Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	$ 3,445,400		$ 3,435,161
Unsecured credit facilities	969,457		484,383
Trading Securities, Change in Unrealized Holding Gain (Loss)	(5,400)	$ 2,200
Impairment of Real Estate	(784)	$ (1,672)
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	34,240		39,599
Available-for-sale Securities	11,061		10,755
Total	45,301		53,341
Interest Rate Cash Flow Hedge Liability at Fair Value	(10,420)		(1,515)
Interest Rate Cash Flow Hedge Liability 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			71,131
Impairment of Real Estate			(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	3,611,169		3,688,604
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	11,061		10,755
Total	11,061		13,742
Interest Rate Cash Flow Hedge Liability at Fair Value	(10,420)		(1,515)
Interest Rate Cash Flow Hedge Liability 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			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	960,796		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	34,240		39,599
Available-for-sale Securities	0		0
Total	34,240		39,599
Interest Rate Cash Flow Hedge Liability at Fair Value	0		0
Interest Rate Cash Flow Hedge Liability at Fair Value			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
Available-for-sale Securities	0		0
Total	0		0
Interest Rate Cash Flow Hedge Liability at Fair Value	$ 0		0
Interest Rate Cash Flow Hedge Liability 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