Notes Payable (Debt Table) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of terms of notes payable
Notes payable	$ 340,374	$ 425,410
The American Cancer Society Center mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.45%
Notes payable	133,479	134,243
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	1.69%
Notes payable	51,000	0
Meridian Mark Plaza mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.00%
Notes payable	26,006	26,194
Points at Waterview mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	5.66%
Notes payable	15,399	15,651
Mahan Village LLC [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	1.84%
Notes payable	14,316	13,027
Callaway Gardens [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.13%
Notes payable	174	172
Terminus 100 mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	5.25%
Notes payable	$ 0	$ 136,123