Notes Payable (Terms of Notes Payable) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total Notes Payable, Gross	$ 1,094,420,000	$ 1,098,764,000
Unamortized premium, net	118,000	219,000
Unamortized loan costs	(5,274,000)	(5,755,000)
Total Notes Payable	$ 1,089,264,000	1,093,228,000
Weighted average maturity period of notes payable outstanding	6 years 2 months 1 day
Term Loan, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	3.29%
Total Notes Payable, Gross	$ 250,000,000	250,000,000
Senior Notes, unsecured | 3.91% Senior Unsecured Notes
Debt Instrument [Line Items]
Interest Rate (percent)	3.91%
Total Notes Payable, Gross	$ 250,000,000	250,000,000
Senior Notes, unsecured | 4.09% Senior Unsecured Notes
Debt Instrument [Line Items]
Interest Rate (percent)	4.09%
Total Notes Payable, Gross	$ 100,000,000	100,000,000
Mortgage Debt | Fifth Third Center
Debt Instrument [Line Items]
Interest Rate (percent)	3.37%
Total Notes Payable, Gross	$ 145,039,000	146,557,000
Mortgage Debt | Colorado Tower
Debt Instrument [Line Items]
Interest Rate (percent)	3.45%
Total Notes Payable, Gross	$ 120,000,000	120,000,000
Mortgage Debt | Promenade
Debt Instrument [Line Items]
Interest Rate (percent)	4.27%
Total Notes Payable, Gross	$ 100,813,000	102,355,000
Mortgage Debt | 816 Congress
Debt Instrument [Line Items]
Interest Rate (percent)	3.75%
Total Notes Payable, Gross	$ 82,498,000	83,304,000
Mortgage Debt | Meridian Mark Plaza
Debt Instrument [Line Items]
Interest Rate (percent)	6.00%
Total Notes Payable, Gross	$ 23,785,000	24,038,000
Mortgage Debt | The Pointe
Debt Instrument [Line Items]
Interest Rate (percent)	4.01%
Total Notes Payable, Gross	$ 22,285,000	22,510,000
Credit Facility, unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	3.14%
Total Notes Payable, Gross	$ 0	$ 0