Notes Payable (Debt Table) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Notes Payable [Line Items]
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2016
Summary of terms of notes payable
Notes payable	$ 789,378	$ 718,810
Long-term Debt, Gross	792,866	721,293
Long-term debt including held for sale	892,054	721,293
Notes payable held for sale	$ (99,188)	0
Post Oak Central Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.26%
Notes payable	$ 179,170	181,770
Fifth Third Center Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	3.37%
Notes payable	$ 150,000	0
The American Cancer Society Center mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.45%
Notes payable	$ 127,989	129,342
Colorado Tower Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	3.45%
Notes payable	$ 120,000	0
Promenade Atlanta Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.27%
Notes payable	$ 106,068	108,203
191 Peachtree Tower Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	3.35%
Notes payable	$ 99,188	100,000
816 Congress Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	3.75%
Notes payable	$ 85,000	85,000
Credit Facility, unsecured Member]
Summary of terms of notes payable
Interest rate on mortgage loan	1.63%
Notes payable	$ 0	92,000
Meridian Mark Plaza mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.00%
Notes payable	$ 24,639	$ 24,978