Notes Payable (Terms of Notes Payable) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Total Notes Payable, Gross	$ 1,098,764	$ 1,378,676
Unamortized premium, net	219	6,792
Unamortized loan costs	(5,755)	(4,548)
Total Notes Payable	$ 1,093,228	$ 1,380,920
Weighted average maturity period of notes payable outstanding	6 years 8 months 12 days
Fifth Third Center
Debt Instrument [Line Items]
Interest Rate (percent)		3.37%
Colorado Tower
Debt Instrument [Line Items]
Interest Rate (percent)		3.45%
One Eleven Congress
Debt Instrument [Line Items]
Interest Rate (percent)		6.08%
Total Notes Payable, Gross	$ 0	$ 128,000
The American Cancer Society Center
Debt Instrument [Line Items]
Interest Rate (percent)		6.45%
Total Notes Payable, Gross	0	$ 127,508
San Jacinto
Debt Instrument [Line Items]
Interest Rate (percent)		6.048%
Total Notes Payable, Gross	0	$ 101,000
3344 Peachtree
Debt Instrument [Line Items]
Interest Rate (percent)		4.75%
Total Notes Payable, Gross	0	$ 78,971
Two Buckhead Plaza
Debt Instrument [Line Items]
Interest Rate (percent)		6.43%
Total Notes Payable, Gross	$ 0	$ 52,000
Term Loan, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	2.76%
Total Notes Payable, Gross	$ 250,000	250,000
Senior Notes, unsecured | 3.91% Senior Unsecured Notes
Debt Instrument [Line Items]
Interest Rate (percent)	3.91%
Total Notes Payable, Gross	$ 250,000	0
Senior Notes, unsecured | 4.09% Senior Unsecured Notes
Debt Instrument [Line Items]
Interest Rate (percent)	4.09%
Total Notes Payable, Gross	$ 100,000	0
Mortgage Debt
Debt Instrument [Line Items]
Total Notes Payable	$ 498,800
Mortgage Debt | Fifth Third Center
Debt Instrument [Line Items]
Interest Rate (percent)	3.37%
Total Notes Payable, Gross	$ 146,557	149,516
Mortgage Debt | Colorado Tower
Debt Instrument [Line Items]
Interest Rate (percent)	3.45%
Total Notes Payable, Gross	$ 120,000	120,000
Mortgage Debt | Promenade
Debt Instrument [Line Items]
Interest Rate (percent)	4.27%
Total Notes Payable, Gross	$ 102,355	105,342
Mortgage Debt | 816 Congress
Debt Instrument [Line Items]
Interest Rate (percent)	3.75%
Total Notes Payable, Gross	$ 83,304	84,872
Mortgage Debt | Meridian Mark Plaza
Debt Instrument [Line Items]
Interest Rate (percent)	6.00%
Total Notes Payable, Gross	$ 24,038	24,522
Mortgage Debt | The Pointe
Debt Instrument [Line Items]
Interest Rate (percent)	4.01%
Total Notes Payable, Gross	$ 22,510	$ 22,945
Credit Facility, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)		2.66%
Total Notes Payable, Gross	$ 0	$ 134,000