Notes Payable (Debt Table) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Notes Payable [Line Items]
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2015
Summary of terms of notes payable
Notes payable	$ 779,570	$ 792,344
Post Oak Central Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.26%
Notes payable	$ 182,618	185,109
The American Cancer Society Center mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.45%
Notes payable	$ 129,794	131,083
Promenade Atlanta Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.27%
Notes payable	$ 108,899	110,946
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	1.29%
Notes payable	$ 134,000	140,200
Meridian Mark Plaza mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.00%
Notes payable	$ 25,088	25,408
Points at Waterview mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	5.66%
Notes payable	$ 14,171	$ 14,598