Fair Value Measurements (Narrative) (Details)	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($) property	Sep. 30, 2021 USD ($) property	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of fixed rate mortgage loans payable	$ 162,500,000	$ 162,500,000		$ 47,000,000.0
Carrying value of fixed rate mortgage payable	$ 157,000,000.0	$ 157,000,000.0		45,400,000
Number of properties | property	53	53
Real estate held for sale	$ 1,876,000	$ 1,876,000		$ 9,498,000
Impairment charges	82,736,000	80,887,000	$ 7,607,000
Aggregate fair value of impairment charges	$ 247,400,000	247,400,000
Dual-track Strategic Alternatives Process [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment charges		$ 82,300,000
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average		6.90%
Level 3 [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capitalization rate		6.00%
Level 3 [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capitalization rate		8.80%
Nonrecurring Basis [Member] | Level 3 [Member] | Assets [Member] | Retail Properties [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of properties | property	1	1
Real estate held for sale	$ 1,900,000	$ 1,900,000
Impairment charges		$ 400,000
Nonrecurring Basis [Member] | Level 3 [Member] | Assets [Member] | Retail Properties [Member] | Direct Approach Valuation Technique | Measurement Input, Cap Rate | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capitalization rate		9.10%