Real Estate Assets (Details) ft² in Thousands, $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2015 USD ($) ft² property	Jun. 30, 2015 USD ($) ft²	Mar. 31, 2015 USD ($) property	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) ft² property	Sep. 30, 2014 USD ($)
Acquisitions [Abstract]
Contingent consideration in connection with the acquisition of land					$ 900	$ 0
Acquisition-related costs	$ 900				1,000
Dispositions [Abstract]
Gain on disposition of property	$ 7,012			$ 36,238	$ 10,581	$ 42,185
Tampa, FL Building Acquisition (09/2015) [Member]
Acquisitions [Abstract]
Acquisition rentable square feet (in sq. ft.) | ft²	528				528
Purchase price of acquisition	$ 113,500
Tampa, FL Land Acquisition (09/2015) [Member]
Acquisitions [Abstract]
Purchase price of acquisition	$ 2,200
Atlanta, GA Building Acquisition (09/2015) [Member]
Acquisitions [Abstract]
Acquisition rentable square feet (in sq. ft.) | ft²	896				896
Number of office properties acquired | property	2				2
Purchase price of acquisition	$ 290,300
Atlanta, GA Land Acquisition [Member] (6/2015) [Member]
Acquisitions [Abstract]
Purchase price of acquisition		$ 5,200
Contingent consideration in connection with the acquisition of land		$ 900
Orlando, FL Building Acquisition [Member] (6/2015) [Member]
Acquisitions [Abstract]
Acquisition rentable square feet (in sq. ft.) | ft²		168
Percentage of equity interest acquired in unconsolidated affiliate (in hundredths)		77.20%
Fair value of debt assumed in connection with the acquisition		$ 19,300
Building Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | property			2
Purchase price of real estate	15,300		$ 3,500
Gain on disposition of property	6,500		400
Land Dispositions [Member]
Dispositions [Abstract]
Purchase price of real estate	1,800	500	2,500
Gain on disposition of property	$ 500	$ 200	$ 800