Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	$ 3,437,959		$ 3,437,959		$ 3,006,478
Unsecured credit facilities	449,309		449,309		708,734
Trading Securities, Change in Unrealized Holding Gain (Loss)	(15,000)	$ 484,000	2,700	$ 400,000
Impairment of Real Estate	14	$ (855)	(12,099)	$ (29,443)
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	37,825		37,825		33,354
Available-for-sale Securities	10,317		10,317		7,933
Interest Rate Derivative Assets, at Fair Value	2,642		2,642		17,482
Total	50,784		50,784		58,769
Interest Rate Cash Flow Hedge Liability at Fair Value	(2,494)		(2,494)		(5,491)
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Operating properties	28,131		28,131		42,760
Impairment of Real Estate			(10,238)		(6,579)
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Payable, Fair Value	3,710,860		3,710,860		2,961,769
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
Available-for-sale Securities	10,317		10,317		7,933
Interest Rate Derivative Assets, at Fair Value	2,642		2,642		17,482
Total	12,959		12,959		25,415
Interest Rate Cash Flow Hedge Liability at Fair Value	(2,494)		(2,494)		(5,491)
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		28,131		42,760
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	450,521		450,521		710,902
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	37,825		37,825		33,354
Available-for-sale Securities	0		0		0
Interest Rate Derivative Assets, at Fair Value	0		0		0
Total	37,825		37,825		33,354
Interest Rate Cash Flow Hedge Liability at Fair Value	0		0		0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Operating properties	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
Available-for-sale Securities	0		0		0
Interest Rate Derivative Assets, at Fair Value	0		0		0
Total	0		0		0
Interest Rate Cash Flow Hedge Liability at Fair Value	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	$ 0		$ 0		$ 0