Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Parent Company [Member]
Cash flows from operating activities:
Net income	$ 38,083	$ 27,067
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	66,062	67,247
Amortization of deferred loan cost and debt premium	6,461	6,140
Accretion of above and below market lease intangibles, net	(437)	(390)
Stock-based compensation, net of capitalization	4,903	5,010
Equity in (income) loss of investments in real estate partnerships	(13,770)	37
Net gain on sale of properties	(10,419)	0
Provision for impairment	23,508	0
Distribution of earnings from operations of investments in real estate partnerships	17,580	22,872
Gain on derivative instruments	(13)	0
Deferred compensation expense	1,073	1,724
Realized and unrealized gains on trading securities held in trust	1,083	903
Change in assets and liabilities:
Restricted cash	(25)	(1,215)
Accounts receivable	(3,084)	7,375
Straight-line rent receivables, net	(3,365)	(2,526)
Deferred leasing costs	(6,146)	(7,003)
Other assets	(2,227)	(6,833)
Accounts payable and other liabilities	(6,393)	(17,220)
Tenants' security and escrow deposits and prepaid rent	563	310
Net cash provided by operating activities	111,271	101,692
Cash flows from investing activities:
Acquisition of operating real estate	(586)	(5,415)
Development of real estate including acquisition of land	(79,755)	(32,066)
Proceeds from sale of real estate investments	48,927	2,067
Issuance of notes receivable	(666)	0
Investments in real estate partnerships	(53,587)	(188,132)
Distributions received from investments in real estate partnerships	12,495	149,357
Dividends on trading securities held in trust	77	98
Acquisition of trading securities held in trust	(11,120)	(10,274)
Proceeds from sale of trading securities held in trust	11,385	8,685
Net cash used in investing activities	(72,830)	(75,680)
Cash flows from financing activities:
Net proceeds from common stock issuance	0	215,369
Net proceeds from issuance of preferred stock	241,386	0
Proceeds from sale of treasury stock	339	2,096
Acquisition of treasury stock	(4)	0
Redemption of preferred stock and partnership units	(248,125)	0
Dividends paid to common stockholders	(82,093)	(78,188)
Dividends paid to preferred stockholders	(6,943)	(9,838)
Proceeds from unsecured credit facilities	450,000	321,790
Repayment of unsecured credit facilities	(185,000)	(300,000)
Repayment of notes payable	(192,375)	(177,251)
Scheduled principal payments	(3,513)	(2,470)
Payment of loan costs	(1,718)	(132)
Net cash (used in) provided by financing activities	(26,813)	(31,279)
Net increase in cash and cash equivalents	11,628	(5,267)
Cash and cash equivalents at beginning of the period	11,402	16,889
Cash and cash equivalents at end of the period	23,030	11,622
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $1,246 and $957 in 2012 and 2011, respectively)	61,901	67,052
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	5,397	0
Real estate received through distribution in kind	0	46,157
Mortgage loans assumed through distribution in kind	0	27,405
Mortgage loans assumed for the acquisition of real estate	11,710	5,937
Real estate acquired through elimination of note receivable	12,585	0
Change in fair value of derivative instruments	(49)	0
Common stock issued for dividend reinvestment plan	495	562
Stock-based compensation capitalized	960	515
Contributions from limited partners in consolidated partnerships, net	940	2,332
Common stock issued for dividend reinvestment in trust	287	323
Contribution of stock awards into trust	806	1,105
Distribution of stock held in trust	1,191	0
Partnership Interest [Member]
Cash flows from operating activities:
Net income	38,083	27,067
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	66,062	67,247
Amortization of deferred loan cost and debt premium	6,461	6,140
Accretion of above and below market lease intangibles, net	(437)	(390)
Stock-based compensation, net of capitalization	4,903	5,010
Equity in (income) loss of investments in real estate partnerships	(13,770)	37
Net gain on sale of properties	(10,419)	0
Provision for impairment	23,508	0
Distribution of earnings from operations of investments in real estate partnerships	17,580	22,872
Gain on derivative instruments	(13)	0
Deferred compensation expense	1,073	1,724
Realized and unrealized gains on trading securities held in trust	(1,083)	(903)
Change in assets and liabilities:
Restricted cash	(25)	(1,215)
Accounts receivable	(3,084)	7,375
Straight-line rent receivables, net	(3,365)	(2,526)
Deferred leasing costs	(6,146)	(7,003)
Other assets	(2,227)	(6,833)
Accounts payable and other liabilities	(6,393)	(17,220)
Tenants' security and escrow deposits and prepaid rent	563	310
Net cash provided by operating activities	111,271	101,692
Cash flows from investing activities:
Acquisition of operating real estate	(586)	(5,415)
Development of real estate including acquisition of land	(79,755)	(32,066)
Proceeds from sale of real estate investments	48,927	2,067
Issuance of notes receivable	(666)	0
Investments in real estate partnerships	(53,587)	(188,132)
Distributions received from investments in real estate partnerships	12,495	149,357
Dividends on trading securities held in trust	77	98
Acquisition of trading securities held in trust	(11,120)	(10,274)
Proceeds from sale of trading securities held in trust	11,385	8,685
Net cash used in investing activities	(72,830)	(75,680)
Cash flows from financing activities:
Net proceeds from common units issued as a result of common stock issued by Parent Company	0	215,369
Net proceeds from issuance of preferred stock	241,386	0
Proceeds from sale of treasury stock	339	2,096
Acquisition of treasury stock	(4)	0
Redemption of preferred stock and partnership units	(248,125)	0
Distributions	1,801	(633)
Dividends paid to common stockholders	(82,257)	(78,348)
Dividends paid to preferred stockholders	(7,347)	(11,700)
Proceeds from unsecured credit facilities	450,000	321,790
Repayment of unsecured credit facilities	(185,000)	(300,000)
Repayment of notes payable	(192,375)	(177,251)
Scheduled principal payments	(3,513)	(2,470)
Payment of loan costs	(1,718)	(132)
Net cash (used in) provided by financing activities	(26,813)	(31,279)
Net increase in cash and cash equivalents	11,628	(5,267)
Cash and cash equivalents at beginning of the period	11,402	16,889
Cash and cash equivalents at end of the period	23,030	11,622
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $1,246 and $957 in 2012 and 2011, respectively)	61,901	67,052
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	5,397	0
Real estate received through distribution in kind	0	46,157
Mortgage loans assumed through distribution in kind	0	27,405
Mortgage loans assumed for the acquisition of real estate	11,710	5,937
Real estate acquired through elimination of note receivable	12,585	0
Change in fair value of derivative instruments	(49)	0
Common stock issued for dividend reinvestment plan	495	562
Stock-based compensation capitalized	960	515
Contributions from limited partners in consolidated partnerships, net	940	2,332
Common stock issued for dividend reinvestment in trust	287	323
Contribution of stock awards into trust	806	1,105
Distribution of stock held in trust	1,191	0
Limited Partner [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions	1,801	(633)
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions	(164)	(160)
Noncontrolling Interests Preferred Units [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions	$ (404)	$ (1,862)