Notes Payable (Terms of Notes Payable) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Notes Payable, Gross	$ 1,024,950	$ 1,378,676
Unamortized premium, net	750	6,792
Unamortized loan costs	(6,081)	(4,548)
Total Notes Payable	$ 1,019,619	1,380,920
Credit Facility, unsecured
Debt Instrument [Line Items]
Interest Rate	2.42%
Notes Payable, Gross	$ 250,000	250,000
Mortgage Debt | Fifth Third Center
Debt Instrument [Line Items]
Interest Rate	3.37%
Notes Payable, Gross	$ 148,049	149,516
Mortgage Debt | Colorado Tower
Debt Instrument [Line Items]
Interest Rate	3.45%
Notes Payable, Gross	$ 120,000	120,000
Mortgage Debt | Promenade
Debt Instrument [Line Items]
Interest Rate	4.27%
Notes Payable, Gross	$ 103,864	105,342
Mortgage Debt | 816 Congress
Debt Instrument [Line Items]
Interest Rate	3.75%
Notes Payable, Gross	$ 84,095	84,872
Mortgage Debt | 3344 Peachtree
Debt Instrument [Line Items]
Interest Rate	4.75%
Notes Payable, Gross	$ 77,928	78,971
Mortgage Debt | Meridian Mark Plaza
Debt Instrument [Line Items]
Interest Rate	6.00%
Notes Payable, Gross	$ 24,284	24,522
Mortgage Debt | The Pointe
Debt Instrument [Line Items]
Interest Rate	4.01%
Notes Payable, Gross	$ 22,730	22,945
Mortgage Debt | One Eleven Congress
Debt Instrument [Line Items]
Interest Rate	6.08%
Notes Payable, Gross	$ 0	128,000
Mortgage Debt | The ACS Center
Debt Instrument [Line Items]
Interest Rate	6.45%
Notes Payable, Gross	$ 0	127,508
Mortgage Debt | San Jacinto Center
Debt Instrument [Line Items]
Interest Rate	6.048%
Notes Payable, Gross	$ 0	101,000
Mortgage Debt | Two Buckhead Plaza
Debt Instrument [Line Items]
Interest Rate	6.43%
Notes Payable, Gross	$ 0	52,000
Senior Note, unsecured
Debt Instrument [Line Items]
Interest Rate	4.09%
Notes Payable, Gross	$ 100,000	0
Credit Facility, unsecured
Debt Instrument [Line Items]
Interest Rate	2.32%
Notes Payable, Gross	$ 94,000	$ 134,000