Fair Value Measurements (Schedule of Impairment of Real Estate Properties) (Details) $ in Thousands	1 Months Ended	3 Months Ended				12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($) mall	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on impairment		$ 18,061			$ 3,263
Number of malls with impairment | mall		1
Fair Value	$ 17,640	$ 17,640				$ 81,350
Janesville Mall
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net sales price	17,640	17,640
Janesville Mall | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on impairment	18,061
Fair Value	$ 17,640	$ 17,640
Percentage of total consolidated revenues		0.70%
Acadiana Mall | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value						$ 67,300
Percentage of total consolidated revenues						1.90%
Equity method investments, fair value disclosure				$ 67,300
Fair value assumptions, expected term				10 years
Fair value inputs, cap rate				15.50%
Fair value inputs, discount rate				15.75%
Hickory Point | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value			$ 14,050			$ 14,050
Percentage of total consolidated revenues						0.50%
Fair value assumptions, expected term			10 years
Fair value inputs, cap rate			18.00%
Fair value inputs, discount rate			19.00%
Outlet Mall and Outparcel Sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on impairment		$ 18,061