Notes Payable, Interest Expense and Commitments and Contingencies (Debt Table) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Summary of terms of notes payable
Notes payable	$ 518,630	$ 539,442
Terminus 100 mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	5.25%
Debt instrument, Term	12 years
Debt instrument, amortization period	30 years
Maturity date of debt instrument	Jan. 01, 2023
Notes payable	136,651	138,194
The American Cancer Society Center mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.45%
Debt instrument, Term	10 years
Debt instrument, amortization period	30 years
Maturity date of debt instrument	Sep. 01, 2017
Notes payable	134,615	135,650
191 Peachtree Tower Mortgage Note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	3.35%
Debt instrument, Term	6 years 6 months
Debt instrument, amortization period	30 years
Maturity date of debt instrument	Oct. 01, 2018
Notes payable	100,000	0
Credit Facility, unsecured Member]
Summary of terms of notes payable
Interest rate on mortgage loan	1.71%
Debt instrument, Term	4 years
Maturity date of debt instrument	Feb. 28, 2016
Notes payable	93,500	198,250
Meridian Mark Plaza mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	6.00%
Debt instrument, Term	10 years
Debt instrument, amortization period	30 years
Maturity date of debt instrument	Aug. 01, 2020
Notes payable	26,286	26,554
100/200 North Point Center East mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	5.39%
Debt instrument, Term	5 years
Debt instrument, amortization period	30 years
Maturity date of debt instrument	Jun. 01, 2012
Notes payable	0	24,478
Points at Waterview mortgage note [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	5.66%
Debt instrument, Term	10 years
Debt instrument, amortization period	25 years
Maturity date of debt instrument	Jan. 01, 2016
Notes payable	15,775	16,135
Mahan Village LLC [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	1.86%
Debt instrument, Term	3 years
Maturity date of debt instrument	Sep. 12, 2014
Notes payable	11,633	1
Callaway Gardens [Member]
Summary of terms of notes payable
Interest rate on mortgage loan	4.13%
Maturity date of debt instrument	Nov. 18, 2013
Notes payable	$ 170	$ 180