Real Estate Assets (Details) ft² in Thousands, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2016 USD ($) a ft²	Mar. 31, 2016 USD ($) ft²	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) a ft²	Jun. 30, 2015 USD ($)
Acquisitions [Abstract]
Acquisition of development land (in acres) | a	500			500
Dispositions [Abstract]
Gains on disposition of discontinued operations	$ 0		$ 0	$ 414,496	$ 0
Gains on disposition of property	$ 5,861		$ 2,412	$ 10,258	$ 3,569
Rentable square feet of building (in sq feet) | ft²	31,100			31,100
Nashville TN Land Acquisition (6/2016) [Member]
Acquisitions [Abstract]
Acquisition of development land (in acres) | a	14			14
Acquisition purchase price	$ 9,100
Division Exit [Member]
Dispositions [Abstract]
Purchase price of real estate		$ 660,000
Gains on disposition of discontinued operations		414,500
Gains on disposition of property		1,300
Disposition closing credits excluded		4,800
Related Party Office Building Disposition [Member]
Dispositions [Abstract]
Purchase price of real estate		4,700
Gains on disposition of property		$ 1,100
Rentable square feet of building (in sq feet) | ft²		32
Buyer occupancy percentage (in hundredths)		79.00%
Disposition closing credits excluded		$ 100
Building Dispositions [Member]
Dispositions [Abstract]
Purchase price of real estate	14,200	6,400
Gains on disposition of property	$ 5,900	2,000
Disposition closing credits excluded		$ 500