84,865
Cash flows from investing activities:
Acquisition of operating real estate	(5,415)	0
Development of real estate including acquisition of land	(32,066)	(22,983)
Proceeds from sale of real estate investments	2,067	34,501
Investments in real estate partnerships	(188,132)	(210,021)
Equity Method Investment, Dividends or Distributions	149,357	79,600
Dividends on trading securities held in trust	98	106
Acquisition of trading securities held in trust	(10,274)	(6,601)
Proceeds from sale of trading securities held in trust	8,685	6,653
Net cash used in investing activities	(75,680)	(118,745)
Cash flows from financing activities:
Net proceeds from common stock issuance	215,369	0
Proceeds from sale of treasury stock	2,096	801
Distributions to limited partners in consolidated partnerships, net	(633)	(1,182)
Distributions to exchangeable operating partnership unit holders	(160)	(290)
Distributions to preferred unit holders/partners	(1,862)	(1,862)
Dividends paid to common stockholders	(78,188)	(74,260)
Dividends paid to preferred stockholders	(9,838)	(9,838)
Repayment of fixed rate unsecured notes	(161,691)	0
Proceeds from Issuance of Unsecured Debt	0	148,949
Proceeds from unsecured credit facilities	320,000	110,000
Repayment of unsecured credit facilities	(300,000)	(110,000)
Proceeds from notes payable	1,790	3,378
Repayment of notes payable	(15,560)	(21,188)
Scheduled principal payments	(2,470)	(2,449)
Payments of Loan Costs	(132)	(1,223)
Net cash (used in) provided by financing activities	(31,279)	40,836
Net (decrease) increase in cash and cash equivalents	(4,052)	6,956
Cash and cash equivalents at beginning of the period	22,460	99,477
Cash and cash equivalents at end of the period	18,408	106,433
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $957 and $3,323 in 2011 and 2010, respectively)	67,052	59,081
Supplemental disclosure of non-cash transactions:
Common stock issued for partnership units exchanged	0	7,311
Real estate received through distribution in kind	46,157	0
Mortgage loans assumed through distribution in kind	27,405	0
Mortgage loans assumed for aquistion of real estate	5,937	0
Real estate received through foreclosure on notes receivable	0	990
Change in fair value of derivative instruments	0	84
Common stock issued for dividend reinvestment plan	562	1,314
Stock-based compensation capitalized	515	333
Contributions from limited partners in consolidated partnerships, net	2,332	98
Common stock issued for dividend reinvestment in trust	323	318
Contribution of stock awards into trust	1,105	1,112
Distribution of stock held in trust	0	51
Partnership Interest [Member]
Cash flows from operating activities:
Net income	27,067	31,143
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	67,247	62,832
Amortization of deferred loan cost and debt premium	6,140	3,185
Accretion of above and below market lease intangibles, net	(390)	(777)
Stock-based compensation, net of capitalization	5,010	3,199
Equity in loss of investments in real estate partnerships	37	2,110
Net gain on sale of properties	0	(7,350)
Provision for doubtful accounts	2,212	2,361
Distribution of earnings from operations of investments in real estate partnerships	22,872	21,694
Settlement of derivative instruments	0	(26,761)
Loss on derivative instruments	0	922
Deferred compensation expense (income)	1,724	(240)
Realized gain on trading securities in held in trust	(172)	(157)
Unrealized (gain) loss on trading securities in held in trust	(731)	517
Changes in assets and liabilities:
Accounts receivable	5,163	5,418
Straight-line rent receivables, net	(2,526)	(2,365)
Deferred leasing costs	(7,003)	(6,278)
Other assets	(6,833)	4,168
Accounts payable and other liabilities	(17,220)	(8,630)
Tenants' security and escrow deposits	310	(126)
Net cash provided by operating activities	102,907	84,865
Cash flows from investing activities:
Acquisition of operating real estate	(5,415)	0
Development of real estate including acquisition of land	(32,066)	(22,983)
Proceeds from sale of real estate investments	2,067	34,501
Investments in real estate partnerships	(188,132)	(210,021)
Equity Method Investment, Dividends or Distributions	149,357	79,600
Dividends on trading securities held in trust	98	106
Acquisition of trading securities held in trust	(10,274)	(6,601)
Proceeds from sale of trading securities held in trust	8,685	6,653
Net cash used in investing activities	(75,680)	(118,745)
Cash flows from financing activities:
Net proceeds from common stock issuance	215,369	0
Proceeds from sale of treasury stock	2,096	801
Distributions to limited partners in consolidated partnerships, net	(633)	(1,182)
Distributions to preferred unit holders/partners	(78,348)	(74,550)
Preferred unit distributions	(11,700)	(11,700)
Repayment of fixed rate unsecured notes	(161,691)	0
Proceeds from Issuance of Unsecured Debt	0	148,949
Proceeds from unsecured credit facilities	320,000	110,000
Repayment of unsecured credit facilities	(300,000)	(110,000)
Proceeds from notes payable	1,790	3,378
Repayment of notes payable	(15,560)	(21,188)
Scheduled principal payments	(2,470)	(2,449)
Payments of Loan Costs	(132)	(1,223)
Net cash (used in) provided by financing activities	(31,279)	40,836
Net (decrease) increase in cash and cash equivalents	(4,052)	6,956
Cash and cash equivalents at beginning of the period	22,460	99,477
Cash and cash equivalents at end of the period	18,408	106,433
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $957 and $3,323 in 2011 and 2010, respectively)	67,052	59,081
Supplemental disclosure of non-cash transactions:
Common stock issued for partnership units exchanged	0	7,311
Real estate received through distribution in kind	46,157	0
Mortgage loans assumed through distribution in kind	27,405	0
Mortgage loans assumed for aquistion of real estate	5,937	0
Real estate received through foreclosure on notes receivable	0	990
Change in fair value of derivative instruments	0	84
Common stock issued for dividend reinvestment plan	562	1,314
Stock-based compensation capitalized	515	333
Contributions from limited partners in consolidated partnerships, net	2,332	98
Common stock issued for dividend reinvestment in trust	323	318
Contribution of stock awards into trust	1,105	1,112
Distribution of stock held in trust	$ 0	$ 51