Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Parent Company [Member]
Net gain on sale of properties	$ 8,135	$ 0
Cash flows from operating activities:
Net income	26,735	8,130
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	32,929	35,190
Amortization of deferred loan cost and debt premium	3,265	3,057
Accretion of above and below market lease intangibles, net	(220)	(195)
Stock-based compensation, net of capitalization	2,447	2,501
Equity in (income) loss of investments in real estate partnerships	(2,966)	2,725
Net gain on sale of properties	(8,135)	0
Gain (Loss) on Sale of Properties	1,834	0
Distribution of earnings from operations of investments in real estate partnerships	8,556	12,735
Gain on derivative instruments	(8)	0
Deferred compensation expense	1,477	1,293
Realized and unrealized gains on trading securities held in trust	(1,528)	(752)
Change in assets and liabilities:
Restricted cash	(356)	(180)
Accounts receivable	(7,913)	7,880
Straight-line rent receivables, net	(1,650)	(1,574)
Deferred leasing costs	(2,467)	(3,272)
Other assets	2,164	(393)
Accounts payable and other liabilities	(8,526)	(17,924)
Tenants' security and escrow deposits and prepaid rent	(598)	86
Net cash provided by operating activities	43,206	49,307
Cash flows from investing activities:
Development of real estate including acquisition of land	(32,352)	(13,135)
Proceeds from sale of real estate investments	28,907	1,303
Issuance of notes receivable	(684)	0
Investments in real estate partnerships	(14,380)	(4,742)
Distributions received from investments in real estate partnerships	0	9,988
Dividends on trading securities held in trust	29	51
Acquisition of trading securities held in trust	(8,392)	(6,479)
Proceeds from sale of trading securities held in trust	8,193	6,409
Net cash used in investing activities	(18,679)	(6,605)
Cash flows from financing activities:
Net proceeds from common stock issuance	0	215,369
Net proceeds from issuance of preferred stock	241,450	0
Redemption of preferred stock	(200,000)	0
Proceeds from sale of treasury stock	339	615
Acquisition of treasury stock	(4)	0
Redemption of preferred partnership units	(48,125)	0
Distributions paid to common stockholders	(41,035)	(37,230)
Dividends paid to preferred stockholders	0	(4,919)
Repayment of fixed rate unsecured notes	(192,375)	(161,691)
Proceeds from unsecured credit facilities	235,000	175,000
Repayment of unsecured credit facilities	(150,000)	(185,000)
Proceeds from notes payable	150,000	829
Repayment of notes payable	0	(20)
Scheduled principal payments	(1,725)	(1,226)
Payment of loan costs	(1,600)	0
Net cash (used in) provided by financing activities	(8,814)	422
Net increase in cash and cash equivalents	15,713	43,124
Cash and cash equivalents at beginning of the period	11,402	16,889
Cash and cash equivalents at end of the period	27,115	60,013
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $371 and $589 in 2012 and 2011, respectively)	25,854	30,992
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	6,944	0
Real estate received through foreclosure on notes receivable	12,585	0
Change in fair value of derivative instruments	(31)	0
Common stock issued for dividend reinvestment plan	256	287
Stock-based compensation capitalized	478	257
Contributions from limited partners in consolidated partnerships, net	42	2,289
Common stock issued for dividend reinvestment in trust	140	161
Contribution of stock awards into trust	381	639
Distribution of stock held in trust	1,191	0
Net Realized and Unrealized Gain (Loss) on Trading Securities	1,528	752
Partnership Interest [Member]
Net gain on sale of properties	8,135	0
Cash flows from operating activities:
Net income	26,735	8,130
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	32,929	35,190
Amortization of deferred loan cost and debt premium	3,265	3,057
Accretion of above and below market lease intangibles, net	(220)	(195)
Stock-based compensation, net of capitalization	2,447	2,501
Equity in (income) loss of investments in real estate partnerships	(2,966)	2,725
Net gain on sale of properties	(8,135)	0
Gain (Loss) on Sale of Properties	1,834	0
Distribution of earnings from operations of investments in real estate partnerships	8,556	12,735
Gain on derivative instruments	(8)	0
Deferred compensation expense	1,477	1,293
Realized and unrealized gains on trading securities held in trust	(1,528)	(752)
Change in assets and liabilities:
Restricted cash	(356)	(180)
Accounts receivable	(7,913)	7,880
Straight-line rent receivables, net	(1,650)	(1,574)
Deferred leasing costs	(2,467)	(3,272)
Other assets	2,164	(393)
Accounts payable and other liabilities	(8,526)	(17,924)
Tenants' security and escrow deposits and prepaid rent	(598)	86
Net cash provided by operating activities	43,206	49,307
Cash flows from investing activities:
Development of real estate including acquisition of land	(32,352)	(13,135)
Proceeds from sale of real estate investments	28,907	1,303
Issuance of notes receivable	(684)	0
Investments in real estate partnerships	(14,380)	(4,742)
Distributions received from investments in real estate partnerships	0	9,988
Dividends on trading securities held in trust	29	51
Acquisition of trading securities held in trust	(8,392)	(6,479)
Proceeds from sale of trading securities held in trust	8,193	6,409
Net cash used in investing activities	(18,679)	(6,605)
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	241,450	0
Proceeds from sale of treasury stock	339	615
Acquisition of treasury stock	(4)	0
Redemption of preferred partnership units	(248,125)	0
Distributions	(249)	(292)
Distributions paid to common stockholders	(41,121)	(37,312)
Dividends paid to preferred stockholders	(404)	(5,850)
Repayment of fixed rate unsecured notes	(192,375)	(161,691)
Proceeds from unsecured credit facilities	235,000	175,000
Repayment of unsecured credit facilities	(150,000)	(185,000)
Proceeds from notes payable	150,000	829
Repayment of notes payable	0	(20)
Scheduled principal payments	(1,725)	(1,226)
Payment of loan costs	(1,600)	0
Net cash (used in) provided by financing activities	(8,814)	422
Net increase in cash and cash equivalents	15,713	43,124
Cash and cash equivalents at beginning of the period	11,402	16,889
Cash and cash equivalents at end of the period	27,115	60,013
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $371 and $589 in 2012 and 2011, respectively)	25,854	30,992
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	6,944	0
Real estate received through foreclosure on notes receivable	12,585	0
Change in fair value of derivative instruments	(31)	0
Common stock issued for dividend reinvestment plan	256	287
Stock-based compensation capitalized	478	257
Contributions from limited partners in consolidated partnerships, net	42	2,289
Common stock issued for dividend reinvestment in trust	140	161
Contribution of stock awards into trust	381	639
Distribution of stock held in trust	1,191	0
Net Realized and Unrealized Gain (Loss) on Trading Securities	1,528	752
Limited Partner [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions	(249)	(292)
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions	(86)	(82)
Noncontrolling Interests Preferred Units [Member] | Parent Company [Member]
Cash flows from financing activities:
Distributions	$ (404)	$ (931)