Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statement of Cash Flows [Abstract]
Net income (loss)	$ 5,535	$ (184)	$ 8,247
Adjustments to reconcile net income (loss) to cash provided by operating activities:
Depreciation and amortization	101,444	92,388	77,894
Stock-based compensation	3,492	2,922	2,411
Straight-line rent receivable	(6,013)	(4,630)	(2,472)
Straight-line rent liability	418	488	92
Gain on sales of real estate properties	(10,874)	(7,035)	(8,352)
Loss on extinguishment of debt		1,986	480
Net gain from mortgage repayment by previously consolidated VIE	(313)
Impairments	14,908	6,697	7,511
Provision for bad debt, net	240	(160)	(409)
Payment of partial pension settlement			(2,582)
Changes in operating assets and liabilities:
Other assets	(3,469)	(5,173)	(2,216)
Accounts payable and accrued liabilities	(712)	10,770	5,834
Other liabilities	11,741	9,783	1,318
Net cash provided by operating activities	116,397	107,852	87,756
INVESTING ACTIVITIES
Acquisitions of real estate	(89,640)	(114,225)	(276,195)
Development of real estate	(7,833)	(83,720)	(60,722)
Tenant improvements and capital additions	(62,251)	(34,306)	(38,825)
Funding of mortgages and notes receivable	(78,297)	(101,931)	(25,109)
Proceeds from sales of real estate	74,817	19,572	34,512
Proceeds from mortgage repayment by previously consolidated VIE	35,057
Proceeds from mortgages and notes receivable repayments	14,893	17,797	9,201
Net cash used in investing activities	(113,254)	(296,813)	(357,138)
FINANCING ACTIVITIES
Net borrowings (repayments) on unsecured credit facility	(102,000)	212,000	(50,000)
Borrowings on notes and bonds payable			396,800
Repayments on notes and bonds payable	(4,990)	(3,703)	(2,516)
Repurchase of notes payable		(280,201)	(8,556)
Dividends paid	(96,356)	(89,270)	(75,821)
Net proceeds from issuance of common stock	202,352	251,916	118,205
Common stock redemptions	(68)	(86)
Capital contributions received from noncontrolling interests			633
Distributions to noncontrolling interest holders	(40)	(284)	(481)
Purchase of noncontrolling interests		(1,591)
Debt issuance and assumption costs	(3)	(8,403)	(1,412)
Net cash provided by (used in) financing activities	(1,105)	80,378	376,852
Increase (decrease) in cash and cash equivalents	2,038	(108,583)	107,470
Cash and cash equivalents, beginning of period	4,738	113,321	5,851
Cash and cash equivalents, end of period	6,776	4,738	113,321
Supplemental Cash Flow Information:
Interest paid	75,348	73,157	62,274
Capitalized interest	5,021	8,531	10,315
Company-financed real estate property sales	11,200	2,700
Invoices accrued for construction, tenant improvement and other capitalized costs	4,297	12,131	13,555
Construction liabilities transferred upon deconsolidation of VIE	3,450
Mortgage notes payable assumed upon acquisition (adjusted to fair value)	5,171	54,392	24,268
Foreclosure of mortgage notes receivable		4,371
Elimination of mortgage note upon consolidation of VIE		$ 21,939