Real Estate Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 acre	Dec. 31, 2010	Dec. 31, 2009
Acquisitions [Abstract]
Acquisition of development land (in acres)	586
Business Acquisition, Purchase Price Allocation [Abstract]
Amount of real estate assets acquired	$ 241,602,000
Amount of acquisition-related intangible assets (in deferred leasing and financing costs) acquired	39,721,000
Amount of furniture, fixtures and equipment (in prepaid and other assets) acquired	1,101,000
Amount of acquisition-related intangible liabilities (in accounts payable, accrued liabilities and other expenses assumed)	(15,627,000)
Total consideration from the acquisition	266,797,000
Business Acquisition, Pro Forma Information [Abstract]
Proforma rental and other revenues	524,480,000	511,861,000	505,840,000
Proforma net income	45,674,000	65,409,000	57,471,000
Proforma earnings per share - basic	$ 0.50	$ 0.77	$ 0.70
Proforma earnings per share - diluted	$ 0.50	$ 0.77	$ 0.70
Dispositions [Abstract]
Number of industrial properties sold	6
Gain on disposition of discontinued operations	2,573,000	(86,000)	21,466,000
Gains on disposition of property	764,000	74,000	266,000
Amount of impairment recorded on disposition	2,429,000	0	2,554,000
Pittsburgh PA Office Property Acquisition [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)	1,540,000
Acquisition purchase price	188,500,000
Fair value of assumed debt from the acquisition	124,500,000
Effective interest rate of assumed secured debt from the acquisition	4.27%
Maturity of assumed debt from the acquisition	Nov. 01, 2017
Acquisition-related costs	4,000,000
Atlanta GA Office Property Acquisition [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)	503,000
Acquisition purchase price	78,300,000
Fair value of assumed debt from the acquisition	67,900,000
Effective interest rate of assumed secured debt from the acquisition	5.45%
Maturity of assumed debt from the acquisition	Jan. 01, 2014
Acquisition-related costs	300,000
Raleigh NC Office Property Acquisition [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)	48,000
Acquisition purchase price	8,900,000
Acquisition-related costs	100,000
Memphis TN Office Property Acquisition [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)		336,000
Acquisition purchase price		52,600,000
Fair value of assumed debt from the acquisition		40,300,000
Effective interest rate of assumed secured debt from the acquisition		6.43%
Maturity of assumed debt from the acquisition		Nov. 01, 2015
Acquisition-related costs		400,000
Tampa FL Office Property and Land Acquisition [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)		117,000
Acquisition of development land (in acres)		32.6
Acquisition purchase price		12,000,000
Acquisition-related costs		200,000
For Sale Residential Condominium JV Interest Acquisition [Member]
Acquisitions [Abstract]
Acquisition purchase price		500,000
Tampa FL Office Property Acquisition (2) [Member]
Acquisitions [Abstract]
Net rentable square feet of acquisition (in sqft)			220,000
Acquisition purchase price			22,300,000
Acquisition-related costs			100,000
Winston-Salem NC Disposition [Member]
Dispositions [Abstract]
Gross proceeds from the sale of real estate	15,000,000
Gain on disposition of discontinued operations	2,600,000
Gains on disposition of property	300,000
Winston-Salem NC and Greensboro NC Office and Industrial Property Disposition [Member]
Dispositions [Abstract]
Number of office properties sold		7
Number of industrial properties sold		6
Gross proceeds from the sale of real estate		24,900,000
Cash received from sale of real estate		7,900,000
Amount of seller financing provided		17,000,000
Additional potential seller financing commitment		1,700,000
Amount of commited additional seller financing funded	200,000
Amount of impairment recorded on disposition		300,000
Deferred gain on sale of property		400,000
Impairments [Abstract]
Impairment of long lived assets in discontinued operations			11,000,000
Raleigh NC Office Property Disposition [Member]
Dispositions [Abstract]
Gains on disposition of property		200,000
Non-Core Retail and Office Property Disposition [Member]
Dispositions [Abstract]
Net rentable square feet of disposition			517,000
Gross proceeds from the sale of real estate			78,200,000
Gains on disposition of property			21,700,000
Orlando FL Office Property Impairment [Member]
Dispositions [Abstract]
Amount of impairment recorded on disposition	2,400,000
Impairments [Abstract]
Number of long lived assets held for use impaired	2
Winston-Salem NC Office Property Impairment [Member]
Dispositions [Abstract]
Amount of impairment recorded on disposition			$ 2,600,000
Impairments [Abstract]
Number of long lived assets held for use impaired			4