Notes Payable (Terms of Notes Payable) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total Notes Payable, Gross	$ 1,070,045,000	$ 1,098,764,000
Unamortized premium, net	0	219,000
Unamortized loan costs	(5,033,000)	(5,755,000)
Total Notes Payable	$ 1,065,012,000	1,093,228,000
Weighted average maturity period of notes payable outstanding	6 years 15 days
Term Loan, Unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	3.46%
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	$ 144,271,000	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	$ 100,030,000	102,355,000
Mortgage Debt | 816 Congress
Debt Instrument [Line Items]
Interest Rate (percent)	3.75%
Total Notes Payable, Gross	$ 82,089,000	83,304,000
Mortgage Debt | Meridian Mark Plaza
Debt Instrument [Line Items]
Interest Rate (percent)	6.00%
Total Notes Payable, Gross	$ 23,655,000	24,038,000
Mortgage Debt | The Pointe
Debt Instrument [Line Items]
Interest Rate (percent)	4.01%
Total Notes Payable, Gross	$ 0	22,510,000
Credit Facility, Unsecured
Debt Instrument [Line Items]
Interest Rate (percent)	3.31%
Total Notes Payable, Gross	$ 0	$ 0