Real Estate Assets (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Raleigh, NC Office Property Acquisition (9/2014) [Member] sqft	Sep. 30, 2013 Orlando, FL Office Properties Acquisition (9/2013) [Member] property sqft	Sep. 30, 2013 Nashville, TN Office Property Acquisition (9/2013) [Member] sqft	Jun. 30, 2013 Atlanta, GA Office Property Acquisition (6/2013) [Member] sqft	Sep. 30, 2014 Raleigh, NC Office Properties Disposition (9/2014) [Member] property	Sep. 30, 2014 Richmond, VA Office Properties Disposition (9/2014) [Member] property	Sep. 30, 2014 Greensboro, NC Office Properties Disposition (9/2014) [Member] property	Sep. 30, 2014 Greenville, SC Office Property Disposition (9/2014) [Member]	Jun. 30, 2014 Atlanta, GA Land Disposition (6/2014) [Member] parcels	Jun. 30, 2014 Greensboro, NC Office Property Impairment (6/2014) [Member]
Acquisitions [Abstract]
Number of office properties acquired						5
Rentable square feet of acquisition (in sq. ft.)					374,000	1,300,000	520,000	553,000
Purchase price of acquisition					$ 83,800,000		$ 150,100,000	$ 140,100,000
Acquisition-related costs					150,000
Percentage of equity interest acquired in unconsolidated affiliate (in hundredths)						60.00%
Percentage of equity interest in unconsolidated affiliate, previously held (in hundredths)						40.00%
Purchase price of acquisition of unconsolidated affiliate						112,800,000
Fair value of assets recorded from the acquisition						188,000,000
Fair value of debt assumed from the acquisition						127,900,000
Effective interest rate of secured debt assumed from the acquisition (in hundredths)						3.11%
Previously held equity interest remeasured at fair value						75,200,000
Gain on acquisition of controlling interest in unconsolidated affiliate	0	7,451,000	0	7,451,000		7,500,000
Dispositions [Abstract]
Number of office properties sold									5	11	6
Number of industrial properties sold											8
Number of land parcels sold													2
Purchase price of real estate									58,700,000	40,700,000	28,200,000	27,200,000	9,500,000
Closing credits excluded for unfunded building and/or tenant improvements											1,200,000	5,800,000
Closing credits excluded for free rent											400,000	1,800,000
Gain on disposition of property	36,238,000	34,000	42,185,000	(3,000)					11,700,000	17,600,000	4,700,000	2,200,000	5,900,000
Impairments [Abstract]
Impairments of real estate assets	$ 0	$ 0	$ 588,000	$ 0										$ 600,000