Fair Value Measurements - Impairment of Real Estate Properties (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment		$ 135,688	$ 14,600	$ 202,121	$ 84,644
Fair Value		160,740	42,100	160,740	42,100
Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment					84,644
Fair Value			42,100		42,100
Greenbrier Mall | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment				22,770
Fair Value		$ 56,300		$ 56,300
Greenbrier Mall | Malls | Expected Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Holding period				10 years
Greenbrier Mall | Malls | Cap Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.110		0.110
Greenbrier Mall | Malls | Discount Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.115		0.115
Honey Creek Mall | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment	$ (239)			$ 2,045
Fair Value		$ 0		0			$ 14,360
The Forum at Grandview | All Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment				8,582
Fair Value		0		0		$ 31,559
EastGate Mall | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment				33,265
Fair Value		$ 25,100		$ 25,100
EastGate Mall | Malls | Expected Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Holding period				10 years
EastGate Mall | Malls | Cap Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.145		0.145
EastGate Mall | Malls | Discount Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.150		0.150
Mid Rivers Mall | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment				$ 83,621
Fair Value		$ 53,340		$ 53,340
Mid Rivers Mall | Malls | Expected Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Holding period				10 years
Mid Rivers Mall | Malls | Cap Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.125		0.125
Mid Rivers Mall | Malls | Discount Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.1325		0.1325
Laurel Park Place | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment				$ 52,067
Fair Value		$ 26,000		$ 26,000
Laurel Park Place | Malls | Expected Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Holding period				10 years
Laurel Park Place | Malls | Cap Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.135		0.135
Laurel Park Place | Malls | Discount Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.140		0.140
Prior period adjustments | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment				$ (229)
Fair Value		$ 0		$ 0
Janesville Mall | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment					18,061
Fair Value			0		0
Fair value of equity method investment			17,640		17,640
Cary Towne Center | Malls
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment					51,985
Fair Value			34,000		34,000
Cary Towne Center | Malls | Expected Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Holding period				10 years
Cary Towne Center | Malls | Cap Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.120		0.120
Cary Towne Center | Malls | Discount Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement input (as a percent)		0.13		0.13
Vacant land | All Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Impairment					14,598
Fair Value			$ 8,100		$ 8,100