Notes Payable (Debt Table) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Document Fiscal Year Focus	2014	2013	2012
Notes Payable	$ 792,344	$ 630,094
Post Oak Central mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	4.26%
Notes Payable	185,109	188,310
Credit Facility, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	1.27%
Notes Payable	140,200	40,075
The American Cancer Society Center mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	6.45%
Notes Payable	131,083	132,714
Promenade mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	4.27%
Notes Payable	110,946	113,573
191 Peachtree Tower mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	3.35%
Notes Payable	100,000	100,000
816 Congress mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	3.75%
Notes Payable	85,000	0
Meridian Mark Plaza mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	6.00%
Notes Payable	25,408	25,813
The Points at Waterview mortgage note
Debt Instrument [Line Items]
Interest Rate (percent)	5.66%
Notes Payable	14,598	15,139
Mahan Village LLC construction facility
Debt Instrument [Line Items]
Interest Rate (percent)	1.80%
Notes Payable	$ 0	$ 14,470