Notes Payable (Debt Table) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Summary of terms of notes payable
Notes payable	$ 642,834	$ 425,410
Post Oak Central Mortgage Note [Member]
Summary of terms of notes payable
Notes payable	188,830	0
The American Cancer Society Center mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.45%
Notes payable	133,112	134,243
Promenade Atlanta Mortgage Note [Member]
Summary of terms of notes payable
Notes payable	114,000	0
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	1.70%
Notes payable	51,075	0
Meridian Mark Plaza mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.00%
Notes payable	25,910	26,194
Points at Waterview mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	5.66%
Notes payable	15,270	15,651
Mahan Village LLC [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	1.85%
Notes payable	14,463	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