Consolidated Statements of Cash Flows (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Parent Company [Member]
Cash flows from operating activities:
Net income	$ 57,903,000	$ 41,608,000
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	96,217,000	100,518,000
Amortization of deferred loan cost and debt premium	9,619,000	9,205,000
Accretion of above and below market lease intangibles, net	(667,000)	(704,000)
Stock-based compensation, net of capitalization	7,332,000	7,363,000
Equity in income of investments in real estate partnerships	(19,173,000)	(4,788,000)
Net gain on sale of properties	(10,646,000)	(2,321,000)
Provision for impairment	24,655,000	6,699,000
Early extinguishment of debt	852,000	0
Distribution of earnings from operations of investments in real estate partnerships	28,550,000	34,557,000
Gain on derivative instruments	(17,000)	0
Deferred compensation expense (income)	1,840,000	(3,204,000)
Realized and unrealized (gains) losses on trading securities held in trust	(1,878,000)	1,373,000
Change in assets and liabilities:
Restricted cash	(83,000)	(1,865,000)
Accounts receivable	5,040,000	5,524,000
Straight-line rent receivables, net	(4,656,000)	(3,257,000)
Deferred leasing costs	(8,869,000)	(6,588,000)
Other assets	(2,140,000)	(4,741,000)
Accounts payable and other liabilities	17,734,000	(4,893,000)
Tenants' security and escrow deposits and prepaid rent	(5,559,000)	8,489,000
Net cash provided by operating activities	196,054,000	182,975,000
Cash flows from investing activities:
Acquisition of operating real estate	(58,015,000)	(70,629,000)
Development of real estate including acquisition of land	(117,550,000)	(57,195,000)
Proceeds from sale of real estate investments	315,235,000	18,348,000
Issuance of notes receivable	(579,000)	0
Investments in real estate partnerships	(53,587,000)	(194,560,000)
Distributions received from investments in real estate partnerships	29,463,000	149,490,000
Dividends on trading securities held in trust	130,000	134,000
Acquisition of trading securities held in trust	(13,635,000)	(10,356,000)
Proceeds from sale of trading securities held in trust	13,900,000	9,057,000
Net cash used in investing activities	115,362,000	(155,711,000)
Cash flows from financing activities:
Net proceeds from common stock issuance	21,677,000	215,369,000
Net proceeds from preferred units issued as a result of preferred stock issued by Parent Company	313,505,000	0
Proceeds from sale of treasury stock	339,000	2,114,000
Acquisition of treasury stock	(4,000)	0
Redemption of preferred stock and partnership units	(323,125,000)	0
Dividends paid to common stockholders	(123,155,000)	(119,170,000)
Dividends paid to preferred stockholders	(13,373,000)	(14,756,000)
Proceeds from unsecured credit facilities	535,000,000	385,000,000
Repayment of unsecured credit facilities	(510,000,000)	(310,000,000)
Proceeds from notes payable	0	1,863,000
Scheduled principal payments	(6,484,000)	(3,900,000)
Payment of loan costs	(4,305,000)	(5,982,000)
Net cash (used in) provided by financing activities	(301,468,000)	(30,394,000)
Net increase (decrease) in cash and cash equivalents	9,948,000	(3,130,000)
Cash and cash equivalents at beginning of the period	11,402,000	16,889,000
Cash and cash equivalents at end of the period	21,350,000	13,759,000
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $2,477 and $1,293 in 2012 and 2011, respectively)	80,742,000	92,103,000
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	4,615,000	0
Real estate received through distribution in kind	0	47,512,000
Mortgage loans assumed through distribution in kind	0	28,760,000
Mortgage loans assumed for the acquisition of real estate	12,810,000	31,186,000
Real estate contributed for investments in real estate partnerships	47,500,000	0
Real estate acquired through elimination of note receivable	12,585,000	0
Change in fair value of derivative instruments	(67,000)	0
Common stock issued for dividend reinvestment plan	740,000	821,000
Stock-based compensation capitalized	1,453,000	930,000
Contributions from limited partners in consolidated partnerships, net	986,000	2,332,000
Common stock issued for dividend reinvestment in trust	439,000	476,000
Contribution of stock awards into trust	821,000	1,117,000
Distribution of stock held in trust	1,191,000	0
Partnership Interest [Member]
Cash flows from operating activities:
Net income	57,903,000	41,608,000
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	96,217,000	100,518,000
Amortization of deferred loan cost and debt premium	9,619,000	9,205,000
Accretion of above and below market lease intangibles, net	(667,000)	(704,000)
Stock-based compensation, net of capitalization	7,332,000	7,363,000
Equity in income of investments in real estate partnerships	(19,173,000)	(4,788,000)
Net gain on sale of properties	(10,646,000)	(2,321,000)
Provision for impairment	24,655,000	6,699,000
Early extinguishment of debt	852,000	0
Distribution of earnings from operations of investments in real estate partnerships	28,550,000	34,557,000
Gain on derivative instruments	(17,000)	0
Deferred compensation expense (income)	1,840,000	(3,204,000)
Realized and unrealized gains on trading securities held in trust	(1,878,000)	1,373,000
Change in assets and liabilities:
Restricted cash	(83,000)	(1,865,000)
Accounts receivable	5,040,000	5,524,000
Straight-line rent receivables, net	(4,656,000)	(3,257,000)
Deferred leasing costs	(8,869,000)	(6,588,000)
Other assets	(2,140,000)	(4,741,000)
Accounts payable and other liabilities	17,734,000	(4,893,000)
Tenants' security and escrow deposits and prepaid rent	(5,559,000)	8,489,000
Net cash provided by operating activities	196,054,000	182,975,000
Cash flows from investing activities:
Acquisition of operating real estate	(58,015,000)	(70,629,000)
Development of real estate including acquisition of land	(117,550,000)	(57,195,000)
Proceeds from sale of real estate investments	315,235,000	18,348,000
Issuance of notes receivable	(579,000)	0
Investments in real estate partnerships	(53,587,000)	(194,560,000)
Distributions received from investments in real estate partnerships	29,463,000	149,490,000
Dividends on trading securities held in trust	130,000	134,000
Acquisition of trading securities held in trust	(13,635,000)	(10,356,000)
Proceeds from sale of trading securities held in trust	13,900,000	9,057,000
Net cash used in investing activities	115,362,000	(155,711,000)
Cash flows from financing activities:
Net proceeds from common units issued as a result of common stock issued by Parent Company	21,677,000	215,369,000
Net proceeds from preferred units issued as a result of preferred stock issued by Parent Company	313,505,000	0
Proceeds from sale of treasury stock	339,000	2,114,000
Acquisition of treasury stock	(4,000)	0
Redemption of preferred stock and partnership units	(323,125,000)	0
Distributions	1,484,000	(551,000)
Dividends paid to common stockholders	(123,401,000)	(119,416,000)
Dividends paid to preferred stockholders	(13,777,000)	(17,550,000)
Proceeds from unsecured credit facilities	535,000,000	385,000,000
Repayment of unsecured credit facilities	(510,000,000)	(310,000,000)
Proceeds from notes payable	0	1,863,000
Scheduled principal payments	(6,484,000)	(3,900,000)
Payment of loan costs	(4,305,000)	(5,982,000)
Net cash (used in) provided by financing activities	(301,468,000)	(30,394,000)
Net increase (decrease) in cash and cash equivalents	9,948,000	(3,130,000)
Cash and cash equivalents at beginning of the period	11,402,000	16,889,000
Cash and cash equivalents at end of the period	21,350,000	13,759,000
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $2,477 and $1,293 in 2012 and 2011, respectively)	80,742,000	92,103,000
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	4,615,000	0
Real estate received through distribution in kind	0	47,512,000
Mortgage loans assumed through distribution in kind	0	28,760,000
Mortgage loans assumed for the acquisition of real estate	12,810,000	31,186,000
Real estate contributed for investments in real estate partnerships	47,500,000	0
Real estate acquired through elimination of note receivable	12,585,000	0
Change in fair value of derivative instruments	(67,000)	0
Common stock issued for dividend reinvestment plan	740,000	821,000
Stock-based compensation capitalized	1,453,000	930,000
Contributions from limited partners in consolidated partnerships, net	986,000	2,332,000
Common stock issued for dividend reinvestment in trust	439,000	476,000
Contribution of stock awards into trust	821,000	1,117,000
Distribution of stock held in trust	1,191,000	0
Limited Partner [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions	1,484,000	(551,000)
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions	(246,000)	(246,000)
Noncontrolling Interests Preferred Units [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions	(404,000)	(2,794,000)
Unsecured Debt [Member] | Parent Company [Member]
Cash flows from financing activities:
Repayment of fixed rate unsecured notes	192,377,000	161,691,000
Unsecured Debt [Member] | Partnership Interest [Member]
Cash flows from financing activities:
Repayment of fixed rate unsecured notes	192,377,000	161,691,000
Secured Debt [Member] | Parent Company [Member]
Cash flows from financing activities:
Repayment of notes payable	0	(15,650,000)
Secured Debt [Member] | Partnership Interest [Member]
Cash flows from financing activities:
Repayment of notes payable	$ 0	$ (15,650,000)