Fair Value Measurements (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	$ 3,658,405	$ 3,435,161
Unsecured credit facilities	264,679	484,383
Trading Securities, Change in Unrealized Holding Gain (Loss)	(3,000)	(3,800)	$ 3,300
Impairment of Real Estate	18,536	54,174	$ 38,437
Impairment of Real Estate	17,500	40,300
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Trading, and Equity Securities, FV-NI	44,986	39,599
Available-for-sale Securities	15,706	10,755
Total	60,692	53,341
Interest Rate Cash Flow Hedge Liability at Fair Value	(9,291)	(1,515)
Interest Rate Derivative Assets, at Fair Value		2,987
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Operating properties	25,000	71,131
Impairment of Real Estate	17,532	50,553
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Payable, Fair Value	4,102,382	3,688,604
Impairment of Real Estate		10,200
Fair Value, Inputs, Level 2 [Member] | Properties Held For Sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of Real Estate		28,100
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Trading, and Equity Securities, FV-NI	0	0
Available-for-sale Securities	15,706	10,755
Total	15,706	13,742
Interest Rate Cash Flow Hedge Liability at Fair Value	(9,291)	(1,515)
Interest Rate Derivative Assets, at Fair Value		2,987
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Operating properties	25,000	28,131
Fair Value, Inputs, Level 2 [Member] | Unsecured Credit Facilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unsecured credit facilities, Fair Value	265,226	489,496
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Trading, and Equity Securities, FV-NI	44,986	39,599
Available-for-sale Securities	0	0
Total	44,986	39,599
Interest Rate Cash Flow Hedge Liability at Fair Value	$ 0	0
Interest Rate Derivative Assets, at Fair Value		0
Fair Value, Inputs, Level 3 [Member] | Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Alternative Investment Rate	0.0858
Fair Value, Inputs, Level 3 [Member] | Terminal Capitalization Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Alternative Investment Rate	0.0475
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Trading, and Equity Securities, FV-NI	$ 0	0
Available-for-sale Securities	0	0
Total	0	0
Interest Rate Cash Flow Hedge Liability at Fair Value	$ 0	0
Interest Rate Derivative Assets, at Fair Value		0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Operating properties		$ 43,000