Notes Payable (Terms of Notes Payable) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total Notes Payable, Gross	$ 1,096,602	$ 1,098,764
Unamortized premium, net	169	219
Unamortized loan costs	(5,513)	(5,755)
Total Notes Payable	$ 1,091,258	1,093,228
Weighted average maturity period of notes payable outstanding	6 years 5 months 12 days
Term Loan, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	3.08%
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	250,000
Senior Notes, unsecured | 4.09% Senior Unsecured Notes
Debt Instrument [Line Items]
Interest Rate (percent)	4.09%
Total Notes Payable, Gross	$ 100,000	100,000
Mortgage Debt | Fifth Third Center
Debt Instrument [Line Items]
Interest Rate (percent)	3.37%
Total Notes Payable, Gross	$ 145,801	146,557
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	$ 101,588	102,355
Mortgage Debt | 816 Congress
Debt Instrument [Line Items]
Interest Rate (percent)	3.75%
Total Notes Payable, Gross	$ 82,903	83,304
Mortgage Debt | Meridian Mark Plaza
Debt Instrument [Line Items]
Interest Rate (percent)	6.00%
Total Notes Payable, Gross	$ 23,912	24,038
Mortgage Debt | The Pointe
Debt Instrument [Line Items]
Interest Rate (percent)	4.01%
Total Notes Payable, Gross	$ 22,398	22,510
Credit Facility, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	2.93%
Total Notes Payable, Gross	$ 0	$ 0