Notes Payable (Debt Table) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Notes Payable [Line Items]
Document Period End Date	Mar. 31, 2016
Document Fiscal Year Focus	2016
Summary of terms of notes payable
Notes payable	$ 767,811	$ 718,810
Long-term Debt, Gross	$ 770,162	$ 721,293	$ 721,293
Post Oak Central Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.26%
Notes payable	$ 180,912		181,770
The American Cancer Society Center mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.45%
Notes payable	$ 128,884		129,342
Promenade Atlanta Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.27%
Notes payable	$ 107,499		108,203
191 Peachtree Tower Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	3.35%
Notes payable	$ 100,000		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	1.54%
Notes payable	$ 143,000		92,000
Meridian Mark Plaza mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.00%
Notes payable	$ 24,867		$ 24,978