Notes Payable (Terms of Notes Payable) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Total Notes Payable, Gross	$ 1,378,676	$ 721,293
Unamortized premium, net	6,792	0
Unamortized loan costs	(4,548)	(2,483)
Total Notes Payable	$ 1,380,920	718,810
Fifth Third Center
Debt Instrument [Line Items]
Interest Rate (percent)	3.37%
Total Notes Payable, Gross	$ 149,516	0
One Eleven Congress
Debt Instrument [Line Items]
Interest Rate (percent)	6.08%
Total Notes Payable, Gross	$ 128,000	0
The American Cancer Society Center
Debt Instrument [Line Items]
Interest Rate (percent)	6.45%
Total Notes Payable, Gross	$ 127,508	129,342
Colorado Tower
Debt Instrument [Line Items]
Interest Rate (percent)	3.45%
Total Notes Payable, Gross	$ 120,000	0
Promenade
Debt Instrument [Line Items]
Interest Rate (percent)	4.27%
Total Notes Payable, Gross	$ 105,342	108,203
San Jacinto
Debt Instrument [Line Items]
Interest Rate (percent)	6.048%
Total Notes Payable, Gross	$ 101,000	0
816 Congress
Debt Instrument [Line Items]
Interest Rate (percent)	3.75%
Total Notes Payable, Gross	$ 84,872	85,000
3344 Peachtree
Debt Instrument [Line Items]
Interest Rate (percent)	4.75%
Total Notes Payable, Gross	$ 78,971	0
Two Buckhead Plaza
Debt Instrument [Line Items]
Interest Rate (percent)	6.43%
Total Notes Payable, Gross	$ 52,000	0
Meridian Mark Plaza
Debt Instrument [Line Items]
Interest Rate (percent)	6.00%
Total Notes Payable, Gross	$ 24,522	24,978
The Pointe
Debt Instrument [Line Items]
Interest Rate (percent)	4.01%
Total Notes Payable, Gross	$ 22,945	$ 0
Post Oak Central
Debt Instrument [Line Items]
Interest Rate (percent)		4.26%
Total Notes Payable, Gross	0	$ 181,770
191 Peachtree Tower
Debt Instrument [Line Items]
Interest Rate (percent)		3.35%
Total Notes Payable, Gross	$ 0	$ 100,000
Term Loan, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	1.97%
Total Notes Payable, Gross	$ 250,000	0
Credit Facility, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	1.87%
Total Notes Payable, Gross	$ 134,000	$ 92,000