Real Estate Assets (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended
	Sep. 30, 2013 acre	Sep. 30, 2012	Sep. 30, 2013 acre	Sep. 30, 2012	Sep. 30, 2013 Pro Forma 2013 Acquisitions [Member]	Sep. 30, 2012 Pro Forma 2013 Acquisitions [Member]	Sep. 30, 2013 Pro Forma 2013 Acquisitions [Member]	Sep. 30, 2012 Pro Forma 2013 Acquisitions [Member]	Sep. 30, 2012 Pro Forma 2012 Acquisitions [Member]	Sep. 30, 2012 Pro Forma 2012 Acquisitions [Member]	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	Sep. 30, 2013 Atlanta, GA Office Properties Acquisition (9/2013) [Member] sqft property	Jun. 30, 2013 Atlanta, GA Office Property Acquisition (6/2013) [Member] sqft	Mar. 31, 2013 Tampa, FL Office Properties Acquisition (3/2013) [Member] sqft property	Mar. 31, 2013 Greensboro, NC Office Properties Acquisition (3/2013) [Member] property sqft	Mar. 31, 2013 Memphis, TN Development Land Acquisition (3/2013) [Member] acre	Sep. 30, 2012 Atlanta, GA Office Property Acquisition (9/2012) [Member] sqft	Sep. 30, 2013 Tampa, FL Office Property Disposition (9/2013) [Member]	Sep. 30, 2013 Atlanta, GA Industrial Properties/Land Disposition (9/2013) [Member] property	Jun. 30, 2013 Atlanta, GA Industrial Property Disposition (6/2013) [Member] property	Jun. 30, 2013 Atlanta, GA Industrial Property/Land Disposition 2 (6/2013) [Member] property	Jun. 30, 2013 Atlanta, GA Industrial Property Disposition 3 (6/2013) [Member] property	Mar. 31, 2013 Orlando, FL Office Properties Disposition (3/2013) [Member] property	Mar. 31, 2013 Jackson, MS Office Property Disposition (9/2012) [Member]	Jun. 30, 2013 Winston-Salem, NC Office Properties (6/2013) [Member] property	Mar. 31, 2013 Atlanta, GA Industrial Properties (3/2013) [Member] property	Mar. 31, 2013 Atlanta, GA Industrial Properties Held For Sale (3/2013) [Member] property
Acquisitions [Abstract]
Percentage of equity interest acquired in unconsolidated affiliate (in hundredths)											60.00%		57.00%
Percentage of equity interest in joint venture, previously held (in hundredths)											40.00%
Number of office properties acquired											5		2		2	2
Rentable square feet of acquisition (in sq. ft)											1,300,000	520,000	505,000	553,000	372,000	195,000		492,000
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					127,891,000		127,891,000				127,900,000		37,600,000
Effective interest rate of secured debt assumed from the acquisition (in hundredths)											3.11%		3.34%
Maturity date of debt assumed from the acquisition											Jul. 01, 2014		Apr. 01, 2015
Previously held equity interest remeasured at fair value											75,200,000
Gain on acquisition of controlling interest in unconsolidated affiliate	7,451,000	0	7,451,000	0							7,500,000
Acquisition purchase price												150,100,000	44,500,000	140,100,000	52,500,000	30,800,000	4,800,000	144,900,000
Acquisition development land (in acres)	592		592														5
Acquisition-related costs	800,000		1,700,000
Business Acquisition, Purchase Price Allocation [Abstract]
Real estate assets					445,396,000		445,396,000		135,128,000	135,128,000
Acquisition-related intangible assets (in deferred financing and leasing costs)					50,595,000		50,595,000		21,637,000	21,637,000
Fair value of debt assumed from the acquisition					(127,891,000)		(127,891,000)				(127,900,000)		(37,600,000)
Acquisition-related below market lease liabilities (in accounts payable, accrued expenses and other liabilities)					(17,818,000)		(17,818,000)		(11,875,000)	(11,875,000)
Total allocation					350,282,000		350,282,000		144,890,000	144,890,000
Business Acquisition, Pro Forma Information [Abstract]
Pro forma, rental and other revenues					152,221,000	162,422,000	451,317,000	410,027,000	127,836,000	381,070,000
Pro forma, net income					49,818,000	23,952,000	90,064,000	56,782,000	35,765,000	68,526,000
Pro forma, earnings per share - basic					$ 0.54	$ 0.28	$ 1.00	$ 0.68	$ 0.43	$ 0.84
Pro forma, earnings per share - diluted					$ 0.54	$ 0.29	$ 1.00	$ 0.69	$ 0.43	$ 0.84
Pro forma, assumption date of acquisition					Jan. 01, 2012				Jan. 01, 2011
Revenues recorded due to acquisitions					10,000,000		10,300,000
Net loss recorded due to acquisitions					(100,000)		(400,000)
Dispositions [Abstract]
Number of office properties sold																								2
Number of industrial real estate properties sold																				16	5	6	2
Proceeds received from sale of real estate																			11,600,000	91,600,000	4,500,000	38,700,000	4,800,000	14,600,000
Closing credits for unfunded tenant improvements																				300,000		1,800,000		800,000
Closing credits for free rent																				300,000	100,000	1,300,000
Gain/(loss) on disposition of discontinued operations	37,946,000	22,936,000	52,353,000	29,455,000															1,200,000	36,700,000	100,000	13,200,000	(100,000)	(300,000)	1,500,000
Gain on disposition of property	34,000	0	(3,000)	0																100,000
Impairments [Abstract]
Number of impaired properties																										4	2	5
Impairments of real estate assets																										1,100,000	400,000
Impairments of real estate assets held for sale	$ 0	$ 0	$ 2,194,000	$ 0																								$ 700,000