Notes Payable (Debt Table) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Notes Payable [Line Items]
Document Period End Date	Jun. 30, 2016
Document Fiscal Year Focus	2016
Summary of terms of notes payable
Notes payable	$ 777,485	$ 718,810
Long-term Debt, Gross	$ 779,704	721,293
Post Oak Central Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.26%
Notes payable	$ 180,046	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	$ 128,440	129,342
Promenade Atlanta Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.27%
Notes payable	$ 106,787	108,203
191 Peachtree Tower Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	3.35%
Notes payable	$ 99,677	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	$ 155,000	92,000
Meridian Mark Plaza mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.00%
Notes payable	$ 24,754	$ 24,978