Fair Value Disclosures (Measurements) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities, unrealized gain		$ 100		$ (200)
Impairment of real estate	$ 5,746		$ 8,170	16,306	$ 17,468
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Terminal capitalization rates	8.00%
Minimum | Discount Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input	8.00%
Northmall Centre
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate investment, fair value	$ 13,900			13,900
Spring Mall
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate investment, fair value	4,900			4,900
Brice Park
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate investment, fair value						$ 9,700
Mohawk Acres Plaza
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate investment, fair value						9,100
Lincoln Plaza
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate investment, fair value						3,400
Lakes Crossing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate investment, fair value						1,300
Fair Value, Measurements, Recurring | Marketable Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	19,987			19,987		18,054
Fair Value, Measurements, Recurring | Interest Rate Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset						3,795
Derivative liability	(31,439)			(31,439)		(13,449)
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Marketable Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	891			891		1,459
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest Rate Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset						0
Derivative liability	0			0		0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Marketable Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	19,096			19,096		16,595
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Interest Rate Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset						3,795
Derivative liability	31,439			31,439		(13,449)
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Marketable Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0			0		0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Interest Rate Derivatives
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset						0
Derivative liability	0			0		0
Fair Value, Measurements, Nonrecurring | Properties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	18,842			18,842		23,533
Impairment of real estate				10,305		7,983
Fair Value, Measurements, Nonrecurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Properties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0			0		0
Fair Value, Measurements, Nonrecurring | Significant Other Observable Inputs (Level 2) | Properties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0			0		0
Fair Value, Measurements, Nonrecurring | Significant Unobservable Inputs (Level 3) | Properties
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	$ 18,842			$ 18,842		$ 23,533