Notes Payable (Terms of Notes Payable) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Notes Payable, Gross	$ 1,939,743	$ 2,218,093
Unamortized premium	10,323	11,239
Unamortized loan costs	(6,032)	(6,357)
Total Notes Payable	$ 1,944,034	2,222,975
Weighted average maturity of notes payable	5 years 8 months 12 days
Unsecured Notes
Debt Instrument [Line Items]
Notes Payable, Gross	$ 1,250,000	$ 1,501,500
Credit Facility, Unsecured
Debt Instrument [Line Items]
Interest Rate		2.04%
Notes Payable, Gross	$ 0	$ 251,500
Term Loan, Unsecured
Debt Instrument [Line Items]
Interest Rate	2.19%
Notes Payable, Gross	$ 250,000	250,000
Senior Notes, Unsecured | 3.95% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.95%
Notes Payable, Gross	$ 275,000	275,000
Senior Notes, Unsecured | 3.91% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.91%
Notes Payable, Gross	$ 250,000	250,000
Senior Notes, Unsecured | 3.86% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.86%
Notes Payable, Gross	$ 250,000	250,000
Senior Notes, Unsecured | 3.78% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	3.78%
Notes Payable, Gross	$ 125,000	125,000
Senior Notes, Unsecured | 4.09% Senior Notes, Unsecured
Debt Instrument [Line Items]
Interest Rate	4.09%
Notes Payable, Gross	$ 100,000	100,000
Secured Mortgage Notes
Debt Instrument [Line Items]
Notes Payable, Gross	$ 689,743	716,593
Secured Mortgage Notes | Fifth Third Center
Debt Instrument [Line Items]
Interest Rate	3.37%
Notes Payable, Gross	$ 139,522	140,332
Secured Mortgage Notes | Terminus 100
Debt Instrument [Line Items]
Interest Rate	5.25%
Notes Payable, Gross	$ 117,375	118,146
Secured Mortgage Notes | Colorado Tower
Debt Instrument [Line Items]
Interest Rate	3.45%
Notes Payable, Gross	$ 116,486	117,085
Secured Mortgage Notes | Promenade
Debt Instrument [Line Items]
Interest Rate	4.27%
Notes Payable, Gross	$ 95,152	95,986
Secured Mortgage Notes | 816 Congress
Debt Instrument [Line Items]
Interest Rate	3.75%
Notes Payable, Gross	$ 79,554	79,987
Secured Mortgage Notes | Terminus 200
Debt Instrument [Line Items]
Interest Rate	3.79%
Notes Payable, Gross	$ 75,654	76,079
Secured Mortgage Notes | Legacy Union One
Debt Instrument [Line Items]
Interest Rate	4.24%
Notes Payable, Gross	$ 66,000	$ 66,000
Secured Mortgage Notes | Meridian Mark Plaza
Debt Instrument [Line Items]
Interest Rate		6.00%
Notes Payable, Gross	$ 0	$ 22,978