Notes Payable (Debt Table) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Notes Payable [Line Items]
Document Fiscal Year Focus	2014
Summary of terms of notes payable
Notes payable	$ 671,074	$ 630,094
Post Oak Central Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.26%
Notes payable	185,922	188,310
The American Cancer Society Center mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.45%
Notes payable	131,507	132,714
Promenade Atlanta Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.27%
Notes payable	111,612	113,573
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
Credit Facility, unsecured Member]
Summary of terms of notes payable
Interest rate on mortgage loan	1.25%
Notes payable	87,700	40,075
Meridian Mark Plaza mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.00%
Notes payable	25,512	25,813
Points at Waterview mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	5.66%
Notes payable	14,736	15,139
Mahan Village LLC [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	1.80%
Notes payable	$ 14,085	$ 14,470