Notes Payable (Debt Table) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Notes Payable	$ 425,410	$ 539,442
Terminus 100 mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	5.25%
Notes Payable	136,123	138,194
The American Cancer Society Center mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	6.45%
Notes Payable	134,243	135,650
191 Peachtree Tower mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	3.35%
Notes Payable	100,000	0
Meridian Mark Plaza mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	6.00%
Notes Payable	26,194	26,554
The Points at Waterview mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	5.66%
Notes Payable	15,651	16,135
Mahan Village LLC contruction facility
Debt Instrument [Line Items]
Interest Rate (percent)	1.86%
Notes Payable	13,027	1
Callaway Gardens mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	4.13%
Notes Payable	172	180
Credit Facility, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	1.71%
Notes Payable	0	198,250
100/200 North Point Center East mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	5.39%
Notes Payable	$ 0	$ 24,478