Investments in Unconsolidated Real Estate Joint Ventures (Unconsolidated Real Estate Joint Ventures) (Details) ft² in Thousands, $ in Thousands	Sep. 30, 2018 USD ($) ft²	Dec. 31, 2017 USD ($) ft²
National Harbor and RioCan Canada [Member]
Schedule of Equity Method Investments [Line Items]
Carrying Value of Investment	$ 111,300
National Harbor [Member]
Schedule of Equity Method Investments [Line Items]
Ownership %	50.00%	50.00%
Square Feet | ft²	341	341
Carrying Value of Investment	$ 600	$ 2,500
RioCan Canda [Member]
Schedule of Equity Method Investments [Line Items]
Ownership %	50.00%	50.00%
Square Feet | ft²	923	923
Carrying Value of Investment	$ 110,700	$ 115,800
Columbus National Harbor RioCan Canada [Member]
Schedule of Equity Method Investments [Line Items]
Carrying Value of Investment		$ 119,400
Columbus, Charlotte and Galveston/Houston [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment Reported In Liabilities	$ (26,700)
Columbus [Member]
Schedule of Equity Method Investments [Line Items]
Ownership %	50.00%	50.00%
Square Feet | ft²	355	355
Carrying Value of Investment		$ 1,100
Equity Method Investment Reported In Liabilities	$ (1,100)
Charlotte [Member]
Schedule of Equity Method Investments [Line Items]
Ownership %	50.00%	50.00%
Square Feet | ft²	398	398
Equity Method Investment Reported In Liabilities	$ (10,300)	$ (4,100)
Galveston/Houston [Member]
Schedule of Equity Method Investments [Line Items]
Ownership %	50.00%	50.00%
Square Feet | ft²	353	353
Equity Method Investment Reported In Liabilities	$ (15,300)	$ (13,000)
Charlotte and Galveston/Houston [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment Reported In Liabilities		(17,100)
Unconsolidated Properties [Member]
Schedule of Equity Method Investments [Line Items]
Total joint venture debt, net	360,600	351,259
Unconsolidated Properties [Member] | National Harbor [Member]
Schedule of Equity Method Investments [Line Items]
Total joint venture debt, net	86,700	86,400
Unconsolidated Properties [Member] | RioCan Canda [Member]
Schedule of Equity Method Investments [Line Items]
Total joint venture debt, net	10,200	11,100
Unconsolidated Properties [Member] | Columbus [Member]
Schedule of Equity Method Investments [Line Items]
Total joint venture debt, net	84,700	84,400
Unconsolidated Properties [Member] | Charlotte [Member]
Schedule of Equity Method Investments [Line Items]
Total joint venture debt, net	99,500	89,800
Unconsolidated Properties [Member] | Galveston/Houston [Member]
Schedule of Equity Method Investments [Line Items]
Total joint venture debt, net	79,500	79,400
Mortgages [Member] | Unconsolidated Properties [Member]
Schedule of Equity Method Investments [Line Items]
Net discount and debt origination costs	$ 1,400	$ 1,400