Consolidated Statements of Cash Flows (USD $) In Thousands	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation, net of capitalization	$ 7,363	$ 4,812
Parent Company [Member]
Cash flows from operating activities:
Net income	41,608	45,017
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	100,518	93,309
Amortization of deferred loan cost and debt premium	9,205	5,410
Accretion of above and below market lease intangibles, net	(704)	(961)
Stock-based compensation, net of capitalization	7,363	4,812
Equity in loss of investments in real estate partnerships	(4,788)	8
Net gain on sale of properties	(1,973)	(7,836)
Provision for doubtful accounts	2,547	2,510
Provision for impairment	6,699	0
Distribution of earnings from operations of investments in real estate partnerships	34,557	32,248
Settlement of derivative instruments	0	(26,761)
Gain on derivative instruments	0	(1,419)
Deferred compensation expense (income)	(3,204)	3,003
Realized gain on trading securities in held in trust	(209)	(346)
Unrealized (gain) loss on trading securities in held in trust	1,582	(699)
Changes in assets and liabilities:
Accounts receivable	2,977	431
Straight-line rent receivables, net	(3,257)	(4,544)
Deferred leasing costs	(6,588)	(9,926)
Other assets	(4,741)	(2,775)
Accounts payable and other liabilities	(5,241)	(911)
Tenants' security and escrow deposits	8,489	(118)
Net cash provided by operating activities	184,840	130,452
Cash flows from investing activities:
Acquisition of operating real estate	(70,629)	(10,058)
Development of real estate including acquisition of land	(57,195)	(40,790)
Proceeds from sale of real estate investments	18,348	34,408
Investments in real estate partnerships	(194,560)	(231,800)
Equity Method Investment, Dividends or Distributions	149,490	79,838
Dividends on trading securities held in trust	134	162
Acquisition of trading securities held in trust	(10,356)	(9,476)
Proceeds from sale of trading securities held in trust	9,057	8,847
Net cash used in investing activities	(155,711)	(168,869)
Cash flows from financing activities:
Net proceeds from common stock issuance	215,369	0
Proceeds from sale of treasury stock	2,114	1,431
Distributions to limited partners in consolidated partnerships, net	(551)	(1,334)
Distributions to exchangeable operating partnership unit holders	(246)	(373)
Distributions to preferred unit holders/partners	(2,794)	(2,794)
Dividends paid to common stockholders	(119,170)	(111,440)
Dividends paid to preferred stockholders	(14,756)	(14,756)
Repayment of fixed rate unsecured notes	(161,691)	(130,445)
Proceeds from Issuance of Unsecured Debt	0	148,949
Proceeds from unsecured credit facilities	385,000	220,000
Repayment of unsecured credit facilities	(310,000)	(110,000)
Proceeds from notes payable	1,863	4,879
Repayment of notes payable	(15,650)	(21,354)
Scheduled principal payments	(3,900)	(3,679)
Payments of Loan Costs	(5,982)	(1,306)
Net cash (used in) provided by financing activities	(30,394)	(22,222)
Net (decrease) increase in cash and cash equivalents	(1,265)	(60,639)
Cash and cash equivalents at beginning of the period	22,460	99,477
Cash and cash equivalents at end of the period	21,195	38,838
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $1,293 and $4,325 in 2011 and 2010, respectively)	92,103	96,240
Supplemental disclosure of non-cash transactions:
Common stock issued for partnership units exchanged	0	7,311
Real estate received through distribution in kind	47,512	0
Mortgage loans assumed through distribution in kind	28,760	0
Mortgage loans assumed for aquistion of real estate	31,186	7,881
Real estate received through foreclosure on notes receivable	0	990
Change in fair value of derivative instruments	0	28,363
Common stock issued for dividend reinvestment plan	821	1,594
Stock-based compensation capitalized	930	499
Contributions from limited partners in consolidated partnerships, net	2,332	132
Common stock issued for dividend reinvestment in trust	476	482
Contribution of stock awards into trust	1,117	1,128
Distribution of stock held in trust	0	51
Partnership Interest [Member]
Cash flows from operating activities:
Net income	41,608	45,017
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	100,518	93,309
Amortization of deferred loan cost and debt premium	9,205	5,410
Accretion of above and below market lease intangibles, net	(704)	(961)
Stock-based compensation, net of capitalization	7,363	4,812
Equity in loss of investments in real estate partnerships	(4,788)	8
Net gain on sale of properties	(1,973)	(7,836)
Provision for doubtful accounts	2,547	2,510
Provision for impairment	6,699	0
Distribution of earnings from operations of investments in real estate partnerships	34,557	32,248
Settlement of derivative instruments	0	(26,761)
Gain on derivative instruments	0	(1,419)
Deferred compensation expense (income)	(3,204)	3,003
Realized gain on trading securities in held in trust	(209)	(346)
Unrealized (gain) loss on trading securities in held in trust	1,582	(699)
Changes in assets and liabilities:
Accounts receivable	2,977	431
Straight-line rent receivables, net	(3,257)	(4,544)
Deferred leasing costs	(6,588)	(9,926)
Other assets	(4,741)	(2,775)
Accounts payable and other liabilities	(5,241)	(911)
Tenants' security and escrow deposits	8,489	(118)
Net cash provided by operating activities	184,840	130,452
Cash flows from investing activities:
Acquisition of operating real estate	(70,629)	(10,058)
Development of real estate including acquisition of land	(57,195)	(40,790)
Proceeds from sale of real estate investments	18,348	34,408
Investments in real estate partnerships	(194,560)	(231,800)
Equity Method Investment, Dividends or Distributions	149,490	79,838
Dividends on trading securities held in trust	134	162
Acquisition of trading securities held in trust	(10,356)	(9,476)
Proceeds from sale of trading securities held in trust	9,057	8,847
Net cash used in investing activities	(155,711)	(168,869)
Cash flows from financing activities:
Net proceeds from common stock issuance	215,369	0
Proceeds from sale of treasury stock	2,114	1,431
Distributions to limited partners in consolidated partnerships, net	(551)	(1,334)
Distributions to preferred unit holders/partners	(119,416)	(111,813)
Preferred unit distributions	(17,550)	(17,550)
Repayment of fixed rate unsecured notes	(161,691)	(130,445)
Proceeds from Issuance of Unsecured Debt	0	148,949
Proceeds from unsecured credit facilities	385,000	220,000
Repayment of unsecured credit facilities	(310,000)	(110,000)
Proceeds from notes payable	1,863	4,879
Repayment of notes payable	(15,650)	(21,354)
Scheduled principal payments	(3,900)	(3,679)
Payments of Loan Costs	(5,982)	(1,306)
Net cash (used in) provided by financing activities	(30,394)	(22,222)
Net (decrease) increase in cash and cash equivalents	(1,265)	(60,639)
Cash and cash equivalents at beginning of the period	22,460	99,477
Cash and cash equivalents at end of the period	21,195	38,838
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $1,293 and $4,325 in 2011 and 2010, respectively)	92,103	96,240
Supplemental disclosure of non-cash transactions:
Common stock issued for partnership units exchanged	0	7,311
Real estate received through distribution in kind	47,512	0
Mortgage loans assumed through distribution in kind	28,760	0
Mortgage loans assumed for aquistion of real estate	31,186	7,881
Real estate received through foreclosure on notes receivable	0	990
Change in fair value of derivative instruments	0	28,363
Common stock issued for dividend reinvestment plan	821	1,594
Stock-based compensation capitalized	930	499
Contributions from limited partners in consolidated partnerships, net	2,332	132
Common stock issued for dividend reinvestment in trust	476	482
Contribution of stock awards into trust	1,117	1,128
Distribution of stock held in trust	$ 0	$ 51