Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	$ 2,994,495		$ 2,994,495		$ 3,006,478
Unsecured credit facilities	683,970		683,970		708,734
Trading Securities, Change in Unrealized Holding Gain (Loss)	525,000	$ 291,000	2,700	$ (93,000)
Impairment of Real Estate	(10,441)	$ (12,533)	(12,113)	$ (28,587)
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	37,093		37,093		33,354
Available-for-sale Securities	8,151		8,151		7,933
Interest Rate Derivative Assets, at Fair Value	4,367		4,367		17,482
Total	49,611		49,611		58,769
Interest Rate Cash Flow Hedge Liability at Fair Value	(1,619)		(1,619)		(5,491)
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Operating properties	41,976		41,976		42,760
Impairment of Real Estate			(10,447)		(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	3,175,311		3,175,311		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	8,151		8,151		7,933
Interest Rate Derivative Assets, at Fair Value	4,367		4,367		17,482
Total	12,518		12,518		25,415
Interest Rate Cash Flow Hedge Liability at Fair Value	(1,619)		(1,619)		(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	41,976		41,976		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	685,705		685,705		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	37,093		37,093		33,354
Available-for-sale Securities	0		0		0
Interest Rate Derivative Assets, at Fair Value	0		0		0
Total	37,093		37,093		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