Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from financing activities:
Net proceeds from common stock issuance	$ 51,600
Parent Company [Member]
Cash flows from operating activities:
Net income	21,134	26,735
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	32,764	32,929
Amortization of deferred loan cost and debt premium	3,087	3,265
Accretion of above and below market lease intangibles, net	(510)	(220)
Stock-based compensation, net of capitalization	3,024	2,447
Equity in income of investments in real estate partnerships	(5,876)	(2,966)
Net gain on sale of properties	0	(8,135)
Distribution of earnings from operations of investments in real estate partnerships	13,859	8,556
Gain on derivative instruments	(5)	(8)
Deferred compensation expense	1,079	1,477
Realized and unrealized gains on trading securities held in trust	(1,079)	(1,528)
Changes in assets and liabilities:
Restricted cash	382	(356)
Accounts receivable	(1,180)	(7,913)
Straight-line rent receivables, net	(1,413)	(1,650)
Deferred leasing costs	(1,983)	(2,467)
Other assets	(1,383)	2,164
Accounts payable and other liabilities	(19,026)	(8,526)
Tenants’ security and escrow deposits and prepaid rent	(3,569)	(598)
Net cash provided by operating activities	39,305	43,206
Cash flows from investing activities:
Development of real estate including acquisition of land	(30,371)	(32,352)
Proceeds from sale of real estate investments	96	28,907
Collection (issuance) of notes receivable	4,024	(684)
Investments in real estate partnerships	(4,060)	(14,380)
Distributions received from investments in real estate partnerships	7,187	0
Dividends on trading securities held in trust	33	29
Acquisition of securities	(7,039)	(8,392)
Proceeds from sale of trading securities	2,019	8,193
Net cash used in investing activities	(28,111)	(18,679)
Cash flows from financing activities:
Net proceeds from common stock issuance	51,608	0
Net proceeds from issuance of preferred stock	0	241,450
Redemption of preferred stock	0	(200,000)
Proceeds from sale of treasury stock	34	339
Acquisition of treasury stock	0	(4)
Redemption of preferred stock and partnership units	0	(48,125)
Dividends paid to common stockholders	(41,290)	(41,035)
Proceeds from notes payable	8,250	150,000
Scheduled principal payments	(2,059)	(1,725)
Payment of loan costs	(111)	(1,600)
Net cash used in financing activities	(11,141)	(8,814)
Net increase in cash and cash equivalents	53	15,713
Cash and cash equivalents at beginning of the period	22,349	11,402
Cash and cash equivalents at end of the period	22,402	27,115
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $1,062 and $371 in 2013 and 2012, respectively)	19,017	25,854
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	5,266	6,944
Real estate received through distribution in kind	7,700	0
Mortgage loans assumed for the acquisition of real estate	7,500	0
Real estate acquired through elimination of note receivable	0	12,585
Change in fair value of derivative instruments	3,385	(31)
Common stock issued for dividend reinvestment plan	288	256
Stock-based compensation capitalized	391	478
Contributions from limited partners in consolidated partnerships, net	0	42
Common stock issued for dividend reinvestment in trust	153	140
Contribution of stock awards into trust	1,068	381
Distribution of stock held in trust	201	1,191
Parent Company [Member] | Unsecured Debt [Member] | Fixed Rate Unsecured Loans [Member]
Cash flows from financing activities:
Repayment of debt	0	(192,375)
Parent Company [Member] | Unsecured Debt [Member] | Unsecured Credit Facilities [Member]
Cash flows from financing activities:
Repayment of debt	(62,000)	(150,000)
Proceeds from unsecured credit facilities	37,000	235,000
Parent Company [Member] | Limited Partner [Member]
Cash flows from financing activities:
Distributions	(2,483)	(249)
Parent Company [Member] | Noncontrolling Interest Exchangeable Operating Partnership Units [Member]
Cash flows from financing activities:
Distributions	(90)	(86)
Parent Company [Member] | Noncontrolling Interests Preferred Units [Member]
Cash flows from financing activities:
Distributions	0	(404)
Partnership Interest [Member]
Cash flows from operating activities:
Net income	21,134	26,735
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	32,764	32,929
Amortization of deferred loan cost and debt premium	3,087	3,265
Accretion of above and below market lease intangibles, net	(510)	(220)
Stock-based compensation, net of capitalization	3,024	2,447
Equity in income of investments in real estate partnerships	(5,876)	(2,966)
Net gain on sale of properties	0	(8,135)
Distribution of earnings from operations of investments in real estate partnerships	13,859	8,556
Gain on derivative instruments	(5)	(8)
Deferred compensation expense	1,079	1,477
Realized and unrealized gains on trading securities held in trust	(1,079)	(1,528)
Changes in assets and liabilities:
Restricted cash	382	(356)
Accounts receivable	(1,180)	(7,913)
Straight-line rent receivables, net	(1,413)	(1,650)
Deferred leasing costs	(1,983)	(2,467)
Other assets	(1,383)	2,164
Accounts payable and other liabilities	(19,026)	(8,526)
Tenants’ security and escrow deposits and prepaid rent	(3,569)	(598)
Net cash provided by operating activities	39,305	43,206
Cash flows from investing activities:
Development of real estate including acquisition of land	(30,371)	(32,352)
Proceeds from sale of real estate investments	96	28,907
Collection (issuance) of notes receivable	4,024	(684)
Investments in real estate partnerships	(4,060)	(14,380)
Distributions received from investments in real estate partnerships	7,187	0
Dividends on trading securities held in trust	33	29
Acquisition of securities	(7,039)	(8,392)
Proceeds from sale of trading securities	2,019	8,193
Net cash used in investing activities	(28,111)	(18,679)
Cash flows from financing activities:
Net proceeds from common units issued as a result of common stock issued by Parent Company	51,608	0
Net proceeds from issuance of preferred stock	0	241,450
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	(2,483)	(249)
Dividends paid to common stockholders	(41,380)	(41,121)
Dividends paid to preferred stockholders	0	(404)
Proceeds from notes payable	8,250	150,000
Scheduled principal payments	(2,059)	(1,725)
Payment of loan costs	(111)	(1,600)
Net cash used in financing activities	(11,141)	(8,814)
Net increase in cash and cash equivalents	53	15,713
Cash and cash equivalents at beginning of the period	22,349	11,402
Cash and cash equivalents at end of the period	22,402	27,115
Supplemental disclosure of cash flow information:
Cash paid for interest (net of capitalized interest of $1,062 and $371 in 2013 and 2012, respectively)	19,017	25,854
Supplemental disclosure of non-cash transactions:
Preferred unit and stock distribution declared and not paid	5,266	6,944
Real estate received through distribution in kind	7,700	0
Mortgage loans assumed for the acquisition of real estate	7,500	0
Real estate acquired through elimination of note receivable	0	12,585
Change in fair value of derivative instruments	3,385	(31)
Common stock issued for dividend reinvestment plan	288	256
Stock-based compensation capitalized	391	478
Contributions from limited partners in consolidated partnerships, net	0	42
Common stock issued for dividend reinvestment in trust	153	140
Contribution of stock awards into trust	1,068	381
Distribution of stock held in trust	201	1,191
Partnership Interest [Member] | Unsecured Debt [Member] | Fixed Rate Unsecured Loans [Member]
Cash flows from financing activities:
Repayment of debt	0	(192,375)
Partnership Interest [Member] | Unsecured Debt [Member] | Unsecured Credit Facilities [Member]
Cash flows from financing activities:
Repayment of debt	(62,000)	(150,000)
Proceeds from unsecured credit facilities	$ 37,000	$ 235,000