Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation, net of capitalization	$ 6,159	$ 4,903
Cash flows from investing activities:
Proceeds from Issuance of Common Stock	99,774
Partnership Interest [Member]
Cash flows from operating activities:
Net income	58,603	38,083
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	65,170	66,062
Amortization of deferred loan cost and debt premium	6,175	6,461
Accretion of above and below market lease intangibles, net	(1,042)	(437)
Stock-based compensation, net of capitalization	6,159	4,903
Equity in income of investments in real estate partnerships	(11,888)	(13,770)
Net gain on sale of properties	(13,127)	(10,419)
Provision for impairment	0	(23,508)
Distribution of earnings from operations of investments in real estate partnerships	(24,376)	(17,580)
Loss on derivative instruments	(9)	(13)
Deferred compensation expense	1,051	1,073
Realized and unrealized gains on trading securities held in trust	(1,051)	(1,083)
Changes in assets and liabilities:
Restricted cash	1,118	(25)
Accounts receivable	(328)	(3,084)
Straight-line rent receivables, net	(2,612)	(3,365)
Deferred leasing costs	(4,212)	(6,146)
Other assets	(3,175)	(2,227)
Accounts payable and other liabilities	(17,286)	(6,393)
Tenants’ security and escrow deposits and prepaid rent	(3,846)	563
Net cash provided by operating activities	104,076	111,271
Cash flows from investing activities:
Acquisition of operating real estate	(26,676)	(586)
Development of real estate, including acquisition of land	(78,951)	(79,755)
Proceeds from sale of real estate investments	84,699	48,927
Payments for (Proceeds from) Loans Receivable	(6,025)	666
Investments in real estate partnerships	(8,060)	(53,587)
Distributions received from investments in real estate partnerships	11,457	12,495
Dividends on trading securities held in trust	70	77
Acquisition of securities	(15,679)	(11,120)
Proceeds from sale of securities	10,632	11,385
Net cash used in investing activities	(16,483)	(72,830)
Cash flows from financing activities:
Net proceeds from common units issued as a result of common stock issued by Parent Company	98,278	0
Net proceeds from issuance of preferred stock	0	241,386
Proceeds from sale of treasury stock	34	339
Acquisition of treasury stock	0	(4)
Redemption of preferred stock and partnership units	0	(248,125)
Distributions	(3,272)	1,801
Dividends paid to common stockholders	(83,467)	(82,257)
Dividends paid to preferred stockholders	(5,265)	(7,347)
Proceeds from Lines of Credit	77,000	450,000
Repayments of Lines of Credit	122,000	185,000
Proceeds from notes payable	8,250	0
Repayments of Notes Payable	(16,349)	(192,375)
Scheduled principal payments	(3,893)	(3,513)
Payment of loan costs	(115)	(1,718)
Net cash used in financing activities	(50,799)	(26,813)
Net increase in cash and cash equivalents	36,794	11,628
Cash and cash equivalents at beginning of the period	22,349	11,402
Cash and cash equivalents at end of the period	59,143	23,030
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $2,305 and $1,246 in 2013 and 2012, respectively)	54,670	61,901
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	5,266	5,397
Real estate received through distribution in kind	7,576	0
Mortgage loans assumed for the acquisition of real estate	0	0
Real estate acquired through elimination of note receivable	0	12,585
Change in fair value of derivative instruments	21,720	(49)
Common stock issued for dividend reinvestment plan	578	495
Stock-based compensation capitalized	948	960
Contributions from limited partners in consolidated partnerships, net	0	940
Common stock issued for dividend reinvestment in trust	320	287
Contribution of stock awards into trust	1,504	806
Distribution of stock held in trust	201	1,191
Parent Company [Member]
Cash flows from operating activities:
Net income	58,603	38,083
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	65,170	66,062
Amortization of deferred loan cost and debt premium	6,175	6,461
Accretion of above and below market lease intangibles, net	(1,042)	(437)
Stock-based compensation, net of capitalization	6,159	4,903
Equity in income of investments in real estate partnerships	(11,888)	(13,770)
Net gain on sale of properties	(13,127)	(10,419)
Provision for impairment	0	(23,508)
Distribution of earnings from operations of investments in real estate partnerships	(24,376)	(17,580)
Loss on derivative instruments	(9)	(13)
Deferred compensation expense	1,051	1,073
Realized and unrealized gains on trading securities held in trust	(1,051)	(1,083)
Changes in assets and liabilities:
Restricted cash	1,118	(25)
Accounts receivable	(328)	(3,084)
Straight-line rent receivables, net	(2,612)	(3,365)
Deferred leasing costs	(4,212)	(6,146)
Other assets	(3,175)	(2,227)
Accounts payable and other liabilities	(17,286)	(6,393)
Tenants’ security and escrow deposits and prepaid rent	(3,846)	563
Net cash provided by operating activities	104,076	111,271
Cash flows from investing activities:
Acquisition of operating real estate	(26,676)	(586)
Development of real estate, including acquisition of land	(78,951)	(79,755)
Proceeds from sale of real estate investments	84,699	48,927
Payments for (Proceeds from) Loans Receivable	(6,025)	666
Investments in real estate partnerships	(8,060)	(53,587)
Distributions received from investments in real estate partnerships	11,457	12,495
Dividends on trading securities held in trust	70	77
Acquisition of securities	(15,679)	(11,120)
Proceeds from sale of securities	10,632	11,385
Net cash used in investing activities	(16,483)	(72,830)
Cash flows from financing activities:
Proceeds from sale of treasury stock	34	339
Acquisition of treasury stock	0	(4)
Redemption of preferred stock and partnership units	0	(248,125)
Dividends, Common Stock, Cash	84,045	82,751
Dividends, Preferred Stock, Cash	10,531	12,744
Dividends paid to common stockholders	(83,296)	(82,093)
Dividends paid to preferred stockholders	(5,265)	(6,943)
Proceeds from Lines of Credit	77,000	450,000
Repayments of Lines of Credit	122,000	185,000
Proceeds from notes payable	8,250	0
Repayments of Notes Payable	(16,349)	(192,375)
Scheduled principal payments	(3,893)	(3,513)
Payment of loan costs	(115)	(1,718)
Net cash used in financing activities	(50,799)	(26,813)
Net increase in cash and cash equivalents	36,794	11,628
Cash and cash equivalents at beginning of the period	22,349	11,402
Cash and cash equivalents at end of the period	59,143	23,030
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $2,305 and $1,246 in 2013 and 2012, respectively)	54,670	61,901
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	5,266	5,397
Real estate received through distribution in kind	7,576	0
Mortgage loans assumed for the acquisition of real estate	0	0
Real estate acquired through elimination of note receivable	0	12,585
Change in fair value of derivative instruments	21,720	(49)
Contributions from limited partners in consolidated partnerships, net	0	940
Common stock issued for dividend reinvestment in trust	320	287
Contribution of stock awards into trust	1,504	806
Distribution of stock held in trust	201	1,191
Parent Company [Member] | Limited Partner [Member]
Cash flows from financing activities:
Distributions	(3,272)	1,801
Grand Ridge Plaza [Member] | Partnership Interest [Member]
Supplemental disclosure of non-cash transactions:
Mortgage loans assumed for the acquisition of real estate		11,710
Grand Ridge Plaza [Member] | Parent Company [Member]
Supplemental disclosure of non-cash transactions:
Mortgage loans assumed for the acquisition of real estate		11,710
Hilltop Village [Member] | Partnership Interest [Member]
Supplemental disclosure of non-cash transactions:
Mortgage loans assumed for the acquisition of real estate	7,500
Hilltop Village [Member] | Parent Company [Member]
Supplemental disclosure of non-cash transactions:
Mortgage loans assumed for the acquisition of real estate	7,500
Total Noncontrolling Interests [Member] | Parent Company [Member]
Cash flows from operating activities:
Net income	654	(128)
Cash flows from financing activities:
Dividends, Common Stock, Cash		164
Dividends, Preferred Stock, Cash	0	404
Total Noncontrolling Interests [Member] | Parent Company [Member] | Noncontrolling Interest Exchangeable Operating Partnership Units [Member]
Cash flows from financing activities:
Dividends, Common Stock, Cash	(171)
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Cash flows from operating activities:
Net income	109	77
Cash flows from financing activities:
Dividends, Common Stock, Cash	171
Dividends, Preferred Stock, Cash	0	0
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member] | Noncontrolling Interest Exchangeable Operating Partnership Units [Member]
Cash flows from financing activities:
Dividends, Common Stock, Cash		(164)
Common Stock [Member] | Parent Company [Member]
Cash flows from investing activities:
Proceeds from Issuance of Common Stock	98,278	0
Supplemental disclosure of non-cash transactions:
Common stock issued for dividend reinvestment plan	578	495
Common Stock [Member] | Total Noncontrolling Interests [Member] | Parent Company [Member]
Supplemental disclosure of non-cash transactions:
Common stock issued for dividend reinvestment plan	0	0
Common Stock [Member] | Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Supplemental disclosure of non-cash transactions:
Common stock issued for dividend reinvestment plan	0	0
Preferred Stock [Member] | Parent Company [Member]
Cash flows from financing activities:
Net proceeds from issuance of preferred stock	$ 0	$ (241,386)