Fair Value Measurements (Details) - USD ($)	3 Months Ended			6 Months Ended			12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	$ 4,028,767,000			$ 4,028,767,000	$ 3,435,161,000		$ 3,435,161,000
Unsecured credit facilities	264,531,000			264,531,000	484,383,000		484,383,000
Trading Securities, Change in Unrealized Holding Gain (Loss)	(4,300,000)		$ 525,000	1,100,000	2,700,000
Impairment of Real Estate	(230,000)	$ (10,441,000)		(1,014,000)		$ (12,113,000)
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	38,677,000			38,677,000	39,599,000		39,599,000
Available-for-sale Securities	11,221,000			11,221,000	10,755,000		10,755,000
Interest Rate Cash Flow Hedge Liability at Fair Value	0			0	2,987,000		2,987,000
Total	49,898,000			49,898,000	53,341,000		53,341,000
Interest Rate Cash Flow Hedge Liability at Fair Value	(11,172,000)			(11,172,000)	(1,515,000)		(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		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	4,321,036,000			4,321,036,000	3,688,604,000		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]
Debt Securities, Trading, and Equity Securities, FV-NI	0			0	0		0
Available-for-sale Securities	11,221,000			11,221,000	10,755,000		10,755,000
Interest Rate Cash Flow Hedge Liability at Fair Value	0			0	2,987,000		2,987,000
Total	11,221,000			11,221,000	13,742,000		13,742,000
Interest Rate Cash Flow Hedge Liability at Fair Value	(11,172,000)			(11,172,000)	(1,515,000)		(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		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	263,891,000			263,891,000	489,496,000		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	38,677,000			38,677,000	39,599,000		39,599,000
Available-for-sale Securities	0			0	0		0
Interest Rate Cash Flow Hedge Liability at Fair Value	0			0	0		0
Total	38,677,000			38,677,000	39,599,000		39,599,000
Interest Rate Cash Flow Hedge Liability at Fair Value	0			0	0		0
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	0		0
Available-for-sale Securities	0			0	0		0
Interest Rate Cash Flow Hedge Liability at Fair Value	0			0	0		0
Total	0			0	0		0
Interest Rate Cash Flow Hedge Liability at Fair Value	$ 0			$ 0	0		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					$ 43,000,000		$ 43,000,000