Fair Value Measurements (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	$ 3,749,273,000		$ 3,749,273,000		$ 3,658,405,000
Unsecured credit facilities					264,679,000
Trading Securities, Change in Unrealized Holding Gain (Loss)	(1,500,000)	$ 1,300,000	217,000	$ 251,000
Impairment of Real Estate	20,000		(115,000)	$ (1,014,000)
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	47,353,000		47,353,000		44,986,000
Available-for-sale Securities	15,605,000		15,605,000		15,706,000
Total	62,958,000		62,958,000		60,692,000
Interest Rate Cash Flow Hedge Liability at Fair Value	(4,265,000)		(4,265,000)		(9,291,000)
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Operating properties					25,000,000
Impairment of Real Estate					(17,532,000)
Fair Value Inputs Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable, Fair Value	4,171,212,000		4,171,212,000		4,102,382,000
Fair Value Inputs Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	15,605,000		15,605,000		15,706,000
Total	15,605,000		15,605,000		15,706,000
Interest Rate Cash Flow Hedge Liability at Fair Value	(4,265,000)		(4,265,000)		(9,291,000)
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					25,000,000
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					265,226,000
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	47,353,000		47,353,000		44,986,000
Total	$ 47,353,000		$ 47,353,000		$ 44,986,000