Fair Value Measurements (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 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]
Notes payable	$ 3,723,923,000		$ 3,723,923,000		$ 3,435,161,000
Unsecured credit facilities	264,605,000		264,605,000		484,383,000
Trading Securities, Change in Unrealized Holding Gain (Loss)	1,300,000	$ (15,000)	251,000	$ 2,700,000
Impairment of Real Estate		$ 14,000	(1,014,000)	$ (12,099,000)
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	40,849,000		40,849,000		39,599,000
Available-for-sale Securities	11,146,000		11,146,000		10,755,000
Interest Rate Cash Flow Hedge Liability at Fair Value					2,987,000
Total	51,995,000		51,995,000		53,341,000
Interest Rate Cash Flow Hedge Liability at Fair Value	(10,417,000)		(10,417,000)		(1,515,000)
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Operating properties					71,131,000
Impairment of Real Estate					(50,553,000)
Fair Value Inputs Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Payable, Fair Value	4,072,114,000		4,072,114,000		3,688,604,000
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	11,146,000		11,146,000		10,755,000
Interest Rate Cash Flow Hedge Liability at Fair Value					2,987,000
Total	11,146,000		11,146,000		13,742,000
Interest Rate Cash Flow Hedge Liability at Fair Value	(10,417,000)		(10,417,000)		(1,515,000)
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					28,131,000
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	264,075,000		264,075,000		489,496,000
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	40,849,000		40,849,000		39,599,000
Total	$ 40,849,000		$ 40,849,000		39,599,000
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					$ 43,000,000